Transcriber’s notes:

Except for the spelling corrections listed below, the text of this book has been preserved as in the original, including inconsistent punctuation, hyphenation and accents.
Lepidotera → Lepidoptera
coccoon → cocoon
subtances → substances
Bütchsli → Bütschli

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THE COCKROACH

STUDIES IN COMPARATIVE ANATOMY—III

THE STRUCTURE AND LIFE-HISTORY OF THE COCKROACH (PERIPLANETA ORIENTALIS) An Introduction to the Study of Insects

BY

L. C. MIALL

PROFESSOR OF BIOLOGY IN THE YORKSHIRE COLLEGE, LEEDS

AND

ALFRED DENNY

LECTURER ON BIOLOGY IN THE FIRTH COLLEGE, SHEFFIELD

LONDON: LOVELL REEVE & CO.

LEEDS: RICHARD JACKSON

1886

STUDIES IN COMPARATIVE ANATOMY.

I.—THE SKULL OF THE CROCODILE. A Manual for Students. By Professor L. C. Miall. 8vo, 2s. 6d.

II.—THE ANATOMY OF THE INDIAN ELEPHANT. By Professor L. C. Miall and F. Greenwood. 8vo, 5s.

III.—THE COCKROACH: An Introduction to the Study of Insects. By Professor L. C. Miall and A. Denny. 8vo, 7s. 6d.

IV.—MEGALICHTHYS; A Ganoid Fish of the Coal Measures. By Professor L. C. Miall (In preparation).

MAY BE HAD OF

LOVELL REEVE & CO., LONDON;

RICHARD JACKSON, LEEDS.

PREFACE.

That the thorough study of concrete animal types is a necessary preliminary to good work in Zoology or Comparative Anatomy will now be granted by all competent judges. At a time when these subjects, though much lectured upon, were rarely taught, Döllinger, of Würzburg, found out the right way. He took young students, often singly, and made them master such animal types as came to hand, thereby teaching them how to work for themselves, and fixing in their minds a nucleus of real knowledge, around which more might crystallise. “What do you want lectures for? Bring any animal and dissect it here,” said he to Baer, then a young doctor longing to work at Comparative Anatomy.[1] It was Döllinger who trained Purkinje, Pander, Baer, and Agassiz, and such fame cannot be heightened by words of praise. In our own time and country Döllinger’s methods have been practised by Professor Huxley, whose descriptive guides, such as the Elementary Biology and the delightful little book on the Crayfish, now make it easy for every teacher to work on the same lines. From the description of the Cockroach in Huxley’s Anatomy of Invertebrated Animals came the impulse which has encouraged us to treat that type at length. It may easily turn out that in adding some facts and a great many words to his account, we have diluted what was valuable for its concentration. But there are students—those, namely, who intend to give serious attention to Entomology—who will find our explanations deficient rather than excessive in detail. It is our belief and hope that naturalists will some day recoil from their extravagant love of words and names, and turn to structure, development, life-history, and other aspects of the animal world which have points of contact with the life of man. We have written for such as desire to study Insects on this side.

Whoever attempts to tell all that is important about a very common animal will feel his dependence upon other workers. Much of what is here printed has been told before. The large number of new figures is, however, some proof that we have worked for ourselves.

It is a pleasant duty to offer our thanks for friendly help received. Professor Félix Plateau, of Ghent; Mr. Joseph Nusbaum, of Warsaw; and Mr. S. H. Scudder, of Cambridge, Massachusetts, have very kindly consented to treat here of those parts of the subject which they have specially illustrated by their own labours.[2] Mr. E. T. Newton, of the Jermyn Street Museum, has lent us the wood blocks used to illustrate one of his papers on the Brain of the Cockroach. A number of the figures have been very carefully and faithfully drawn for us by Miss Beatrice Boyle, a student in the Yorkshire College. We are much indebted to Dr. Murie, the Librarian of the Linnean Society, for procuring us access to the extensive literature of Insect Anatomy, and for answering not a few troublesome questions.

Five articles on the Cockroach were contributed by us to Science Gossip in 1884, and some of the figures were then engraved and published.

In issuing a book which has been long in hand, but which can never hope to be complete, we venture to adopt the words already used by Leydig concerning his Lehrbuch der Histologie:—“Die eigentlich nie fertig wird, die man aber für fertig erklären muss, wenn man nach Zeit und Umständen das Möglichste gethan hat.”

CONTENTS.

*_** Where the species is not named, it is to be understood that the figures are drawn from the Cockroach.

Leeds:

McCorquodale & Co. Limited,

Basinghall Street.

STUDIES IN COMPARATIVE ANATOMY.—No. III.

THE COCKROACH.

CHAPTER I.

Writings on Insect Anatomy.

Marcello Malpighi. 1628–1694.

Jan Swammerdam. 1637–1680.

Pierre Lyonnet. 1707–1789.

Hercule Straus-Dürckheim. 1790–1865.

The lovers of minute anatomy have always been specially attracted to Insects; and it is not hard to tell why. No other animals, perhaps, exhibit so complex an organisation condensed into so small a body. We possess, accordingly, a remarkable succession of memoirs on the structure of single Insects, beginning with the revival of Anatomy in the 17th century and extending to our own times. The most memorable of these Insect-monographs bear the names of Malpighi, Swammerdam, Lyonnet, and Straus-Dürckheim.

Malpighi on the Silkworm.

Malpighi’s treatise on the Silkworm (1669) is an almost faultless essay in a new field. No Insect—hardly, indeed, any animal—had then been carefully described, and all the methods of work had to be discovered. “This research,” says Malpighi, “was extremely laborious and tedious” (it occupied about a year) “on account of its novelty, as well as the minuteness, fragility, and intricacy of the parts, which required a special manipulation; so that when I had toiled for many months at this incessant and fatiguing task, I was plagued next autumn with fevers and inflammation of the eyes. Nevertheless, such was my delight in the work, so many unsuspected wonders of nature revealing themselves to me, that I cannot tell it in words.” We must recall the complete ignorance of Insect-anatomy which then prevailed, and remember that now for the first time the dorsal vessel, the tracheal system, the tubular appendages of the stomach, the reproductive organs, and the structural changes which accompany transformation were observed, to give any adequate credit to the writer of this masterly study. Treading a new path, he walks steadily forward, trusting to his own sure eyes and cautious judgment. The descriptions are brief and simple, the figures clear, but not rich in detail. There would now be much to add to Malpighi’s account, but hardly anything to correct. The only positive mistakes which meet the eye relate to the number of spiracles and nervous ganglia—mistakes promptly corrected by Swammerdam. Had the tract De Bombycibus been the one work of its author, this would have kept his memory bright, but it hardly adds to the fame of the anatomist who discovered the cellular structure of the lung, the glandular structure of the liver and kidney, and the sensory papillæ of the skin, who first saw the blood-corpuscles stream along a vessel, who studied very early and very completely the minute structure of plants and the development of the chick, and whose name is rightfully associated with the mucous layer of the epidermis, the vascular tufts of the kidney, and the follicles of the spleen, as well as with the urinary tubules of Insects.

All that we know of Malpighi commands our respect. Precise and rapid in his work, keen to discover points of real interest, never losing himself in details, but knowing when he had done enough, he stands pre-eminent in the crowd of minute anatomists, who are generally faithful in a few things, but very unfit to be made rulers over many things. The last distinct glimpse which we get of him is interesting. Dr. Tancred Robinson, writing to John Ray, from Geneva, April 18th, 1684, tells how he met Malpighi at Bologna. They talked of the origin of fossils, and Malpighi could not contain himself about Martin Lister’s foolish hypothesis that fossils were sports of nature. “Just as I left Bononia,” he continues, “I had a lamentable spectacle of Malpighi’s house all in flames, occasioned by the negligence of his old wife. All his pictures, furniture, books, and manuscripts were burnt. I saw him in the very heat of the calamity, and methought I never beheld so much Christian patience and philosophy in any man before; for he comforted his wife, and condoled nothing but the loss of his papers, which are more lamented than the Alexandrian Library, or Bartholine’s Bibliothece, at Copenhagen.”[3]

Swammerdam on the Honey Bee.

Swammerdam’s great posthumous work, the Biblia Naturæ, contains about a dozen life-histories of Insects worked out in more or less detail. Of these the May-fly (published during the author’s life-time, in 1675) is the most famous; that on the Honey Bee the most elaborate. Swammerdam was ten years younger than Malpighi, and knew Malpighi’s treatise on the Silkworm—a not inconsiderable advantage. His working-life as a naturalist comes within the ten years between 1663 and 1673; and this short space of time was darkened by anxiety about money, as well as by the religious fanaticism, which in the end completely extinguished his activity. The vast amount of highly-finished work which he accomplished in these ten years justifies Boerhaave’s rather rhetorical account of his industry. Unfortunately, Boerhaave, whom we have to thank not only for a useful sketch of Swammerdam’s life, but also for the preservation of most of his writings, was only twelve years old when the great naturalist died, and his account cannot be taken as personal testimony. Swammerdam, he tells us, worked with a simple microscope and several powers. His great skill lay in his dexterous use of scissors. Sometimes he employed tools so fine as to require whetting under the microscope. He was famous for inflated and injected preparations. As to his patience, it is enough to say that he would spend whole days in clearing a single caterpillar. Boerhaave gives us a picture of Swammerdam at work which the reader does not soon forget. “His labours were superhuman. Through the day he observed incessantly, and at night he described and drew what he had seen. By six o’clock in the morning in summer he began to find enough light to enable him to trace the minutiæ of natural objects. He was hard at work till noon, in full sunlight, and bareheaded, so as not to obstruct the light; and his head streamed with profuse sweat. His eyes, by reason of the blaze of light and microscopic toil, became so weakened that he could not observe minute objects in the afternoon, though the light was not less bright than in the morning, for his eyes were weary, and could no longer perceive readily.”

Comparing Swammerdam’s account of the Bee with the useful and amply illustrated memoir of Girdwoyn (Paris, 1876), it is plain that two centuries have added little to our knowledge of the structure of this type. Much has been made out since 1675 concerning the life-history of Bees, but of what was to be discovered by lens and scalpel, Swammerdam left little indeed to others. It is needless to dwell upon the omissions of so early an explorer. Swammerdam proved by dissection that the queen is the mother of the colony, that the drones are males, and the working-bees neuters; but he did not find out that the neuters are only imperfect females. In this instance, as in some others, Swammerdam’s authority served, long after his death, to delay acceptance of the truth. It is far from a reproach to him that in the Honey Bee he lit upon an almost inexhaustible subject. In the 17th century no one suspected that the sexual economy of any animal could be so complicated as that which has been demonstrated, step by step, in the Honey Bee.

Lyonnet on the Goat Moth.

In Lyonnet’s memoir on the larva of the Goat Moth (Traité Anatomique de la Chenille qui ronge le bois de Saule, 1760[4]) we must not look for the originality of Malpighi, nor for the wide range of Swammerdam. One small thing is attempted, and this is accomplished with unerring fidelity and skill. There is something of display in the delineation of the four thousand and forty-one muscles of the Caterpillar, and the author’s skill as a dissector is far beyond his knowledge of animals, whether live or dead. The dissections of the head are perhaps the most extraordinary feat, and will never be surpassed. Modern treatises on Comparative Anatomy continue to reproduce some of these figures, such as the general view of the viscera, the structure of the leg, and the digestive tract. Nearly the whole interest of the volume lies in the plates, for the text is little more than a voluminous explanation of the figures.

It is not without surprise that we find that Lyonnet was an amateur, who had received no regular training either in anatomy or engraving, and that he had many pursuits besides the delineation of natural objects. He was brought up for the Protestant ministry, turned to the bar, and finally became cipher-secretary and confidential translator to the United Provinces of Holland. He is said to have been skilled in eight languages. His first published work in Natural History consisted of remarks and drawings contributed to Lesser’s Insect Theology (1742). About the same time, Trembley was prosecuting at the Hague his studies on the freshwater Polyp, and Lyonnet gave him some friendly help in the work. Those who care to turn to the preface of Trembley’s famous treatise (Mémoires pour servir à l’histoire des Polypes d’eau douce, 1744) will see how warmly Lyonnet’s services are acknowledged. He made all the drawings, and engraved eight of them himself, while Trembley is careful to note that he was not only a skilful draughtsman, but an acute and experienced observer. When the work was begun, Lyonnet had never even seen the operation of engraving a plate. Wandelaar, struck by the beauty of his drawings, persuaded him to try what he could do with a burin. His first essay was made upon the figure of a Dragon-fly, next he engraved three Butterflies, and then, without longer apprenticeship, he proceeded to engrave the plates still required to complete the memoir on Hydra.

Lyonnet tells us that the larva of the Goat Moth was not quite his earliest attempt in Insect Anatomy. He began with the Sheep Tick, but suspecting that the subject would not be popular, he made a fresh choice for his first memoir. Enough interest was excited by the Traité Anatomique to call for the fulfilment of a promise made in the preface that the description of the pupa and imago should follow. But though Lyonnet continued for some time to fill his portfolio with drawings and notes, he never published again. Failing eyesight was one ground of his retirement from work. What he had been able to finish, together with a considerable mass of miscellaneous notes, illustrated by fifty-four plates from his own hand, was published, long after his death, in the Mémoires du Muséum (XVIII.–XX.).

Straus-Dürckheim on the Cockchafer.

In beauty and exact fidelity Straus-Dürckheim’s memoir on the Cockchafer (Considérations Générales sur l’Anatomie Comparée des Animaux Articulés, auxquelles on a joint l’Anatomie Descriptive du Melolontha vulgaris, 1828) rivals the work of Lyonnet. Insect Anatomy was no longer a novel subject in 1828, but Straus-Dürckheim was able to treat it in a new way. Writing under the immediate influence of Cuvier, he sought to apply that comparative method, which had proved so fertile in the hands of the master, to the Articulate sub-kingdom. This conception was realised as fully as the state of zoology at that time allowed, and the Considérations Générales count as an important step towards a complete comparative anatomy of Arthropoda. Straus-Dürckheim had at command a great mass of anatomical facts, much of which had been accumulated by his own observations. He systematically compares Insects with other Articulata, Coleoptera with other Insects, and the Cockchafer with other Coleoptera. Perhaps no one before him had been perfectly clear as to the morphological equivalence of the appendages in all parts of the body of Arthropods, and here he was able to extend the teaching of Savigny. His limitations are those of his time. If in certain sections we find his collection of facts to be meagre, and his generalisations nugatory, we must allow for the progress of the last sixty years—a progress in which Straus-Dürckheim has his share. It is the work of science continually to remake its syntheses, and no work becomes antiquated sooner than morphological generalisation.

It is therefore no reproach to Straus-Dürckheim that his treatise should now be chiefly valuable, not as “Considérations Générales,” but as the anatomy of the Cockchafer. Long after his theories and explanations have ceased to be instructive, when the morphology and physiology of 1828 have become as obsolete as the Ptolemaic astronomy, the naturalist will study these exquisite delineations of Insect-structure with something of the pleasure to be found in examining for the hundredth time a delicate organism familiar to many generations of microscopic observers.

The fidelity and love of anatomical detail which characterise the description of the Cockchafer are not less conspicuous in Straus-Dürckheim’s Anatomie Descriptive du Chat (1846). Both treatises have become classical.

We have seen how, in Straus-Dürckheim’s hands, Insect anatomy became comparative. New studies—histology, embryonic development, and palæontology—have since arisen to complicate the task of the descriptive anatomist, and it appears to be no longer possible for one man to complete the history of any animal of elaborate structure and ancient pedigree. As a method of research the monograph has had its day. The path of biological discovery now follows an organ or a function across all zoological boundaries, and it is in the humbler office of biological teaching that the monograph finds its proper use.

Later Insect Anatomists.

It is impossible even to glance at the many anatomists who have illustrated the structure of Insects by studies, less simple in plan, but not less profitable to science, than those of the monographers. If we attempt to select two or three names for express mention, it is with a conviction that others are left whom the student is bound to hold in equal honour.

Dufour[5] laboured, not unsuccessfully, to construct a General Anatomy of Insects, which should combine into one view a crowd of particular facts. The modern reader will gratefully acknowledge his industry and the beauty of his drawings, but will now and then complain that his sagacity does not do justice to his diligence.

Newport,[6] a naturalist of greater weight and interest, is memorable for his skill in minute dissection, for his many curious observations upon the life-history of Insects (see, for example, his memoir on the Oil-beetle), and especially for his early appreciation of the value of embryological study.

Leydig[7] was the first to occupy fully the new field of Insect histology, and point out its resources to the physiologist. In all his works the student finds beauty and exactness of delineation, suggestiveness in explanation. Leydig’s contributions to Insect anatomy and physiology, valuable as they are to the specialist, are not isolated researches, but form part of a new comparative anatomy, based upon histology. Incomplete so vast a work must necessarily remain, but it already extends over considerable sections of the animal kingdom.

CHAPTER II.

The Zoological Position of the Cockroach.

The place of the Cockroach in the Animal Kingdom is illustrated by the above table. It belongs to the sub-kingdom Arthropoda, to the class Insecta, and to the order Orthoptera.

Characters of Arthropoda.

Arthropoda are in general readily distinguished from other animals by their jointed body and limbs. In many Annelids the body is ringed, and each segment bears a pair of appendages, but these appendages are soft, and never articulated. The integument of an Arthropod is stiffened by a deposit of the tough, elastic substance known as Chitin, which resembles horn in appearance, though very different in its chemical composition. In marine Arthropoda, as well as in many Myriopoda and Insects, additional firmness may be gained by the incorporation of carbonate and phosphate of lime with the chitin. However rigid the integument may be, it is rendered compatible with energetic movements by its unequal thickening. Along defined, usually transverse lines it remains thin, the chitinous layer, though perfectly continuous, becoming extremely flexible, and allowing a certain amount of deflection or retraction (fig. 1). The joints of the trunk and limbs may thus resemble stiff tubes. Muscles are attached to their inner surface, and are therefore enclosed by the system of levers upon which they act (fig. 2B). In Vertebrate animals, on the contrary, which possess a true internal skeleton, the muscles clothe the levers (bones) to which they are attached (fig. 2A). The whole outer surface of an Arthropod, including the eyes, auditory membrane (if there is one), and surface-hairs, is chitinised. Chitin may also stiffen the larger tendons, internal ridges and partitions, and the lining membrane of extensive internal cavities, such as the alimentary canal, and the air-tubes of Insects.

Fig. 1.—Diagram of Arthropod limb extended, retracted, and flexed. Graber has given a similar figure (Insekten, fig. 8*).

Fig. 2.—Vertebrate and Arthropod joints. A, Vertebrate joint, the skeleton clothed with muscles. B, Arthropod joint, the skeleton enclosing the muscles.

In most Arthropoda the body is provided with many appendages. In Crustacea there are often twenty pairs, but some Myriopoda have not far from two hundred pairs. Some of these may be converted to very peculiar functions; in particular, several pairs adjacent to the mouth are usually appropriated to mastication. One or more pairs of appendages are often transformed into antennæ.

The relative position of the chief organs of the body, viz.:—heart, nerve-cord, and alimentary canal, is constant in Arthropoda. The heart is dorsal, the nerve-cord ventral, the alimentary canal intermediate. (See fig. [3].) The œsophagus passes between the connectives of the nerve-cord. Not a few other animals, such as Annelids and Mollusca, exhibit the same arrangement.

Arthropoda are not known to be ciliated in any part of the body, or in any stage of growth. Another histological peculiarity, not quite so universal, is the striation of the muscular fibres throughout the body. In many Invertebrates there are no striated muscles at all, while in Vertebrates only voluntary muscles, as a rule, are striated.

The circulatory organs of Arthropoda vary greatly in plan and degree of complication, but there is never a completely closed circulation.

The development of Arthropoda may be accompanied by striking metamorphosis, e.g., in many marine Crustacea, but, as in other animals, the terrestrial and fluviatile forms usually develop directly. Even in Insects, which appear to contradict this rule flatly, the exception is more apparent than real. The Insect emerges from the egg as a fully formed larva, and so far its development is direct. It is the full-grown larva, however, which corresponds most nearly to the adult Myriopod, while the pupa and imago are stages peculiar to the Insect. It is not by any process of embryonic development, but by a secondary metamorphosis of the adult that the Insect acquires the power of flight necessary for the deposit of eggs in a new site.

Fig. 3.—Longitudinal section of Female Cockroach, to show the position of the prin­ci­pal or­gans. Oe, œ­sopha­gus; S.gl, salivary gland; S.r, sal­iv­ary res­er­voir; Cr, crop; G, gizzard; St, chyl­ific stom­ach; R, rec­tum; Ht, heart; N.C, nerve-cord. × 7.

Characters of Insects.

Insects are distinguished from other Arthropoda by the arrangement of the segments of the body into three plainly marked regions—head, thorax, and abdomen; by the three pairs of ambulatory legs carried upon the thorax; by the single pair of antennæ; and by the tracheal respiration. Myriopods and Arachnida have no distinct thorax. Most Crustacea have two pairs of antennæ, while in Arachnida antennæ are wanting altogether. Crustacea, if they possess special respiratory organs at all, have branchiæ (gills) in place of tracheæ (air-tubes). In Arachnida, Myriopoda, and Crustacea there are usually more than three pairs of ambulatory legs in the adult.

The appendages of an Insect’s head (antennæ, mandibles, maxillæ) are appropriated to special senses, or to the operations of feeding, and have lost that obvious correspondence with walking legs which they still retain in some lower Arthropoda (Peripatus, Limulus, Arachnida). The thorax consists of three[8] segments, each of which carries a pair of ambulatory legs. No abdominal legs are found in any adult insect. The middle thoracic segment may carry a pair of wings or wing-covers, and the third segment a pair of wings.

The lower or less-specialised Insects, such as the Cockroach, have nearly as many nerve-ganglia as segments, and the longitudinal connectives of the nerve-cord are double. In the adult of certain higher Insects[9] (e.g., many Coleoptera, and some Diptera) the nerve-ganglia are concentrated, reduced in number, and restricted to the head and thorax; while all the connectives, except those of the œsophageal ring, may be outwardly single.

The heart, or dorsal vessel, is subdivided by constrictions into a series of chambers, from which an aorta passes forwards to the head.

Air is usually taken into the body by stigmata or breathing-pores,[10] which lie along the sides of the thorax and abdomen. It circulates through repeatedly-branching tracheal tubes, whose lining is strengthened by a spiral coil. Air-sacs (dilated portions of the air-tubes) occur in Insects of powerful flight.

The generative organs are placed near the hinder end of the body.[11] Most Insects are oviparous.[12] The sexes are always distinct; but imperfect females (“neuters”) occur in some kinds of social Insects. Agamogenesis (reproduction by unfertilised eggs) is not uncommon.

Orders of Insects.

The orders of Insects are usually defined with reference to the degree of metamorphosis and the structure of the parts of the mouth. Five of the orders (3, 5–8) in the table on page [9] undergo complete metamorphosis, and during the time of most rapid change the insect is motionless. In the remaining orders (1, 2, 4) there is either no metamorphosis (Thysanura), or it is incomplete—i.e., the insect is active in all stages of growth. Among these three orders we readily distinguish the minute and wingless Thysanura. Two orders remain, in which the adult is commonly provided with wings; of these, the Orthoptera have biting jaws, the Hemiptera, jaws adapted for piercing and sucking.

The name of Black Beetle, often given to the Cockroach, is therefore technically wrong. True Beetles have a resting or chrysalis stage, and may further be recognised in the adult state by the dense wing-covers, meeting along a straight line down the middle of the back, and by the transversely folded wings. Cockroaches have no resting stage, the wing-covers overlap, and the wings fold up fan-wise.

Further Definition of Cockroaches.

In the large order of Orthoptera, which includes Earwigs, Praying Insects, Walking Sticks, Grasshoppers, Locusts, Crickets, White Ants, Day-flies, and Dragon-flies, the family of Cockroaches is defined as follows:—

Family Blattina. Body usually depressed, oval. Pronotum shield-like. Legs adapted for running only. Wing-covers usually leathery, opaque, overlapping (if well developed) when at rest, anal area defined by a furrow (fig. 4). Head declivent, or sloped backwards, retractile beneath the pronotum. Eyes large, ocelli rudimentary, usually two, antennæ long and slender.

Fig. 4.—Generalised sketch of Cockroach wing-cover.

About eight hundred species of Cockroaches have been defined, and to facilitate their arrangement, three groups have been proposed, under which the different genera are ranked.[13]

Group 1. Both sexes wingless (Polyzosteria).

Group 2. Males winged, females wingless (Perisphæria, Heterogamia).

Group 3. Both sexes with more or less developed wings (about 7 genera).

In Group 3 occur the only two genera which we shall find it necessary to describe—viz., Blatta, which includes the European Cockroaches, and Periplaneta, to which belong the Cockroaches of tropical Asia and America.

Genus Blatta. A pulvillus between the claws of the feet. The seventh sternum of the abdomen entire in both sexes; sub-anal styles rudimentary in the male.

Genus Periplaneta. Readily distinguished from Blatta by the divided seventh abdominal sternum of the female, and the sub-anal styles of the male.

Two species of Periplaneta have been introduced into Europe. These are—

1. P. orientalis (Common Cockroach, Black Beetle). Wing-covers and wings not reaching the end of the abdomen in the male; rudimentary in the female.

2. P. americana (American Cockroach). Wing-covers and wings longer than the body in both sexes.

CHAPTER III.

The Natural History of the Cockroach.

SPECIAL REFERENCES.

Hummel. Essais Entomologiques, No. 1 (1821).

Cornelius. Beiträge zur nähern Kenntniss von Periplaneta orientalis (1853.)

Girard. La domestication des Blattes. Bull. Soc. d’Acclimatisation, 3^e Sér., Tom. IV., p. 296 (1877).

Range.

The common Cockroach is native to tropical Asia,[14] and long ago made its way by the old trade-routes to the Mediterranean countries. At the end of the sixteenth century it appears to have got access to England and Holland, and has gradually spread thence to every part of the world.

Perhaps the first mention of this insect in zoological literature occurs in Moufet’s Insectorum Theatrum (1634), where he speaks of the Blattæ as occurring in wine cellars, flour mills, &c., in England. It is hard to determine in all cases of what insects he is speaking, since one of his rude woodcuts of a “Blatta” is plainly Blaps mortisaga; another is, however, recognisable as the female of P. orientalis; a third, more doubtfully, as the male of the same species. He tells how Sir Francis Drake took the ship “Philip,”[15] laden with spices, and found a great multitude of winged Blattæ on board, “which were a little larger, softer, and darker than ours.” Perhaps these belonged to the American species, but the description is obscure. Swammerdam also was acquainted with our Cockroach as an inhabitant of Holland early in the seventeenth century. He speaks of it as “insectum illud Indicum, sub nomine Kakkerlak satis notum,” and very properly distinguishes from it “the species of Scarabæus” (Blaps), which Moufet had taken for a Blatta.[16]

The American Cockroach is native to tropical America, but has now become widely spread by commerce. An Australian species also (P. australasiæ) has begun to extend its native limits, having been observed in Sweden,[17] Belgium, Madeira, the East and West Indies,[18] Florida,[19] &c. In Florida it is said to be the torment of housekeepers.

To the genus Blatta belong a number of small European species, which mostly lurk in woods and thickets. Some of these are found in the south of England. B. lapponica is one of the commonest and most widely distributed. It is smaller than the common Cockroach, and both sexes have long wings and wing-cases. The males are black and the females yellow. It is found on the mountains of Norway and Switzerland as high as shrubs extend, and when sheltered by human dwellings, can endure the extreme cold of the most northern parts of Europe. This is the insect of which Linnæus tells, that in company with Silpha lapponica it has been known to devour in one day the whole stock of dried but unsalted fish of a Lapland village. B. germanica also has the wings and wing-cases well developed in both sexes. Two longitudinal stripes on the pronotum, or first dorsal plate of the thorax, are the readiest mark of this species, which is smaller and lighter in colour than the common Cockroach. It is plentiful in most German towns, and has been introduced from Germany into many other countries;[20] but it appears to be native, not to Germany alone, but to Asia and all parts of central and southern Europe. Where and how it first became domesticated we do not know.

The other species of Cockroaches which have been met with in Europe are Panchlora maderæ, said by Stephens to be occasionally seen in London, and Blabera gigantea the Drummer of the West Indies, which has often been found alive in ships in the London Docks.

Blatta germanica, Periplaneta orientalis, and P. americana, are so similar in habits and mode of life as to be interchangeable, and each is known to maintain itself in particular houses or towns within the territory of another species, though usually without spreading.

Orientalis is, for example, the common Cockroach of England, but germanica frequently gets a settlement and remains long in the same quarters. H. C. R., in Science-Gossip for 1868, p. 15, speaks of it as swarming in an hotel near Covent Garden, where it can be traced back as far as 1857. In Leeds, one baker’s shop is infested by this species; it is believed to have been brought by soldiers to the barracks, after the Crimean war, and to have been carried to the baker’s in bread-baskets. We have met with no instance in which it has continued to gain ground at the expense of orientalis. Americana also seems well established in particular houses or districts in England. H. C. R. (loc. cit.) mentions warehouses near the Thames, Red Lion and Bloomsbury Squares, and the Zoological Gardens, Regent’s Park. It frequents one single warehouse in Bradford, and is similarly local in other towns with foreign trade.

Many cases are recorded in which germanica has been replaced by orientalis, as in parts of Russia and Western Germany, but detailed and authenticated accounts are still desired. On the whole orientalis seems to be dominant over both germanica and americana.

The slow spread of the Cockroaches in Europe is noteworthy, not as exceptional among invading species, but as one more illustration of the length of time requisite for changes of the equilibrium of nature. It took two centuries from the first introduction of orientalis into England for it to spread far from London. Gilbert White, writing, as it would appear, at some date before 1790, speaks of the appearance of “an unusual insect,” which proved to be the Cockroach, at Selborne, and says: “How long they have abounded in England I cannot say; but have never observed them in my house till lately.”[21] It is probable that many English villages are still clear of the pest. The House Cricket, which the Cockroaches seem destined to supplant, still dwells in our houses, often side by side with its rival, sharing the same warm crannies, and the same food. The other imported species, though there is reason to suppose that they cannot permanently withstand orientalis, are by no means beaten out of the field; they retreat slowly where they retreat at all, and display inferiority chiefly in this, that in countries where both are found, they do not spread, while their competitor does. It may yet require some centuries to settle the petty wars of the Cockroaches.

It is also worth notice that in this, as in most other cases, the causes of such dominance over the rest as one species enjoys are very hard to discover. We cannot explain what peculiarities enable Cockroaches to invade ground thoroughly occupied by the House Cricket, an insect of quite similar mode of life: and it is equally hard to account for the superiority of orientalis over the other species. It is neither the largest nor the smallest; it is not perceptibly more prolific, or more voracious, or fonder of warmth, or swifter than its rivals, nor is it easy to see how the one conspicuous structural difference—viz., the rudimentary state of the wings of the female, can greatly favour orientalis. Some slight advantage seems to lie in characteristics too subtle for our detection or comprehension.

Food and Habits.

As to the food of Cockroaches, we can hardly except any animal or vegetable substance from the long list of their depredations. Bark, leaves, the pith of living cycads, paper, woollen clothes, sugar, cheese, bread, blacking, oil, lemons, ink, flesh, fish, leather, the dead bodies of other Cockroaches, their own cast skins and empty egg-capsules, all are greedily consumed. Cucumber, too, they will eat, though it disagrees with them horribly.

In the matter of temperature they are less easy to please. They are extremely fond of warmth, lurking in nooks near the oven, and abounding in bakehouses, distilleries, and all kinds of factories which provide a steady heat together with a supply of something eatable. Cold is the only check, and an unwarmed room during an English winter is more than they can endure. They are strictly nocturnal, and shun the light, although when long unmolested they become bolder. The flattened body enables the Cockroach to creep into very narrow crevices, and during cold weather it takes refuge beneath the flags of a kitchen floor, or in other very confined spaces.

The Cockroach belongs to a miscellaneous group of animals, which may be described as in various degrees parasitic upon men. These are all in a vague sense domestic species, but have not, like the ox, sheep, goat, or pig, been forcibly reduced to servitude; they have rather attached themselves to man in various degrees of intimacy. The dog has slowly won his place as our companion; the cat is tolerated and even caressed, but her attachment is to the dwelling and not to us; the jackal and rat are scavengers and thieves; the weasel, jackdaw, and magpie are wild species which show a slight preference for the neighbourhood of man. All of these, except the cat, which holds a very peculiar place, possess in a considerable degree qualities which bring success in the great competitive examination. They are not eminently specialised, their diet is mixed, their range as natural species is wide. Apart from man, they would have become numerous and strong, but those qualities which fit them so well to shift for themselves, have had full play in the dwellings of a wealthy and careless host. Of these domestic parasites at least two are insects, the House-fly and the Cockroach; and the Cockroach in particular is eminent in its peculiar sphere of activity. The successful competition of Cockroaches with other insects under natural conditions is sufficiently proved by the fact that about nine hundred species have already been described,[22] while their rapid multiplication and almost worldwide dissemination in the dwellings of man is an equally striking proof of their versatility and readiness to adapt themselves to artificial circumstances. In numerical frequency they probably exceed all domestic animals of larger size, while in geographical range the five species, lapponica, germanica, orientalis, americana, and australasiæ, are together comparable to the dog or pig, which have been multiplied and transported by man for his own purposes, and which cover the habitable globe.

The Cockroach a persistent type.

The Cockroach is historically one of the most ancient, and structurally one of the most primitive, of our surviving insects. Its immense antiquity is shown by the fact that so many Cockroaches have been found in the Coal Measures, where about eighty species have been met with. The absence of well-defined stages of growth, such as the soft-bodied larva or inactive pupa, the little specialised wings and jaws, the simple structure of the thorax, the jointed appendages carried on the end of the abdomen, and the unconcentrated nervous system, are marks of the most primitive insect-types. The order Orthoptera is undeniably the least specialised among winged insects at least, and within this order none are more simple in structure, or reach farther back in the geological record than the Cockroaches. The wingless Thysanura are even more generalised, but their geological history is illegible.[23]

Life-History.

The eggs of the Cockroach are laid sixteen together in a large horny capsule. This capsule is oval, with roundish ends, and has a longitudinal serrated ridge, which is uppermost while in position within the body of the female. The capsule is formed by the secretion of a “colleterial” gland, poured out upon the inner surface of a chamber (vulva) into which the oviducts lead. The secretion is at first fluid and white, but hardens and turns brown on exposure to the air. In this way a sort of mould of the vulva is formed, which is hollow, and opens forwards towards the outlet of the common oviduct. Eggs are now passed one by one into the capsule; and as it becomes full, its length is gradually increased by fresh additions, while the first-formed portion begins to protrude from the body of the female. When sixteen eggs have descended, the capsule is closed in front, and after an interval of seven or eight days, is dropped in a warm and sheltered crevice. In Periplaneta orientalis it measures about ·45 in. by ·25 in. (fig. 5). The ova develop within the capsule, and when ready to escape are of elongate-oval shape, resembling mummies in their wrappings. Eight embryos in one row face eight others on the opposite side, being alternated for close packing. Their ventral surfaces, which are afterwards turned towards the ground, are opposed, and their rounded dorsal surfaces are turned towards the wall of capsule; their heads are all directed towards the serrated edge. The ripe embryos are said by Westwood to discharge a fluid (saliva?) which softens the cement along the dorsal edge, and enables them to escape from their prison. In Blatta germanica the female is believed to help in the process of extrication.[24] The larvæ are at first white, with black eyes, but soon darken. They run about with great activity, feeding upon any starchy food which they can find.

Fig. 5.—Egg-capsule of P. orientalis (magnified). A, external view; B, opened; C, end view.

The larvæ of the Cockroach hardly differ outwardly from the adult, except in the absence of wings. The tenth tergum is notched in both sexes, as in the adult female. The sub-anal styles of the male are developed in the larva.

Cornelius, in his Beiträge zur nähern Kenntniss von Periplaneta orientalis (1853), gives the following account of the moults of the Cockroach. The first change of skin occurs immediately after escape from the egg-capsule, the second four weeks later, the third at the end of the first year, and each succeeding moult after a year’s interval. At the sixth moult the insect becomes a pupa,[25] and at the seventh (being now four years old) it assumes the form of the perfect Insect. The changes of skin are annual, and like fertilisation and oviposition, take place in the summer months only. He tells us further that the ova require about a year for their development. These statements are partly based upon observation of captive Cockroaches, and are the only ones accessible; but they require confirmation by independent observers, especially as they altogether differ from Hummel’s account of the life-history of Blatta germanica, and are at variance with the popular belief that new generations of the Cockroach are produced with great rapidity.

Fig. 6.—Young nymph (male). × 6.

Fig. 7.—Older nymph (male) with rudiments of wings. × 2 1/2.

The antennæ of the male nymph resemble those of the adult female. Wings and wing-covers appear first in the later larval stages, but are then rudimentary, and constitute a mere prolongation of the margins of the thoracic rings. Cornelius says that the round white spot internal to the antenna first appears plainly in the pupa, but we have readily found it in a very young larva. The Insect is active in all its stages, and is therefore, with other Orthoptera, described as undergoing “incomplete metamorphosis.” After each moult it is for a few hours nearly pure white. Of the duration of life in this species we have no certain information, and there is great difficulty in procuring any.

Sexual Differences.

Male Cockroaches are readily distinguished from the females by the well-developed wings and wing-covers. They are also slighter and weaker than the females; their terga and sterna are not so much thickened; their alimentary canal is more slender, and they feed less greedily (the crop of the male is usually only half-full of food). They stand higher on their legs than the females, whose abdomen trails on the ground. The external anatomical differences of the sexes may be tabulated thus:—

Parasites.

We have before us a long list of parasites[26] which infest the Cockroach. There is a conferva, an amœba, several infusoria, nematoid worms (one of which migrates to and fro between the rat and the Cockroach), a mite, as well as hymenopterous and coleopterous Insects. The Cockroach has a still longer array of foes, which includes monkeys, hedgehogs, pole-cats, cats, rats, birds, chamæleons, frogs, and wasps, but no single friend, unless those are reckoned as friends which are the foes of its foes.

Names in common use.

A few lines must be added upon the popular and scientific names of this insect. Etymologists have found it hard to explain the common English name, which seems to be related to cock and roach, but has really nothing to do with either. The lexicographers usually hold their peace about it, or give derivations which are absurd. Mr. James M. Miall informs us that “Cockroach can be traced to the Spanish cucarácha, a diminutive form of cuco or coco (Lat. coccum, a berry). Cucarácha is used also of the woodlouse, which, when rolled up, resembles a berry. The termination -ácha (Ital. -accio, -accia) signifies mean or contemptible. The Spanish word has also taken a French form; at least coqueraches has some currency (see, for example, Tylor’s Anahuac, p. 325).” In provincial English Black Clock is a common name. The German word Schabe, often turned into Schwabe, means perhaps Suabian, as Moufet, quoting Cordus, seems to explain.[27] Franzose and Däne are other German words for the insect, applied specially to Blatta germanica; and all seem to imply some popular theory as to the native country of the Cockroach.[28] This etymology of Schabe is not free from suspicion, particularly as the same term is commonly applied to the clothes-moth. Kakerlac, much used in France and French-speaking colonies, is a Dutch word of unknown signification. P. Americana is usually named Kakerlac or Cancrelat by the French; while orientalis has many names, such as Cafard, Ravet, and Bête noire.[29] The name Blatta was applied by the ancients to quite different insects, of which Virgil and Pliny make mention; Periplaneta is a modern generic term, coined by Burmeister.

Uses.

Of the uses to which Cockroaches have been put we have little to say. They constitute a popular remedy for dropsy in Russia, and both cockroach-tea and cockroach-pills are known in the medical practice of Philadelphia. Salted Cockroaches are said to have an agreeable flavour which is apparent in certain popular sauces.

CHAPTER IV.

The Outer Skeleton.

SPECIAL REFERENCES.

Krukenberg. Vergleichend-Physiologische Vorträge. IV.—Vergl. Physiologie der Thierischen Gerüstsubstanzen. (1885.) [Chemical Relations of Chitin.]

Graber. Ueber eine Art fibrilloiden Bindegewebes der Insectenhaut. Arch. f. mikr. Anat. Bd. X. (1874.) [Minute Structure of Integument.] Also,

Viallanes. Recherches sur l’Histologie des Insectes. Ann. Sci. Nat., Zool. VI^e Série, Tom. XIV. (1882).

Audouin. Recherches anatomiques sur le thorax des Insectes, &c. Ann. Sci. Nat. Tom. I. (1824.) [Theoretical Composition of Insect Segments.] Also,

Milne-Edwards. Leçons sur la Physiologie et l’Anatomie Comparée. Tom. X. (1874.)

Savigny. Mémoires sur les animaux sans vertèbres. Partie I^e. Théorie des organes de la bouche des Crustacées et des Insectes. (1816.) [Comparative Anatomy of the Mouth-parts.]

Muhr. Ueber die Mundtheile der Orthopteren. Prag. 1877. [Mouth-parts of Orthoptera.]

Chitin.

When the skin of an Insect is boiled successively in acids, alkalies, alcohol, and ether, an insoluble residue known as Chitin (C₁₅H₂₆N₂O₁₀) is obtained. It may be recognised and sufficiently separated by its resistance to boiling liquor potassæ. Chitin forms less than one-half by weight of the integument, but it is so coherent and uniformly distributed that when isolated by chemical reagents, and even when cautiously calcined, it retains its original organised form. The colour which it frequently exhibits is not due to any essential ingredient; it may be diminished or even destroyed by various bleaching processes. The colouring-matter of the chitin of the Cockroach, which is amber-yellow in thin sheets and blackish-brown in dense masses, is particularly stable and difficult of removal. Its composition does not appear to have been ascertained; it is white when first secreted, but darkens on exposure to air. Fresh-moulted Cockroaches are white, but gradually darken in three or four hours. Lowne[30] observes that in the Blow-fly the pigment is “first to be met with in the fat-bodies of the larvæ. These are perfectly white, but when cut from the larva, and exposed to the air, they rapidly assume an inky blackness.... When the perfect insect emerges from the pupa, and respiration again commences, the integument is nearly white, or a faint ashy colour prevails. This soon gives place to the characteristic blue or violet tint, first immediately around those portions most largely supplied with air vessels.” Professor Moseley[31] tells us that, thinking it just within the limits of possibility that the brown coloration of the Cockroach might be due to the presence of silver, he analysed one pound weight of Blatta. He found no silver, but plenty of iron, and a remarkable quantity of manganese. That light has some action upon the colouring matter seems to be indicated by the fact that in a newly-moulted Cockroach the dorsal surface darkens first.

Chitin is not peculiar to Insects, nor even to Arthropoda. The pen of cuttle-fishes and the shell of Lingula contain the same substance,[32] which is also proved, or suspected, to occur in many other animals.

The chemical stability of chitin is so remarkable that we might well expect it to accumulate like the inorganic constituents of animal skeletons, and form permanent deposits. Schlossberger[33] has, however, shown that it changes slowly under the action of water. Chitin kept for a year under water partially dissolved, turned into a slimy mass, and gave off a peculiar smell. This looks as if it were liable to putrefaction. The minute proportion of nitrogen in its composition may explain the complete disappearance of chitin in nature.

The Chitinous Cuticle.

Fig. 8.—Diagram of Insect integument, in section. bm, basement membrane; hyp, hypodermis, or chitinogenous layer; ct, ct′, chitinous cuticle; s, a seta.

The chitinous exoskeleton is rather an exudation than a true tissue. It is not made up of cells, but of many superposed laminæ, secreted by an underlying epithelium, or “chitinogenous layer.” This consists of a single layer of flattened cells, resting upon a basement membrane. A cross-section of the chitinous layer, or “cuticle,” examined with a high power shows extremely close and fine lines perpendicular to the laminæ. The cells commonly form a mosaic pattern, as if altered in shape by mutual pressure. The free surface of the integument of the Cockroach is divided into polygonal, raised spaces. Here and there an unusually long, flask-shaped, epithelial cell projects through the cuticle, and forms for itself an elongate chitinous sheath, commonly articulated at the base; such hollow sheaths form the hairs or setæ of Insects—structures quite different histologically from the hairs of Vertebrates.

The polygonal areas of the cuticle correspond each to a chitinogenous cell. Larger areas, around which the surrounding ones are radiately grouped, are discerned at intervals, and these carry hairs, or give attachment to muscular fibres.

Viallanes (loc. cit.) has added some interesting details to what was previously known of Insect-hairs. There are, he points out, two kinds of hairs, distinguished by their size and structure. The smaller spring from the boundary between contiguous polygonal areas, and have no sensory character. The larger ones occupy unusually large areas, surmount chitinogenous cells of corresponding size, and receive a special nervous supply. The nerve[34] expands at the base of the hair into a spindle-shaped, nucleated mass (bipolar ganglion-cell), from which issues a filament which traverses the axis of the hair, piercing the chitinogenous cell, whose protoplasm surrounds it with a sheath which is continued to the tip of the hair. Such sensory hairs are abundant in parts which are endowed with special sensibility.

Fig. 9.—Nerve-end­ing in skin of Stratio­mys larva. h, hairs; b, their chi­tin­ous base; c, nuc­leus of gen­er­at­ing cell; g, gan­glion cell. × 250. Copied from Viallanes.

Fig. 10.—Diagram of sen­sory hair of Insect. Cc, chi­ti­nous cu­ti­cle; h, hair; c, its gen­er­at­ing cell; g, gan­glion cell; bm, base­ment-mem­brane.

The chitinous cuticle is often folded in so as to form a deep pit, which, looked at from the inside of the body, resembles a lever, or a hook. Such inward-directed processes serve chiefly for the attachment of muscles, and are termed apodemes (apodemata). A simple example is afforded by the two glove-tips which lie in the middle line of the under-surface of the thorax (p. 58, and fig. [27]). In other cases the pit is closed from the first, and the apodeme is formed in the midst of a group of chitinogenous cells distant from the superficial layer, though continuous therewith. Many tendons of insertion are formed in this way. The two forked processes in the floor of the thorax (p. 58, and fig. [27]) are unusually large and complex structures of the same kind. In the tentorium of the head (p. 39, and fig. [17]) a pair of apodemes are supposed to unite and form an extensive platform which supports the brain and gullet.

Fig. 11.—Nymph (in last larval stage) escaping from old skin. × 2 1/2.

Like other Arthropoda, Insects shed their chitinous cuticle from time to time. A new cuticle, at first soft and colourless, is previously secreted, and from it the old one gradually becomes detached. The setæ probably serve the same purpose as the “casting-hairs” described by Braun in the crayfish, and by Cartier in certain reptiles,[35] that is, they mechanically loosen the old skin by pushing beneath it. In many soft-bodied nymphs the new skin can be gathered up into a multitude of fine wrinkles, which facilitate separation, but we have not found such wrinkles in the Cockroach, except in the wings. The integument about to be shed splits along the back of the Cockroach, from the head to the end of the thorax,[36] and the animal draws its limbs out of their discarded sheaths with much effort. It is remarkable that the long, tapering, and many-jointed antennæ are drawn out from an entire sheath. At the same time the chitinous lining of the tracheal tubes is cast, while that of the alimentary canal is broken up and passed through the body.

Fig. 12.—Cast skin of older nymph (“pupa”). × 2 1/2.

Prolonged boiling in caustic potash, though it dissolves the viscera, does not disintegrate the exoskeleton. This shows that the segments of the integument are not separate chitinous rings, but thickenings of a continuous chitinous investment. Nevertheless, their constancy in position and their conformity in structure often enable us to trace homologies between different segments and different species as certainly as between corresponding elements of the osseous vertebrate skeleton.

Parts of a Somite.

Audouin’s laborious researches into the exoskeleton of Insects[37] resulted in a nomenclature which has been generally adopted. He divides each somite (segment) into eight pieces, grouped in pairs—viz., terga (dorsal plates), sterna (ventral plates), epimera (adjacent to the terga), and episterna (adjacent to the sterna).

While admitting the usefulness of these terms, we must warn the reader not to suppose that this subdivision is either normal or primitive. The eight-parted segment exists in no single larval or adult Arthropod. Lower forms and younger stages take us further from such a type, instead of nearer to it; and Audouin’s theoretical conception is most fully realised in the thorax of an adult Insect with well-developed legs and wings.

The morphologist would derive all the varieties of Arthropod segments from the very simple and uniform chitinous cuticle found in Annelids and many Insect-larvæ. This becomes differentiated by unequal thickening and folding in, and a series of rings connected by flexible membranes is produced. Locomotive and respiratory activity commonly lead to the definition of terga and sterna, which are similarly attached to each other by flexible membranes. A pair of limbs may next be inserted between the terga and sterna, and the simple segment thus composed occurs so extensively in the less modified regions and in early stages that it may well be considered the typical Arthropod somite.

Special needs may lead to the division of the sterna into lateral halves, but this is purely an adaptive change. The third thoracic sternum of the male Cockroach, and the second and third of the female are thus divided, as is also the hinder part of the seventh abdominal sternum of the female.

In an early stage every somite has its tergal region divided into lateral halves, owing to the late completion of the body on this side. Traces of this division may survive even in the imago. There is often a conspicuous dorsal groove in the thoracic terga, and at the time of moult the terga split along an accurately median line (see fig. [12]).

Additional pieces may be developed between the terga and sterna, and these have long been termed pleural.[38] There may be, for example, single stigmatic plates, as in the abdomen of the Cockroach, pieces to support the thoracic legs, and pieces to support the wings; but the number and position of these plates depends upon their immediate use, and their homologies become very uncertain when Insects of different orders are compared. In general, the pleural elements of the segment are late in development, variable, and highly adaptive.

Somites of the Cockroach.

The exoskeleton of the Cockroach is divisible into about seventeen segments, which are grouped into three regions, as follows:—

Head		Procephalic lobes	3
		Post-oral segments
Thorax			3	[39]
Abdomen			11
			—
			17
			—

It is a strong argument in favour of this estimate that many Insects, at the time when segmentation first appears, possess seventeen segments.[40] The procephalic lobes, from which a great part of the head, including the antennæ, is developed, are often counted as an additional segment.[41]

The limbs, which in less specialised Arthropoda are carried with great regularity on every segment of the body, are greatly reduced in Insects. Those borne by the head are converted into sensory and masticatory organs; those on the abdomen are either totally suppressed, or extremely modified, and only the thoracic limbs remain capable of aiding in locomotion.

The primitive structure of the Arthropod limb is adapted to locomotion in water, and persists, with little modification, in most Crustacea. Here we find in most of the appendages[42] a basal stalk (protopodite), often two-jointed, an inner terminal branch (endopodite), and an outer terminal branch (exopodite), each of the latter commonly consisting of several joints. It does not appear that the appendages of Insects conform to the biramous Crustacean type, though the ends of the maxillæ are often divided into an outer and an inner portion.

We shall now proceed to describe, in some detail, the regions of the body of the adult Cockroach.

Head; Central Parts.

Fig. 13.—Front of Head. × 10.

The head of the Cockroach, as seen from the front, is pear-shaped, having a semi-circular outline above, and narrowing downwards. A side-view shows that the front and back are flattish, while the top and sides are regularly rounded. In the living animal the face is usually inclined downwards, but it can be tilted till the lower end projects considerably forward. The mouth, surrounded by gnathites or jaws, opens below. On the hinder surface is the occipital foramen, by which the head communicates with the thorax. A rather long neck allows the head to be retracted beneath the pronotum (first dorsal shield of the thorax) or protruded beyond it.

On the front of the head we observe the clypeus, which occupies a large central tract, extending almost completely across the widest part of the face. It is divided above by a sharply bent suture from the two epicranial plates, which form the top of the head as well as a great part of its back and sides. The labrum hangs like a flap from its lower edge. A little above the articulation of the labrum the width of the clypeus is suddenly reduced, as if a squarish piece had been cut out of each lower corner. In the re-entrant angle so formed, the ginglymus, or anterior articulation of the mandible, is situated.

The labrum is narrower than the clypeus, and of squarish shape, the lower angles being rounded. It hangs downwards, with a slight inclination backwards towards the mouth, whose front wall it forms. On each side, about halfway between the lateral margin and the middle line, the posterior surface of the labrum is strengthened by a vertical chitinous slip set with large setæ. Each of these plates passes above into a ring, from the upper and outer part of which a short lever passes upwards, and gives attachment to a muscle (levator menti).

Fig. 14.—Top of Head. ep, epi­cra­nial plate; oc, eye; ge, gena. × 10.

The top and back of the head are defended by the two epicranial plates, which meet along the middle line, but diverge widely as they descend upon the posterior surface, thus enclosing a large opening, the occipital foramen. Beyond the foramen, they pass still further downwards, their inner edges receding in a sharp curve from the vertical line, and end below in cavities for the articulation of the mandibular condyles.[43]

The sides of the head are completed by the eyes and the genæ. The large compound eye is bounded above by the epicranium; in front by a narrow band which connects the epicranium with the clypeus; behind, by the gena. The gena passes downwards between the eye and the epicranial plate, then curves forwards beneath the eye, and just appears upon the front of the face, being loosely connected at this point with the clypeus. Its lower edge overlaps the base of the mandible, and encloses the extensor mandibulæ.

Fig. 15.—Side of Head. oc, eye; ge, gena; mn, man­dible. × 10.

Fig. 16.—Back of Head. ca, cardo; st, stipes; ga, galea; la, lacinia; pa, palp; sm, sub­mentum; m, men­tum; pg, para­glossa. × 10.

The occipital foramen has the form of an heraldic shield. Its lateral margin is strengthened by a rim continuous with the tentorium, or internal skeleton of the head. Below, the foramen is completed by the upper edge of the tentorial plate, which nearly coincides with the upper edge of the submentum (basal piece of the second pair of maxillæ); a cleft, however, divides the two, through which nerve-commissures pass from the sub-œsophageal to the first thoracic ganglion. Through the occipital foramen pass the œsophagus, the salivary ducts, the aorta, and the tracheal tubes for the supply of air to the head.

Fig. 17.—Fore-half of Head, with ten­tor­ium, seen from be­hind. × 12.

The internal skeleton of the head consists of a nearly transparent chitinous septum, named tentorium by Burmeister, which extends downwards and forwards from the lower border of the occipital foramen. In front it gives off two long crura, or props, which pass to the ginglymus, and are reflected thence upon the inner surface of the clypeus, ascending as high as the antennary socket, round which they form a kind of rim. Each crus is twisted, so that the front surface becomes first internal and then posterior as it passes towards the clypeus. The form of the tentorium is in other respects readily understood from the figure (fig. 17). Its lower surface is strengthened by a median keel which gives attachment to muscles. The œsophagus passes upwards between its anterior crura, the long flexor of the mandible lies on each side of the central plate; the supra-œsophageal ganglion rests on the plate above, and the sub-œsophageal ganglion lies below it, the nerve-cords which unite the two passing through the circular aperture. A similar internal chitinous skeleton occurs in the heads of other Orthoptera, as well as in Neuroptera and Lepidoptera. Palmén[44] thinks that it represents a pair of stigmata or spiracles, which have thus become modified for muscular attachment, their respiratory function being wholly lost. In Ephemera he finds that the tentorium breaks across the middle when the skin is changed, and each half is drawn out from the head like the chitinous lining of a tracheal tube.

Antennæ; Eyes.

Fig. 18.—Base of Antenna of Male (to left) and Fe­male (to right). × 24.

A pair of antennæ spring from the front of the head. In the male of the common Cockroach they are a little longer than the body; in the female rather shorter. From seventy-five to ninety joints are usually found, and the three basal joints are larger than the rest. Up to about the thirtieth, the joints are about twice as wide as long; from this point they become more elongate. The joints are connected by flexible membranes, and provided with stiff, forward-directed bristles. The ordinary position of the antennæ is forwards and outwards.

Each antenna is attached to a relatively large socket (fig. [15]), which lies between the epicranium and clypeus, to the front and inner side of the compound eyes. A flexible membrane unites the antenna to the margin of the socket, from the lower part of which a chitinous pin projects upwards and supports the basal joint.

It is well known that in many Crustacea two pairs of antennæ are developed, the foremost pair (antennules) bearing two complete filaments. Some writers have suggested that both pairs may be present in Insects, though not simultaneously, the Crustacean antennule being found in the larva, and the Crustacean antenna in the adult. This view was supported by the familiar fact that in many larvæ the antennæ are placed further forward than in the adult. The three large joints at the base of Orthopterous antennæ have been taken to correspond with those of Crustacean antennules, and it has been inferred that in Insects with incomplete metamorphosis, only antennules or larval antennæ are developed.[45] This reasoning was never very cogent, and it has been impaired by further inquiry. Weismann has shown that in Corethra plumicornis, the adult antenna, though inserted much further back than that of the larva, is developed within it,[46] and Graber has described a still more striking case of the same thing in a White Butterfly.[47] There is, therefore, no reason to suppose that Insects possess more than one pair of antennæ, which is probably preoral, not corresponding with either of the Crustacean pairs.

We have already noticed (p. 26) the superficial points in which the antenna of the male Cockroach differs from that of the female.

The eyes of some Crustacea are carried upon jointed appendages, but this is never the case in Insects, though the eye-bearing surface may project from the head, as in Diopsis and Stylops. Professor Huxley[48] supposes that the head of an Insect may contain six somites, the eyes representing one pair of appendages. The various positions in which the eyes of Arthropoda may be developed weakens the argument drawn from the stalk-eyed Crustacea. Claus and Fritz Müller go so far on the other side as to deny the existence of an eye-segment even in Crustacea.

Mouth-parts of the Cockroach.

Before entering upon a full description of the mouth-parts of the Cockroach, which present some technical difficulties, the beginner in Insect anatomy will find it useful to get a few points of nomenclature fixed in his memory. Unfortunately, the terms employed by entomologists are at times neither convenient nor philosophical.

There are three pairs of jaws, disposed behind the labrum, as in the diagram:—

Labrum.

1st pair of Jaws

(Mandibles).

2nd  "

(Maxillæ).

3rd  "

(Labium, or 2nd pair of Maxillæ).

Fig. 19.—Diagram of Cock­roach Jaws, in hori­zon­tal sec­tion.

The mandible is undivided in all, or nearly all, Insects. Each maxilla may consist of

A palp on the outer side,
A galea (hood),
A lacinia (blade), on the inner side.

The galea (hood) of the 3rd pair of jaws is sometimes called the paraglossa.

A tongue-like process may be developed from the front wall of the mouth (epipharynx), or from the back wall (hypopharynx or lingua).[49] Both epipharynx and hypopharynx project into the mouth, and, in some Diptera, far beyond it.

The tip of the labium is sometimes produced into a long tongue, called the ligula (strap).

The mouths of Insects may be classed as:—

Biting.—Orthoptera, Neuroptera, Coleoptera (in some Coleoptera a licking tongue is developed), most Hymenoptera.

Licking and Sucking.—Some Hymenoptera—e.g., Honey Bee.

Sucking.—(a) With lancets—Diptera, Hemiptera. (b) Without lancets—Lepidoptera.

The reference of these to a common plan, and the determination of the constituent parts, is mainly the work of Savigny. Mouth-parts were made the basis of the classification of Insects by Fabricius (1745–1808).