Printed by H. Howard, RA.         Engraved by W. T. Fry.
William Kirby. M.A. F.R. L.S. &c

Published by Longman & C^o. London, July, 1825.

AN

INTRODUCTION

TO

ENTOMOLOGY:

OR

ELEMENTS

OF THE

NATURAL HISTORY OF INSECTS:

WITH PLATES.

By WILLIAM KIRBY, M.A. F.R. and L.S.

RECTOR OF BARHAM,
AND

WILLIAM SPENCE, Esq. F.L.S.

VOL. III.

LONDON:

PRINTED FOR

LONGMAN, REES, ORME, BROWN, AND GREEN,

PATERNOSTER ROW.

1826.

PRINTED BY RICHARD TAYLOR,
SHOE LANE, LONDON.

ADVERTISEMENT.

The publication of the concluding volumes of the "Introduction to Entomology" has been unavoidably delayed by the continued ill health of one of the Authors, which has devolved upon the other a considerable increase of labour, and demanded a greater expenditure of time than would otherwise have been required: for though Mr. Spence put every facility in Mr. Kirby's power, and had drawn up a rough copy of every Letter belonging to his department; yet, as most of them had been written several years ago, many curious facts, and a great variety of interesting information subsequently derived from various sources, were necessarily to be inserted, and the whole to be prepared for the press.

When the thousands of objects that were to be examined, and many of them repeatedly, in composing the Letters on the External Anatomy of Insects, are considered, it will not appear surprising if some errors should have crept in; especially as Mr. Kirby was deprived of the effectual help formerly derived from the acumen, learning, and judgement of his esteemed coadjutor, by his lamented and protracted indisposition: but it is hoped that these errors will be found of minor importance, and not to affect any general principles advanced. The same remarks are also in part applicable to the Anatomical and Orismological Tables (Vol. III. p. 354-393, and Vol. IV. p. 257-354), which were drawn up by the Authors jointly many years ago, before any other portion of the work was composed, but which have, especially the former, required considerable alterations and additions in consequence of subsequent observations and information.

It will not be amiss here to state, in order to obviate any charge of inconsistency in the possible event of Mr. Kirby's adverting in any other work to this subject, that though on every material point the authors have agreed in opinion, their views of the theory of instinct do not precisely accord. That given in the second and fourth volumes is from the pen of Mr. Spence.

It was originally intended, as mentioned in the Preface, to have given a complete list of Entomological works, of which a large portion was drawn up; but the great length to which more important matters have extended, has rendered necessary the intire omission of this list,—an omission in some degree compensated by the catalogue of Authors quoted, which comprises most of the standard Entomological works.

ERRATA.

DIRECTIONS TO THE BINDER.

The Synoptical Table of the Nomenclature of the Parts of the External Crust of Insects should be placed opposite to page 354.

Plates VI-XX. should be placed in this Volume, and the remainder in the Fourth.

It is however suggested to Purchasers, that in binding complete Sets of the Work, a separate Volume may be formed of the Synoptical Table, the Plates and their Explanations, and the Indexes.

[CONTENTS OF VOL. III.]

[NOTICE RESPECTING VOL. I. and II.]

It being judged expedient, since the publication of the last Edition of the first and second Volumes of this Work, to adopt a new plan with respect to the reference letters of the Plates, the Reader is requested to make the following corrections in those Volumes.

AN INTRODUCTION TO ENTOMOLOGY.

[LETTER XXVIII.]

DEFINITION OF THE TERM INSECT.

What is an insect? This may seem a strange question after such copious details as have been given in my former Letters of their history and economy, in which it appears to have been taken for granted that you can answer this question. Yet in the scientific road which you are now about to enter, to be able to define these creatures technically is an important first step which calls for attention. You know already that a butterfly is an insect—that a fly, a beetle, a grasshopper, a bug, a bee, a louse, and flea, are insects—that a spider also and centipede go under that name; and this knowledge, which every child likewise possesses, was sufficient for comprehending the subjects upon which I have hitherto written. But now that we are about to take a nearer view of them—to investigate their anatomical and physiological characters more closely—these vague and popular ideas are insufficient. In common language, not only the tribes above mentioned, but most small animals—as worms, slugs, leeches, and many similar creatures, are known by the name of insects. Such latitude, however, cannot be admitted in a scientific view of the subject, in which the class of insects is distinguished from these animals just as strictly as beasts from birds, and birds from reptiles and amphibia, and these again from fishes. Not, indeed, that the just limits of the class have always been clearly understood and marked out. Even when our correspondence first commenced, animals were regarded as belonging to it, which since their internal organization has been more fully explained, are properly separated from it. But it is now agreed on all hands, that an earthworm, a leech, or a slug, is not an insect; and a Naturalist seems almost as much inclined to smile at those who confound them, as Captain Cook at the islanders who confessed their entire ignorance of the nature of cows and horses, but gave him to understand that they knew his sheep and goats to be birds.

You will better comprehend the subsequent definition of the term Insect, after attending to a slight sketch of the chief classifications of the animal kingdom, more especially of the creatures in question, that have been proposed. That of Aristotle stands first. He divides animals into two grand sections, corresponding with the Vertebrata and Invertebrata of modern Zoologists: those, namely, that have blood, and those that have it not[1]:—by this it appears that he only regarded red blood as real blood; and probably did not suspect that there was a true circulation in his Mollusca and other white-blooded animals. His Enaima, or animals that have blood, he divides into Quadrupeds, Birds, Fishes, Cetacea, and Apods or reptiles; though he includes the latter, where they have four legs, amongst the quadrupeds[2]; and his Anaima, or animals without blood, into Malachia, Malacostraca, Ostracoderma, and Entoma. The first of these, the Malachia, he defines as animals that are externally fleshy and internally solid, like the Enaima; and he gives the Sepia as the type of this class, which answers to the Cephalopoda of the moderns. The next, the Malacostraca, synonymous with the Crustacea of Cuvier and Lamarck, are those, he says, which have their solid part without and the fleshy within, and whose shell will not break, but splits, upon collision[3]. The Ostracoderma, corresponding with the Testacea of Linné, he also defines as having their fleshy substance within, and the solid without; but whose shell, as to its fracture, reverses the character of the Malacostraca. He defines his last class Entoma, in Latin Insecta, with which we are principally concerned, as animals whose body is distinguished by incisures, either on its upper or under side, or on both, and has no solid or fleshy substance separate, but something intermediate, their body being equally hard both within and without[4]. This definition would include the Annelida and most other Vermes of Linné, except the Testacea, which accordingly were considered as insects by those Zoologists that intervened between Aristotle and the latter author. The Stagyrite, however, in another place, has expressly excluded all apods[5]. From other passages in his works, it appears that he regarded the Vermes, &c. either as larvæ, or as produced spontaneously and not ex ovo[6].

This definition of an insect, though partly founded on misconception, as well as his primary division of animals in general, is by no means contemptible. If you look at a bee or a fly, you will observe at first sight that its body is insected, being divided as it were into three principal pieces—head, trunk, and abdomen[7]; and if you examine it more narrowly, you will find that the two last of these parts, especially the abdomen, are further subdivided. And this character of insection, or division into segments, more or less present in almost every insect[8], is not to be found (with the exception of the Crustacea, which Aristotle distinguishes by the nature of their integument and its contents) in any of the other classes into which he divided animals without blood. It was on account of this most obvious of their characters, that these little creatures were in Greek named Entoma, and in Latin Insecta; and from the former word, as you know, our favourite science takes the name of Entomology.

Pliny adhering to the definition of Aristotle, as far as it relates to the insection of the animals we are speaking of, expressly includes Apods, as well as Aptera, amongst them[9]; and in this was followed, without any attempt at improvement, by all the entomological writers that intervened between him and the great Aristotle of the moderns, Linné.

This illustrious naturalist, aware of the incorrectness of the primary divisions of the animal kingdom founded upon the presence or absence of blood, establishes his system upon the structure of the heart, and upon the temperature and colour of the circulating fluid. He divided animals into two great sections or sub-kingdoms, each comprising two classes. His first section included those having a heart with two ventricles, two auricles, and warm and red blood, viz. the Mammalia or beasts, and the Aves or birds. His second, those having a heart with one ventricle, one auricle, and cold and red blood, namely, the classes Amphibia, which included reptiles, serpents, &c. and Pisces or fish. His third, those having a heart with one ventricle and no auricle, and cold white sanies in the place of blood, namely, his classes Insecta et Vermes, including the Invertebrate animals of Lamarck. Thus the first of Aristotle's great divisions he increased by the addition of a new and very distinct class, the Amphibia, by which some ground was gained in the science; but as much was lost by his compressing the four classes of which the last consisted into two, by which the natural classes of Cephalopoda and Crustacea merged under Insecta and Vermes. Linné was not aware of the extraordinary fact, that the Cephalopoda have three hearts; and that though the Crustacea and Arachnida have a circulation, Insects have none, or he would never have taken this retrograde step.

Indeed Linné's definition of an Insect is, in many most material points, inapplicable, not only to the Crustacea, but to many other animals included under that denomination. This will appear evident from a very slight examination. Thus it runs: "Polypod animalcula, breathing by lateral spiracles, armed every where with an osseous skin, whose head is furnished with movable sensitive antennæ[10]." Now of this definition only the first member can be applied to the whole class which it is meant to designate; for the entire genus Cancer L., which, with some others, forms the class Crustacea of the moderns, does not respire by spiracles at all, but by gills; and the same in some degree may be said of spiders, scorpions, &c. With the last member of the definition Linné himself must have been aware that a large number of what he conceived to be insects were at variance, as mites, spiders, and many other of his apterous tribes: though from some very recent observations of M. Latreille[11], there seems some ground for thinking, that in these the antennæ are represented by the mandibles, palpi, &c.[12], and to the soft flexible, coriaceous or membranous skin of a vast number of insects, the term cutis ossea is by no means applicable.

Evident as these incongruities are, when the Herculean task which Linné imposed upon himself, and the vastness and variety of his labours, are considered, they become very venial. Indeed, unless he had divided his class Insecta into two or more, it was impossible to define it intelligibly to ordinary readers, otherwise than nearly in the terms which he actually employed; and these characters, restricted and amended by qualifying clauses, are still those to which recurrence must be had in a popular definition of the class, when separated as it ought to be from the Crustacea and Arachnida.

Pennant, Brisson, and other zoologists, who, attending to nature rather than system, saw the impropriety of uniting a crab or a lobster in the same class with a bee or a beetle, long since assigned the Crustacea their ancient distinct rank. "But these changes," as Latreille observes[13], "being only founded upon external characters, might be deemed arbitrary; and to fix our opinion, it was necessary to have recourse to a decisive authority—the internal and comparative organization of these animals." It results from the observations of the most profound comparative anatomist of our age, M. Cuvier, that the Crustacea and Arachnida differ from insects properly so called, and particularly from those that are furnished with wings, in having a complete system of circulation, a different mode of respiration, and that they have a more perfect organization. Influenced by these motives, both Cuvier and Lamarck have considered them as forming two classes separate from insects. Treviranus, led by considerations founded on the organs of circulation, of respiration, and of generation, is of opinion that spiders and scorpions ought to form one class with the Crustacea: he observes, however, that the nervous system of all three is very dissimilar; and that in an arrangement founded on this circumstance, the organs of motion, and the external shape, even spiders and scorpions must be placed in different classes[14].

It is to be observed with regard to the Arachnida of the French school, that the class as laid down by them includes several animals that have no circulation, and breathe by tracheæ, of which description are the mites (Acarus L.), and the harvest-men (Phalangium L.) &c.; and therefore it has been divided into two orders, Pulmonaria and Tracheana; but if the definition from the internal organization be adhered to, the latter should either remain with the class Insecta, or form a new one by themselves. Yet the animals that compose the Trachean order of Arachnida, their external form considered, are certainly much more nearly related to the spiders and scorpions than to any members of the class Insecta at present known. This circumstance, perhaps, may seem to throw some doubt upon the modern system of classification.

I must further observe, that the assertion of Treviranus, which appears to intimate that the respiration of the pulmonary Arachnida is the same with that of the Crustacea, is not quite correct, since in the latter the branchiæ or gills are external, and in the former internal, the air entering by spiracles before it acts upon them[15].

It may not be amiss in this place to lay before you the principal points in which the Crustacea and Arachnida agree with Insecta, and also those in which they differ.

The Crustacea agree with Insecta in having a body divided into segments, furnished with jointed legs, compound eyes, and antennæ. Their nervous system also is not materially different, and they are both oviparous. They differ from them in having the greater insections of the body less strongly marked; in the greater number of legs on the trunk, the anterior ones performing the office of maxillæ: in their eyes usually on a moveable footstalk: their palpigerous mandibles; and their four antennæ at least in the great majority. But the principal difference consists in the internal organization and the fountains of vitality; for the Crustacea have a double circulation, the fountain of which is a heart in the middle of their thorax[16]. They have too a kind of gizzard and liver, at least the Decapods[17], and their respiration is by gills. Genuine insects terminate their existence after they have laid their eggs[18]; but the Crustacea live longer, and lay more than once.

The Arachnida will be found to differ from insects more widely than even the Crustacea. They agree in their jointed legs and palpi; immoveable eyes; and in being covered with a coriaceous or corneous integument: but they differ in having a system of circulation; gills instead of tracheæ; their organs of generation double; and the females lay more than once in their lives. Their head also is not distinct from the trunk as in insects; they have no compound eyes; and their antennæ, if we admit the opinion on this head of MM. Latreille and Treviranus, that they have representatives of these organs, differ totally in structure, situation, and use, from those of the great body of insects. In the Araneidæ or Spiders, their body seems to have no segments or incisure but that which separates the abdomen from the trunk; and in the Scorpionidæ they are observable only in the abdomen. Other particulars might be enumerated in which these two classes differ from insects; but these will be sufficient to convince you that Aristotle and MM. Cuvier and Lamarck were justified in separating them.

The two last-mentioned authors made further improvements in Zoology. The latter, from the consideration of the general structure of animals, perceiving that Aristotle's Enaima were distinguished from his Anaima, by being built as it were upon a vertebral column, very judiciously changed the denomination, which was indeed improper, of "The Philosopher's" two sub-kingdoms, into that of Vertebrata or animals that have a vertebral column, and Invertebrata or those that have no vertebral column. These he distributes into three primary divisions according to their supposed degrees of intelligence—Thus:

Profiting by the light afforded by the Aristotelian system, this eminent zoologist improved, we see, upon that of Linné, by resolving his Insecta into three classes, and his Vermes into seven, interposing the Linnean Insecta between the four first and three last, in which he was not so happy, since as to sense insects should certainly occupy the place he has here assigned to the Mollusca.

In the work from which I have taken this statement of Lamarck's system, that acute writer has given a sketch of another method of arrangement, in which he has made the first deviation from the beaten track of an unbroken and unbranching series. In the Supplement to the first volume, he has distributed the Invertebrata in a double subramose series—one consisting of articulate, and the other of inarticulate animals[20].

Upon Lamarck's system, most of the modern ones, with some variation, are founded. There is one, however, by a learned countryman of ours, that is more unique, sui generis, and I may add profound, than any that has yet appeared. I am speaking of that, you will perceive, of which our friend Mr. Wm. MacLeay has given a detailed statement in his Horæ Entomologicæ. In this he goes even far beyond what Lamarck has attempted in the above sketch, and substantiates his claim to be considered as one of those original thinkers, rari nantes in gurgite vasto, that do not appear every day. The following are the principal bases of his system.

1. That all natural groups, whether kingdoms or any subdivision of them, return into themselves; a distribution which he expresses by circles.

2. That each of these circles is formed precisely of five groups, each of which is resolvable into five other smaller groups, and so on till you reach the extreme term of such division.

3. That proximate circles or larger groups are connected by the intervention of lesser groups, which he denominates osculant.

4. That there are relations of analogy between the corresponding points of contiguous circles.

This system he has represented by tables of circles inscribed with the five primary divisions of each group. His first table exhibits a general view of organized matter as distributed in the animal and vegetable kingdoms—Thus:

Our learned author here divides the animal kingdom into what may be denominated five sub-kingdoms or provinces, in three of which (with the exception of the Crustacea and Arachnida belonging to his Annulosa) no circulation of blood is visible, but which obtains in the rest. These he names—

1. Acrita, consisting of the Infusory Animals, the Polypi, the Corallines, the Tæniæ, and the least organized of the Intestinal Worms.

2. Radiata, including the Jelly-fish, Star-fish, Echini, and some others.

3. Annulosa, consisting of Insecta, Arachnida, and Crustacea.

4. Vertebrata, consisting of Beasts, Birds, Reptiles, Amphibia, and Fishes.

5. Mollusca, including the numerous tribes of shell-fish, land-shells, slugs, &c., which, from their mucous or gelatinous substance, from their nervous system and the imperfection of their senses, return again to the Acrita, though connected with the Vertebrata by having a heart and circulation.

His next set of circles shows the sub-division of these five sub-kingdoms into classes—Thus:

In this scheme the osculant classes are those placed between the circles. In the Mollusca circle two classes are still wanting to complete the quinary arrangement of that sub-kingdom. I am not sufficiently conversant with the details of the animal kingdom at large to hazard any decided opinion upon Mr. MacLeay's whole system, or to ascertain whether all these classes are sufficiently distinct[21]. My sentiments with regard to those of the Annulosa I shall state to you hereafter.

Upon a future occasion I shall consider more at large the station to which insects seem entitled in a system of invertebrate animals, which will not accord exactly with that assigned by MM. Cuvier and Lamarck. But I am now in a field in which I have no intention to expatiate further, than as it is connected with the subject of the present letter. I shall therefore confine myself in what I have more to say to the definitions of Insecta that have been given by modern authors, beginning with that of the zoologist last mentioned. Insects form a part of his second group, which he terms sensitive animals (animaux sensibles), which group he thus defines: "They are sentient, but obtain from their sensations only perceptions of objects—a kind of simple ideas which they cannot combine to obtain complex ones. Charact. No vertebral column; a brain, and most commonly an elongated medullary mass; some distinct senses; the organs of movement attached under the skin: form symmetrical, by parts, in pairs[22]." This division of animals, from the kind and degree of sense and intelligence that they possess, seems rather fanciful than founded in nature, since many insects show a greater portion of them than many vertebrate animals. Compare in this respect a bee with a tortoise[23]. Lamarck divides his group of animaux sensibles into two sections, namely, Articulated animals, exhibiting segments or articulations in all or some of their parts; and Inarticulatcd animals, exhibiting neither segments nor articulations in any of their parts. Insecta, Arachnida, and Crustacea, belong to the first of these sections, which he defines as "those whose body is divided into segments, and which are furnished with jointed legs bent at the articulations[24]." Insecta he defines—"Articulate animals, undergoing various metamorphoses, or acquiring new kinds of parts—having, in their perfect state, six feet, two antennæ, two compound eyes, and a corneous skin. The majority acquiring wings. Respiration by spiracles (stigmates), and two vascular opposite chords, divided by plexus, and constituting aeriferous tracheæ, which extend every where. A small brain at the anterior extremity of a longitudinal knotty marrow, with nerves. No system of circulation, no conglomerate glands. Generation oviparous: two distinct sexes. A single sexual union in the whole course of life[25]." Arachnida he defines—"Oviparous animals, having at all times jointed legs, undergoing no metamorphosis, and never acquiring new kinds of parts. Respiration tracheal or branchial: the openings for the entrance of the air spiraculiform (stigmatiformes). A heart and circulation beginning in many. The majority couple often in the course of life[26]." I shall next add his definition of Crustacea: "Oviparous, articulated, apterous animals, with a crustaceous integument more or less solid, having jointed legs; eyes either pedunculate or sessile, and most commonly four antennæ, with a maxilliferous mouth seldom rostriform; maxillæ in many pairs placed one over the other; scarcely any under-lip; no spiraculiform openings for respiration; five or seven pair of legs; a longitudinal knotty marrow terminated anteriorly by a small brain. A heart and vessels for circulation. Respiration branchial with external branchiæ, sometimes hid under the sides of the shell of the thorax, or shut in prominent parts; sometimes uncovered, and in general adhering to particular legs or to the tail. Each sex usually double[27]."

I have given Lamarck's definitions of these three classes, all considered as Insecta by Linné, that by comparing them together you may be better enabled to appreciate the system of this author. On looking over the characters of the Arachnida as here given, you will see at once that it consists of heterogeneous animals—for in fact he includes in this class not only the Trachean Arachnida of Latreille, but the Ametabolia of Dr. Leach, or the Hexapod Aptera, and the Myriapoda.

I shall next copy for you Latreille's latest definition of Insecta and Arachnida.

"Insecta: A single dorsal vessel representing the heart: two trunks of tracheæ running the whole length of the body, and opening externally by numerous spiracles; two antennæ; very often upper appendages for flight, indicating the metamorphosis to which the animal is subject when young; legs most commonly reduced to six. Arachnida: Distinguished from Crustacea by having their respiratory organs always internal, opening on the sides of the abdomen or thorax to receive the respirable fluid. Sometimes these organs perform the office of lungs, and then the circulation takes place by means of a dorsal vessel, which sends forth arterial, and receives venose branches. Sometimes they are tracheæ or air-vessels, which, as in the class Insecta, replace those of circulation. These have only the vestige of a heart, or a dorsal vessel alternately contracting and sending forth no branch. The absence of antennæ, the reunion of the head with the thorax, a simple trachea but ramified and almost radiating, serve to distinguish these last Arachnida, or the most imperfect of insects, which respire only by tracheæ[28]." Under this head he observes—"Of all these characters, the most easy to seize and the most certain would doubtless be, if there were no mistake in it, that of the absence of antennæ; but later and comparative researches, confirmed by analogy, have convinced me, that these organs, under particular modifications it is true, and which have misled the attention of naturalists, do exist[29]:" and he supposes, from the situation and direction of the mandibles of the Arachnida, corresponding with that of the intermediate pair of antennæ in Crustacea, that they really represent the latter organs. If this supposition be admitted, their use is wholly changed; the palpi, in fact, executing the functions of antennæ, which probably induced Treviranus to call them Fühlhörner (Feelinghorns). Perhaps these last may be regarded as in some sort representing the external antennæ of the Crustacea? With regard to Insecta, their antennæ seem to disappear in the Pupiparæ Latr., or the genus Hippobosca L.

The above definitions of the Arachnida by these two celebrated authors, appear to me the reverse of satisfactory. When we are told of animals included in it, that some breathe by gills and others by tracheæ, that some have a heart and circulation and others not, we are immediately struck by the incongruity, and are led to suspect that animals differing so widely in the fountains of life ought not to be associated in the same class. A learned zoologist of our own country, Dr. Leach, seems to have made a nearer approach to a classification in accordance with the internal organization, by excluding from Arachnida the Acari and Myriapoda.

Sub-kingdom Annulata Cuv.

Mr. MacLeay, on whose system I shall now say a few words, divides his sub-kingdom Annulosa into five classes, namely, Crustacea, Ametabola, Mandibulata, Haustellata, Arachnida. From the Crustacea he goes by the genus Porcellio Latr. to Iulus[31], which begins his Ametabola: these he connects with the Mandibulata, by Nirmus, which he thinks approaches some of the corticarious Coleoptera[32]. This class he appears to leave by the Trichoptera Kirby, and so enters his Haustellata by the Lepidoptera[33], and leaves it again by the Diptera by means of the Pupiparæ Latr., especially Nycteribia, connecting this class with the Arachnida, which he enters by the Hexapod Acari L.[34], and these last he appears to leave by the Araneidæ, and to enter the Crustacea by the Decapods[35]: thus making good his circle of classes, or a series of Annulose animals returning into itself. Mr. MacLeay's whole system upon paper appears very harmonious and consistent, and bears a most seducing aspect of verisimilitude; but it has not yet been so thoroughly weighed, discussed, and sifted, as to justify our adopting it in toto at present: should it, however, upon an impartial and thorough investigation, come forth from the furnace as gold, and be found to correspond with the actual state of things in nature, my objections, which rest only upon some parts of his arrangement of Annulosa, would soon vanish. Some of those objections I will state here, and some will come in better when I treat of the Systems of Entomology. My first objection is, that his Ametabola, Mandibulata, and Haustellata, approach much nearer to each other than they do to the other two classes of his circle, or than even these last to each other; so that under this view it should primarily consist of three greater groups, resolvable, it may be, into five smaller ones. My next objection is, that he has also considered the Trachean and Pulmonary Arachnida as forming one class. Whether an animal breathes by gills or tracheæ, or has a circulation or not, is surely as strong a reason for considering those so distinguished as belonging to different classes, as the taking of their food by suction or by manducation is, for separating others to the full as much or more nearly related as to their external structure. But of this more hereafter. I cannot help, as a last objection, lamenting that our learned author has rejected from his system a term consecrated from the most remote antiquity, and which, even admitting his arrangement, might have been substituted for Annulosa, a name borrowed by Scaliger from Albertus Magnus, neither of whom, in Entomology, is an authority to weigh against Aristotle, from whom we derive the term Insecta, in Greek Εντομα.

As Fabricius did not alter Linné's class Insecta, but merely broke up his orders into new ones, which he named classes, I shall give you a detail of the alterations he introduced into the science in a future letter.

Having stated what my predecessors have done in classification, I shall next proceed to lay before you my own sentiments as to—What is an insect. Since our correspondence commenced, the Arachnida, principally on account of their internal organization, have been excluded from bearing that name, carrying with them, as we have seen, several tribes, which as yet have not been discovered to differ materially in that respect from the present Insecta: for the sake, therefore, of convenience and consistency, that I may, as far as the case will admit, adhere to the Horatian maxim

—— Servetur ad imum
Qualis ab incepto processerit et sibi constet,

I shall regard as Insects all those Annulosa that respire by tracheæ[36] and have no circulation, considering the Trachean Arachnida and the Myriapoda for the present as sub-classes, the one bordering upon the Arachnida, and the other upon the Crustacea. Some of these I am ready to own seem separated by an interval sufficiently wide from the Hexapods, which may be regarded as more peculiarly entitled to the denomination of Insects. The most striking differences will be found in the coalition of the head with the trunk in some (Phalangidæ), and the disappearance of the annulose form of the body in others (Acarus L.), so that the legs only are jointed[37]. Yet an approach to such structure may be traced in some Hexapods; for instance, the coalition of the head and trunk in Melophagus, Latr., and that of the trunk and abdomen in Sminthurus, Latr.[38] The Myriapoda exhibit other remarkable differences; though their head and trunk are distinct, the former antenniferous, and their body annulose, the abdomen as well as the trunk is furnished with legs, sometimes amounting to hundreds; but even to this a tendency has been observed in some Hexapods[39]. If you examine a specimen of Machilis polypoda, an insect related to the common sugar-louse (Lepisma saccharina), you will find that the abdomen is furnished with a double series of elastic appendages, which, being instruments of motion, may be regarded as representing legs. It is worthy of notice, that the Myriapoda when first disclosed from the egg have never more than six legs[40], and keep acquiring additional pairs of them and additional segments to their abdomen as they change their skins: and it is equally remarkable, that many Hexapods are subject to a law in some degree the very reverse of this, having many abdominal legs in their first state, and losing them all in their last. The union of the head with the trunk in the Trachean Arachnida has been regarded as almost an unanswerable argument, in spite of their different internal organization, for including them in the same class with the Pulmonary Arachnida; but the case of Galeodes, which, though furnished with gills, (as an eminent Russian Entomologist Dr. G. Fischer is reported to have discovered,) implying also a circulation, and evidently belonging to the last-mentioned class, has nevertheless a distinct thorax consisting of more than one piece, to which are affixed only six legs[41], proves that even this circumstance possesses no weight when set against the organization. If it was a difference in this respect, that proved the Crustacea classically distinct from Insecta—that likewise was the principal reason for the separation also of the Arachnida—it seems to follow that it ought also to furnish an argument equally cogent for considering the Trachean Arachnida, as well as the Myriapoda, distinct from the Pulmonary.

Another difference between the tribes in question is that of their metamorphosis; and this appears to have had great weight with Lamarck, inducing him to include in his Arachnida, not only the Tracheans and Myriapods, but even the apterous Hexapods, except Pulex, or the Anoplura and Thysanura of modern authors. But the metamorphosis alone, unless supported by the internal organization, will I think scarcely be deemed a sufficient reason for separating from each other tribes agreeing in that respect, and placing them with others with which they disagree. The metamorphosis in some of the Hexapods (Lepidoptera) consists in the loss of legs, the acquisition of wings, a great change in the oral organs and in the general form; in others (some Coleoptera), in the acquisition only of wings and a change of shape, the oral organs remaining much the same; in others again (Curculio L.), in the acquisition of six legs and wings and a change of form; in the flea, in the acquisition of six legs and a change of form only; in the Orthoptera, Hemiptera, &c. in the mere acquisition of wings; in the Libellulidæ, in the loss of the mask that covers the mouth and the acquisition of wings; in the Diptera, in the acquisition of six legs, wings, a change of the oral organs and of the form; in some of the Octopods (Acarus L.), in the acquisition of a pair of legs; and in others (Phalangium and Aranea L.), solely in a modification of them as to their proportions; in the Myriapods, the alteration that takes place in this respect is considerable; a large number of pairs of legs is acquired and many additional abdominal segments, and the proportion which the abdomen bears to the whole insect is quite altered. In all these cases there is a change more or less, either partial or general, of the original shape or organs of the animal; and with regard to their metamorphosis, there is a greater difference between a young and adult Iulus than between a young and adult grasshopper or bug: so that if the metamorphosis, per se, be assumed as a principal regulator of the class, the grasshopper or bug have as little claim to belong to it as the Iulus.

M. Lamarck lays considerable stress upon another character—That Insecta engender only once in the course of their lives, and Arachnida more than once. But this, if examined, will be found to be confined chiefly to the Pulmonary Arachnida, the Tracheans following the law of Insecta in this respect[42].

You may perhaps object that the bringing of the Trachean Arachnida and the Myriapoda into the class Insecta will render the approximation of them to a natural arrangement more difficult, since it will be impossible at the same time to connect the Myriapods with the Crustacea, and the Trachean with the genuine Arachnida. I admit the validity of your objection, but by no arrangement of insects in a simple series can we attain this object: the difficulty, however, may perhaps be obviated in this way. The distribution of organized matter, to adopt Mr. Wm. MacLeay's metaphor[43], begins in a dichotomy, constituting the animal and vegetable branches of the great tree of nature, and from these two great branches, by means of infinite ramifications, the whole system is formed, and, what is remarkable, these branches unite again so as to represent a series returning into itself, a discovery due to the patient investigation and acumen of our learned friend just mentioned. Now, in considering the Aptera order, we find at first setting out from the Hexapods, a dichotomy, where the Anoplura Leach branch off on the one side, and the Thysanura Latr. on the other—the former, by means of the Pediculidæ, taking their food by suction, particularly Phthirus Leach, or the Morpion (in which the segments of the trunk and abdomen become indistinct[44]) approach the Octopods by the hexapod Acari L.—the latter by Machilis polypoda tending towards the Myriapods. In the Octopod branch a further dichotomy takes place, from which you proceed on one side to the Araneidæ in the Arachnida, by Phalangium, &c.; and in the other by Chelifer, &c. to Scorpio. Again, the Myriapod branch also divides, going by the Iulidæ to one branch of the Isopod Crustacea, and by the Scolopendridæ to another.

But there is another view of this subject before alluded to, which may be repeated here, and which seems to prove that the types of form in one natural group or class are reproduced in another; this appears to result from the following parallel series:

No type representing the Myriapoda has yet been discovered in the Arachnida class; but I have little doubt of its existence. You will observe that the analogies between the larvæ of the winged orders and the Aptera were first noticed by Mr. W. MacLeay[45]. It is probable that these parallel series of representatives of each other might be increased, as well as the numbers in the respective columns.

What I have said will, I trust, sufficiently justify me for making at present no more material alterations in the classification I long since proposed to you[46]; I shall, therefore, now proceed to define the objects I consider as Insecta; but I shall first observe—that as Latreille considers the branchiopod Crustacea or Entomostraca of Müller as entitled to the denomination of Crustaceo-Arachnida[47]; so his Trachean Arachnida might be called Arachnida-Insecta, and his Myriapoda, Crustaceo-Insecta.

Sub-kingdom—Annulosa[48].

Class—Insecta.

First Definition—From their external Organization.

• Body—divided into Head—Trunk—Abdomen.
  • Head.—Principal seat of the organs of sensation.
    
    • Organs of sight. Immoveable eyes, simple or compound, varying in number.
    • Organs of hearing uncertain, probably connected with the antennæ.
    • Organ of taste. Ligula or palate within the mouth, accompanied by the organs of manducation—a pair of mandibles and maxillæ and an upper and lower lip, or their representatives.
    • Organs of touch. Principally two jointed antennæ or their representatives, and four jointed feelers—two maxillary and two labial.
  • Trunk. Principal seat of the organs of motion.
    
    • Organs of walking, running, or jumping. Six or eight jointed thoracic legs, in pairs.
    • Organs of flight. Four wings or their representatives, mostly with branching nervures containing air-vessels; found in the majority of the class.
    • Organs (external) of respiration. A double set of lateral spiracles, some for expiration.
  • Abdomen. Principal seat of the organs of generation.
    
    • Organs of motion. In the Myriapods many pairs of acquired legs; in the Thysanura elastic ventral or caudal appendages.
    • Organs of respiration. A double series of lateral spiracles for inspiration in the majority; in some only a single series, and in others only a single pair.
    • Organs of generation those common to the Vertebrata, but retractile within the body, attended usually by various anal appendages, particularly a forceps in the males, and an ovipositor in the females.

Second Definition—From their internal Organization.

• Sensation.
  • Nervous System. A small brain usually subbilobed, crowning a knotty double medullary chord; nerves proceeding from the brain and other ganglions to all parts of the body.
• Circulation.
  • Heart replaced by a simple alternately contracting dorsal vessel or pseudocardia, without arteries or veins, but filled with a white cold sanies.
• Respiration.
  • Lungs replaced by tracheæ, which receive the air from the spiracles, and distribute it by bronchiæ infinitely ramified.
• Digestion.
  • Liver and biliary vessels in most replaced by from 2 to + 150 floating hepatic filaments opening into the space between the two skins of the intestinal canal below the pylorus.
• Generation.
  • Internal organs. Males—Vasa deferentia, and vesiculæ seminales, and the other ordinary organs. Females—Ovary usually bipartite, with palmate lobes; genital organs single and mostly anal; one sexual union impregnates the female for her life.
  • Development. In their passage to their adult state, after they have left the egg, insects undergo several simultaneous changes of their integument or successive moults, and the majority assume three distinct forms, with distinct organs, which appear as rudiments in their second state, and are completely developed in their last.

In defining the Arachnida I shall only mention those particulars in which they differ from Insectæ in their external anatomy.

Class—Arachnida.

• Body.
  • Head and Trunk usually not separated by a suture.
    
    • Eyes. Two to eight, not lateral.
    • Mandibles cheliform or unguiculate, representing the interior pair of the antennæ of the Crustacea.
    • Palpi pediform or cheliform.
    • Trunk. Legs eight or their representatives: tibiæ mostly consisting of two joints.
  • Abdomen with from two to eight spiracles.
    
• Sensation.
  • Nervous System. A small bilobed brain crowning a double, knotty, medullary chord; nerves proceeding from the brain and other ganglions to all parts of the body.
• Circulation.
  • Heart unilocular, inaurite, with a system of circulation by arteries and veins; blood a cold white sanies.
• Respiration.
  • Lungs replaced by internal gills receiving the air by spiracles.
• Digestion.
  • Liver, consisting of conglomerate glands, and enveloping the intestines[49]; hepatic ducts.
• Generation.
  • Genital organs double, ventral; more than one sexual union in the course of life.

The external characters in this class are the same almost in every respect as those which distinguish the Phalangidæ, the whole difference consisting almost in the systems of circulation, respiration, and digestion. Perhaps some future anatomist may discover in the tribe just mentioned, that there is a nearer agreement between them and the Arachnida in these systems than is at present suspected, which would prove them true Arachnida. I am inclined to think that Phrynus and Gonyleptes, &c. breathe by branchial spiracles; but having no opportunity of examining living specimens, I dare not speak with any confidence on the subject.

Having thus given you a view of the most important diagnostics by which what we have all along called Insects may scientifically be distinguished from other invertebrate animals, it may not be without use, if, under this head, I take a more popular and familiar view of the subject, and say something upon those distinctions which may attract the attention of the more common observer.

The notion of diminutive size, particularly as compared with vertebrate animals, seems more frequently attached to the idea of an insect than any other; and this notion is generally correct, for one insect that is bigger than the least of the above animals, thousands and thousands are vastly smaller: but there exist some that are considerably larger, whether we take length or bulk into consideration, and this in almost every order. To prove this most effectually, and that you may have a synoptical view of the comparative size of the larger insects of the different orders and tribes, I now lay before you a table of the dimensions of such of the largest as I have had an opportunity of measuring, including particularly those giants that are natives of the British isles.

From this table you see that several insects included in it exceed some of the smallest Vertebrata in bulk. In the Mammalia, the Sorex Araneus, called by the common people here the Ranny, is not more than two inches long excluding the tail; and the Mus messorius, or harvest-mouse, peculiar to the southern counties of England, is still more diminutive: so that to these little animals, the larger Dynastidæ, Goliathi, and Prioni, &c., appear giants, and may compete with the mole in size. Even some of the beetles of our own country, as the great Hydrophilus, the stag-beetle, &c., are more bulky than the two first-named quadrupeds. Amongst the birds, many Picæ, Passeres, &c., yield to several insects in dimensions, and their wings when expanded do not extend so far as those of not a few Lepidoptera. The great owl-moth of Brazil (Erebus Strix) in this respect is a larger fowl than the quail. Those beautiful little creatures, the humming-birds (Trochilus L.), the peculiar ornament and life of tropical gardens, which emulate the most splendid butterflies in the brilliancy of their plumage, are smaller than a considerable number of insects in almost every order, and even than some of those that are natives of Britain. Various reptiles also are much inferior in size to many of the insects of the above table. The smallest lizard of this country would be outweighed by the great British beetles lately mentioned, and the mole-cricket (Gryllotalpa vulgaris); and some of the serpent tribe are smaller than the larger Scolopendræ and Iuli. Amongst the fishes also, though some are so enormous in bulk, others in this respect yield the palm to several insects. The minnow and the stickleback that frequent our own pools and streams are considerably inferior in size to some of our water beetles.

In looking over the table, and comparing the different species that compose it with each other, you will perceive that the largest insects of the two sections of Hemiptera, of the Lepidoptera as to their body merely, of the Hymenoptera and Diptera, in general size fall considerably short of those of the other orders; and that certain individuals of the Orthoptera and Aptera bear away the palm in this respect from all the rest. In the Coleoptera the giants, with the exception of the Goliathi, are chiefly to be found amongst the timber devourers in the Lamellicorn and Capricorn tribes. Of orthopterous insects the Phasmidæ present the most striking examples of magnitude; and in the Neuroptera, the Agrionidæ of great length.

It is worthy of remark here, that although the tropical species of a genus usually exceed those of colder climates in size, the Gryllotalpa of Brazil is very considerably smaller than that of Europe: whether this is the case with the rest of the cricket tribe I have not had an opportunity of ascertaining. The Lepidoptera, though often remarkable for the vast expansion of their "sail-broad vans," if you consider only their bodies, never attain to gigantic bulk. Even the hawk-moths (Sphinx L.), though usually very robust, make no approach to the size of the great beetles, or the length of some of the spectres (Phasma) and dragon-flies (Agrionidæ). With regard to the superficial contents of their wings, a considerable difference obtains in different species where they expand to the same length—for the secondary wings are sometimes smaller than the primary, and sometimes they equal them in size. In some instances, also, the latter although long are narrow, and in others they are nearly as wide as long: regard, therefore, should be had to their expansion both ways. In the Hymenoptera and Diptera, the principal giants are to be found in the predaceous or blood-sucking tribes, as Scolia, the Sphecidæ, Pompilidæ, Vespidæ, &c., belonging to the former order; and the Asilidæ and Tabanidæ to the latter. The true and false humble bees (Bombus and Xylocopa) and the fly tribe (Muscidæ), though they sometimes attain to considerable size, scarcely afford an exception to this observation. Amongst the Aptera none of the Hexapods strike us by their magnitude, and few of the Octopods, though the legs of some of the Phalangidæ inclose a vast area. That in the table would with them describe a circle of six inches diameter, though its body is little more than a quarter of an inch in length. The Myriapods exceed most insects in the vast elongation of their body, which with their motion gives them no slight resemblance to the serpents. In the class Arachnida, the bird-spiders (Mygale) are amongst the principal giants, nor do the Scorpions fall far short of them—both of them when alive often alarming the beholder as much by their size as by their aspect.

But as I have before observed, generally speaking, one of the most remarkable characters of the insect world, is the little space they occupy; for though they touch the vertebrate animals and even quadrupeds by their giants, yet more commonly in this feature they go the contrary way, and by their smallest species reach the confines of those microscopic tribes that are at the bottom of the scale of animal life. I possess an undescribed beetle, allied to Silpha minutissima E. B.[50] which, though furnished with elytra, wings, antennæ, legs, and every other organ usually found in the order it belongs to, is absolutely not bigger than the full stop that closes this period. In several other coleopterous genera there are also very minute species, as in Cryptophagus, Anisotoma, Agathidium, &c. I know no orthopterous insect that can be called extremely minute, except that remarkable one found on the Continent in the nests of ants, the Blatta Acervorum of Panzer[51], but now called, I believe, Myrmecophilus: nor indeed any in the Hemiptera, Neuroptera, and Diptera, that approach the extreme limits of visibility: but in the Lepidoptera, the pygmy Tinea occultella is almost invisible except in flight, being scarcely thicker than a horse's hair, and proportionably short; indeed, many others of those lovely Lilliputians, the subcutaneous Tineæ, decorated with bands of gold and silver, and studded with gems and pearls, that in larger species would dazzle the beholder's eye, are in size not much more conspicuous. In the Hymenoptera order, Ichneumon Punctum of Dr. Shaw, which forms so striking a contrast to his giant Phasma dilatatum, being placed together in the same plate; and another that I possess, under the trivial name of Atomos, would elude the searching eye of the entomologist unless when moving upon glass. Linné named the tribe of parasites to which these belong, Minuti, on account of their generally diminutive size. But these little minims, under the superintendence of Providence, are amongst the greatest benefactors of the human race, since they keep within due bounds the various destroyers of our produce.

The number of minute species of insects seems greatly to exceed that of large ones, at least in Europe, of which it may be asserted probably with truth, that two-thirds are under a quarter of an inch in length, and one-third not exceeding much a duodecimal of it. It might hold good perhaps in Coleoptera, Hymenoptera, Diptera, and Aptera: but in Orthoptera, Hemiptera, Neuroptera, and especially Lepidoptera, a large proportion would be found to exceed three lines in length. Neither can it be affirmed of extra-European species, of those at least preserved in cabinets, amongst which it is rare to find an insect less than the fourth of an inch long. This, however, must probably be attributed to the inattention of collectors, who neglect the more minute species.

Though size forms a pretty accurate distinction between insects and the great bulk of vertebrate animals, it affords less assistance in separating them from the invertebrate classes, which are of every size, from the monstrous bulk of some Cephalopoda (cuttle-fish) and Mollusca (shell-fish, &c.) to the invisible infusory animalcule: but external characters, abundantly sufficient for this purpose, may be drawn from the general covering, substance, form, parts, and organs of the body. As I shall enter into pretty full details upon this subject when I come to treat of the external anatomy of insects, I shall here, therefore, only give such a slight and general sketch of the distinctions just mentioned, as will answer the end I have in view. I must here repeat what I have before observed, and what it is necessary that you should always bear in mind, namely, that at the limits of classes and of every other natural group, the characters begin to change, those peculiar to the one group beginning gradually to disappear, and those of the other to show themselves; so that it is impossible almost to draw up a set of characters so precise as exactly in every respect to suit all the members of any natural group.

Whichever way we turn our eyes on the objects of creation, above—below—athwart, analogies meet us in every direction, and it appears clear, that the Book of Nature is a Book of Symbols, in which one thing represents another in endless alternation. And not only does one animal, &c. symbolize another, but even between the parts and organs of one set of animals there is often an analogy as to their situation and use, when there is little or no affinity as to their structure—or again, the analogy is in their situation, without affinity in either structure or use. Thus certain parts in one tribe represent other certain parts of another tribe, though as to their structure there is often a striking disagreement. This is particularly observable between the vertebrate and invertebrate animals. I shall therefore, in my remarks on the general and particular structure of insects, contrast it in its most important points with that of the first-mentioned tribe.

The first thing that strikes us when we look at an insect is its outside covering, or the case that incloses its muscles and internal organs. If we examine it attentively, we find that it is not like the skin of quadrupeds and other Vertebrata, covering the whole external surface of the body; but that in the large majority it consists of several pieces or joints, in this respect resembling the skeleton of the animals just named; and that even in those in which the body appears to have no such segments, as in many of the Mites (Acarus L.), they are to be found in the limbs. This last circumstance, to have externally jointed legs, is the peculiar and most general distinction by which the Insecta of Linné, including the Crustacea, may always be known from the other invertebrate animals[52].

If we proceed further to examine the substance of this crust or covering, though varying in hardness, we shall find it in most cases, if we exclude from our consideration the shells of the Mollusca, &c., better calculated to resist pressure than that of the majority of animals that have no spine. In all the invertebrate tribes, indeed, the muscles, there being no internal skeleton, are attached to this skin or its processes, which of course is firmer than the internal substance; but in insects it is very often rigid and horny, and partially difficult to perforate, seldom exhibiting that softness and flexibility which is found in the cuticle of birds and most quadrupeds. From this conformation it has been sometimes said, that insects carry their bones on the outside of their body, or have an external skeleton. This idea, though not correct in all respects, is strictly so in this—that it affords a general point of support to the muscles, and the whole structure is erected upon it, or rather I should say within it. The difference here between Insects and the Vertebrata seems very wide; but some of the latter make an approach towards it. I allude to the Chelonian Reptiles (Testudo L.), in which the vertebral column becomes external or merges in the upper shell. The cyclostomous fishes also are not very wide of insects as to their integument. But on this subject I shall be more full hereafter.

The forms of insects are so infinitely diversified that they almost distance our powers of conception: in this respect they seem to exceed the fishes and other inhabitants of the ocean, so that endless diversity may be regarded as one of their distinctions. But on all their variations of form the Creator has set his seal of symmetry; so that, if we meet with an animal in the lower orders in which the parts are not symmetrical, we may conclude in general that it is no insect.

But it is by their parts and organs that insects may be most readily distinguished. In the vertebrate animals, the body is usually considered as divided into head, trunk, and limbs, the abdomen forming no part of the skeleton; but in the insect tribes, besides the organs of sense and motion, the body consists of three principal parts—Head, Trunk, and Abdomen—the first, as was before observed, bearing the principal organs of sense and manducation; the second most commonly those of motion; and the third those of generation—the organs of respiration being usually common to both trunk and abdomen. These three primary parts,—though in some insects the head is not separated from the trunk by any suture, as for instance in the Arachnida; and in others, head, trunk, and abdomen form only one piece, as in some mites,—still exist in all, and in the great majority they are separated by incisures more or less deeply marked: this is particularly visible in the Hymenoptera and Diptera, which, in this respect, are formed upon a common model; and in the rest, with the above exceptions, it may be distinctly traced.

The head of insects is clearly analogous to that of vertebrate animals, except in one respect, that they do not breathe by it. It is the seat probably of the same senses as seeing, hearing, smelling, tasting—and more peculiarly perhaps of that of touch. The eyes of insects, though allowed on all hands to be organs of sight, are differently circumstanced in many particulars from those of the animals last mentioned; they are fixed, have neither iris nor pupil, are often compound, and are without eyelids to cover them during sleep or repose; there are usually two compound ones composed of hexagonal facets, but in some instances there are four; and from one to three simple in particular orders. The antennæ of insects in some respects correspond with the ears of the animals we are comparing with them; but whether they convey the vibrations of sound has not been ascertained: that they receive pulses of some kind from the atmosphere I shall prove to you hereafter—so that if insects do not hear with them in one sense, they may, by communicating information, and by aëroscopy, to use Lehman's term, not directly in his sense[53], supply the place of ears, which would render them properly analogous to those organs. That in numbers these remarkable organs are tactors is generally agreed, but this is not their universal use. That insects smell has been often proved; but the organ of this sense has not been ascertained. What has improperly been called the clypeus, or the part terminating the face above the upper lip (labrum), is in the situation of the nose of the Vertebrata, and therefore so far analogous to it, and in some cases even in form: I therefore call it the nose. Whether this part represents the nose by being furnished with what answer the purpose of nostrils, residing somewhere at or above the suture that joins it to the upper lip, I cannot positively affirm; but from the observations of M. P. Huber, with regard to the hive-bee, it appears that at least these insects have the organ of the sense in question somewhere in the vicinity of the mouth, and above the tongue[54]: analogy, therefore, would lead us to look for its site somewhere between the apex of the nose and the upper lip; and in some other cases, which I shall hereafter advert to, there is further reason for thinking that it actually resides at the apex of the nose. The organ of taste in insects, though some have advanced their palpi to that honour, is doubtless in some part within the mouth analogous in a degree to the tongue and palate of the higher animals. The organs of manducation, in what may be deemed the most perfect description of mouth, consist of an upper lip closing the mouth above, a pair of mandibles moving horizontally that close its upper sides, and a lower lip with a pair of maxillæ attached to it, which close the mouth below and on the under sides, both labium and maxillæ being furnished with jointed moveable organs peculiar to annulose pedate animals, called palpi. In some tribes these organs assume a different form, that they may serve for suction; but though in many cases some receive an increment at the expense of others, and a variation in form takes place, none, as M. Savigny has elaborately proved, are totally obliterated or without some representative[55]. The organs now described, except the upper lip, are formed after a quite different type from those of Vertebrata, with which they agree only in their oral situation and use.

The second portion of the body is the Trunk, which is interposed between the head and abdomen, and in most insects consists of three principal segments, subdivided into several pieces, which I shall afterwards explain to you. I shall only observe, that some slight analogy may perhaps be traced between these pieces and the vertebræ and ribs of vertebrate animals, particularly the Chelonian reptiles. This is most observable in Gryllus L. and Libellula L., in which the lateral pieces of the trunk are parallel to each other[56]. In the Diptera and many of the Aptera most of these pieces are not separated by sutures. Each of the segments into which the trunk is resolvable bears a pair of jointed legs, the first pair pointing to the head, and the two last to the anus. These legs in their composition bear a considerable analogy to those of quadrupeds, &c., consisting of hip, thigh, leg, and foot; but the last of these, the foot or Tarsus, is almost universally monodactyle, unless we regard the Calcaria that arm the end of the tibia, as representing fingers or toes, an idea which their use seems to justify. Acheta monstrosa and Tridactylus paradoxus, however[57], exhibit some appearance of a phalanx of these organs. They differ from them first in number, the thoracic legs being invariably six in all insects, with the exception of the Octopods or most of the Trachean Arachnida, which have usually eight. In the Myriapods, though there are hundreds of abdominal legs, only six are affixed to the trunk. Next they differ with regard to the situation of their legs; for though the anterior pair or arms are analogous in that respect, the posterior pair are not, since in quadrupeds these legs are placed behind the abdomen, but in insects before it—in fact, in the former the legs may be considered as placed at each end of the body, excluding only the head and tail, but in the latter in the middle. Though they correspond with those of quadrupeds in being in pairs or opposite to each other, yet their direction with respect to the body is different, the legs of quadrupeds, &c. being nearly straight, whereas in insects they are bent or form an angle, often very obtuse at the principal articulations, which occasions them to extend far beyond the body, and when long to inclose a proportionally greater space. The wings are the organs of motion with which the upper side of the trunk is furnished; and these, though they are the instruments of flight, are in no other respect analogous to those of birds, which replace the anterior legs of quadrupeds, but approach nearer, both in substance and situation, to the fins of some fishes, and perhaps in some respects even to the leaves of plants. M. Latreille is of opinion, That the four wings or their representatives replace the four thoracic legs of the decapod Crustacea[58]. Upon this opinion, which shows great depth of research and practical acumen, I shall have occasion to express my sentiments when I come to treat more at large on the anatomy of the trunk and its members; at any rate they do not replace the two anterior pair of legs of the hexapod Aptera. When merely used as wings, they commonly consist of a fine transparent double membrane, strengthened by various longitudinal and transverse nervures, or bones as some regard them, accompanied by air-vessels, of which more hereafter, as well as of their kind and characters. I shall only observe, that insects are known from all other winged animals, by having four wings, or what represent them, and this even generally in those that are supposed to have only a pair. Another peculiarity distinguishes the trunk of insects that you will in vain look for in the vertebrate animals—these are one or two pair of lateral spiracles or breathing pores. Though the respiratory sacs, &c. of birds are almost as widely dispersed as the tracheæ and bronchiæ of insects[59], yet their respiration is perfectly pulmonary, and nothing like these pores is to be discovered in them.

The principal peculiarity of the third part of the body, the abdomen, is its situation behind the posterior pair of thoracic legs, and its rank as forming a distinct portion of what represents the skeleton. In most insects it is so closely affixed to the posterior part of the trunk as to appear like a continuation of it, but in the majority of the Hymenoptera and Diptera, and in the Araneidan Arachnida, or spiders, it is separated by a deep incisure; and in the first-mentioned tribe is mostly suspended to the trunk by a footstalk, sometimes of wonderful length and tenuity. In the Mammalia the male genital organs are partly external; but in insects as well as in many of the vertebrate animals, except when employed, they are retracted within the body. This part is the principal seat of the respiratory pores or spiracles, many having eight in each side, while others have only one.

Such are the principal external characters which distinguish Insecta and Arachnida, or what we have heretofore regarded as insects, to which here may be added another connected with their internal organization. The union of the sexes takes place in the same manner as amongst larger animals; and the females with very few exceptions, more apparent than real, are oviparous. They are, however, distinguished by this remarkable peculiarity already alluded to, that, except in the case of the Arachnida, one impregnation fertilizes all the eggs they are destined to produce. In most cases, after these are laid, the females die immediately, and the males after they have performed their office, though they will sometimes unite themselves to more than one female. One other circumstance may be named here—that no genuine insect or Arachnidan has yet been found to inhabit the ocean.

Before I conclude this letter, it is necessary to apprize you, that every thing which it contains relative to the characters of insects, has reference to them only in their last or perfect state, not in those preparatory ones through which you are aware that the majority of them must pass. The peculiar characteristics of them in these states—in the egg, the larva, and the pupa, will be the subjects of my next letters, which will be devoted to a more detailed view of the metamorphosis of insects than I gave you before when adverting to this subject[60].

[LETTER XXIX.]

STATES OF INSECTS.

EGG STATE.

On a former occasion I gave you a general idea of what has been called, perhaps not improperly, the metamorphosis of insects[61]; but since that time much novel and interesting speculation on the subject has employed the pens of many eminent Physiologists; and besides this, the doctrine then advanced of successive developments has been altogether denied by a very able Anatomist, Dr. Herold, who, with a hand, eye, and pencil, second only to those of Lyonnet, has traced the changes that gradually take place in the structure of the cabbage-butterfly (Pieris Brassicæ) on passing through its several states of larva, pupa, and imago. It is necessary, therefore, that previously to considering separately and in detail the states of insects, I should again call your attention to this subject, and endeavour to ascertain whether Dr. Herold's hypothesis rests upon a solid foundation; or whether that adopted from Swammerdam by all the most eminent Entomologists and Physiologists since his time can be maintained against it.

I shall first give you a short abstract of the new hypothesis.

According to Dr. Herold—The successive skins of the caterpillar, the pupa-case, the future butterfly, and its parts and organs, except those of sex which he discovered in the newly excluded larva, do not preexist as germes, but are formed successively from the rete mucosum, which itself is formed anew upon every change of skin from what he denominates the blood, or the chyle after it has passed through the pores of the intestinal canal into the general cavity of the body, where, being oxygenated by the air-vessels, it performs the nutritive functions of blood. He attributes these formations to a vis formatrix (Bildende Kraft).

The caul or epiploon (Fett-masse), the corps graisseux of Reaumur, &c., which he supposes to be formed from the superfluous blood, he allows, with most physiologists, to be stored up in the larva, that in the pupa state it may serve for the development of the imago. But he differs from them in asserting that in this state it is destined to two distinct purposes—first, for the production of the muscles of the butterfly, which he affirms are generated from it in the shape of slender bundles of fibres;—and secondly, for the development and nutrition of the organs formed in the larva, to effect which, he says, it is dissolved again into the mass of blood, and being oxygenated by the air-vessels, becomes fit for nutrition, whence the epiploon appears to be a kind of concrete chyle[62].

Need I repeat to you the hypothesis to which this stands opposed—That every caterpillar at its first exclusion contains within itself the germe of the future butterfly and of all its envelopes, which successively presenting themselves are thrown off, till it appear in perfection and beauty, with all its parts and organs, when no further development takes place.

I believe you will agree with me, when you have read and considered the above abstract of Dr. Herold's hypothesis, that in it he substitutes a name for knowledge, talks of a vis formatrix because his assisted eye cannot penetrate to the primordial essence or state of the germes of being, and denies the existence of what he cannot discover[63]. From ancient ages philosophers have done the same, to conceal their own ignorance of causes under a sounding name, when they have endeavoured to penetrate within the veil of the sanctum sanctorum, which it is not permitted to vain man to enter. This has occasioned the invention, not only of the term in question, but of many others, as little meriting the appellation of Signs of ideas; such as Plastic Nature, Epigenesis, Panspermia, Idea seminalis, Nisus formativus, &c. But upon this subject you cannot do better than consult what the learned Dr. Barclay has said in his admirable work On Life and Organization[64], in which he has placed the inanity, the vox et præterea nihil, of such high-sounding terms in their true light. The processes of nature in the formation and development of the fœtus in utero, of the chick in the egg, of the butterfly in the caterpillar, we in vain attempt fully to investigate; yet we can easily comprehend that pre-existent germes, by the constant accretion of new matter in a proper state, may be gradually developed, but we find it impossible to conceive how, by the action of second causes, without the intervention of the first cause, the butterfly should be formed in the caterpillar, unless it preexists there as a germe or fœtus. "Is it not clear," asks Dr. Virey in his lively manner, "as Blumenbach and other Physiologists maintain, that there is a formative power, a nisus formativus, which organizes the embryo? Admirable discovery!" says he, "which teaches us that the fœtus forms itself because it forms itself! As if you should affirm that the stone falls because it falls[65]!" Had Dr. Herold considered what Bonnet says with as much good sense as modesty, he would never have imagined that his discovering the organs of the butterfly one after the other at certain periods in the caterpillar, was any sound argument against their preexistence and coexistence as germes. "Organs," says that amiable and excellent Physiologist, "that have no existence as to us, exist as they respect the embryo, and perform their essential functions; the term of their becoming visible is that which has been erroneously mistaken for the period of their existence[66]." This has been Dr. Herold's grand error; he mistook the commencement of the appearance of the organs of the butterfly for that of their existence, and yet the early appearance of the sexual organs ought to have led him to a conclusion the reverse of that which he has adopted.

Dr. Virey has observed with great truth—that "Every being has a peculiar and unique nature, which would be impossible if the body was composed of parts made at several intervals, and without a uniform power that acts by concert[67]:" and every Physiologist acquainted with the history of insects that undergo a complete metamorphosis will allow, that their developments and acquisition of new parts and organs take place according to a law which regulates the number, kind, and times of them, differing in different species, and which has had an invariable operation, since the first creation, upon every sound individual that has been produced into the world.

In consequence of this law, one species changes its skin only four times, and another five or six;—in some cases the first skins shall be covered or bristled with hairs or spines, and the last be naked and without arms;—that which forms the case of the pupæ shall differ in form and substance from the preceding skins, varying in both respects in different species; and finally the butterfly shall invariably follow, when no other change but the last mortal one shall take place. Can this law, so constantly observed, be the result of a blind power? Or are we to suppose that the Deity himself is always at work to create the necessary organs in their time and place? Is it not much more consonant to reason and the general analogy of nature, to suppose that these parts and organs exist in embryo in the newly-hatched caterpillar, and grow and are successively developed by the action of the nutritive fluid? In the pupa of many Diptera the inclosed animal, even under the microscope, appears without parts or organs, like a mere pulp; but Bonnet tells us, that if boiled, all the parts of the pupa appear[68], which proves the preexistence of these parts even when not to be discerned, and that nothing but the evaporation of the fluids in which they swim is wanted to render them visible.

Mr. William MacLeay has with great truth observed: "The true criterion of animal as well as vegetable perfection is the ability to continue the species[69];" and in their progress to this state certain changes take place in the parts and organs of all animals and vegetables: there is, therefore, an analogy in this respect between them; and this analogy also furnishes another argument against Dr. Herold's hypothesis, as we shall presently see. These changes are of three kinds: In the vegetable kingdom, at least in the phænogamous classes, there is a succession of developments terminating in the appearance of the generative organs, inclosed in the flower; in this kind the integuments, or most of them, are usually persistent. In insects and other annulose and some vertebrate animals, there is a succession of spoliations, or simultaneous changes of the whole integument, till the animal appears in its perfect form with powers of reproduction; in this kind the integuments are caducous.—In man and most of the vertebrate animals there is a gradual action of the vital forces in different organs till they are fitted for reproduction; accompanied, as progress is made to the adult state, by the acquisition of certain organs, &c. as of teeth, horns, pubes, feathers, &c.[70] Let us now consider a little in detail the analogies that appear to exist between the second and the first and third kinds. I shall first consider the latter as the least obvious. That able, judicious, and learned physiologist, Dr. Virey, has pointed out no inconsiderable resemblance between the metamorphosis of the insect, and the changes, which he denominates a metamorphosis by metastasis, to which most vertebrate animals are subject. In them, he observes, a state analogous to the larva state begins at the exclusion of the fœtus from the womb; it is deprived of teeth, and its viscera are only accommodated to milk: in the cornute species the horns are in embryo: the digestive system now preponderates, and the great enjoyment is eating. A second state, in a degree analogous to that of pupa, commences at the period of dentition—the teeth now produce another modification in the intestinal canal, which becomes capable of receiving and digesting solid food: during this period the vital forces are all tending to produce the perfect state of the animal; and in this state, in man especially, the individual is educated and fitted to discharge the duties of active life. Again, analogous to the imago state is the age of puberty, in which the complete development of the sexual powers takes place in both sexes, and the animal has arrived at its acme, and can continue its kind[71]: now the digestive powers diminish in their activity, and love reigns paramount. When this state is fully attained, no further or higher change is to be expected, and the progress is soon towards decay and the termination of the animal's mortal career. So we see that in fact man and other mammalia, though they do not simultaneously cast their skins like the insect; or pass into a state of intermediate repose, before they attain the perfection of their nature, like the caterpillar; have their three states, in each of which they acquire new parts, powers, and appetites.

But a more striking analogy has been traced between the insects that undergo a complete metamorphosis and the vegetable kingdom; for though the primary analogy seems to be between the Polypus and the Plant, yet the secondary one with the Insect is not by any means remote. There are circumstances to which I shall have occasion hereafter to call your attention, which afford some ground for supposing, that the substance of the insect and the vegetable partakes of the same nature, at least approximates more nearly, than that of the insect and the vertebrate animal; and every one who has observed these little creatures with any attention, will have observed amongst them forms and organs borrowed as it were from the kingdom of Flora; and vice versâ the Botanist, if he makes the comparison, will find amongst his favourite tribes many striking resemblances of certain insects. But the analogy does not stop here; for the butterfly and the plant appear to have been created with a particular reference to each other, both in the epoch of their appearance and the changes that take place in them. Thus, as Dr. Virey has observed, the caterpillar is simultaneous with the leaf of the tree or plant on which it feeds, and the butterfly with the flowers of which it imbibes the nectar[72]. Swammerdam, I believe, was the first who noticed the analogy between the changes of the insect and the vegetable, and has given a table in which he has contrasted their developments, including other animals that undergo a metamorphosis[73]: an idea which has been generalized by Bonnet[74], and adopted and enlarged by Dr. Virey[74]. A state analogous to that of the larva in the insect begins in the plant when it is disclosed from the seed, or springs from its hybernaculum in the bulb, &c., or is evolved from the gemma; integument after integument, often in various forms, as cotyledon, radical, cauline, or floral leaves, expands as the stem rises, all which envelopes incase the true representative of the plant, the fructification, as the various skins do the future butterfly. When these integuments are all expanded, the fructification appears inclosed by the calyx or corolla as the case may be, in which the generative organs are matured for their office—this is the bud, which is clearly analogous to the pupa state of the insect. Next the calyx and corolla expand, the impregnation of the germen takes place, and the seed being ripened, and dispersed by the opening of the seed-vessel or ovary of the plant, the individual dies: thus the imago state of the insect has its representative in the plant. "If we place," says Dr. Virey, "here the egg of the insect, next its caterpillar, a little further the chrysalis, and lastly the butterfly—what is this but an animal stem—an elongation perfectly similar to that of the plant issuing from the seed to attain its blossoming and propagation?"[75]

There being, therefore, this general analogy in their progress to that state in which they can continue their species between every part of animated nature, it holds good, I think, that the same analogy should take place in their developments. If the adult man or quadruped, &c. is evidently an evolution of the fœtus, as from microscopical observations it appears that they are[76], if the teeth, horns, and other parts, &c. to be acquired in his progress to that state are already in him in their embryos, we may also conclude that the butterfly and its organs, &c. are all in the newly-hatched caterpillar. Again, if the blossom and its envelopes are contained in the gemma, the bulb, &c. where they have been discovered[77], it follows analogically that the butterfly and its integuments all preexist in its forerunner.

Perhaps after this view of the objections to Dr. Herold's hypothesis, it will not be necessary to say much with regard to the argument he draws from the change of organs—the loss of some and the acquisition of others—since this may readily be conceived to be the natural consequence of the vital forces tending more and more to the formation of the butterfly, and the withdrawing of their action more and more from the caterpillar; I shall not, therefore, enter further into the question, especially since the change of organs will come more regularly under our notice upon a future occasion.

Winged insects, many branchiopod Crustacea, and the Batracian reptiles, have been observed by Dr. Virey to bear some analogy to the mammalia, aves, &c. in another respect. In leaving their egg, they only quit their first integument, answering to the chorion or external envelope of the human fœtus; they therefore still continue a kind of fœtus, so to speak, more or less enveloped under other tunics, and principally in their amnios, or the covering in which the fœtus floats in the liquor amnii[78]. This the butterfly does in the pupa case; and its birth from this, under this view, will be the true birth of the animal. In the human subject, the ova upon impregnation are said to pass from the ovary through the Fallopian tube into the uterus. In the insect world, upon impregnation, the eggs pass first from the ovaries into the oviduct, answering to the Fallopian tube, which in them terminates in the ovipositor, or the instrument by which the parent animal conveys the eggs to their proper station: there is, therefore, nothing properly analogous to the uterus in the insect, and the substance upon which the larva feeds upon exclusion answers the purpose of a placenta.

After this general view of the most modern theories with regard to the metamorphosis of insects, I shall in the present and some following letters, treat separately of the different states through which these little beings successively pass.

The first of these is the Egg state, the whole class of insects being strictly oviparous. Some few tribes indeed bring into the world living young ones, and have on that account been considered as viviparous, but incorrectly, for the embryos of none of these are nourished, as in the true viviparous animals, within a uterus by means of a placenta, but receive their development within true eggs which are hatched in the body of the mother. This is proved by the observations of Leeuwenhoek, who found eggs in the abdomen of a female scorpion[79]; and of Reaumur, with regard to the flesh-fly (Musca carnaria) and other viviparous flies as they have been called[80]. A similar mode of production takes place in vipers and some other reptiles, which have hence been denominated ovo-viviparous, to distinguish them from the true viviparous animals—the class Mammalia. By far the larger portion of insects is oviparous in the ordinary acceptation of the term. The ovo-viviparous tribes at present known are scorpions; the flesh-fly and several other flies; a minute gnat belonging to Latreille's family of Tipulariæ[81]; some species of Coccus; some bugs (Cimicidæ)[82]; and most Aphides, which last also exhibit the singular fact of individuals of the same species being some oviparous and others ovo-viviparous, the former being longer in proportion than the latter.—Bonnet, however, is of opinion that the eggs of the first are not perfect eggs, but a kind of cocoon, which defends the larva, already formed in some degree, from the cold of winter[83].

When excluded from the body of the mother, or from the egg, as has been before observed, some insects appear nearly in the form of their parents, which, with a very slight alteration, they always retain; others, and the greater number, assume an appearance totally different from that of their parents, which they acquire only after passing through various changes. It is to these last, which have chiefly engaged the attention of Entomologists, that the title of metamorphoses has been often restricted. As, however, those insects which undergo the slightest change of form, as spiders do, undergo some change, and almost all insects cast their skins several times[84] before they attain maturity, Linné and most Entomologists, till very recently, have regarded the whole class as undergoing metamorphoses, and as passing through four different states, viz. the Egg—the Larva—the Pupa—and the Imago.

It is obvious, however, that in ovo-viviparous species three states of their existence only come under our cognizance, as these, being hatched in the body of the mother, come forth first under the form of larvæ. There is even one tribe of insects which presents the strange anomaly of being born in the pupa state. This is the Linnean genus Hippobosca (Pupipara fam. Latr.), to which our forest-fly belongs, the females of which lay bodies so much resembling eggs, that they were long considered as such until their true nature was ascertained by Reaumur (most of whose observations were confirmed by De Geer), who, from their size, which nearly equals that of the parent fly—from their slight motion when first extruded—from spiraculiform points which run down each side of them—and lastly, from their producing not a larva, as all other insects' eggs do, but perfect flies in the winged state—inferred, and doubtless with reason, that they are not real eggs, but pupæ, or larvæ just ready to assume the pupa state, which, however strange it may seem, have passed the egg and larva states in the body of the mother[85].

Insects, therefore, as to their mode of birth, may be divided into—

I. Ovo-viviparous, subdivided into—

1. Larviparous, coming forth from the matrix of the mother in the state of larvæ, as the Scorpion (Scorpio), the Flesh-fly (Musca), the Plant-louse (Aphis), &c.

2. Pupiparous, continuing in the matrix of the mother during the larva state, and coming forth in that of pupa, as the Forest-fly (Hippobosca equina), the Sheep-louse (Melophagus ovinus), the Bat-louse (Nycteribia Vespertilionis), &c.

II. Oviparous. All other insects.

Our business for the remainder of this letter will be with the latter description of these little animals.

The unerring foresight with which the female deposits her eggs in the precise place where the larvæ, when excluded, are sure to find suitable food; and the singular instruments with which, for this purpose, the extremity of their abdomen is furnished, have been noticed in a former letter[86], and those last mentioned will be adverted to in a future one. I shall now, therefore, confine myself to other circumstances connected with the subject, arranged for the sake of order under several distinct heads, as—their exclusion—situation—substance—number—size—figure—colour—and period of hatching.

i. Exclusion. The exclusion or extrusion of the impregnated eggs takes place, when, passing from the ovary into the oviduct, they are conducted by means of the ovipositor, in which it terminates, to their proper situation. By far the greater number of insects extrude them singly, a longer interval elapsing between the passage of each egg in some than in others. In those tribes which place their eggs in groups, such as most butterflies and moths, and many beetles, they pass from the ovaries usually with great rapidity; while in the Ichneumonidæ, Sphegidæ, Œstri, and other parasitic genera, which usually deposit their eggs singly, an interval of some minutes, hours, or perhaps even days, intervenes between the extrusion of each egg. One remarkable instance of the former mode I noticed in my letter on the Perfect Societies of Insects[87]; another may be cited, to which you may yourself be a witness—I allude to that common moth, vulgarly called the Ghost (Hepialus Humuli), which lays a large number of minute black eggs, resembling grains of gunpowder, and ejects them so fast that, according to De Geer, they may be said to run from the oviduct, and are sometimes expelled with the force of a popgun[88]. A Tetrapterous insect, the genus of which is uncertain, is said, when it is taken, to discharge its eggs like shot from a gun[89]. And a friend of mine, who had observed with attention the proceedings of a common crane-fly (Tipula), assured me that several females which he caught projected their eggs to the distance of more than ten inches.

A few Diptera extrude them in a sort of chain or necklace, each egg being connected by a glutinous matter with that which precedes and follows it. In a small species of a genus allied to Psychoda (a kind of midge), which one season was abundant in a window of my house, this necklace is composed of eggs joined by their sides, not unlike those strung by children of the seeds of the mallow[90]. Other Tipulidæ on the contrary extrude their eggs joined end to end, so as to resemble a necklace of oval beads. Beris clavipes and Sciura Thomæ, two other flies, produce a chain about an inch long, consisting of oval eggs connected, in an oblique position, side by side; an arrangement very similar prevails in the ribband of eggs which drop from some of the Ephemeræ[91].

These eggs, like those of the insects first mentioned, though connected, are expelled in succession; but other tribes, as the Libellulidæ, with the exception of Agrion, many Ephemeræ, Trichopterous insects, &c. expel the whole at once, as it were in a mass. In those first mentioned they are gummed together in an oblong cluster[92]. In one Ephemera mentioned by Reaumur[93], they formed two oblong masses, each containing from three to four hundred eggs, and three and a half or four lines long. These animals as soon as their wings are developed eject these masses by two orifices, and are aided in the process by two vesicles full of air, wherever they happen to alight or to fall; in most instances it is the water, their proper element, that receives them, but the animal does not appear to know the difference between a solid and a liquid, and seems only anxious how to free herself from a burthen that oppresses her; all has been contrived that an insect so short-lived may finish her different operations with the utmost celerity: the term of her existence would not have admitted the leisurely extrusion of such a number of eggs in succession[94]. Some Trichoptera, or May-flies, as Phryganea grandis L., exclude their eggs in a double packet, enveloped in a mass of jelly, (a circumstance often attending the eggs that produce aquatic larvæ,) upon the leaves of willows[95]. A similar double packet in the year 1810 I observed appended to the anus of a black species with long antennæ, probably Phryganea atrata F.[96] Upon taking several of the females I was surprised to find in the above situation a seemingly fleshy substance of a dirty yellow. At first, from its annular appearance, I conceived it to be some parasitic larva, but was not a little surprised upon pulling it away that it was full of globular transparent dusky eggs: it was about two lines and a quarter in length and nearly one in breadth. Being bent double it was attached to the animal by the intermediate angle, and when unfolded was constricted in the middle[97]. Each half, which was roundish, had about ten sharp transverse ridges, the interstices of which appeared as if crenated, an appearance produced by the eggs which it contained. Upon more than gentle pressure it burst and let out the eggs. Though resembling the packet of P. grandis in shape and other circumstances, it was nothing like jelly, but had rather a waxy appearance, and seems to have been covered by a membrane: so that the excluded larvæ must probably have eaten their way out of it. I have still by me, in 1822, specimens of these egg-packets, which, after the lapse of so many years, retain their original form and colour. It is not improbable that other species extrude their eggs in a similar case. Scopoli says of P. bicaudata L., that the female carries about under her belly her eggs united into a globe, like Lycosa saccata[98]. The eggs of Geometra Potamogata F. are also enveloped in a gelatinous substance, and the mass is covered with leaves[99].

Insects of the Diptera order also, like frogs and toads, commit their eggs to the water imbedded in masses of jelly. Dr. Derham describes two different kinds of them, in one of which the eggs were laid in parallel rows end to end, and in another in a single row, in which the sides were parallel[100]. But the most remarkable and beautiful specimen of this kind that I ever saw was one that, many years ago, I took out of a pond at Wittersham in Kent, from which I requested a young lady to make the drawing I send you[101]. The mass of jelly, about an inch and a quarter long, and rather widest in the middle, was attached by one end to some aquatic grass, and from one end to the other ran a spiral thread of very minute eggs, the turns of the screw being alternately on each side.

The mode of exclusion of the eggs of the Blattæ, which are engaged for a whole week in the business of oviposition, is very singular: the female deposits one or two large suboviform capsules, as large as half their abdomen, rounded on one side, and on the other straight and serrated, which at first is white and soft, but soon becomes brown and hard. This egg-case, as it may be called, contains sixteen or eighteen eggs arranged in a double series, and the cock-roaches when hatched make their escape through a cleft in its straight side, which shuts so accurately when they have quitted it, that at first it appears as entire as before[102]. The insects of the genus Mantis also, or what are called the praying insects, when they deposit their eggs, eject with them a soft substance, which hardens in the air and forms a long kind of envelope resembling parchment, in which the eggs are arranged also in a double series. And the Locusts (Gryllus Locusta L.) are said by Morier[103] to deposit in the ground an oblong substance, of the shape of their abdomen, which contains a considerable number of eggs arranged neatly in rows. The peristaltic motion observed in the females of some insects during oviposition has been before described[104].

ii. Situation. Under this head I include the situation in which the female insect places her eggs when extruded, whether she continues her care of them and carries them about till they hatch, or whether she entirely deserts them, placing them either without a covering within reach of their food, or enveloping them in hair or otherwise protecting them from accident or the attack of enemies. I shall consider them under two views: first, as depositing their eggs in groups, whether covered or naked; and secondly, as depositing them singly.

1. Those that deposit their eggs in groups are first to be considered. I shall begin with those that protect them with some kind of covering.

I have already mentioned in a former letter[105] the silken bag with which Lycosa saccata Latr., a kind of spider, surrounds her eggs, and in which she constantly carries them about with her, defending them to the last extremity. Many other spiders, indeed nearly the whole tribe, fabricate similar pouches, but of various sizes, forms, texture, and colours. Some are scarcely so big as a pea, others of the size of a large gooseberry; some globular, some bell-shaped; others, the genus Thomisus Walck. in particular, depressed like a lupine; some of a close texture like silk; others of a looser fabric resembling wool: some consisting of a single pellicle, but most of a double, of which the interior is finer and softer[106]; some white; others inclining to blue; others again yellow or reddish; most of them are of a whole colour, but that of Epeira fasciata is gray varied with black[107]. And while the parent spider of some kinds (the Lupi) always carries her egg-bag attached to her anus, others hold them by their palpi and maxillæ; and others suspend them by a long thread, or simply fasten them in different situations, either constantly remaining near them (the Telariæ), or wholly deserting them (the Retiariæ). The eggs of one of these last Lister describes as often fixed in a very singular situation—the cavity at the end of a ripe cherry; and thus, as he expresses it—"Stomachi maxime delicatuli quoties hanc innocuam buccam non minus ignoranter quam avide devorarunt[108]."

Herman informs us, that the species of the genus Chelifer carry their eggs in a mass under their belly[109].

Madam Merian gives an account of two species of Blatta, which she affirms carry an egg-pouch about with them—one species (B. gigantea?) she describes as carrying its eggs in a globular pouch of web like certain spiders, and the other in a brown bag, which, when alarmed, it drops and makes off[110]. But this admirable paintress of natural objects was not always correct in her statements[111]: it seems very improbable, from the habits of those species of which we know the history, that any of them should spin a pouch of web for their eggs.

The only insects certainly known to spin an egg-pouch like the spiders, are the Hydrophili, a kind of water-beetles. Some of these, as H. lividus, carry them about with them, like Lycosa saccata, attached to the under side of their body, as M. Miger observed[112]; and others when they are finished desert them. That of the great water-beetle (Hydrophilus piceus) was long ago described and figured by Lyonnet[113]; and a more detailed account of it has since been given by M. Miger[114]. In form it somewhat resembles a turnip when reversed, since it consists of a pouch of the shape of an oblate spheroid, the great diameter of which is three quarters of an inch; and the small, half an inch, from which rises a curved horn, about an inch long and terminating in a point[115]. The animal is furnished with a pair of anal spinners, which move from right to left, and up and down, with much quickness and agility: from these spinners a white and glutinous fluid appears to issue, that forms the pouch, which it takes the animal about three hours to construct. The exterior tissue is produced by a kind of liquid and glutinous paste, which by desiccation becomes a flexible covering impermeable to water; the second, which envelops the eggs, is a kind of light down of great whiteness, that keeps them from injuring each other. The tissue of the horn is of a silky nature, porous and shining, and greatly resembling the cocoons of Lepidoptera. This part, contrary to what Lyonnet supposes, appears calculated to admit the air, the water soon penetrating it when submerged. At its base is the opening prepared for the egress of the larvæ, when hatched, which is closed by some threads, that, by means of the air confined in the cocoon or pouch, hinder the water from getting in[116]. This nidus does not float at liberty in the water till after the eggs are hatched, the parent animal always attaching it to some plant. By means of this anomalous process for a beetle, which this insect is instructed by Providence thus to perfect, the precious contents of its little ark are secured from the action of the element which is to be the theatre of their first state of existence, from the voracity of fishes, or the more rapacious larvæ of its own tribe, until the included eggs are hatched, and emerge from their curious cradle.

I shall next amuse you with a few instances, in which the Allwise Creator instructs the parent insect, instead of defending her eggs with a covering furnished by her internal organs, to provide it from without, either from her own body or from some other substance. Most commonly, indeed, the female leaves her cluster of eggs without any other covering than the varnish with which in this case they are usually besmeared. Either they are deposited in summer and will soon be hatched, or they are of a substance calculated to encounter and resist the severities of the season. But many species, whose eggs are more tender or have to resist the cold and wet of winter, defend them in the most ingenious manner with a clothing of different kinds of substance.

Cassida viridis, a tortoise beetle, Rösel tells us, covers her group of eggs with a partially transparent membrane. Arctia Salicis F., a moth, common on willows, wholly conceals hers with a white frothy substance, which when dry is partly friable and partly cottony, and being insoluble in water effectually protects them from the weather[117]. The female of Lophyrus Pini (a saw-fly), having by means of her double saw made a suitable longitudinal incision in the leaf of a fir, and placed in it her eggs in a single row end to end, stops it up with a green frothy fluid mixed with the small pieces of leaf detached by her saws, which when dry becomes friable: a necessary precaution, since these eggs are extremely brittle[118]. Arctia chrysorhœa, Hypogymna dispar, and several other moths, surround theirs with an equally impervious and more singular clothing—hair stripped from their own bodies. With this material, which they pluck by means of their pincer-like ovipositor, they first form a soft couch on the surface of some leaf: they then place upon it successively layers of eggs, and surround them with a similar downy coating, and when the whole number is deposited cover the surface with a roof of hairs, which cannot be too much admired; for those used for the interior of the nest are placed without order, but those employed externally are arranged with as much art and skill as the tiles of a roof, and as effectually keep out the water, one layer resting partly on the other, and all having the same direction, so that the whole resembles a well-brushed piece of shaggy cloth or fur. When the mother has finished this labour, which often occupies her for twenty-four hours, and sometimes even twice that period, her body, which before was extremely hairy, is almost wholly naked—she has stripped herself to supply clothing to her offspring, and having performed this last duty she expires. The female moths which thus protect their eggs are often furnished with an extraordinary quantity of hair about the anus for this express purpose; and Reaumur conjectures, that the singular anal patch of scales resembling those of the wings, but considerably larger, which is found in the female of Lasiocampa Pityocampa, is destined for the same purpose[119].

Reaumur had once brought to him a nidus of eggs clothed still more curiously: they surrounded a twig in a spiral direction, like those of Lasiocampa Neustria, but were much more numerous, and were thickly covered with fine down, not pressed close, but standing off horizontally, which assumed much the same appearance as a fox's tail would if twisted spirally round a branch[120].

A procedure nearly similar was observed by De Geer in some species of Aphides (A. Alni and A. Pruni), which covered their eggs with a white cottony down detached from their belly by means of their hind legs[121]. In this case, however, the eggs were separately coated with the down, but there was no general covering to the group.

Several insects make the leaves and other parts of plants serve as coverings for their eggs. Tenthredo Rosæ L., a saw-fly, and other species of the same genus, with their saws make an incision in the green twigs of shrubs and trees, and fill it with a line of eggs placed end to end, taking care that, as the eggs grow after they are laid, they are placed at such distances as to leave room for their expansion[122]. Rhynchites Bacchus, a brilliant weevil, well known to the vine-dressers for the injury it does[123], rolls with much art the leaves of the vine, so as to form a cavity, in which it places its eggs; other species practise similar manœuvres; and some probably place their young progeny in the interior of twigs, making an opening for that purpose with their rostrum—at least, I once saw Rhynchites Alliariæ L. with its rostrum plunged up to the antennæ in the twig of a crab-tree. Others of this tribe, as we know, place their eggs in the interior of fruits and grain, as the nut, acorn, and common weevils.

It is probable that most of the above coverings serve another purpose besides the protection of the eggs from wet and cold—that of sheltering them from the action of too great light, which, as Dr. Michellotti by numerous experiments has ascertained, is fatal to the included germe[124]. On this account it is perhaps that so many insects fasten their eggs to the under side of leaves. Those exposed in full day have usually an opaque and horny texture.

Some insects are spared all trouble in providing a covering for their eggs, their own bodies furnishing one in every respect adapted to this purpose. Not to mention the Onisci, or wood-lice, since they rather belong to the Crustacea, which have a four-valved cell under the breast, in which they carry their eggs, as the kangaroo does its young in its abdominal pouch, the whole body of the female of those strange animals the Cocci becomes a covering for her eggs, which it incloses on every side. To make this intelligible to you, further explanation is necessary. You must have noticed those singular immovable tortoise-shaped insects, which are such pests to myrtles and other greenhouse plants. These are the young of a species of Coccus (C. Hesperidum L.), and their history is that of the whole race. Part of them never become much bigger than the size of which you ordinarily see them, and when full-grown disclose minute two-winged flies, which are the males. The size of the females, which glue themselves to a twig or leaf as if lifeless, now augments prodigiously, and the whole body, distended with the thousands of eggs which it includes to the bigness of a large pea, without any vestige of head or limb, resembles a vegetable excrescence or gall-apple rather than an insect. If you remove one of them, you will perceive that the under part of its abdomen is flat and closely applied to the surface of the branch on which it rests, only a thin layer of a sort of cotton being interposed between them. In laying her eggs the female Coccus does not, like most insects, protrude them beyond her body into day-light; but as soon as the first egg has passed the orifice of her oviduct, she pushes it between her belly and the cottony stratum just mentioned, and the succeeding eggs are deposited in the same manner until the whole are excluded. You will ask how there can be found space between the insect's belly and the cotton, to which at first it was closely applied, for so large a mass of eggs? To comprehend this, you must consider that nearly the whole contents of its abdomen were eggs; that in proportion as these are extruded a void space is left, which allows the skin of the under side of the body to be pushed upwards, or towards that of the back, affording room between it and the cottony web for their convenient stowage. If you examine the insect after its eggs are all laid, you will find that they have merely changed their situation; instead of being on the upper side of the skin forming the belly, and within the body, they now are placed between it (now become concave and nearly touching the back) and the layer of cotton. As soon as the female Coccus has finished her singular operation she dies; but her body, retaining its shape, remains glued upon the eggs, to which it forms an arched covering, effectually protecting them, until they are hatched, from every external injury. Some species lay so many eggs, that the abdomen is not sufficiently large to cover the whole mass, but merely one side of it, the remainder being enveloped in cottony web[125].

I am next to consider the situation of those eggs that are excluded by the mother in groups without any other covering than the varnish with which they are usually besmeared in their passage from the oviduct. The females only place them upon or near the food appropriated to the young larvæ, to which they adhere by means of the varnish just mentioned. These groups consist of a greater or less number; and when the eggs are hatched by the heat of the sun, the larvæ begin to disperse and attack with voracity the food that surrounds them. It is thus that most butterflies and moths attach their eggs to the stems, twigs, and leaves of plants; that the lady birds (Coccinellæ), the aphidivorous flies (Syrphi &c.), and the lace-winged flies (Hemerobii), deposit them in the midst of plant-lice (Aphides); that the eggs of some flesh-flies are gummed upon flesh; those of crickets and grasshoppers buried in the earth; those of gnats and other Tipulidans set afloat upon, or submerged in, the water.

Frequently the whole number of eggs laid by one female is placed in one large group, more commonly, however, in several smaller ones, either at a distance from each other on the same plant, or on distinct plants. The object in the latter case seems to be, in some instances, to avoid crowding too many guests at one table, in others to protect the unhatched eggs from the voracity of the larvæ first excluded, which would often devour them if in their immediate neighbourhood.

In the disposition of the eggs which compose these groups much diversity prevails. Sometimes they are placed without order in a confused mass: more frequently, however, they are arranged in different, and often in very beautiful modes. The common cabbage-butterfly (Pieris Brassicæ) and many other insects place theirs upon one end, side by side, so as, comparing small things with great, to resemble a close column of soldiers, in consequence of which those larvæ which, on hatching, proceed from the upper end, cannot disturb the adjoining eggs. Many indeed have a conformation purposely adapted to this position, as the hemisphærical eggs of the puss-moth (Cerura Vinula), which have the base by which they are gummed membranous and transparent, while the rest is corneous and opaque. The same ready exit to the larva is provided for in the oblong eggs of the emperor moth (Saturnia Pavonia), which are piled on their sides in two or more lines like bottles of wine in a bin[126].

Where the larva does not emerge exactly from the end of the egg other arrangements take place. The whirlwig-beetle (Gyrinus natator) and the saw-fly of the gooseberry &c. (Tenthredo flava L.) dispose theirs end to end in several rows; the former upon the leaf of some aquatic grass, the rows being parallel[127], the latter gummed to the main nerves of gooseberry or currant leaves, the direction of which they follow[128].

But the lackey-moths (Lasiocampa Neustria, castrensis, &c.) adopt a different procedure. As their eggs, which are laid in the autumn, are not to be hatched until the spring, the female does not, like most other moths, place them upon a leaf, with which they might be blown by the winter's storms far from their destined food, but upon the twig of some tree, round which she ranges them in numerous circles. If you examine your fruit-trees, you can scarcely fail to find upon the young twigs collections of these eggs, which are disposed with such admirable art, that you would take them rather for pearls, set by the skilful hand of a jeweller, than for the eggs of an insect. Each of these bracelets, as the French gardeners aptly call them, is composed of from 200 to 300 pyramidal eggs with flattened tops[129], having their axes perpendicular to the circumference of the twig to which they are fastened, surrounding it in a series of from fifteen to seventeen close spiral circles, and having their interstices filled up with a tenacious brown gum, which, while it secures them alike from the wintry blast and the attack of voracious insects, serves as a foil to the white enamel of the eggs that it encompasses. It is not easy to conceive how these moths contrive to accomplish so accurately with their tail and hind feet an arrangement which would require nicety from the hands of an artist; nor could Reaumur, with all his efforts and by any contrivance, satisfy himself upon this head. He bred numbers of the fly from the egg, and supplied the females after impregnation with appropriate twigs; but these, as though resolved that imprisonment should not force from them the secret of their art, laid their eggs at random, and made no attempt to place them symmetrically[130].

This illustrious Entomologist was more successful in discovering the mode in which another insect, the common gnat, whose group of eggs is, in some respects, as extraordinary as that last described, performs its operations. The eggs of this insect, of a long phial-like form, are glued together, side by side, to the number of from 250 to 300, into an oblong mass, pointed and more elevated at each end, so as considerably to resemble a little boat in shape. You must not here suppose that I use the term boat by way of illustration merely; for it has all the essential properties of a boat. In shape it pretty accurately resembles a London wherry, being sharp and higher, to use a nautical phrase, fore and aft; convex below and concave above; floating, moreover, constantly on the keel or convex part. But this is not all. It is besides a life-boat, more buoyant than even Mr. Greathead's: the most violent agitation of the water cannot sink it; and what is more extraordinary, and a property still a desideratum in our life-boats, though hollow it never becomes filled with water, even though exposed to the torrents that often accompany a thunder-storm. To put this to the test, I yesterday (July 25, 1811) placed half a dozen of these boats upon the surface of a tumbler half full of water; I then poured upon them a stream of that element from the mouth of a quart bottle held a foot above them. Yet after this treatment, which was so rough as actually to project one out of the glass, I found them floating as before upon their bottoms, and not a drop of water within their cavity.

This boat, which floats upon the surface of the water until the larvæ are disclosed, is placed there by the female gnat. But how? Her eggs, as in other insects, are extruded one by one. They are so small at the base in proportion to their length that it would be difficult to make them stand singly upright on a solid surface, much more on the water. How then does the gnat contrive to support the first egg perpendicularly until she has glued another to it—these two until she has fixed a third, and so on until a sufficient number is fastened together to form a base capable of sustaining them in their perpendicular position? This is her process. She fixes her four anterior legs upon a piece of leaf, or a blade of grass, and projects her tail over the water. She then crosses her two hind legs, and in the inner angle which they form, retains and supports the first laid egg, as it proceeds from the anus. In like manner she also supports the second, third, &c., all of which adhere to each other by means of their glutinous coating, until she feels that a sufficient number are united to give a stable base to her little bark; she then uncrosses her legs, and merely employs them to retain the mass until it is of the required size and shape, when she flies away, and leaves it to its fate floating upon the water[131].

It may not be out of place to mention here a remarkable circumstance which not seldom attends a kind of water-scorpion (Naucoris F.) occasionally to be met with in collections of Chinese insects. Its back is often covered with a group of rather large eggs, closely arranged; but whether these are its own eggs or those of some large species of water-mite (Hydrachna Maïll.) has not been clearly ascertained. On the former supposition, the ovipositor must be remarkably long and flexile to enable the animal to place the eggs on its back. In confirmation of the latter it may be observed, that the species of the genus Hydrachna usually attach their eggs to the body and legs of aquatic insects, as for instance H. abstergens to the water-scorpion (Nepa cinerea), &c.[132]

2. After having thus laid before you some of the procedures of those insects that usually deposit their eggs in groups, either naked or defended by coverings of various kinds, I next proceed to a rapid survey of those of the species that commonly deposit them singly. Some of these, as for instance the Admiral Butterfly (Vanessa Atalanta), glue each egg carefully to its destined leaf by alighting on it for a moment. Another butterfly (Hipparchia Hyperanthus) whose caterpillar is polyphagous, drops hers at random on different plants. In general it may be observed, that all those larvæ which live in solitude, as in the interior of wood, leaves, fruits, grain, animals, &c., proceed from eggs laid singly by the female, which is usually provided with an appropriate instrument for depositing them in their proper situation. Thus the nut-weevil (Balaninus Nucum Germ.) and also that of the acorn (B. Glandium) pierce a nut or an acorn with their long beak, and then deposit in the hole an egg, from which proceeds the maggot that destroys those fruits. Leeuwenhoek asserts that the common weevil (Calandra granaria) adopts the same process, boring a hole in every single grain of corn before it commits an egg to it, and at the same time, by this manœuvre, prepares a small quantity of flour to serve for the food of the tender grub when it is first hatched[133]. It is probable that the Rhyncophorous or weevil tribe in general chiefly use their beaks for the purpose of depositing their eggs in different vegetable substances, and perhaps principally in fruit or grain. The tribe of gall-flies (Cynips) on the contrary, whose economy, detailed in a former letter[134], interested you so much, bore an opening for the egg with their spiral oviduct, which also conveys it.

Another large tribe of insects depositing their eggs singly, are those which feed upon the bodies of other animals, into the flesh of which they are either inserted, or placed so as speedily to find their way into it. Some of these introduce them into living animals, and then leave them to their fate, as the Ichneumons and gad-flies: others deposit them along with the dead body of an insect interred in a hole, often prepared with great labour, as the different species of sand-wasps (Sphecidæ), spider-wasps (Pompilidæ), &c.: the manners of the latter of these tribes have been already adverted to[135], and those of the Ichneumonidæ will come more fully under consideration when I treat of the diseases of insects.

A similar labour in providing suitable habitations for their eggs is undergone by various other insects whose larvæ live chiefly on vegetable food, some inserting their egg within the substance the larva devours, as those that prey on timber, twigs, roots, or the like, and others on its surface. One would suppose at first, that the exceedingly small egg which produces the subcutaneous larvæ would, by the parent moth, be imbedded in the substance of the leaf which is to exhibit hereafter their serpentine galleries: but this is not the case, for she merely glues it on the outside; at least such was the situation of the only egg of these very minute moths Reaumur had ever an opportunity to observe[136].

Other insects, belonging to the tribe which lay their eggs singly, bury them in the ground. Of this description are many of the lamellicorn insects, the dung-chafers (Scarabæidæ MacLeay) particularly, which, inclosing their eggs in a pellet of dung, deposit them in deep cylindrical cavities. Concerning the proceedings of some of these, as well as of the whole race of bees, wasps, &c., which all lay single eggs, I have before detailed to you many interesting particulars[137]. I must not conclude this subject without observing, that the female Pycnogonidæ, an osculant tribe between Insects and Crustacea, carry their eggs upon two pair of false legs[138].

iii. Substance. From this long dissertation on the situation of the eggs of insects and matters connected with it, I pass on to their substance or their external and internal composition, giving at the same time some account of the embryo included in them. The eggs of insects, like those of birds, consist in the first place of an external coat or shell, varying greatly, as to substance, in different genera. Most commonly, particularly in those which deposit their eggs in moist situations, as in dung, earth, and the like, it is a mere membrane, often thin and transparent, and showing, as in spiders, all the changes that take place in the inclosed embryo, as the formation of the head, trunk, and limbs[139]. This membrane is sometimes so delicate as to yield to the slightest pressure, and insufficient to protect the included fluids from too rapid an evaporation, if the eggs be exposed to the full action of the atmosphere. In most Lepidoptera, and several other tribes, this integument is considerably stronger, in those moths whose eggs are exposed throughout the winter, as Lasiocampa Neustria, &c., so hard as not to yield easily to the knife. Even in these, however, its substance is more analogous to horn or a stiff membrane than to the shell of the eggs of birds. Nothing calcareous enters into its composition, and it is not perceptibly acted upon by diluted sulphuric acid. The eggs of birds are lined by a fine membrane; but I have examined several of those of insects, and have been able to discover nothing of the kind in them. I will not, however, affirm that it does not exist, though the shell of the insect egg appears more analogous to the membrane that lines that of the bird than to the outside shell itself.

Within this integument is included a fluid, on the precise nature of which, except that it is an aqueous whitish fluid, few or no observations have been made, or indeed are practicable; but it is reasonable to suppose, that like the white and yolk of the bird's egg, it serves for the development of the organs of the germe of the future insect.

But few observations are recorded that relate to the embryo included in the egg. It is stated, that it is invested with an extremely fine and delicate pellicle—supposed by some analogous to the Chorion and Amnios of the human fœtus, though others think the shell of the egg to correspond with the Chorion, and the successive integuments of the larva with the Amnios[140]. When the egg is first laid, nothing indeed is to be seen in it but the fluid just mentioned; the first change in this fluid is the appearance of the head of the embryo, more particularly in Coleoptera, of two points, the rudiments of the mandibles, and of those apertures into the tracheæ which I have called spiracles[141]; the little animal we may suppose then assumes its form and limbs. The embryo is usually so folded in the egg that the head and tail meet[142], and the head, annuli, and other parts of the larva are often visible through the shell[143]. Swammerdam even saw the pulsation of the great dorsal vessel through the shell of the egg of Oryctes nasicornis.

Under this head I must notice another singular circumstance peculiar I believe to the eggs of insects, that sometimes, though rarely, they are covered with down or hair. Those of a singular little hemipterous insect, of a genus I believe at present undescribed, the ravages of which upon the larch have been before noticed[144], are covered by a downy web, as is the case with the animal itself. De Geer has described the eggs of a bug, not uncommon in this country (Pentatoma juniperina Latr.), which are reticulated with black veins, in which very short bristles are planted[145]. I possess also a nest of brown eggs, probably of a species of the same genus, found upon furze, which appear to be covered with very short downy hairs. The top of these is flat, and surrounded by a coronet of short bristles, each surmounted by a small white ball, so as to wear the appearance of a beautiful little Mucor. But hairy eggs are not confined to the Hemiptera Order, for, according to Sepp, those of the figure-of-eight moth (Bombyx cæruleocephala) are of this description[146].

iv. Number. The fertility of insects far exceeds that of birds, and is surpassed only by that of fishes[147]. But the number of eggs laid by different species, sometimes even of the same natural family, is extremely various. Thus the pupiparous insects may be regarded as producing only a single egg; Musca Meridiana L., a common fly, lays two[148], other flies six or eight; the flea twelve; the burying beetle (Necrophorus Vespillo[149]) thirty; May-flies (Trichoptera K.) under a hundred; the silk-worm moth about 500; the great goat-moth(Cossus ligniperda) 1,000; Acarus americanus more than 1,000[150]; the tiger-moth (Callimorpha Caja) 1,600; some Cocci 2,000, others 4,000; the female wasp at least 30,000[151]; the queen bee varies considerably in the number of eggs that she produces in one season, in some cases it may amount to 40,000 or 50,000 or more[152]; a small hemipterous insect, resembling a little moth (Aleyrodes proletella Latr.) 200,000. But all these are left far behind by one of the white ants (Termes fatale F. bellicosus Smeath.)—the female of this insect, as was before observed[153], extruding from her enormous matrix not less than 60 eggs in a minute, which gives 3,600 in an hour, 86,400 in a day, 2,419,200 in a lunar month, and the enormous number of 211,449,600 in a year: probably she does not always continue laying at this rate; but if the sum be set as low as possible, it will exceed that produced by any other known animal in the creation.

v. Size. The size of the eggs is in proportion to that of the insect producing them, though in some instances small ones produce larger eggs than those laid by bigger species. Thus the eggs of many Aptera, as those of that singular mite Uropoda vegetans, and of the bird-louse found in the golden pheasant, are nearly as large, it is probable, as the parent insect; while those of the ghost-moth (Hepialus Humuli) and many other Lepidoptera, &c. are vastly smaller. This circumstance perhaps depends principally on the number they produce: the majority of them, however, are small. The largest egg known, if it be not rather an egg-case, is that of a spectre insect (Phasma dilatatum), figured in the Linnean Transactions[154], being five lines in length and three in width, which probably approaches near the size of that of some humming-birds. The largest egg of any British insect I ever saw was that of the common black rove-beetle (Staphylinus olens) sent me by Mr. Sheppard—this is a line and half long by a line in width. But we do not often meet with insect-eggs exceeding a line in length. A vast number are much smaller: those of Ephemeræ are more minute than the smallest grains of sand[155], and some almost imperceptible, as those of the subcutaneous moths, to the naked eye. Commonly the eggs laid by one female are all of the same size; but in several tribes, those containing the germe of the female are larger than those that are to give birth to a male. This appears to be the case with those of the Rhinoceros beetle (Oryctes nasicornis[156]), and according to Gould with those of ants[157]. As the female in a vast number of instances is much bigger than the male, it is not improbable that this law may hold very extensively. It is stated, however, by Reaumur[158], that the reverse of this takes place in the eggs of the hive-bee, those that are to produce males being larger than the rest.

Another peculiarity connected with the present head is the augmentation in bulk which takes place, after exclusion, in the eggs of the great tribe of saw-flies (Tenthredo L.), the gall-flies (Cynips L.), the ants (Formica L.) and the water-mites (Hydrachna Maïll. Atax F.). Those of the two former, which are usually deposited in the parenchymous substance of the leaves, or of the young twigs, of various plants, imbibe nutriment in some unknown manner, through their membranous skins, from the vegetable juices which surround them[159], and when they have attained their full size are nearly twice as large as when first laid. Except in the eggs of fishes, whose volume in like manner is said to augment previously to the extrusion of the young, there is nothing analogous to this singular fact in any other of the oviparous tribes of animals, the eggs of which have always attained their full size when they are laid.

It is to M. P. Huber that we are indebted for the knowledge of the fact that the eggs of ants grow after being laid, a circumstance favoured probably by the moist situation in which the workers are always careful to keep them. By an accurate admeasurement he found that those nearly ready to be hatched were almost twice as big as those just laid[160]. A similar observation was made on the red eggs of a water-mite (Hydrachna abstergens) by Rösel, who conjectured that they draw their means of increase from the body of the water-scorpions (Nepæ), of which they form so singular an appendage[161], which opinion is confirmed by De Geer, who observes that when the water-scorpions are covered by an unusual number of the eggs of the water-mites, they grow weak and languid, and endeavour to rid themselves of their parasitic appendages[162]. It is most probable that the mite lately named (Uropoda vegetans), which is often found planted as it were upon the bodies of various beetles, by means of a long pedicle, through which, as the fœtus by an umbilical chord and placenta, it derives its nutriment from the above animals, is at first so fixed in the egg state, though before it is disengaged from the pedicle it is hatched, since it is often found with its legs displayed and quite active—this is the more probable, as the eggs of the water-mite are fixed by a pedicle to the animals to which they are attached[163]. I have met with a remarkable instance, in which pedunculated eggs seem to draw nutriment from the mother, which brings the pedicle still near to the nature of the umbilical chord. Those of the small hemipterous insect which infests the larch before alluded to, are attached to the anal end of the mother by a short foot-stalk not longer than the egg.

Dr. Derham seems to have observed, that the eggs of some Diptera, of the tribe of Tipulidæ, also increase in size before the larva is excluded[164]. It seems to me likely enough, that in this and many of the above cases in which the egg is supposed to grow, it is rather an extension of the flexile membrane that forms their exterior proportioned to the growth of the included embryo from food it finds within the egg, than from any absorption from without.

vi. Shape. We are accustomed to see the eggs of different species of oviparous animals so nearly resembling each other in form, that the very term egg-shaped has been appropriated to a particular figure. Amongst those of birds, with which we are most familiar, the sole variations are shades of difference between a globular and oval or ovate figure. The eggs of insects, however, are confined by no such limited model. They differ often as much, both as to their shape, sculpture, and appendages, as one seed does from another; and it is not improbable that, if duly studied, they would furnish as good indications of generic distinctions as Gærtner has discovered in those of plants. Their most usual form indeed is globular, oval, or oblong, with various intermediate modifications. We meet with them ovate, or of the shape of the common hen's egg, flat and orbicular, elliptical, conical, cylindrical, hemispherical, lenticular, pyramidal, square, turban-shaped, pear-shaped, melon-shaped, boat-shaped, of the shape of an ale-stand, of a drum, &c.[165], and sometimes of shapes so strange and peculiar, that we can scarcely credit their claim to the name of eggs. Thus the eggs of the gnat are oblong and narrow, or nearly cylindrical, having at the top a cylindrical knob[166], so as to give them the precise form of the round-bottomed phial sometimes used by chemists: those of the common water-scorpion (Nepa cinerea) are oblong, and at the upper end are surrounded by a sort of coronet, consisting of seven slender rays or bristles of the length of the egg[167], so as to resemble somewhat the seeds of Carduus benedictus (Cnicus acarna[168]) of the old botanists. One would think this spinous circlet a very awkward appendage to bodies which are to be gradually extruded through the fine membranous ovaries and oviduct which inclose them: but they are so admirably packed, the unarmed end of each egg fitting closely into the space inclosed by the spines of the one next below it, or, rather, the spines which are moveable, embracing it closely, that not only is no room lost, but the ovaries are perfectly secure from injury. The eggs of another species of this tribe (Ranatra linearis) have only two of these spines or bristles—they are inserted in the stem of a water-rush (Scirpus) or other aquatic plant, so as to be quite concealed, and are only to be detected by the two bristles which stand out from it[169]. The eggs of the beautiful lace-winged flies (Hemerobius), those golden-eyed insects so serviceable in destroying the plant-lice (Aphides[170]), are still more singular. Those of H. Perla are oval, and each of them attached to a filiform pedicle not thicker than a hair, and seven or eight times as long as the egg. By this pedicle (which is supposed to be formed by a glutinous matter attached to one end, which the female draws out by abstracting her ovipositor with the egg partly in it from the leaf, to which she has previously applied it, to a proper length, when the gluten becoming sufficiently solid she wholly quits the egg,) the eggs are planted in groups of ten or twelve on the surface of leaves and twigs, from which they project like so many small fungi, to some of which they have a remarkable resemblance. When the included larva has made its way out of them by forcing open the top, they look like little vases, and were actually once figured by a Naturalist, as we learn from Reaumur, as singular parasitic flowers growing upon the leaves of the elder, for the origin of which he was extremely puzzled to account[171]. Eggs similarly furnished with a pedicle are also laid by other insects; but as most of these have been before alluded to, it is not necessary to describe them here[172]. The cause of these differences of form is for the most part concealed from us: in many instances it may perhaps be referred to that will to vary forms, and so to glorify his wisdom[173] and power, independently of other considerations, which, as Dr. Paley has well remarked[174], seems often to have guided the Great Author of Nature. But in some cases the end to be answered is sufficiently evident. The long footstalks of the eggs of the Hemerobius just mentioned, there can be little doubt, are meant to place them out of the reach of the hosts of predaceous insects which roam around them, from whose jaws, thus elevated on their slender shaft, they are as safe as the eggs of the tailor bird in its twig-suspended nest from the attack of snakes. Reaumur has described the eggs of a kind of fly, common upon the excrements of the horse and other animals (Scatophaga vulgaris Latr.), or one related to it, that requires to be immersed in the dung to which it is committed, on which the future grubs are to feed. He found that if not thus surrounded with moisture, they infallibly shrivelled up and came to nothing; but it is equally necessary that they should not be wholly covered: if they were, the young larva would be suffocated at its first exit from the egg. In what way is this nice point secured? In this manner. Each egg is provided at its upper end, at which the animal when hatched comes out, with two diverging horns[175]; these prevent it from being stuck into the excrement, in which the female deposits the eggs one by one, more than three-fourths of its length: and when examined they resemble not badly, as Reaumur remarks (except that their colour is white), a parcel of cloves stuck into a pudding, as they are neatly inserted at due distances in the disgusting mass[176]. The French Naturalists found these eggs in swine's dung; I have observed them in cow-dung. Latreille thinks that the bristles above described attached to the eggs of Nepa and Ranatra have a similar use, as the female plunges them all but these bristles into the stems of aquatic plants[177]: but may not this have something to do with their oxygenation? Reaumur has figured another egg of a dipterous insect which has a longitudinal wing or lateral margin attached to it, giving it the form of an oblong square, the object of which, he conceives, is to give a greater surface by which it may be more firmly fixed to the substance against which the fly attaches it[178].

Besides these more striking variations in figure, their surface, though often smooth, is frequently curiously and most elegantly sculptured, a circumstance that distinguishes the eggs of no other oviparous animals. Some, as the margined egg just mentioned, are only sculptured on one side, the other being plain; or, as those of the Tusseh silk-worm[179] (Attacus Paphia) and other Bombyces, which have orbicular depressed eggs with a central cavity above and below, have their circumference crossed with wrinkles corresponding with the rings of the inclosed embryo[180]. Others again are sculptured all over. Of these, in some, the sculpture of the two sides is not symmetrical, as in those of a fly figured by Reaumur[181]: but in general there is a correspondence in this respect between the different parts of the egg. In those elegant ones before alluded to of some bird-louse attached to the golden pheasant, the shell resembles the purest wax, and is scored with longitudinal striæ, each distinguished by a series of impressed points, which give it a beautiful appearance of net-work. In the others, as in a common butterfly (Hipparchia Ægeria) and moth (Geometra cratægata), the whole surface is covered with hexagonal reticulations[182]. Others, as those of another butterfly (Hipparchia Hyperanthus), are beset with minute granules or tubercles[183]. Others again, like those of the cabbage and hawthorn butterflies (Pieris Brassicæ and Cratægi), are remarkable for beautiful longitudinal ribs, often connected by elevated lines crossing them at right angles[184]; and in some, as in another butterfly (Hipparchia Jurtina), crowned by imbricated scales[185]. Many other minor differences in this respect might be noticed, but these will suffice to give some idea of the infinite variety exhibited in this respect by these little atoms. If the Creator has wrought them with so much art and skill, can it be beneath his reasonable creatures to examine and admire them, that they may glorify those attributes which they serve to illustrate?

Some eggs after exclusion occasionally become slightly corrugated: Malpighi supposed that this occurs only when the eggs are barren, having observed that those of the moth of the silk-worm which preserved their plumpness always produced caterpillars, while those which lost their original rotundity and became wrinkled were constantly unprolific. Bonnet, however, found exactly the reverse take place in another moth[186], so that these appearances are scarcely to be depended upon. Kuhn asserts, that a virgin female of the puss-moth (Cerura Vinula) having begun to lay eggs, which were yellow above, green below, and depressed, he introduced to her an hour afterwards a male, and some minutes subsequently to the union, she again deposited eggs, which were wholly of a dark brown and convex[187].

vii. Colour. The colour of the eggs of insects is as various as their shape and sculpture. They are very often white, those of some spiders like minute pearls[188]; some are yellow, as those of the silk-worm; others orange, such are the eggs of the bloody-nosed beetle (Timarcha tenebricosa); others again of a golden hue; sometimes they are of a sanguine red. I remember once being much surprised at seeing the water at one end of a canal in my garden as red as blood: upon examining it further I found it discoloured by an infinite number of minute red eggs, belonging probably to some dipterous insect of the Tipulidan tribe. There are also eggs of every intermediate shade between red and black; some again are blue and others green. They are not always of whole colours, for some are speckled like those of many birds, of which I can show you specimens, that are also shaped like birds' eggs; these I think were laid by a common moth (Odenesis potatoria); others are banded with different colours—thus the blue eggs of the lappet-moth (Gastropacha quercifolia) are encircled by three brown zones[189]; others are brown with a white zone[190].

Many eggs assume a very different colour after being laid a few days. In general upon their first exclusion they are white. Those of the chameleon-fly (Stratyomis Chamæleon) which I once found in great numbers, arranged like tiles on a roof one laid partly over another, on the under side of the leaves of the water-plantain, from white become green, and then change to olive green. Those of the hemipterous enemy of the larch, more than once mentioned in this letter, are first mouse-coloured, then they assume a reddish hue, and lastly a blackish one. Those of the gnat from white in a short time assume a shade of green, in a few hours they are entirely green, and at length become gray[191]. Those of the silk-worm, which at first are of a yellow or sulphur colour, acquire a violet shade. The eggs of that rare moth Endromis versicolor, are at first sulphur-coloured, then green, next rose-coloured, and lastly blackish. The colour of almost all eggs changes when they are near hatching; but this change depends more frequently upon the colour of the included larva, which appears through the transparent shell of the egg, than upon any actual alteration in the egg itself.

viii. Period of hatching. The general rule for the hatching of the eggs of insects is the absorption by the embryo of all the superabundant moisture included in them; but the time varies according to the state of the atmosphere, to the action of which they are subjected. Like those of other animals, they require a certain degree of heat for the due evolution of the included larva. This heat in much the greater number of instances is derived from the temperature of the air, but often also from other sources. The eggs of the gad-fly tribe are hatched principally by the heat of the body of the animal to which they are committed; and doubtless the vital heat of various larvæ, small as it may be, must contribute something to the hatching of the eggs deposited in them by various Ichneumons. In the fermenting bark in which the instinct of the rhinoceros beetles (Oryctes nasicornis &c.) impels them to place theirs, the dung which the Scarabæidæ select for that purpose, and the decaying vegetables chosen by many other insects, a degree of artificial heat must exist: and the eggs, or rather egg-like pupæ, of the spider-fly of the swallow (Ornithomyia Hirundinis) are hatched by the heat of those birds which sit upon them along with their own eggs.

Fabricius says, "Insects never sit upon their eggs[192];" but certainly, as I formerly related to you[193], the female earwig does this, and one would be induced to suppose, from the circumstance of the young ones following their mother, as chickens do the hen, that Pentatoma grisea (Cimex Linn.), formerly mentioned, may do the same[194].

With these exceptions, the eggs of all insects are hatched by atmospheric heat alone, the variations in which determine the more speedy or more tardy disclosure of the included insect. The eggs of such species as have several broods in the year, as the nettle butterfly (Vanessa Urticæ) when laid in summer are hatched in a few days; but if not laid till the close of autumn, they remain dormant through the winter, and are only hatched at the return of spring. That this difference is to be attributed to the influence of heat has been often proved by experiment: the autumnal eggs if brought into a warm room may be hatched as soon as those laid in the height of summer. Silk-worms' eggs naturally are not hatched till they have been laid six weeks, but in countries where they are reared, the women effect their exclusion in a much shorter period by carrying them in their bosoms: yet to retard their hatching with particular views is in many circumstances impossible. When the heat of the atmosphere has reached a certain point, the hatching cannot be retarded by cellars; and M. Faujas has remarked, that in June the silk-worm's eggs would hatch in an ice-house[195].

The period of exclusion does not, however, depend solely upon temperature: the hardness or softness of the shell, and possibly differences in the consistence of the included fluid, intended to serve this very purpose, cause some eggs to be hatched much sooner than others exposed to the same degree of heat. Thus the eggs of many flesh-flies are hatched in twenty-four hours[196]; those of bees and some other insects in three days; those of a common lady-bird (Coccinella bipunctata) in five or six days; those of spiders in about three weeks; those of the mole-cricket in a month; while those of many Lepidoptera and Coleoptera require a longer period for exclusion. The hard eggs of Lasiocampa Neustria and castrensis, noticed above, remain full nine months before being hatched[197], as do those of another moth (Hypogymna dispar), which, though laid in the beginning of the warm month of August, do not send forth the included caterpillar till the April following[198]. We know no more of the cause of this difference than of that which takes place in the period of exclusion of the eggs of the different species of birds.

Some eggs change considerably both their form and consistence previously to being hatched. M. P. Huber found that those of different species of ants when newly laid are cylindrical, opaque, and of a milky white; but just before hatching their extremities are arched, and they become transparent with only a single opaque whitish point, cloud, or zone, in their interior[199]. An analogous change takes place in the eggs of many spiders, which just before hatching exhibit a change of form corresponding with that which the included spider receives when its parts begin to be developed, the thin and flexible skin of the egg moulding itself to the body it incloses[200].

In proportion as the germe included in the egg is expanded, it becomes visible through the shell when transparent: this is particularly the case with spiders, in which, as was before observed, every part is very distinctly seen. At length, when all the parts are consolidated so as to be capable of motion, which in spiders takes place in four or five days after they begin to be visible in the egg, the animal breaks the pellicle by the swelling of its body and the movement of its legs, and then quits it, and disengages all its parts one after the other[201]. In general, at least where the shell is harder than that of spiders, insects make their way out by gnawing an opening with their mandibles in the part nearest their head, which, when the shell is very strong (as in Lasiocampa Neustria, &c.), it is often several hours in accomplishing[202]. In many instances, however, the larva is spared this trouble, one end of the egg being furnished with a little lid or trap-door, which it has but to force up, and it can then emerge at pleasure: such lids are to be found in the eggs of several butterflies and moths, as Satyrus Mæra, Saturnia pavonia major, &c. and the common louse[203]. In those exquisitely elegant eggs, before described, of some kind of bird-louse (Nirmus) found adhering to the base of the neck feathers of the golden pheasant[204], there is a lid or cap of this kind of a hemispherical form terminating in a tortuous style. Those of a species of bug (Pentatoma Latr.), found by our friend the Rev. R. Sheppard, besides a convex lid are furnished with a very curious machine, as it should seem, for throwing it off. This machine is dark-brown, of a corneous substance, and of the shape of a cross-bow[205], the bow part being attached to the lid or pushing against it, and the handle, by means of a membrane, to the upper end of the side of the egg.

When the included animal has made its way out of the egg, it enters upon a new state of existence, that of Larva, to which I shall direct your attention in the following letter.

[LETTER XXX.]

STATES OF INSECTS.

LARVA STATE.

The Larva state is that in which insects exist immediately after their exclusion from the egg (or from the mother in ovo-viviparous species), in which they usually eat voraciously, change their skin several times, and have the power of locomotion, but do not propagate.

Almost all larvæ, at their birth, are for a time in a very feeble and languid state, the duration of which differs in different species. In most it continues for a very short time, a few minutes or perhaps hours, after which they revive and betake themselves to their appropriate food. In others, as in the generality of spiders, this debility lasts for seven or eight days, and in some species even a month, during which the young ones remain inactive in the egg-pouch[206], and it is not till they have cast their first skin that their active state of existence commences.

All larvæ may be divided into two great divisions:—

I. Those which in general form more or less resemble the perfect insect.

II. Those which are wholly unlike the perfect insect.

I shall begin by calling your attention to the characters of the first of these divisions: the second, which is by far the most numerous, will be afterwards considered.

I. The first division includes the larvæ of Scorpions, Spiders, Cockroaches, Grasshoppers, Lanthorn-flies, Bugs, &c.; or generally, with the exception of the Flea and Crustacea, the whole of the Linnean Orders Aptera and Hemiptera. All these larvæ, however remotely allied in other respects, agree in the general similarity which they bear to the perfect insects which proceed from them. The most acute entomologist, untaught by experience, could not even guess what would be the form of the perfect insects to be produced from larvæ of the second division, while they can recognise the form of the spider, the cricket, the cockroach, the bug, and the frog-hopper, in that of the larvæ. There are, however, differences in the degrees of this resemblance, according to which we may, perhaps, divide this tribe in their second state as follows:—

i. Those that resemble the perfect insect, except in the relative proportions and number of some of their parts.

ii. Those which resemble the perfect insect, except that they are apterous, or not yet furnished with organs of flight.

i. Spiders, Phalangia, scorpions, lice, Poduræ, sugar-lice (Lepisma), mites, centipedes, millepedes, &c. come under the first subdivision. The larvæ of the first six tribes here mentioned differ at their birth from the perfect insect, only in size and the proportions of their parts. Thus the larvæ of spiders have their legs of a different relative length from that which they subsequently acquire; and the palpi in the males, which previously to the discoveries of Treviranus were regarded as their sexual organs, are not yet fully developed[207]: and a similar difference takes place in the legs of Phalangia. The general form too of the body undergoes slight alterations, and the colour very considerable ones, with each change of the skin—a change to which all these tribes are subject.

The larvæ of the three last-mentioned tribes (the mites, centipedes, and millepedes) differ from the perfect insect not only in the proportion but also in the number of their parts. Leeuwenhoeck states (and De Geer confirms his assertion, extending it to other species of mites[208]), that the common cheese-mite, which in its perfect state has eight legs, when first excluded from the egg has but six, the third pair being wanting[209]. Some however are born with eight legs, for instance A. eruditus of Schrank, which he saw come from the egg itself with that number[210]. Others again have never more than six legs: this is the case with Latreille's genera—Caris, Leptus, Atoma, and Ocypetes of Dr. Leach[211]. In the centipedes (Scolopendridæ) and millepedes (Iulidæ) differences still more remarkable, as I have stated in a former letter, have been observed by De Geer; these animals, in their progress to the perfect state, not only gain several additional pairs of legs, but also several additional segments of the body. This illustrious Entomologist found that Pollyxenus lagurus (Scolopendra L.) was born a hexapod, with but three segments and as many pairs of feet, but successively acquired five additional segments with other appendages, and nine more pairs of feet[212]. A species of millepede (Iulus terrestris L.), which he also traced from its birth, and which begins the world at first with only eight segments and six feet, by a successive development at length acquires, in its perfect state, 50 segments and not less than 200 feet[213]. The nature of these very singular accretions, which Latreille and Mr. Wm. MacLeay have also observed in the centipedes[214], seems not well understood. If, as is most probable, though De Geer could not find any exuviæ[215], the larvæ cast a skin before each change, they do not essentially differ from the metamorphosis of other insects. The legs that these insects thus acquire are affixed to the abdomen, the six that they set out with being attached to the part representing the trunk, so that the former may be regarded as analogous to the prolegs of caterpillars. These animals therefore, as I have before intimated, invert the order of Nature, and from perfect degenerate into imperfect insects.

ii. If you examine the cockroach, cricket, or grasshopper, in different stages of their growth, you will find that the larva does not vary essentially from the perfect insect, except in wanting wings and elytra. The case is the same in almost all the Linnean genera of the modern order—Hemiptera; and with Raphidia, Termes, and Psocus, in the Neuroptera. Some of these, however, exhibit slighter discrepancies in the proportion of some of their parts, but without affecting the general resemblance. Thus the larvæ of the common ear-wig have at first only eight, and subsequently nine joints to their antennæ, whereas the perfect insect has fourteen[216]; and the forceps is quite different, resembling rather two straight styles than what its name implies. In those also of many bugs (Coreus marginatus F. &c.), the joints of the antennæ are of a shape dissimilar to that which obtains in the perfect insect. In that of the common water-scorpion, the anal air-tube, which is so long in the imago, is scarcely visible[217]. In the Cicada tribe, so celebrated for their song[218], neither the larva nor the imago have the enormous thigh armed below with strong teeth, the tibiæ terminating in a fixed incurved claw, probably for the purpose of digging the holes into which they retire till they disclose the fly, which distinguish the pupæ of some species, and is particularly conspicuous in one commonly brought from China[219]. These often exhibit also other minor differences.

II. In treating of the second great division of larvæ, those that are wholly unlike the parent insect,—which includes, with few exceptions[220], the whole of the Linnean orders, Coleoptera, Lepidoptera, Hymenoptera, Diptera, the majority of the Neuroptera, Coccus and Aleyrodes in Hemiptera, and the genus Pulex in Aptera,—I shall advert to their characters, under several distinct heads; and to avoid unnecessary circumlocution, I shall in what follows wholly leave out of consideration the first division already explained, and use the term larvæ with reference only to those of the second. The heads under which I propose to treat of them are: The substance of their body, its parts, shape, or figure, clothing, colour. Also the Economy or mode of life of these creatures: their food, moultings, growth, age, sex, and their preparations for assuming the Pupæ.

i. Substance, with the exception of the head and six fore-feet, which are usually corneous, the exterior integument or skin of larvæ is commonly of a membranous texture, and the body is of a much softer consistence than in the perfect insect. In those, however, of some Staphylinidæ and other Coleoptera, the dorsal part of the three first pieces, which represent the trunk of the perfect insect, is hard and horny. Some also have their whole skin coriaceous, as the tortoise-shell butterfly (Vanessa polychloros); and some few, as the wire-worm (Elater segetum), and other Elateres, very hard. I possess a very remarkable larva from Brazil, from the extreme flatness of its body, and from its having cavities to receive its legs when unemployed, probably living under bark, the skin of which is still harder than that of the grub of the Elaters. Perhaps it has to resist great pressure; and on that account is gifted with this quality, so seldom to be met with in other kinds of larvæ. The interior of the body of these animals is generally of a softer consistence than in the perfect insect. Their intestines, and other internal organs, are usually wrapped in a voluminous substance of a fatty nature, which is regarded as analogous to the epiploon, omentum, or caul, which envelops the viscera of quadrupeds, &c., and is called by Reaumur the corps graisseux. The use of this general flexibility of larvæ is obvious; for, their bodies being mostly long and narrow, a hard rigid covering would have been very inconvenient, and a considerable impediment to their motions. When a caterpillar is feeding, it has occasion to apply its body to any part of the margin of a leaf so as to support itself by its prolegs, and when moving it wants to give it all the curves that are necessary to enable it to avoid obstacles, and thread its way through the sinuous labyrinths which it must often traverse. On the other hand, the hardness of the substance of its head affords a strong fulcrum to the muscles which keep its powerful jaws in constant play. The larvæ, indeed, of some Diptera have a membranous head; but their mandibles, which serve also as legs, are not grinders, but merely claws, the muscles of which require less powerful support[221]. Under this head it may be proper to observe, that generally larvæ are opaque; but some, as those of ants, and a few Lepidoptera[222], are diaphanous. That of Corethra crystallina (Tipula De Geer) is so beautifully transparent as to resemble a piece of crystal, and scarcely to be distinguished from the water in which it lives[223].

ii. Parts. The body of each larva consists of the head, including its different organs, and of the succeeding segments, of which the three first may usually be denominated the trunk, and have the six anterior feet, when present, attached to their under side: the remainder is the abdomen. The latter includes in some species a variable number of membranous feet, as well as various appendages affixed usually to its tail and sides. No larva is ever furnished with wings[224]. Each of these greater divisions, and the organs which they include, require separate consideration.

1. Head. This, as was lately observed, is exteriorly of a horny substance, or at least harder than the rest of the body, in most larvæ; and on this account, though rarely separated from it by any visible distinct neck[225], is, if the larva be of a tolerable size, distinguished at the first view. In those of many Dipterous insects, however, the head is covered with the same flexible membranous skin with the rest of the body, from which it is often scarcely to be distinguished. In these, except that it contains the organs of manducation, it wears no more the appearance of a head than any other segment of the body, and scarcely so much as the last or anal one. The head of these larvæ is also remarkable for another peculiarity,—that it is capable of being extended or contracted, and assuming different forms at the will of the insect: a property which the head of no superior animal can boast. It is probable that there is a considerable variety in the shape and circumstances of the heads of larvæ; but since, with the exception of those of Lepidoptera, they have had less attention paid to them than they deserve (indeed in a vast number of cases, from the difficulty of meeting with them, these variations, except in a few instances, have not been described), I will here mention a few of the most remarkable. The head of the young larva at its first exclusion from the egg is usually the most dilated part of the body, but it does not often continue so. In that of Cicindela campestris, however,—the beautiful green beetle sometimes found in sandy banks,—and also in several caterpillars of Lepidoptera, it is much larger than any of the following segments[226], which, in conjunction with the animal's formidable jaws, gives it a most ferocious appearance. In some lepidopterous larvæ the head is of the same diameter with the rest of the body, but in insects in general it may, I think, be stated as less; and occasionally it bears no proportion whatever to it. This is the case with the subcortical one from Brazil lately mentioned. It is more commonly longer than broad; but in some, as in the larvæ of carrion beetles (Silphæ), the reverse of this takes place. Its shape varies from triangular to orbicular, the mouth of the animal forming the vertex of the triangle. In some larvæ of Hemerobii, however, the head is narrowest behind. That of the grub of a gnat noticed above (Corethra crystallina) forms a kind of sharp horn or claw, terminating the body anteriorly[227]. The contour of the head of larvæ is usually intire and unbroken; but in the caterpillars of some Lepidoptera, as the butterfly called the grand admiral (Vanessa Atalanta), the Glanville fritillary (Melitæa Cinxia), &c. it is divided into two lobes[228]. In the Brazil flat larvæ it is trilobed, each lateral lobe being divided into three smaller ones: in which circumstance it somewhat resembles the head of some subcortical Cimicidæ. Although the part we are treating of is generally without horns, yet in some tropical butterflies of the tribe of Nymphales, it is singularly armed with them. Thus Papilio Anchises is distinguished, according to Madame Merian[229], by two in the occiput, which it has the power of retracting. In the purple highflier (Apatura Iris), a British species, the two lobes of the head, I am informed, terminate behind in two horns; as they do likewise in the brilliant Morpho Menelaus[230], the lobes assuming the form of a pear, and the horn representing the stalk. In a caterpillar I found amongst Mr. Francillon's larvæ, the head is bilobed, with a very long recurving subcapitate subramose spine. In Satyrus Cassiæ, the head is armed with three occipital stout spines[231]. The larva of Nymphalis Amphinome Latr. (Limenitis F.) is crowned with a coronet of eight occipital stout acute spines, the intermediate ones being the longest[232]; and that of Morpho Teucer has a similar coronet, consisting of only seven blunt rays, seemingly, rather than spines[233]. With regard to the articulation of the head with the trunk, it is generally by its whole diameter; but in some instances, only by a part of it. This is the case with one of a sphinx figured by Mad. Merian[234]; and I have another, probably belonging to the nocturnal Lepidoptera (Phalæna L.)[235]. In both these, the head is vertical and triangular; and in the latter (which is a remarkable creature, the tail itself being more like a head, and furnished with what resemble two prominent black eyes) the vertex of the triangle is considerably higher than the back of the animal. Whatever may be the clothing of the body, the head is usually naked. Sometimes, however, it is itself beset with very small simple spines, as in the butterfly of the mallow (Hesperia Malvæ); or with longer compound ones, such as are found on the rest of the body. This is the case with one of a butterfly named by Rösel Papilio morsa. The most common colour of the head of larvæ, where it differs from the rest of the body, is a darker or lighter reddish brown, or piceous. This is particularly observable in those of Coleopterous insects, but it is very commonly in other tribes of the same hue. Sometimes, amongst the Lepidoptera, the head is of a different colour from the rest of the body; especially where a contrast renders it striking. I can show the caterpillar of some insect, probably of the hawk-moth tribe (Sphingidæ), from Georgia, remarkable for the length of its anal spine, in which the body is black, and the head red: another has a white head and a brown body. In the larvæ of some Lepidoptera, Coleoptera, and Diptera, the head can be wholly or nearly withdrawn within the first segment of the body. This may be readily seen in that of the common glow-worm; and that of a small gnat (Tipula replicata De Geer) withdraws it so completely that the anterior margin of that segment closes the orifice, so that the animal appears to have no head[236].—The parts of the head which require distinct consideration are, the eyes, antennæ, and the mouth: consisting of various organs, which will be specified. Some of these parts and organs are peculiar to larvæ of one order, others to those of another, and some are furnished with them all.

Eyes. The larvæ of many insects have no eyes. Those with antennæ which terminate in a lamellated clava (Scarabæus L.), and capricorn beetles also (Cerambyx L.), amongst the Coleoptera, are without them, and probably several others; and amongst the Diptera, all those with a membranous or variable head. Those of the remaining orders, with the exception, perhaps, of some Hymenoptera and Lepidoptera, are furnished with these organs; and in the Coleoptera all the predaceous tribes, as well as most of those that are herbivorous or granivorous, and the Gnats and other Tipulidans (Tipulariæ Latr.) in the Diptera, are also distinguished by them. In the larvæ of the dragon-flies (Libellula L.), and other Neuroptera, they are composed of many facets as in those of the perfect insect, from which they differ chiefly in being smaller. But in the other insects of this description they are simple, and resemble those of the Arachnida, and many aptera. These simple eyes vary in their number, in different genera and tribes, from one to six on each side of the head. Thus the larva of Telephorus, and the saw-flies, has only one[237]; that of Cicindela three, the two posterior ones being large with a red pupil surrounded by a paler iris, which adds to the fierce aspect of this animal; and the anterior one very minute. Those of the tortoise-beetles also (Cassida) have three[238]; of Staphylinus, four; of Timarcha (the bloody-nosed beetle) five; of Carabus, and the Lepidoptera in general, six. In the last they are of different sizes, and generally arranged in a circle: in that of Hemerobius there are five in a circle, with one central one[239]. The appearance of these globules, which are often not visible but under a powerful lens, is so different from that of the eyes of a butterfly or moth, or other perfect insect, that it has been doubted whether they actually perform the office of eyes, but without reason. They occupy the usual station of those organs, being situated in many instances upon a protuberance which appears to incase them; and seem of a construction closely analogous to that of the eyes of spiders, and the stemmata or ocelli of Hymenoptera, which have been satisfactorily proved to be organs of vision. In the larva of a moth not yet ascertained to exist in this country, Attacus Tau, and probably other species, the eyes, after the skin has been changed a few times, are no longer to be seen[240].

Antennæ. Most larvæ are provided with organs near the base of the mandibles, which from their situation and figure may be regarded as antennæ. Fabricius has asserted that the larvæ of the saw-flies (Tenthredo L.) have no antennæ; but in this he was mistaken, for though very short, they are discoverable in them, as he might have learned by consulting De Geer[241]. In the majority of Neuropterous larvæ, they almost precisely resemble those of the perfect insect. In all the rest they are very different. The antennæ of Coleopterous larvæ are usually either filiform or setaceous, consisting of four or five joints, nearly equal in length. Those of Lepidopterous larvæ are commonly conical, as are those likewise of Chrysomela and Coccinella &c. amongst the Coleoptera, and very short, composed of two or three joints, of which the last is much thinner than the first, and ends in one or two hairs or bristles. These antennæ the larva has the power of protruding or retracting at pleasure. Lyonnet informs us, that the caterpillar of the great goat-moth (Cossus ligniperda) can draw the joints of its antennæ one within the other, so as nearly to conceal the whole[242]. The larva of the common gnat has two long incurved setaceous antennæ, fringed with hairs at some distance from their apex, which consist only of a single joint[243]. The greater number of Dipterous larvæ, however, all indeed except the Tipulidans (Tipulariæ Latr.), and many belonging to the Coleoptera and Hymenoptera orders (as those of Curculio, Apion, Apis, &c.), are wholly deprived of antennæ. It is a general rule, that the antennæ of larvæ are shorter than the same organs in the perfect insect, the tribe Ephemerina perhaps affording the only example in which the reverse of this takes place[244].

Mouth. All larvæ have a mouth situated in the head, by which they receive their food, and furnished with one or more instruments for the purpose of mastication and deglutition. These instruments, in all the orders except Lepidoptera, some Neuroptera and Diptera, bear a general resemblance to the same parts in the perfect insect. In larvæ of the Coleopterous, Lepidopterous, and Hymenopterous orders, we can distinguish for the most part an upper and under lip; two pairs of jaws answering to the mandibulæ and maxillæ; and two, four, or six palpi[245]: and some of these instruments may be found in most Diptera. Each of these parts require separate notice.

Upper-lip (Labrum). The mouth of almost all larvæ, except some of the order Diptera, are provided with a distinct upper-lip, for retaining their food during mastication. As the construction of this part does not widely differ from that of the perfect insect, which will hereafter be more fully described, it is only necessary to observe, that it is usually a transverse moveable plate, attached posteriorly to the nasus (clypeus F.), and situated just above the mandibles[246].

Upper-jaws (Mandibulæ). The most usual figure of these, which are of a hard horny consistence[247], is that of two slightly concave, oblong, or triangular plates, often at their lower extremity of considerable thickness, and of very irregular form, the base of which is filled with powerful muscles, and planted in the side of the mouth so as to move transversely. The other extremity can be made to meet or diverge like the claws of pincers, and are divided into one or more tooth-like indentations, by means of which the food of the larva is cut[248]. This is their construction in the larvæ of all Lepidoptera, and in many of those of the other orders. They frequently, however, assume a different form, though their situation is always the same. Thus in the larvæ of the capricorn beetles (Cerambyx L.) and of other wood-boring species, they are shaped like the half of a cone, the inner sides of which, applying close to each other, form a pair of powerful grindstones, capable of comminuting the hardest timber[249]. M. Cuvier has observed, with regard to the mandibulæ of those of stag-beetles (Lucanus), that besides their teeth at the extremity, they have towards their base a flat striated molary surface; so that they both cut and grind their ligneous food[250]. It seems to have escaped him, that a similar structure takes place in many perfect insects of the lamellicorn tribe, as I shall hereafter show you. In the larvæ of the water-beetles (Dytiscus L.), ant-lions (Myrmeleon L.), and lace-winged flies (Hemerobius L.), they resemble somewhat the forceps at the tail of an ear-wig, being long and incurved; and, what is more remarkable, hollow and perforated at the end, so as to serve as a channel for conveying into the larva's mouth the juices of the prey which by their aid it has seized. Reaumur even asserts, that the larva of Myrmeleon has no other entrance into its throat than through these tubular mandibles[251]. That of the rove-beetles (Staphylinus L.), and of many other Coleopterous genera, have these organs of this forcipate construction, without being perforated[252]. In the larva of the carnivorous flies, and many other Diptera, are two black incurved subulate parts, connected at the base, and capable of being protruded out of, and retracted into, the head, through the skin of which they are usually visible. As I informed you in a former letter[253], these mandibles are used for walking as well as feeding: they are parallel to each other, and are neither formed for cutting nor grinding like the mandibles of other insects, but merely detach particles of food by digging into it and tearing the fibres asunder. In this operation they are probably assisted by an acutely triangular dart-like instrument of a horny substance, which in some species (Musca vomitoria) is placed between the two. In others this part is wanting. Some Dipterous larvæ have two similar mandibles, but instead of being parallel, they are placed one above the other; others (Musca domestica and meridiana) have but one such mandible, and some have no perceptible mandible of any kind. The mandibles of the larva of the crane-flies (Tipula), which are transverse and unguiform, do not act against each other, but against two other fixed, internally concave and externally convex, and dentated pieces[254].

Under-jaws (Maxillæ). These are a pair of organs, usually of a softer consistence, placed immediately under the upper-jaws; but as they are usually so formed and situated as not to have any action upon each other, it is probable that in general they rather assist in submitting the food to the action of the mandibulæ, than in the comminution of it. In Lepidopterous larvæ they appear to be conical or cylindrical (at least in that of the cossus so admirably figured by Lyonnet[255]), and to consist of two joints; which may, I imagine, be analogous to the upper and lower portions of which the maxillæ of perfect insects usually consist. The last of these joints is surmounted by two smaller jointed palpiform organs. If any part of the maxillæ can act upon each other, it is these organs or palpi; but it is evident they are not calculated for mastication, although they may assist in the retention of the substance to be masticated. In a figure given by Reaumur of the under side of the head of another lepidopterous larva (Erminea Pomonella), the maxillæ consist of a single joint, and appear to be crowned by chelate palpi[256]: a circumstance which is also observable in that of a common species of stag-beetle (Lucanus parallelipipedus), the weevil of the water-hemlock (Lixus paraplecticus[257]), and other insects. In general the maxillæ of larvæ are without the lobe or lobes discoverable in those of most perfect insects, this part being usually represented by a kind of nipple, or palpiform jointed process, strictly analogous to the interior maxillary palpi of the predaceous coleoptera; but in most of the lamellicorn beetles the lobe exists in its proper form[258], as it does likewise in that of the capricorn-beetle before noticed (Callidium violaceum[259]). In the former instance, it is armed with spines or claws; but in the latter it is unarmed, and rounded at the end. In the larva of Cicindela campestris, the base of the maxilla runs in a transverse direction from the mentum, to which, as is usually the case, it is attached. From this at right angles proceeds the lobe, from the outer side of which the feeler emerges; and the inner part terminates in an unguiform joint, ending in two or three bristles. The structure in the larvæ of water-beetles (Dytiscus L.) is different, for they appear to be without maxillæ[260]; but the case really seems to be, that these organs are represented by the first joint of what M. Cuvier calls their palpi[261]; from which proceed the real palpi, the interior one being very short, and consisting only of a single joint. These maxillæ of larvæ were regarded by Reaumur and other writers as parts of the under-lip, on each side of which they are situated; and indeed, as well as those in the perfect insect, they form a part of the same machine, being connected by their base with the mentum, which is part of the labium, but they are clearly analogous to the maxillæ of the imago. They are not to be found in the larvæ of many Dipterous insects, and perhaps in some species belonging to other orders. In some Neuropterous larvæ, as those of the Libellulina MacLeay, the maxillæ are of a substance quite as solid and horny as the mandibles, which in every respect they resemble[262].

Under-lip (Labium). Between the two maxillæ in the larvæ of most of the insects under consideration is a part termed by Reaumur the middle division of the under-lip, but which is in fact analogous to the whole of that organ in the imago. This organ varies in shape, being sometimes quadrangular, often conical, &c. Interiorly it is frequently connected with a more fleshy protuberance, called the tongue by Reaumur[263], and supplying the place of the ligula in the perfect insect. On each side of the apex of the under-lip is a minute feeler, and in the middle between these in the Lepidoptera and many others, is a filiform organ, which I shall call the spinneret (Fusulus), through which the larva draws the silken thread employed in fabricating its cocoon, preparatory to assuming the pupa state, and for other purposes[264]. This organ is found only in those larvæ which have the power of spinning silk; that is, in all Lepidoptera, most Hymenoptera, Trichoptera, some Neuroptera, and even a Dipterous insect[265]. This tube, Lyonnet had reason to believe, is composed of longitudinal slips, alternately corneous and membranous, so as to give the insect the power of contracting its diameter, and thus making the thread thicker or smaller. There is only a single orifice at the end, which is cut obliquely, somewhat like a pen, only with less obliquity, and without a point, the opening being below, so as to be conveniently applicable to the bodies on which the larva is placed. Reaumur conceived that this spinneret had two orifices; but Lyonnet ascertained this to be a mistake, the two silk tubes uniting into one before they reach the orifice. From the contractile nature of the sides and the form of the orifice, combined with the power the insect has of moving it in every direction, results the great difference which we see in the breadth and form of the threads, some being seven or eight times as thick as others, some cylindrical, others flat, others channelled, and others of different thickness in different parts[266]. In the larvæ of many Diptera the under-lip is merely a small tubercle, which can be protruded from the insect's mouth by pressure[267].

One of the most remarkable prepensile instruments, in which the art and skill of a Divine Mechanician are singularly conspicuous, and which appears to be without a parallel in the insect world, may be seen in the under-lip of the various species of dragon-fly (Libellula L.). In other larvæ this part is usually small and inconspicuous, and serves merely for retaining the food and assisting in its deglutition; but in these it is by far the largest organ of the mouth, which when closed it entirely conceals; and it not only retains but actually seizes the animal's prey, by means of a very singular pair of jaws with which it is furnished. Conceive your under-lip (to have recourse, as Reaumur on another occasion[268], to such comparison,) to be horny instead of fleshy, and to be elongated perpendicularly downwards[269], so as to wrap over your chin and extend to its bottom,—that this elongation is there expanded into a triangular convex plate[270], attached to it by a joint[271], so as to bend upwards again and fold over the face as high as the nose, concealing not only the chin and the first-mentioned elongation, but the mouth and part of the cheeks[272]: conceive, moreover, that to the end of this last-mentioned plate are fixed two other convex ones, so broad as to cover the whole nose and temples[273],—that these can open at pleasure, transversely like a pair of jaws, so as to expose the nose and mouth, and that their inner edges where they meet are cut into numerous sharp teeth or spines, or armed with one or more long and sharp claws[274]:—you will then have as accurate an idea as my powers of description can give, of the strange conformation of the under-lip in the larvæ of the tribes of Libellulina; which conceals the mouth and face precisely as I have supposed a similar construction of your lip would do yours. You will probably admit that your own visage would present an appearance not very engaging while concealed by such a mask; but it would strike still more awe into the spectators, were they to see you first open the two upper jaw-like plates, which would project from each temple like the blinders of a horse; and next, having by means of the joint at your chin let down the whole apparatus and uncovered your face, employ them in seizing any food that presented itself, and conveying it to your mouth. Yet this procedure is that adopted by the larvæ provided with this strange organ. While it is at rest, it applies close to and covers the face. When the insects would make use of it, they unfold it like an arm, catch the prey at which they aim by means of the mandibuliform plates, and then partly refold it so as to hold the prey to the mouth in a convenient position for the operation of the two pairs of jaws with which they are provided. Reaumur once found one of them thus holding and devouring a large tadpole;—a sufficient proof that Swammerdam was greatly deceived in imagining earth to be the food of animals so tremendously armed and fitted for carnivorous purposes. Such an under-lip as I have described is found in the tribe of dragon-flies (Libellulina); varied, however, considerably in its figure in the different genera. In the larva of Libellula Fab., such as Libellula depressa, &c. it is of the shape above described; so exactly resembling a mask, that if Entomologists ever went to masquerades, they could not more effectually relieve the insipidity of such amusements and attract the attention of the demoiselles, than by appearing at the supper table with a mask of this construction, and serving themselves by its assistance. It would be difficult, to be sure, by mechanism to supply the place of the muscles with which in the insect it is amply provided: but Merlin, or his successor, has surmounted greater obstacles. In the larva of the Fabrician Æshnæ (Libellula grandis, &c. L.), this apparatus is not convex but flat: so that, though it equally conceals the face, it does not so accurately resemble a mask; and the jaws at its apex are not convex plates, but rather two single conical teeth[275]. It is, as to its general shape, similarly constructed in Agrion Fab. (L. Virgo, &c. L.); but the first joint is more remarkably elongated, the jaws more precisely resemble jaws than in any of the rest, and are armed with three long, very sharp teeth: between them also there is a lozenge-shaped opening, through which, when the apparatus is closed, is protruded a circular sort of nipple, apparently analogous to the ligula[276]. Libellula ænea, L., which is the type of another tribe (Cordulia Leach), has a mask somewhat different from all the above, the jaws being armed with a moveable claw and an internal tooth[277]. You will admire the wisdom of this admirable contrivance, when you reflect that these larvæ are not fitted to pursue their prey with rapidity, like most predaceous animals; but that they steal upon them, as De Geer observes[278], as a cat does upon a bird, very slowly, and as if they counted their steps; and then, by a sudden evolution of this machine, take them as it were by surprise, when they think themselves safe. De Geer says, it is very difficult for other insects to elude their attacks, and that he has even seen them devour very small fishes[279]. As these animals are found in almost every ditch, you will doubtless lose no time in examining for yourself an instance of so singular a construction.

Feelers (Palpi). In the orders Diptera and Hymenoptera are many larvæ in which these organs have not been certainly discovered; yet Reaumur in that of a common fly (M. meridiana L.) found four retractile nipples[280] which seem analogous to them; and Latreille has observed, that below the mandibles of those of ants are four minute points, two on each side[281]: but in all other larvæ their existence is more clearly ascertained. The maxillary palpi vary in number, many having two on each maxilla and others only one. In the perfect insect the former is one of the distinguishing characters of the predaceous beetles (Entomophagi Latr.), but in the larvæ it is more widely extended; since even in the caterpillars of Lepidoptera the inner lobe of the maxilla which represents this feeler is jointed, which is precisely the case with the beetles just named. Cuvier has observed this circumstance in the larva of the stag-beetle[282]; and it belongs to many other Coleoptera that have only a pair of maxillary palpi in the perfect state. The labial palpi are always two, emerging usually one on each side from the apex of the under-lip. With regard to the form of the palpi, those of the Lepidoptera are mostly conical; in other orders they are sometimes setaceous and sometimes filiform. Their termination is generally simple, but sometimes the last joint is divided. They are for the most part very short, and the labial shorter than the maxillary. The latter never exceed four joints[283], which seems the most natural number; and the former are limited to three. Both vary between these numbers, and one joint. The joints, though commonly simple, are sometimes branched. This is the case with one I met with in considerable numbers upon the Turnip, in October 1808, the second joint of the palpi of which sends forth near the apex an internal branch. In the larva of the Cossus, as Lyonnet informs us[284], the joints of the palpi are retractile, so that the whole of the organ may be nearly withdrawn.

After thus describing the head of larvæ, and its principal organs, we must next say something upon the remainder of the body, or what constitutes the

2. Trunk and Abdomen: which I shall consider under one article. These are composed of several segments or rings, to which the feet and other appendages of the body are fixed. The form of these segments, or that of their vertical section, varies considerably: in many Lepidoptera, the wire-worm, &c., it would be nearly circular; in others a greater or less segment of a circle would represent it; and in some, perhaps, it would consist of two such segments applied together. Their lower surface is generally nearly plane. Their most natural number, without the head and including the anal segment, is twelve: this they seldom exceed, and perhaps never fourteen. The three first segments are those which represent the trunk of the perfect insect, and to which the six anterior legs when present are affixed. In general, they differ from the remaining segments only in being shorter, and in many cases less distinctly characterized; but in Neuropterous larvæ, those of Dytisci, and some other Coleoptera, they are longer than the succeeding ones, and pretty nearly resemble the trunk of the animal in its last state. The surface of the trunk and abdomen will be considered under a subsequent head; I shall not, therefore, describe it here. The conformation of the different segments varies but little, except of the terminal one, or tail, which in different larvæ takes various figures. In most, this part is obtuse and rounded; in others acute or acuminate; in others truncate; and in others emarginate, or with a wider sinus, and with intermediate modifications of shape which it would be endless to particularize. In some, also, it is simple and unarmed; in others beset with horns, spines, radii, and tubercles of different forms, some of which will come under future consideration. The parts connected with the trunk and abdomen which will require separate consideration, are the legs, the spiracles, and various appendages.

Legs. It may be stated generally that the larvæ of the orders Coleoptera, Lepidoptera, and Neuroptera, have legs; and that those of the orders Hymenoptera and Diptera have none. This must be understood, however, with some exceptions. Thus the larvæ of some Coleoptera, as the weevil tribes (Curculio L.) have no legs, unless we may call by that name certain fleshy tubercles besmeared with gluten, which assist them in their motions[285]; while those of Tenthredo and Sirex in the order Hymenoptera are furnished with these organs. At present I know no Dipterous larva that may be said to have real legs, unless we are to regard as such certain tentacula formed upon a different model from the legs of other larvæ[286]. Rösel has, I think, figured a Lepidopterous apode. No Neuropterous one has yet been discovered.

The legs of larvæ are of two kinds; either horny and composed of joints, or fleshy and without joints[287]. The first of these, as I observed in a former letter[288], are the principal instruments of locomotion, and the last are to be regarded chiefly as props and stays by which the animal keeps its long body from trailing, or by which it takes hold of surfaces; while the other legs, or where there are none, the annuli of its body, regulate its motions. The former have been commonly called true legs (pedes veri), because they are persistent, being found in the perfect insect as well as in the larva; and the latter spurious legs (pedes spurii), because they are caducous, being found in the larva only. Instead of these not very appropriate names, I shall employ for the former the simple term legs, and for the latter prolegs (propedes)[289].

The legs, when present, are always in number six, and attached by pairs to the underside of the three first segments of the trunk. They are of a horny substance, and consist usually of the same parts as those of the perfect insect; namely, coxa, trochanter, femur, tibia, and tarsus, suspended to each other by membranous ligaments: these parts are less distinctly marked in some than in others. Thus in the legs of a caterpillar, or the grub of a capricorn-beetle, at first you would think there were only three or four joints besides the claw; but upon a nearer inspection, you would discover at the base of the leg the rudiments of two others[290], in the latter represented indeed by the fleshy protuberance from which the legs emerge. In the larvæ of the predaceous Coleoptera, the hip and trochanter are as conspicuous nearly as in the perfect insect; and the tarsus, which still consists of only a single joint, is armed with two claws[291]. In those of the Neuroptera order, in which all the joints are very conspicuous, the tarsi are jointed, as well as two-clawed[292]. The legs of larvæ are usually shorter than those of the perfect insect, and scarcely differ from each other in shape, for they all gradually decrease in diameter from the base to the apex. This is the most usual conformation of them in Lepidopterous, Hymenopterous, and some Coleopterous larvæ, (those of the capricorn-beetles are very short and minute, so as to be scarcely visible,) in which they are so small as to be concealed by the body of the insect[293]. In Neuropterous larvæ, however, and several Coleoptera, as those of Dytiscus, Staphylinus, Coccinella, &c., they more resemble the legs of the perfect insect, the joints being more elongated, and the femoral one projecting beyond the body[294].

You will find no other than true legs in most Coleopterous, Neuropterous, and Hymenopterous larvæ. But those of the saw-flies (Tenthredo L.), and all caterpillars, have besides a number of prolegs: a few Dipterous larvæ also, are provided with some organs nearly analogous to them. These prolegs are fleshy, commonly conical or cylindrical, and sometimes retractile protuberances, usually attached by pairs to the underside of that part of the body that represents the abdomen of the future fly[295]. They vary in conformation and in number; some having but one, others as many as eighteen.

With regard to their conformation, they may be divided into two principal sections: first, those furnished with terminal claws; and secondly, those deprived of them. Each of which may be divided into smaller sections, founded on the general figure of the prolegs, and arrangement of the claws or hooks.

i. The prolegs of almost all Lepidopterous larvæ are furnished with a set of minute slender horny hooks, crotchets, or claws, of different lengths, somewhat resembling fish-hooks; which either partially or wholly surround the apex like a pallisade. By means of these claws, of which there are from forty to sixty in each proleg, a short and a long one arranged alternately, the insect is enabled to cling to smooth surfaces, to grasp the smallest twigs to which the legs could not possibly adhere: a circumstance which the flexible nature of the prolegs greatly facilitates[296]. Claws nearly similar are found on the prolegs of some Dipterous larvæ[297], but not in any of those of the other orders. These last, however, are seldom either so numerous, or arranged in the same manner, as in caterpillars. When the sole of the foot is open, the claws with which it is more or less surrounded are turned outwards, and are in a situation to lay hold of any surface; but when the animal wishes to let go its hold, it begins to draw in the skin of the sole, and in proportion as this is retracted, the claws turn their points inwards, so as not to impede its motion[298].

The prolegs with claws may be further divided into four different kinds.

1. In the larvæ of the great majority of butterflies and moths they assume the form of a truncated cone, the lower and smaller end of which is expanded into a semicircular or subtriangular plate, having the inner half of its circumference beset with the claws above mentioned; and, from its great power of dilating and contracting, admirably adapted for performing the offices of a foot. Jungius calls these legs pedes elephantini[299]; and the term is not altogether inapplicable, since they exhibit considerable resemblance to the clumsy but accommodating leg and foot of the gigantic animal he alludes to.

2. The larvæ of many minute moths, particularly of the Fabrician genera Tortrix and Tinea—those which live in convoluted leaves, the interior of fruits, &c., as well as the Cossus, and some other large moths,—have their prolegs of a form not very unlike those of the preceding class, but shorter, and without any terminal expansion; the apex, moreover, is wholly, instead of half, surrounded with claws[300]; the additional provision of which, together with a centrical kind of nipple capable of being protruded or retracted, in some measure, though imperfectly, supplies the place of the more flexible plate-like expansion present in the first class.

3. The third class is composed of a very few Lepidopterous larvæ which have their prolegs very thick and conical at the base, but afterwards remarkably slender, long, and cylindrical, so as exactly to assume the shape of a wooden leg[301]. These, as in the first class, are expanded at the end into a flat plate: but this is wholly circular, is surrounded with claws, and has also in the middle a retractile nipple, as in the preceding class. In Cossus, at least in an American species (Cossus Robiniæ), described by Professor Peck[302], the anal prolegs have the claws only on their exterior half.

4. The remaining description of unguiferous prolegs, if they may not rather be deemed a kind of tentacula, are those of certain Diptera, provided with no true legs; which differ from the three preceding classes, either in their shape, or the arrangement of their claws. In one kind of those remarkable larvæ, which from their long respiratory anal tubes Reaumur denominates "rat-tailed," that of Elophilus pendulus, there are fourteen of these prolegs, affixed by pairs to the ventral segments, the twelve posterior ones of which are subconical, and truncate at the apex, which is surrounded with two circles of very minute claws, those of the inner being much more numerous and shorter than those of the exterior circle; while the anterior pair terminate in a flat expansion, and in shape almost exactly resemble those of a mole[303]. The prolegs of the larvæ of a kind of gnat called by De Geer Tipula amphibia, and of Syrphus mystaceus F., (Musca plumata De Geer,) are nearly of a similar construction, but in the last are armed with three claws only[304]. Long moveable claws also distinguish the singular prolegs before described[305] of another gnat (Tanypus maculatus Meig., Tipula De Geer). The case-worms (Trichoptera K.) and some others, have two prolegs at the anus, each furnished with a single claw[306].

ii. The prolegs deprived of claws are found in the larva of the Hymenopterous tribe of saw-flies (Tenthredo L.), in those of some Lepidoptera (Hepialus F. &c.), and in some few Coleopterous and Dipterous genera. Those of the former are of the shape of a truncated cone, and resemble the second class of unguiculate prolegs, except in the defect of claws. In the latter they are a mere retractile nipple-like protuberance, in some species so small as scarcely to be perceptible. In all they aid in progressive motion; but it is by laying hold of surfaces, and so enabling the body more readily to push itself forward by annular contraction and dilatation, and not by taking steps, of which all prolegs are incapable: to assist in this purpose the protuberance sometimes secretes a gluten[307], which supplies the place of claws. Some larvæ have the power of voluntarily dilating certain portions of the underside of their body, so as to assume nearly the shape and to perform the functions of prolegs. In a Coleopterous (?) subcortical one from Brazil, before alluded to, there are four round and nearly flat areas in each ventral segment of the abdomen, but the last very little raised above the surface, and rough, somewhat like a file; and besides these, the base of the anal segment has ten of these little rough spaces, but of a different shape, being nearly linear, placed in a double series, five on each side. Doubtless these may be regarded as a kind of prolegs, which enable the animal to push itself along between the bark and the wood[308].

In considering, in the next place, the number and situation of the prolegs, it will contribute to distinctness to advert to these circumstances as they occur in the different orders furnished with these organs.

To begin with the Lepidoptera.—Lepidopterous larvæ have either ten, eight, six, or two prolegs, seldom more[309], and never fewer. Of these, with a very few exceptions, two are attached to the last or anal, and the rest, when present, to one or more of the sixth, seventh, eighth, and ninth segments of the body: none are ever found on the fourth, fifth, tenth, or eleventh segments.

1. Where ten prolegs are present, as is the case in by far the greatest proportion of Lepidopterous larvæ, there is constantly an anal pair, and a pair on each of the four intermediate segments just mentioned.

2. In caterpillars, which like those of a few species of the genera Sphinx, Pyralis, and of the Bombycidæ, &c. have eight legs, they are placed in three different ways. In those which have an anal pair, the remaining six are in some fixed to the sixth, seventh, and eighth; in others, to the seventh, eighth, and ninth segments. In those which, like Cerura Vinula, and several other species of the same family, have no anal prolegs; the whole eight emerge from the sixth, seventh, eighth, and ninth segments.

3. The Hemigeometers, as Noctua Gamma, &c. have only six legs: namely, an anal pair, and two ventral ones, situated on the eighth and ninth segments.

4. The larvæ of the Geometers (Geometræ F.) have but four prolegs; of which two are anal, and two spring from the ninth segment. It should be observed, however, that the larvæ of Hemigeometers, and even of some of those that have ten prolegs, where the four anterior ones are much shorter than the rest, move in the same way as the Geometers. This even prevails in a few where these organs are all of equal length.

5. Many of the larvæ of Tinea L. which live in the interior of fruits, seeds, &c., have but one pair of prolegs, which are attached to the anal segment.

6. The larvæ of Haworth's genus Apoda (Hepialus Testudo and Asellus F.), remarkable for their slug-like shape and appearance, move by the aid of two lateral longitudinal pustule-like protuberances, which leave a trace of a gummy slime in their course.

Hymenoptera.—The larvæ of the different tribes of Tenthredo L., almost the only Hymenopterous insects in which prolegs are present, have a variable number of these organs; some sixteen, as the saw-fly of the willow (T. lutea L.), and this is the most numerous tribe of them, including the modern genera, Cimbex F., Pterophorus, &c. Others have fourteen, as that of the cherry (T. cerasi L.); and many others with only nine joints to their antennæ. A third class have only twelve, as that of the rose (T. Rosæ L.), but this contains but few species. The last class contains those that have no prolegs at all, but only the six horny ones appended to the trunk. Of this tribe, the caterpillars of which have a very different aspect from the preceding, are those of the genus Lyda F. (T. crythrocephala L.)[310]. Two of the prolegs are anal, and the rest intermediate, and none are furnished with claws. This circumstance, in conjunction with the greater number of prolegs, except in the case of Lyda, will always serve as a mark to distinguish these fausses chenilles, as the French call the larvæ of saw-flies, from true caterpillars. The dorsal prolegs of a species of Cynips described by Reaumur have been before noticed.

Coleoptera.—The larvæ of insects of this order are so little known or attended to, that no very accurate generalization of them in this respect is practicable. Many of them, in addition to their six horny legs, have a proleg at the anus; which in many cases appears to be the last segment of the abdomen, forming an obtuse angle with the remainder of it, so as to support that part of the body, and prevent it from trailing; and in some instances, as in Chrysomela Populi, a common beetle, secreting a slimy matter to fix itself[311]. In the larvæ of Staphylinidæ this proleg is very long and cylindrical; in that of Cicindela it is shorter, and in shape a truncated cone rather compressed; it is very short, also, in those of the Silphæ that I have seen. In the wire-worm (Elater Segetum) it is a minute retractile tubercle, placed in a nearly semicircular space, shut in by the last dorsal segment, which becomes also ventral at the anus. This space is in fact the last ventral segment. This seems characteristic of the genus[312]. From the underside of the body of the common meal-worm (Tenebrio Molitor), at the junction of the two last segments, when the animal walks, there issues a fleshy part, furnished below with two rather hard, long, and moveable pediform pieces, which the animal uses in walking[313]. In the larva of another beetle, whose ravages have been before noticed, under the name of the cadelle[314] (Trogosita mauritanica), a pair of prolegs are said to be found under the anal segment; and in that of the bloody-nose beetle (Timarcha tenebricosa), that segment is bifid. That of the weevil of the common water-hemlock (Lixus paraplecticus F.) exhibits a singular anomaly: prolegs occupy the usual station of the true legs, being attached to the three segments representing the trunk[315]. This insect, however, does not appear to use them in moving. A pair in each of the twelve segments of the body are found in the grub of another weevil (Hypera Rumicis Germ.), the nine last pair being the shortest, which all assist the insect in walking[316]. But the greatest number of prolegs is to be found in the Brazil subcortical larva lately mentioned. Besides the six horny legs of the trunk, this remarkable animal has four prolegs on each of the seven intermediate abdominal segments, and five on each side of the base of the last, making the whole number of prolegs, if so they may be called, amount to forty-four: a far greater number than is to be found in any larva at present known. When I wrote to you upon the motions of insects, I informed you that some larvæ moved by means of legs upon their back[317], but I was not then aware that any were furnished with them both on the back and the belly at the same time. By the kindness of Mr. Joseph Sparshall of Norwich, a very ardent and indefatigable entomologist, I am in possession of the larva of Rhagium fasciatum, a timber-feeding beetle. This animal on the ten intermediate segments of the underside of the body, which in the centre form a fleshy protuberance, has on it a double series of rasps, as it were, consisting each of two rows of oblique oblong prominences; and on the seven intermediate dorsal segments there are also in the centre seven rasps of three or four rows each, of similar prominences: so that this animal at the same time can push itself along both by dorsal and ventral prolegs. It is worthy of observation, that a pair of these rasps is between the second and third pair of true legs.

Diptera.—The larva of a little gnat, Tipula stercoraria De Geer[318] (Chironomus Meig.?), drags itself along by the assistance of a single tubercle, placed on the underside of the first segment of the body, which the animal has the power of lengthening or contracting[319]. That of another beautiful Chironomus (C. plumosus), remarkable for the feathered antennæ of the male[320], has two short prolegs, or pediform but not retractile tentacula in the same situation[321]. Others, as that of Tanypus maculatus, &c. have two pairs, one attached to the anal and the other to the first segment[322]. Tipula amphibia De Geer in this state has ten prolegs, placed by pairs on the fourth, fifth, eighth, ninth, and tenth dorsal segments[323]; and Scæva Pyrastri F., one of the aphidivorous flies, has not fewer than forty-two, arranged in a sextuple series, seven in each row[324].

It may not be useless to close this long description of the legs of larvæ with a tabular view of them, founded chiefly upon these organs; which afford very obvious marks of distinction.

1. Larvæ without legs.
   1. With a corneous head of determinate shape (coleopterous and hymenopterous apods—Culicidæ, some Tipulidæ, &c. amongst the Diptera).
   2. With a membranaceous head of indeterminate shape (Muscidæ, Syrphidæ, and other Diptera).
2. Larvæ with legs.
   1. With legs only, and with or without an anal proleg (Neuroptera, and many Coleoptera).
      1. Joints short and conical (Elater, Cerambycidæ, &c.).
      2. Joints long and subfiliform (Staphylinus, Coccinella, Cicindela, &c.).
   2. Prolegs only (many Tipulidæ, and some subcutaneous Lepidopterous larvæ, &c.).
   3. Both legs and prolegs (Lepidoptera, Tenthredinidæ, and some Coleoptera).
      1. Without claws (Tenthredinidæ, &c.).
      2. With claws (Lepidoptera, &c.).

I should next say something upon the spiracles, or breathing-pores, or any other external apparatus for the purpose of respiration, in larvæ; but I think it will be best to reserve the consideration of these for a subsequent Letter. We will therefore conclude this detailed description of their parts in their first state, with some account of their other.

iii. Appendages. The generality of larvæ have no other external organs than those already described; but in several of them we observe various kinds of retractile ones and others—protuberances—horn-like processes—rays, &c.; which, though not properly coming either under any of the above parts, or under the clothing of these animals, yet require to be noticed. Upon these I shall now enlarge a little.

You must have observed upon the back of the last segment but one of the caterpillar of the silk-worm a horn-like process, rising at first nearly perpendicularly, and then bending forward. A similar horn, though confined in the genus Bombyx to the silk-worm and a few others, if we may believe Madame Merian, who, however, often makes great mistakes, is found in the beautiful caterpillar of one of the largest and finest moths that we know (Erebus Strix[325]), the glory of the Noctuidæ, and in most of those of the hawk-moths (Sphinx F.) [S. Porcellus, Vitis, and a few others excepted; in some of which, as S. Labruscæ, &c., this anal horn is replaced by a gibbosity, and in others, as S. Œnotheræ, by a callous eye-like plate[326]] in the same situation, but much longer[327], and commonly curving backwards over the tail[328]. Sometimes, however, as in S. ocellata and S. Stellatarum, it is perfectly straight. These organs towards the apex are horny, and often end in a sharp point; nearer the base they are fleshy. They are without any true joint[329], yet the insect can elevate or depress them at pleasure. Under a lens, they usually appear covered with spinous eminences, arranged like scales. The use of these horns is quite unknown: Goedart fancies that they secrete a potent poison, and are intended as instruments of defence; but both suppositions are altogether unfounded. It has been remarked, that the body of those caterpillars which have these horns, is firmer, and yields less to the touch than that of those which have no such appendages[330]. The larva of a small timber-devouring beetle (Lymexylon dermestoides F.) has, like the above caterpillars, a long horn, and in the same situation: it has also a singular protuberance on the first segment[331]. Upon some other caterpillars, as in Bombyx Stigma F., a singular pair of horn-like appendages arises from the back of the second segment of the body, excluding the head. In a tawny-coloured one from Georgia, with a transverse row of short black spines on each segment, these horns are half an inch long, black, covered with spinous eminences, rather thickest at the base, and terminate in a little knob. They appear to articulate with the body at the lower extremity. I have another species, black, with narrow longitudinal yellow stripes, in which these horns are of equal thickness at base and apex, but with the same terminal knob. Danais Archippus has a pair of tentacula at the head, and another pair, but shorter, at the tail; and D. Gylippus has, besides these, two in the middle of the body[332].