The Insect World / Being a Popular Account of the Orders of Insects; Together with a Description of the Habits and Economy of Some of the Most Interesting Species

The Dragon-fly (Libellula depressa).
a. Perfect Insect. b. Perfect Insect emerging from the Pupa. c. d. Larvæ and Pupæ.

THE

INSECT WORLD:

BEING

A Popular Account of the Orders of Insects;

TOGETHER WITH

A DESCRIPTION OF THE HABITS AND ECONOMY OF
SOME OF THE MOST INTERESTING SPECIES.

BY
LOUIS FIGUIER.

A New Edition,

REVISED AND CORRECTED BY P. MARTIN DUNCAN, F.R.S.

—————
WITH 579 ILLUSTRATIONS.
—————

D. APPLETON AND CO.,

NEW YORK.

[PREFACE.]

This popular French book on Insects has been placed in my hands in order that the scientific portions of it should be examined and, if necessary, corrected. This task has been a light one, for the book had already passed through the able editorship of Mr. Jansen. But I have added a short notice of the Thysanoptera, which did not appear in M. Figuier's original work, and also the necessary information respecting the evolution of Stylops.

P. MARTIN DUNCAN.
Lee, 1872.

[CONTENTS.]

[The Insect World.]

[INTRODUCTION.]

It is not intended to investigate the anatomy of insects in this work thoroughly; but, as we are about to treat of the habits and economy of certain created beings, it is necessary first to explain the principal parts of their structure, and the stages which every perfect insect or imago has undergone before arriving at that state.

We, therefore, proceed to explain, as simply as possible, the anatomy of an insect, and the functions of its organs.

Fig. 1.—Head of an Insect

If we take an insect, and turn it over, and examine it carefully, the first thing that strikes us is that it is divided into three parts: the head; the thorax, or chest; and the abdomen, or stomach.

The head ([Fig. 1]) is a kind of box, formed of a single piece, having here and there joints more or less strongly marked, sometimes scarcely visible. It is furnished in front with an opening—often very small—which is the mouth; and with some for the eyes, and with others for the insertion of the antennæ or horns.

The integuments of the head are generally harder than the other parts of the body. It is necessary that this should be so. Insects often live and die in the midst of substances which offer some resistance. It is necessary, therefore, that the head should be strong enough to overcome such resistance. The head contains the masticatory organs, which, frequently having to attack hard substances, must be strongly supported. The exception to this rule is among insects which live by suction.

Fig. 2.—A Compound Cornea

It would be out of place here to mention the numerous modifi cations of the head which are presented in the immense class of insects.

The eyes of insects are of two kinds. There are compound eyes, or eyes composed of many lenses, united by their margins and forming hexagonal facettes; and there are also simple eyes, or ocelli.

The exterior of the eye is called the cornea ([Fig. 2]), each facette being a cornea; and the facettes, which vary in size even in the same eye, unite and form a common cornea, which is represented by the entire figure.

In order to show the immense number of the facettes possessed by many insects, we give the following list:—

The facettes appear to be most numerous in insects of the genus Scarabæus (a genus of beetles). They are so minute, that they can only be detected with a magnifying glass.

Looked at in front, a compound eye may be considered an agglomeration of simple eyes; but internally this is hardly correct.

On the under side of each facette we find a body of a gelatinous appearance, transparent, and usually conical; the base of this occupies the centre of the facette in such a manner as to leave around it a ring to receive some colouring matter. This body diminishes in thickness towards its other extremity, and terminates in a point where it joins a nervous filament proceeding from the optic nerve. These cones, agreeing in number with the facettes, play the part of the crystalline lens in the eyes of animals. They are straight and parallel with each other. A pigment fills all the spaces between the cones, and between the nervous filaments, and covers the under side of each cornea, except at the centre. This pigment varies much in colour. There are almost always two layers, of which the exterior one is the more brilliant. In fact, these eyes often sparkle with fire, like precious stones.

M. Lacordaire, in his "Introduction à l'Entomologie," from which we borrow the greater part of this information, has summed up as follows, the manner in which, according to M. Müller, the visual organs of insects operate:—

"Each facette, with its lens and nervous filament, separated from those surrounding them by the pigment in which they are enclosed, form an isolated apparatus, impenetrable to all rays of light, except those which fall perpendicularly on the centre of the facette, which alone is devoid of pigment. All rays falling obliquely are absorbed by that pigment which surrounds the gelatinous cone. It results partly from this, and partly from the immobility of the eye, that the field of vision of each facette is very limited, and that there are as many objects reflected on the optic filaments as there are corneæ. The extent, then, of the field of vision will be determined, not by the diameter of these last, but by the diameter of the entire eye, and will be in proportion to its size and convexity. But whatever may be the size of the eyes, like their fields of vision, they are independent of each other; there is always a space, greater or less, between them; and the insect cannot see objects in front of this space without turning its head. What a peculiar sensation must result from the multiplicity of images on the optic filaments! This is not more easily explained than that which happens with animals which, having two eyes, see only one image; and probably the same is the case with insects. But these eyes usually look in opposite directions, and should see two images, as in the chameleon, whose eyes move independently of each other. The clearness and length of vision will depend, continues M. Müller, on the diameter of the sphere of which the entire eye forms a segment, on the number and size of the facettes, and the length of the cones or lenses. The larger each facette, taken separately, and the more brilliant the pigment placed between the lenses, the more distinct will be the image of objects at a distance, and the less distinct that of objects near. With the latter the luminous rays diverge considerably; while those from the former are more parallel. In the first case, in traversing the pigment, they impinge obliquely on the crystalline, and consequently confuse the vision; in the second, they fall more perpendicularly on each facette.

"Objects do not appear of the same size to each optic filament, unless the eye is a perfect section of a sphere, and its convexity concentric with that of the optic nerve. Whenever it is otherwise, the image corresponds more or less imperfectly with the size of the object, and is more or less incorrect. Hence it follows, that elliptical or conical eyes, which one generally finds among insects, are less perfect than those referred to above.

"The differences which exist in the organisation of the eye among insects are explicable, to a certain point, on the theory which we are about to explain in a few words. Those species which live in the same substances on which they feed, and those which are parasitical, have small and flattened eyes; those, on the contrary, which have to seek their food, and which need to see objects at a distance, have large or very convex eyes. For the same reason the males, which have to seek their females, have larger eyes than the latter. The position of the eyes depends also on their size and shape; those which are flat, and have consequently a short field of vision, are placed close together, and rather in front than at the sides of the head, and often adjoining. Spherical and convex eyes, on the contrary, are placed on the sides, and their axes are opposite. But the greater field of vision which they are able to take in makes up for this position."

Almost all insects are provided with a pair of compound eyes, which are placed on the sides of the head. The size and form of these organs are very variable, as we shall presently see. They are generally placed behind the antennæ.

Although simple eyes (ocelli or stemmata) are common, they do not exist in all the orders of insects. They are generally round, and more or less convex and black, and there are three in the majority of cases. When there is this number they are most frequently placed in a triangle behind, and at a greater or less distance from the antennæ. Under the cornea, which varies in convexity, is found a transparent, rather hard, and nearly globular body, which is the true crystalline resting on a mass, which represents the vitreous body. This vitreous body is enclosed in an expansion of the optic nerve. Besides these, there is a pigment, most frequently red-brown, sometimes black, or blood-red. The organisation of these eyes is analogous to the eyes of fishes, and their refractive power is very great.

With these eyes insects can only see such objects as are at a short distance. Of what use then can stemmata be to insects also provided with compound eyes? It has been remarked that most insects having this arrangement of eyes feed on the pollen of plants, and it has been surmised that the stemmata enable them to distinguish the parts of the flowers.

The antennæ, commonly called horns, are two flexible appendages, of very variable form, which are joined to different parts of the head, and are always two in number. The joints of which they are made up have the power of motion, which enables the insect to move them in any direction.

The antennæ consist of three parts: the basal joint, commonly distinguished by its form, length, and colour; the club, formed by a gradual or sudden thickening of the terminal joints, of which the number, form, and size present great variations; lastly, the stalk, formed by all the joints of the antennæ, except the basal, when no club exists, and in case of the existence of a club, of all those between it and the basal one.

We give as examples the antennæ of two beetles, one of the genus Asida, the other of the genus Zygia (Figs. [3] and [4]).

Insects, for the most part, while in repose, place their antennæ on their backs, or along the sides of the head, or even on the thorax. Others are provided with cavities in which the antennæ repose either wholly or in part.

During their different movements, insects move their antennæ more or less, sometimes slowly and with regularity, at other times in all directions. Some insects impart to their antennæ a perpetual vibration. During flight they are directed in front, perpendicular to the axis of the body, or else they repose on the back.

What is the use of the antennæ, resembling as they do, feathers, saws, clubs, &c.? Everything indicates that these organs play a very important part in the life of insects, but their functions are imperfectly understood. Experience has shown that they only play a subordinate part as feelers, and have nothing to do with the senses of taste or smell. There is no other function for them to fulfil, except that of hearing.

On this hypothesis the antennæ will be the principal instruments for the transmission of sound-waves. The membrane at their base represents a trace of the tympanum which exists among the higher animals. This membrane then will have some connection with an auditory nerve.

The mouth of insects is formed after two general types, which correspond to two kinds of requirements. It is suited in the one case to break solid substances, in the other to imbibe liquids.

At first sight there seems no similarity between the mouth of a biting insect and of one living by suction. But on examination it is found that the parts of the mouth in the one are exactly analogous to the same parts in the other, and that they have only modifications suiting them to the different purposes which they have to fulfil.

The mouth of a biting insect is composed of an upper lip, a pair of mandibles, a pair of jaws, and a lower lip ([Fig. 5]).

The lower lip and the jaws carry on the outside certain appendages or filaments which have received the name of palpi.

When speaking of sucking insects, and in general of the various orders of insects, we shall speak more in detail of the various parts of the mouth.

The thorax ([Fig. 6]), the second primary division of the body of insects, plays almost as important a part as the head. It consists of three segments or rings, which are in general joined together—the prothorax, the mesothorax, and the metathorax, each of which bears a pair of legs. The wings are attached to the two posterior segments.

All insects have six true legs. There is no exception whatever to this rule, though some may not be developed.

From the segments to which they are attached, the legs are called anterior, posterior, and intermediate. The legs are composed of four parts: the trochanter, a short joint which unites the thigh to the body; the thigh or femur; the tibia, answering to the shank in animals; and the tarsus, or foot, composed of a variable number of pieces placed end to end, and called the phalanges.

We take as examples the hind leg of a Heterocerus ([Fig. 7]), and the front leg of a Zophosis ([Fig. 8]) (genera of beetles).

We shall not dwell on the different parts, as they perform functions which will occupy us later, when speaking of the various species of the great class of insects.

Fig. 7. Hind leg of a Heterocerus.		Fig. 8. Front leg of a Zophosis.
Fig. 9.—Posterior leg of a jumping insect.

The functions which the legs of insects have to perform consist in walking, swimming, or jumping.

In walking, says M. Lacordaire, insects move their legs in different ways. Some move their six legs successively, or only two or three at a time without distinction, but never both legs of the same pair together, consequently one step is not the same as another. The walk of insects is sometimes very irregular, especially when the legs are long; and they often hop rather than walk. Others have one kind of step, and walk very regularly. They commence by moving the posterior and anterior legs on the same side and the intermediate ones on the opposite side. The first step made, these legs are put down, and the others raised in their turn to make a second.

Running does not change the order of the movements, it only makes them quicker—very rapid in some species, and surpassing in proportion that of all other animals; but in others the pace is slow. Some insects rather crawl than walk.

In swimming, the posterior legs play the principal part. The other legs striking the water upwards or downwards, produce an upward or downward motion. The animal changes its course at will by using the legs on one side only, in the same way as one turns a rowing boat with one oar without the aid of a rudder. Swimming differs essentially from walking, for the foot being surrounded by a resisting medium, the legs on both sides are moved at the same time.

The act of jumping is principally performed by the hind legs. Insects which jump have these legs very largely developed, as in [Fig. 9]. When about to jump they bring the tibia into contact with the thigh, which is often furnished with a groove to receive it, having on each side a row of spines. The leg then suddenly straightens like a spring, and the foot being placed firmly on the ground, sends the insect into the air, and at the same time propels forward. The jump is greater in proportion as the leg is longer.

To treat here in a general manner of the wings of insects would be useless. We shall refer to them at length in their proper place, when treating of the various types of winged insects.

In the perfect insect the abdomen does not carry either the wings or the legs. It is formed of nine segments, which are without appendages, with the exception of the posterior ones, which often carry small organs differing much in form and function. These are saws, probes, forceps, stings, augers, &c. We shall consider these different organs in their proper places.

With vertebrate animals, which have an interior skeleton suited to furnish points of resistance for their various movements, the skin is a more or less soft covering, uniformly diffused over the exterior of the body, and intended only to protect it against external injury. In insects the points of resistance are changed from the interior to the exterior. The skin is altered by Nature to fit it to this purpose. It is hard, and presents between the segments only membranous intervals, which allow the hard parts to move in all directions.

We are examining a perfect insect; we have glanced at its skeleton, and the different appendages which spring from it. The principal organs which are contained in the body remain to be examined.

We will first study the digestive apparatus. This apparatus consists of a lengthened tubular organ, swollen at certain points, forming more or less numerous convolutions, and provided with two distinct orifices. This alimentary canal is always situated in the median line of the body, traverses its whole length, and is at first surrounded by, and then passes above, the nervous ganglia. [1]

Fig. 10.—Digestive apparatus of Carabus auratus.

In its most complicated form the alimentary canal is composed of an œsophagus, or gullet, of a crop, of a gizzard, of a chylific ventricle or stomach, a small intestine, a large intestine, divers appendages, salivary, biliary, and urinary glands. The œsophagus is often not wider than a hair, and part of it in many species is enlarged into a pouch, which is called the crop, because it occupies the same position, and performs analogous functions with that organ in birds. It is enough to say that the food remains there some time before passing on to the other parts of the intestinal canal, and undergoes a certain amount of preparation. It is in the gizzard, when one exists, that the food, separated by the masticatory organs of the mouth, undergoes another and more complete grinding. Its structure is suited to its office. It is, in fact, very muscular, often half cartilaginous, and strongly contractile. Its interior walls are provided with a grinding apparatus, which varies according to the species, and consists of teeth, plates, spines, and notches, which convert the food into pulp. It only exists among insects which live on solid matters, hard vegetables, small animals, tough skin, &c. This apparatus is absent in sucking insects and those which live on soft substances, such as the pollen of flowers, &c.

The chylific ventricle or stomach is never absent; it is the organ which performs the principal part in the act of digestion.

Two kinds of appendages belong to the chylific ventricle, but only in certain families. The first are papillæ, in the form of the fingers of a glove, which bristle over the exterior of this organ, and in which it is believed that the food begins to be converted into chyle. The second are cæca, and larger and less numerous.

They have been considered as secretory organs, answering to the pancreas in vertebrate animals.

[Fig. 10], which represents the digestive apparatus of Carabus auratus, a common beetle, presents to the eyes of the reader the different organs of which we are speaking.

Fig. 11.—Posterior extremity of the chylific ventricle, surrounded by the Malpighian vessels.

A is the mouth of the insect, B the œsophagus, C the crop, D the gizzard, E the chylific ventricle, F and G the small and large intestines, and H the anus.

It is not necessary to consider the other parts of the alimentary canal in insects, but only to refer to some of the appendages of this apparatus.

The salivary glands pour into the digestive tube a liquid, generally colourless, which, from the place where it is secreted, and its alkaline nature, corresponds to the saliva in vertebrate animals. It is this liquid which comes from the tongue of sucking insects in the form of drops.

These glands are always two in number. Their form is as variable as complicated. The most simple is that of a closed flexible tube, generally rolled into a ball, and opening on the sides of the œsophagus.

At the posterior extremity of the chylific ventricle are inserted a variable number of fine tubes, usually elongated and flexible, and terminating in culs-de-sac at one end. Their colour, which depends on the liquid they may contain, is sometimes white, but more frequently brown, blackish, or green. They appear to be composed of a very slight and delicate membrane, as they are very easily torn, and nothing is more difficult than to unroll and to disengage them from the fatty or other tissues by which they are enveloped.

The function of these vessels is uncertain. Cuvier and Léon Dufour supposed them to be analogous to the liver, and on that account they have been called biliary vessels; and they are often termed the Malpighian vessels, after the name of their discoverer.

According to M. Lacordaire, their functions vary with their position. When they enter the chylific ventricle, they furnish only bile; bile and a urinary liquid when they enter the posterior part of the ventricle and the intestine; and urine alone when they are placed near the posterior extremity of the alimentary canal.

[Fig. 11] represents part of the preceding figure more highly magnified, showing the manner in which these tubes enter the chylific ventricle.

In our rapid description of the digestive apparatus of insects, it only remains for us to mention certain purifying organs which secrete those fluids, generally blackish, caustic, or of peculiar smell, which some insects emit when they are irritated, and which cause a smarting when they get into one's eyes.

Less well developed than the salivary organs, they are often of a very complicated structure. In [Fig. 12] is represented the secretory apparatus of the Carabus auratus, which will serve for an example: A represents the secretory sacs aggregated together like a bunch of grapes, B the canal, C the pouch which receives the secretion, D the excretory duct.

Fig. 12.
Secretory apparatus of Carabus auratus.

Sometimes the secretion is liquid, and has a fœtid or ammoniacal odour; sometimes, as in the Bombardier beetle (Brachinus crepitans), it is gaseous, and is emitted, with an explosion, in the form of a whitish vapour, having a strong pungent odour analogous to that of nitric acid, and the same properties. It reddens litmus paper, and burns and reddens the skin, which after a time becomes brown, and continues so for a considerable time.

About the middle of the seventeenth century Malpighi at Bologna, and Swammerdam at Utrecht, discovered a pulsatory organ occupying a median line of the back, which appeared to them to be a heart, in different insects. Nevertheless, Cuvier, having declared some time afterwards that there was no circula tion, properly so called, among insects, his opinion was universally adopted.

But in 1827 a German naturalist named Carus discovered that there were real currents of blood circulating throughout the body, and returning to their point of departure. The observations of Carus were repeated and confirmed by many other naturalists, and we are thus enabled to form a sufficiently exact idea of the manner in which the blood circulates.

The following summary of the phenomena of circulation among insects is borrowed from "Leçons sur la Physiologie et l'Anatomie comparée," by M. Milne-Edwards:—

The tube which passes under the skin of the back of the head, and front part of the body, above the alimentary canal, has been known for a long time as the dorsal vessel. It is composed of two very distinct portions: the anterior, which is tubular and not contractile; and the posterior, which is larger, of more complicated structure, and which contracts and dilates at regular intervals.

This latter part constitutes, then, more particularly the heart of the insect. Generally it occupies the whole length of the abdomen, and is fixed to the vault of the tegumentary skeleton by membranous expansions, in such a manner as to leave a free space around it, but shut above and below, so as to form a reservoir into which the blood pours before penetrating to the heart. This reservoir is often called the auricle, for it seems to act as an instrument of impulsion, and to drive the blood into the ventricle or heart, properly so called.

The heart is fusiform, and is divided by numerous constrictions into chambers. These chambers have exits placed in pairs, and membranous folds which divide the cavity in the manner of a portcullis. The lips of the orifices, instead of terminating in a clean edge, penetrate into the interior of the heart in the form of the mouth-piece of a flute. The double membranous folds thus formed on each side of the dorsal vessel are in the shape of a half moon, and separate from each other when this organ dilates; but the contrary movement taking place, the passage is closed.

By the aid of this valvular apparatus, the blood can penetrate into the heart from the pericardic chamber, the empty space surrounding the heart, but cannot flow back from the heart into that reservoir.

The anterior or aortic portion of the dorsal vessels shows neither fan-shaped lateral expansions, nor orifices, and consists of a single membranous tube. The whole of the blood set in motion by the contractions of the cardial portion of the dorsal vessel runs into the cavity of the head, and circulates afterwards in irregular channels formed by the empty spaces left between the different organs. It is the unoccupied portions of the great visceral cavity which serve as channels for the blood, and through them run the main currents to the lateral and lower parts of the body. These currents regain the back part of the abdomen, and enter the heart after having passed over the internal organs. These principal channels are in continuity with other gaps between the muscles, or between the bundles of fibres of which these muscles are composed.

The principal currents send into the network thus formed, minor branches, which having ramified in their turn among the principal parts of the organism, re-enter some main current to regain the dorsal vessel.

In the transparent parts of the body the blood may be seen circulating in this way to a number of inter-organic channels, penetrating the limbs and the wings, when these appendages are not horny, and, in short, diffusing itself everywhere. "If, by means of coloured injections," says M. Milne-Edwards, "one studies the connections which exist between the cavities in which sanguineous currents have been found to exist and the rest of the economy, it is easy to see that the irrigatory system thus formed penetrates to the full depth of every organ, and should cause the rapid renewal of the nourishing fluid in all the parts where the process of vitality renders the passage of this fluid necessary."

We shall see presently, in speaking of respiration, that the relations between the nourishing fluid and the atmospheric air are more direct and regular than was for a long time supposed.

In short, insects possess an active circulation, although we find neither arteries nor veins, and although the blood put in motion by the contractions of the heart, and carried to the head by the aortic portion of the dorsal vessel, can only distribute itself in the different parts of the system to return to the heart, by the gaps left between the different organs, or between the membranes and fibres of which these organs are composed.

[Fig. 13] (page 14), which shows both the circulating and breathing systems of an insect, enables us to recognise the different organs which we have described, as helping to keep up both respiration and circulation.

The knowledge of the respiration of the insect is comparatively a modern scientific acquisition. Malpighi was the first to prove, in 1669, that insects are provided with organs of respiration, and that air is as indispensable to them as it is to other living beings. But the opinion of this celebrated naturalist has been contradicted, and his views were long contested. Now, however, one can easily recognise the apparatus by the aid of which the respiration of the insect is effected.

Fig. 13.—Organs of circulation and breathing in an insect.
A, abdominal portion of the dorsal vessel. B, aortic or thoracic portion. C, air-vessels of the head; D, of the abdomen.

The respiratory apparatus is essentially composed of membranous ducts of great tenuity, their ramifications spread everywhere in incalculable numbers, and bury themselves in the different organs, much in the same way as the fibrous roots of plants bury themselves in the soil. These vessels are called tracheæ. Their communications with the air are established externally in different ways, according to the character of the medium in which the insect lives.

It is well known that a vast number of insects live in the air. The air penetrates into the tracheæ by a number of orifices placed at the sides of the body, which are termed spiracles. On close examination these may be seen in the shape of button-holes in a number of different species. Let us dwell for a moment on the breathing apparatus of the insect, that is to say, on the tracheæ.

This apparatus is sometimes composed of elastic tubes only, sometimes of a collection of tubes and membranous pouches. We will first treat of the former.

The coats of these breathing tubes are very elastic, and always preserve a cylindrical form, even when not distended. This state of things is maintained by the existence, throughout the whole length of the tracheæ, of a thread of half horny consistency, rolled up in a spiral, and covered externally by a very delicate membranous sheath. The external membrane is thin, smooth, and generally colourless, or of a pearly white. The cartilaginous spiral is sometimes cylindrical and sometimes flat. It only adheres slightly to the external membrane, but is, on the other hand, closely united to the internal one. This spiral thread is only continuous in the same trunk; it breaks off when it branches, and each branch then possesses its own thread, in such a way that it is not joined to the thread of the trunk from which it issued, except by continuity, just as the branch of a tree is attached to the stem which supports it. This thread is prolonged, without interruption, to the extreme points of the finest ramifications.

The number of tracheæ in the body of an insect is very great. That patient anatomist, Lyonet, has proved this in his great work on the Goat-moth Caterpillar, Cossus ligniperda. Lyonet, who congratulated himself with having finished his long labours without having had to destroy more than eight or nine of the species he wished to describe, had the patience to count the different air-tubes in that caterpillar. He found that there were 256 longitudinal and 1,336 transverse branches; in short, that the body of this creature is traversed in all directions by 1,572 aeriferous tubes which are visible to the eye by the aid of a magnifying glass, without taking into account those which may be imperceptible.

The complicated system of the breathing apparatus which we are describing is sometimes composed of an assemblage of tubes and membranous pouches, besides the elastic tubes which we have already mentioned. These pouches vary in size, and are very elastic, expanding when the air enters, and contracting when it leaves them, as they are altogether without the species of framework formed by the spiral thread of the tubular tracheæ, of which they are only enlargements.

[Fig. 13] is explanatory of these organs of respiration.

The respiratory mechanism of an insect is easily understood. "The abdominal cavity," says M. Milne-Edwards, "in which is placed the greater part of the respiratory apparatus, is susceptible of being contracted and dilated alternately by the play of the different segments of which the skeleton is composed, and which are placed in such a manner that they can be drawn into each other to a greater or less extent. When the insect contracts its body, the tracheæ are compressed and the air driven out. But when, on the other hand, the visceral cavity assumes its normal size, or dilates, these channels become larger, and the air with which they are filled being rarefied by this expansion, is no longer in equilibrium with the outer air with which it is in communication through the medium of the spiracles. The exterior air is then impelled into the interior of the respiratory tubes, and the inspiration is effected."

The respiratory movements can be accelerated or diminished, according to the wants of the animal; in general, there are from thirty to fifty to the minute. In a state of repose the spiracles are open, and all the tracheæ are free to receive air whenever the visceral cavity is dilated, but those orifices may be closed, and the insect thus possesses the faculty of stopping all communication between the respiratory apparatus and the surrounding atmosphere.

Some insects live in the water; they are therefore obliged to come to the surface to take the air they are in need of, or else to possess themselves of the small amount contained in the water. Both these methods of respiration exist under different forms in aquatic insects.

Fig. 14.
Branchiæ, or gills, of an aquatic larva
(Ephemera).
A, foliaceous laminæ, or gills.

To inhale atmospheric air, which is necessary for respiration, above the water, certain insects employ their elytra [2] as a sort of reservoir; others make use of their antennæ, the hairs of which retain the globules of air. In this case it is brought under the thorax, whence a groove carries it to the spiracles. Sometimes the same result is obtained by a more complicated arrangement, consisting of respiratory tubes which can be thrust into the air, which it is their function to introduce into the organisation.

Insects which breathe in the water without rising to the surface are provided with gills—organs which, though variable in form, generally consist of foliaceous or fringed expansions, in the midst of which the tracheæ ramify in considerable numbers. These vessels are filled with air, but it does not disseminate itself in them directly, and it is only through the walls of these tubes that the contained gas is exchanged for the air held in suspension by the surrounding water. The oxygen contained in the water passes through certain very permeable membranes of the gill, and penetrates the tracheæ, which discharge, in exchange, carbonic acid, which is the gaseous product of respiration.

[Fig. 14] represents the gills or breathing apparatus in an aquatic insect. We take as an example Ephemera. [3] It may be observed that the gills or foliaceous laminæ are placed at the circumference of the body, and at its smallest parts.

We have now seen that the respiratory apparatus is considerably developed in insects; it is, therefore, easy to foresee that those functions are most actively employed by them. In fact, if one compares the oxygen they imbibe with the heavy organic matter of which their body is composed, the amount is enormous.

Before finishing this rapid examination of the body of an insect, we shall have to say a few words on the nervous system.

This system is chiefly composed of a double series of ganglions, or collections of nerves, which are united together by longitudinal cords. The number of these ganglions corresponds with that of the segments. Sometimes they are at equal distances, and extend in a chain from one end of the body to the other; at others they are many of them close together, so as to form a single mass.

The cephalic ganglions are two in number; they have been described by anatomists under the name of brain. "This expression," says M. Lacordaire, "would be apt to mislead the reader, as it would induce him to suppose the existence of a concentration of faculties to control the feelings and excite the movements, which is not the case." [4] The same naturalist observes, "All the ganglions of the ventral chain are endowed with nearly the same properties, and represent each other uniformly."

The ganglion situated above the œsophagus gives rise to the optic nerves, which are the most considerable of all those of the body, and to the nerves of the antennæ. The ganglion beneath the œsophagus provides the nerves of the mandibles, of the jaws, and of the lower lip. The three pairs of ganglions which follow those placed immediately below the œsophagus, belong to the three segments of the thorax, and give rise to the nerves of the feet and wings. They are in general more voluminous than the following pairs, which occupy the abdomen.

[Fig. 15] represents the nervous system of the Carabus auratus: A is the cephalic ganglion; B, the sub-œsophagian ganglion; C, the prothoracic ganglion; D and E are the ganglions of the mesothorax and metathorax. The remainder, F F, are the abdominal ganglions.

Fig. 15.—Nervous system of Carabus auratus.

Before finishing these preliminary observations, it is necessary to say that the preceding remarks only apply absolutely to insects arrived at the perfect state. It is important to make this remark, as insects, before arriving at that state, pass through various other stages. These stages are often so different from each other, that it would be difficult to imagine that they are only modifications of the same animal; one would suppose that they were as many different kinds of animals, if there was not abundant proof of the contrary.

The successive stages through which an insect passes are four in number:—the egg; the larva; the pupa, nymph, or chrysalis; and the perfect insect, or imago.

The egg state, which is common to them, as to all other articulate animals, it is unnecessary to explain. Nearly all insects lay eggs, though some few are viviparous. There often exists in the extremity of the abdomen of the female a peculiar organ, called the ovipositor, which is destined to make holes for the reception of the eggs. By a wonderful instinct the mother always lays her eggs in a place where her young, on being hatched, can find an abundance of nutritious substances. It will not be needless to observe that in most cases, these aliments are quite different to those which the mother seeks for herself.

In the second stage, that is to say, on leaving the egg—the larva period—the insect presents itself in a soft state, without wings, and resembles a worm. In ordinary language, it is nearly always called a worm, or grub, and in certain cases, a caterpillar.

Linnæus was the first to use the term "larva"—taken from the Latin word larva, "a mask"—as he considered that, in this form, the insect was as it were masked. During this period of its life the insect eats voraciously, and often changes its skin. At a certain period it ceases to eat, retires to some hidden spot, and, after changing its skin for the last time, enters the third stage of its existence, and becomes a chrysalis. In this state it resembles a mummy enveloped in bandages, or a child in its swaddling clothes. It is generally incapable of either moving or nourishing itself. It continues so for days, weeks, months, and sometimes even for years.

While the insect is thus apparently dead, a slow but certain change is going on in the interior of its body. A marvellous work, though not visible outside, is being effected, for the different organs of the insect are developing by degrees under the covering which surrounds them. When their formation is complete, the insect disengages itself from the narrow prison in which it was enclosed, and makes its appearance, provided with wings, and capable of propagating its kind; in short, of enjoying all the faculties which Nature has accorded to its species. It has thrown off the mask; the larva and pupa has disappeared, and given place to the perfect insect.

To show the reader the four states through which the insect passes in succession, in [Fig. 16] is represented the insect known as the Hydrophilus, [5] firstly, in the egg state; secondly, as the larva, or caterpillar; thirdly in the pupa; and fourthly as the perfect insect or imago. The different degrees of transformation and evolution which we have just described, are those which take place either completely or incompletely in all insects. Their metamorphoses are then at an end. There are certain insects, however, that show no difference in their various stages, except by absence of wings in the larva; and in these the chrysalis is only characterised by the growth of the wings, which, at first folded back and hidden under the skin, afterwards become free, but are not wholly developed till the last skin is cast. These insects are said to undergo incomplete metamorphoses, the former complete metamorphoses. Some never possess wings; indeed, there are others which undergo no metamorphosis, and are born possessed of all the organs with which it is necessary they should be provided.

Fig. 16.—Hydrophilus in its four states.
A, eggs; B, larva; C, pupa; D, imago, or perfect insect.

Some curious researches have been lately made on the strength of insects. M. Felix Plateau, of Brussels, has published some observations on this point, which we think of sufficient interest to reproduce here.

In order to measure the muscular strength of man, or of animals—as the horse, for instance—many different dynamometric apparatuses have been invented, composed of springs, or systems of unequal levers. The Turks' heads which are seen at fairs, or in the Champs Élysées, at Paris, and on which the person who wishes to try his strength gives a strong blow with his fist, represent a dynamometer of this kind. The one which Buffon had constructed by Régnier the mechanician, and which is known by the name of Régnier's Dynamometer, is much more precise. It consists of an oval spring, of which the two ends approach each other; when they are pulled in opposite directions, a needle, which works on a dial marked with figures, indicates the force exercised on the spring. It has been proved, with this instrument, that the muscular effort of a man pulling with both hands is about 124 lbs., and that of a woman only 74 lbs. The ordinary effort of strength of a man in lifting a weight is 292 lbs.; and a horse, in pulling, shows a strength of 675 lbs.; a man, under the same circumstances, exhibiting a strength of 90 lbs.

Physiologists have not as yet given their attention to the strength of invertebrate animals. It is, relatively speaking, immense. Many people have observed how out of proportion a jump of a flea is to its size. A flea is not more than an eighth of an inch in length, and it jumps a yard; in proportion, a lion ought to jump two-thirds of a mile. Pliny shows, in his "Natural History," that the weights carried by ants appear exceedingly great when they are compared with the size of these indefatigable labourers. The strength of these insects is still more striking, when one considers the edifices they are able to construct, and the devastations they occasion. The Termes, or White Ant,[6] constructs habitations many yards in height, which are so firmly and solidly built, that the buffaloes are able to mount them, and use them as observatories; they are made of particles of wood joined together by a gummy substance, and are able to resist even the force of a hurricane.

There is another circumstance which is worth being noted. Man is proud of his works; but what are they, after all, in comparison with those of the ant, taking the relative heights into consideration? The largest pyramid in Egypt is only 146 yards high, that is, about ninety times the average height of man; whereas, the nests of the Termites are a thousand times the height of the insects which construct them. Their habitations are thus twelve times higher than the largest specimen of architecture raised by human hands. We are, therefore, far beneath these little insects, as far as strength and the spirit of working go.

The destructive power of these creatures, so insignificant in appearance, are still more surprising. During the spring of a single year they can effect the ruin of a house by destroying the beams and planks. The town of La Rochelle, to which the Termites were imported by an American ship, is menaced with being eventually suspended on catacombs, like the town of Valencia in New Grenada. It is well known what destruction is caused when a swarm of locusts alight in a cultivated field; and it is certain that even their larvæ do as severe injury as the perfect insect. All this sufficiently proves the destructive capabilities of these little animals, which we are accustomed to despise.

M. Plateau has studied the power of traction in some insects, the power of pushing in the digging insects, and the lifting power of others during flight. He has thus been able to make some most interesting comparisons, of some of which we will relate the results.

The average weight of man being 142 lbs., and his power of traction, according to Régnier, being 124 lbs., the proportion of the weight he can draw to the weight of his body is only as 87 to 100. With the horse the proportion is not more than 67 to 100, a horse 1,350 lbs. in weight only drawing about 900 lbs. The horse, therefore, can draw little more than half his own weight, and a man cannot draw the weight of his own body.

This is a very poor result, if compared with the strength of the cockchafer. This insect, in fact, possesses a power of traction equal to more than fourteen times its own weight. If you amuse yourself with the children's game of making a cockchafer draw small cargoes of stones, you will be surprised at the great weight which this insignificant looking animal is able to manage.

To test the power of traction in insects, M. Plateau attached them to a weight by means of a thread fastened to one of their feet. The Coleoptera (Beetles) are the best adapted for these experiments.

The following are some of the results obtained by the Belgian physician:—Carabus auratus can draw seven times the weight of its body; Nebria brevicollis, twenty-five times; Necrophorus vespillo, fifteen times; Trichius fasciatus, forty-one times; and Oryctes nasicornis, four times only. The bee can draw twenty times the weight of its body; Donacia nymphæ [7] forty-two times its own weight.

From this it follows that if the horse possessed the same strength as this last insect, or if the insect were the size of a horse, they would either of them be able to draw 155,250 lbs. M. Plateau has ascertained the pushing power in insects, by introducing them into a pasteboard tube, the interior of which was made rough, and in which was fixed a glass plate, which allowed the light to penetrate into the prison. The animal, if excited, struggled with all its strength against the transparent plate, which, on being pushed forward, turned a lever adapted to a miniature dynamometer, which indicated the amount of effort exercised.

The results thus obtained prove that the pushing power, like the power of traction, is greater in inverse proportion to the size and weight of the animal. A few figures will better explain this curious law. In Oryctes nasicornis the proportion of the pushing power to the weight of the insect is only three to two; in Geotrupes stercorarius it is sixteen to two; and in Onthophagus nuchicornis seventy-nine to six.

Experiments have been made on the lifting power of insects by fastening a ball of soft wax to a thread attached to the hind legs. The proportion of the weight lifted has been found equal to that of the body. That is to say, that the insect, when flying, can lift its own weight. This is proved by the following calculations:—In the Neuroptera the proportion is 1 in the Dragon-fly (Libellula vulgata), ·7 in Lestes sponsa. In the order Hymenoptera it is ·78 in the bee, and ·63 in Bombus terrestris, the humble-bee. In the Diptera it is ·9 in Calliphora vomitoria, [8] 1·84 in the Syrphus corollæ, and 1·77 in the house-fly.

These results show that insects have only sufficient power to sustain their own weight when flying, as the above calculations exhibit the maximum of which they are capable, and at the utmost this strength would only compensate for the fatigue occasioned by the action of flight.

At the same time it is to be observed that the Diptera, and among others the house-fly, can sustain their flight longer than the Hymenoptera and Neuroptera, although one would not think so from their appearance. In conclusion, if an insect's power of flying is not considerable, its power of traction and propulsion are immense, compared with the vertebrate animals; and, in the same group of insects, those that are the smallest and lightest are the strongest. The proportion between the muscular strength of insects and the dimensions of their bodies, would not appear to be on account of their muscles being more numerous than those of vertebrate animals, but on account of greater intrinsic energy and muscular activity. The articulations of insects may be considered as solid cases which envelop the muscles, and the thickness of these cases appears to decrease in a singular manner according to the size of the creature. The relative bulk of the muscles being less in the smaller species than in the larger, it is necessary to explain the superior relative strength of the former by supposing them to possess a greater amount of vital energy.

These astonishing phenomena will perhaps be better understood if we consider the obstacles which insects have to overcome to satisfy their wants, to seek their food, to defend themselves against their enemies, &c.

To meet these requirements they are marvellously constructed for both labour and warfare, and their strength is superior to that displayed by all other animals. It is also much greater than that of the machines we construct to replace manual labour. They represent strength itself. God's workmen are infinitely more powerful than those invented by the genius of man, which we call machines.

We think it necessary, in closing this chapter, to give a sort of general outline of the great class of animals which we are about to study. If we wished to characterise insects by their exterior aspect, we might consider them as articulate animals, whose bodies, covered with tough and membranous integuments, are divided into three distinct parts: the head, provided with two antennæ, and eyes and mouth of very variable form; a trunk or thorax, composed of three segments, which has underneath it always six articulated limbs, and often above it two or four wings; and an abdomen, composed of nine segments, although some may not appear to exist at first sight.

If, in addition to these characteristics, one considers that these animals are not provided with interior skeletons—that their nervous system is formed of a double cord, swelling at intervals, and placed along the under-side of the body, with the exception of the first swellings or ganglions which are under the head—that they are not provided with a complete circulating system—that they breathe by particular organs, termed tracheæ, extending parallel to each other along each side of the body, and communicating with the exterior air by lateral openings termed spiracles—that their sexes are distinct—that they are reproduced from eggs—and, in conclusion, that the different parts we have mentioned are not complete until the creature has passed through several successive changes, called metamorphoses, a general idea may be formed of what is meant in zoology by the word "insect."

Insects, whose general organisation we have briefly traced, have been classed by naturalists as follows:—

1. Aptera (Fleas and Lice).
2. Diptera (Gnats, Flies, &c.)
3. Hemiptera (Bugs, &c.)
4. Lepidoptera (Butterflies and Moths).
5. Orthoptera (Grasshoppers, Crickets, Cockroaches, &c.)
6. Hymenoptera (Bees, Wasps, &c.)
7. Thysanoptera (Thrips cerealium).
8. Neuroptera (Libellula, or Dragon-fly; Ephemera, or May-fly; Phryganea, or Alder-fly).
9. Coleoptera (Beetles).

We shall commence the history of the various orders by examining the Aptera.

[I.]

APTERA.

Insects of this order are without wings, and the name is derived from two Greek words, α, privative, and πτερον, wing, indicating the negative character which constitutes this order. [9] It consists of Fleas and Lice. The Flea (Pulex), of which De Geer formed a separate group, and called Suctoria, includes several species.

The common flea (Pulex irritans, [Fig. 17]) has a body of oval form, somewhat flattened, covered with a rather hard horny skin of a brilliant chestnut brown colour. It is the breaking of this hard skin which produces the little crack which is heard when, after a successful hunt, one has the happiness to crush one of these parasites between one's nails.

Fig. 17.
Flea (Pulex irritans).

Its head, small in proportion to the body, is compressed, and carries two small antennæ, of cylindrical form, composed of four joints, which the animal shakes continually when in motion, but which it lowers and rests in front of its head when in a state of repose. The eyes are simple, large, and round. The beak is composed of an exterior jointed sheath, having inside it a tube, and carrying underneath two long sharp lancets, with cutting and saw-like edges. It is with this instrument that the flea pierces the skin, irritates it, and causes the blood on which it lives to flow.

This bite, as every one knows, is easily recognised by the presence of small darkish red spots, surrounded by a circle of a paler colour. The quantity of blood absorbed by this little creature is enormous, when compared with its size.

The body of the flea is divided into thirteen segments, of which one forms the head; three the thorax, which is short, and the remainder the abdomen.

The limbs are long, strong, and spiny. The tarsus, or foot, has five joints, and terminates in hooks turned in opposite directions. The two anterior limbs are separated from the others, and are inserted nearly under the head; the posterior ones are particularly large and strong.

The jumps which fleas are able to make are really gigantic, and the strength of these little animals quite herculean, when compared with the size of their bodies. The reader may be inclined to smile at the assertion that the flea possesses herculean strength; but let him wait a little, and he will find that it is no exaggeration.

To give some idea of the strength, the docility, and the goodwill of the fleas, some wonderful little things have been made, which have served at the same time to show the astonishing skill of certain workmen.

In his "Histoire abrégée des Insectes," published in the seventh year of the French Republic, Geoffroy relates that a certain Mark, an Englishman, had succeeded, by dint of patience and art, in making a gold chain the length of a finger, with a padlock and a key to fasten it, not exceeding a single grain in weight. A flea attached to the chain pulled it easily. The same learned writer relates a still more surprising fact. An English workman constructed a carriage and six horses of ivory. The coachman was on the box, with a dog between his legs, there were also a postillion, four persons in the carriage, and two servants behind, and the whole of this was drawn by one flea.

In his "Histoire Naturelle des Insectes Aptères," Baron Walckenaer relates the following marvellous instance of industry, patience, and dexterity:—

"I think it is about fifteen years ago, that the whole population of Paris could see the following wonders exhibited on the Place de la Bourse for sixty centimes. They were the learned fleas. I have seen and examined them with entomological eyes, assisted by a glass.

"Thirty fleas went through military exercise, and stood upon their hind legs, armed with pikes, formed of very small splinters of wood.

"Two fleas were harnessed to and drew a golden carriage with four wheels and a postillion. A third flea was seated on the coach-box, and held a splinter of wood for a whip. Two other fleas drew a cannon on its carriage; this little trinket was admirably finished, not a screw or a nut was wanting. These and other wonders were performed on polished glass. The flea-horses were fastened by a gold chain attached to the thighs of their hind legs, which I was told was never taken off. They had lived thus for two years and a half, not one having died during the period. To be fed, they were placed on a man's arm, which they sucked. When they were unwilling to draw the cannon or the carriage, the man took a burning coal, and on it being moved about near them, they were at once roused, and recommenced the performances."

The learned fleas were the admiration and amazement of Paris, Lyons, and the chief provincial towns of France, in 1825.

But how, one will ask, was it possible in a large public room to see this wonderful sight? And it is necessary that this should be explained. The spectators were seated in front of a curtain, provided with magnifying glasses, through which they looked, as they would at a diorama of landscapes or buildings.

But let us return to the natural history of our insect. The female flea lays from eight to twelve eggs, which are of oval shape, smooth, viscous, and white.

Contrary to what one might think, à priori, the flea does not fix its eggs to the skin of its victims. She lets them drop on the ground, between the boards of floors, or old furniture, and among dirty linen and rubbish.

M. Defrance has remarked that there are always found mixed with the eggs a certain number of grains of a brilliant black colour, which are simply dried blood. This is a provision which the foreseeing mother has prepared at our expense to nourish her young offspring.

In four or five days in summer, and in eleven days in winter, one may see coming out of these eggs small, elongated larvæ, of cylindrical form, covered with hair, and divided into three parts, the last provided with two small hooks. The head is scaly above, has two small antennæ, and is without eyes. These larvæ are without limbs, but they can twist about, roll themselves over and over, and even advance pretty fast by raising their heads. Though at first white, they become afterwards of a reddish colour.

About a fortnight after they are hatched they cease to eat, and are immovable, as if about to die. They then commence to make a small, whitish, silky cocoon, in which they are transformed into pupæ. In another fortnight these pupæ become perfect insects.

A most remarkable trait, and unique among insects, has been observed in the flea. The mother disgorges into the mouths of the larvæ the blood with which she is filled.

The flea is most abundant in Europe and the North of Africa. Certain circumstances particularly favour its multiplication; being most abundant in dirty houses, in barracks, and in camps; in deserted buildings, in ruins, and in places frequented by people of uncleanly habits.

Other kinds of fleas live on animals, as, for example, the cat flea, the dog flea, and those of the pigeon and poultry.

We shall say a few words about a peculiar species which abounds in all the hot parts of America, but principally in the Brazils and the neighbouring countries. This formidable species is the Chigo (Pulex penetrans).

The chigo, called also the tick, is smaller than the common flea. It is flat, brown with a white spot on the back, and is armed with a strong pointed stiff beak, provided with three lancets. It is with this instrument that the female attacks man with the intention of lodging in his skin and bringing forth her young there.

The chigo attacks chiefly the feet. It slips in between the flesh and the nails, or gets under the skin of the heel. Notwithstanding the length of the animal's beak, introducing itself beneath the skin does not at first cause any pain; but after a few days one is made aware of its presence by an itching, which, though at first slight, gradually increases, and ends by becoming unbearable.

The chigo, when under the skin, betrays itself by a bump outside. Its body has now become as large as a pea; in the attacked skin a large brown bag containing matter is formed. In this bag are collected the eggs, which issue from an orifice in the posterior extremity, and are not hatched in the wound itself, as was long thought to be the case.

The chigoes are an object of terror to the Brazilian negroes. These formidable parasites sometimes attack the whole of the foot, which they devour, and thus bring on mortification; many negroes losing the bones of some of their toes by the ravages of these dangerous creatures. To guard against their attacks, they wear thick shoes, and examine their feet carefully every day. The plan usually followed in the Brazils to prevent the chigoes from injuring the feet, is to employ children, who, by their sharpness of sight, can easily perceive the red spot on the skin where the chigo has entered. These children are in the habit of extracting the insect from the wound by means of a needle. But this is not without risk; as, if any portion of the insect remains in the wound, a dangerous inflammation may ensue. For this reason, operators who are renowned for their skill are much sought after, flattered, and rewarded by the poor negroes of the plantations.

Fig. 18.—Louse (Pediculus capitis) magnified.

The Head Louse (Pediculus capitis, [Fig. 18]) is an insect with a flat body, slightly transparent, and of greyish colour, spotted with black on the spiracles, soft in the middle, and rather hard at the sides. The head, which is oval, is furnished with two thread-like antennæ, composed of five joints, which are constantly in motion while the creature is walking; it is also furnished with two simple, round, black eyes; and lastly, with a mouth. In the front of the head is a short, conical, fleshy nipple. This nipple contains a sucker, or rostrum, which the animal can put out when it likes, and which, when extended, represents a tubular body, terminating in six little pointed hooks, bent back, and serving to retain the instrument in the skin. This organ is surmounted by four fine hairs, fixed to one another, and seated in its interior. It is by means of this complicated apparatus that the louse pricks and sucks the skin of the head. The thorax is nearly square, and divided into three parts by deep incisions. The abdomen, strongly lobed at the sides, is composed of eight rings, and is provided with sixteen spiracles. The limbs consist of a trochanter, a thigh, a shank, and a tarsus of a single joint, and are very thick. A strong nail, which folds back on an indented projection, thus forming a pincer, terminates the tarsus. It is with this pincer that the louse fastens itself to the hair.

Lice are oviparous. Their eggs, which remain sticking to the hair, are long and white, and are commonly called "nits." The young are hatched in the course of five or six days; and in eighteen days are able to reproduce their kind. Leuwenhoek calculated that in two months two female lice could produce ten thousand! Other naturalists have asserted that the second generation of a single individual can amount to two thousand five hundred, and the third, to a hundred and twenty-five thousand! Happily for the victims of these disgusting parasites, their reproduction is not generally to this prodigious extent.

Many means are employed to kill lice. Lotions of the smaller centaury or of stavesacre, and pomatum mixed with mercurial ointment, are very efficacious. But the surest and easiest remedy is to put plenty of oil on the head. The oil kills the lice by obstructing their tracheæ, and thus stopping respiration.

There are other kinds of lice, but we will only mention the louse which infests beggars and people of unclean habits, Pediculus humanus corporis, producing the complaint called phthiriasis. In the victims of this disease these parasites increase with fearful rapidity. This dreadful disorder is often mentioned by the ancients. King Antiochus, the philosopher Pherecydes of Scyros, the contemporary and friend of Thales, the dictator Sylla, Agrippa, and Valerius Maximus, are said to have been attacked by phthiriasis, and even to have died of it. Amatus Lusitanus, a Portuguese doctor of the sixteenth century, relates that lice increased so quickly and to such an extent on a rich nobleman attacked with phthiriasis, that the whole duty of two of his servants consisted in carrying away, and throwing into the sea, whole basketfuls of the vermin, which were continually escaping from the person of their noble master.

Little is known at the present day of the details of this complaint, though it is observed frequently enough in some parts of the south of Europe, where the dirty and miserable inhabitants are a prey to poverty and uncleanliness—two misfortunes which often go together. In Gallicia, in Poland, in the Asturias, and in Spain, we may find many victims of phthiriasis.

Lice increase with such rapidity on persons thus attacked, that it is common to attribute their appearance to spontaneous generation alone. But the prodigious rapidity of reproduction in these insects sufficiently explains their increase, especially when it is admitted that it is possible for the female louse to reproduce young without the agency of the male.

The Thysanura or "Skip Tail" tribe are small insects, which are better known on account of the beauty of their microscopic body scales than for any interesting habits or instincts. They do not undergo metamorphosis.

The Fish Scale or Lepisma saccharina, and the Skip Tail or Podura plumbea belong to the Thysanura.

[II.]

DIPTERA.

All suctorial insects which in the perfect state possess only two membranous wings, are called Diptera, from two Greek words—δις, twice, and πτερον, wing.

The Diptera were known and scientifically described at a very early date. They are frequently mentioned by Aristotle in his "History of Animals;" and he applied the term to the same insects as now constitute the order.

The absence of the second wings, common to other insects, which are in this case replaced by two appendages, which have received the name of balancers, [10] because they serve to regulate the action of flight, constitutes the chief characteristic of the Diptera. Let us, however, give a glance at their other organs, which have more or less affinity with those which exist in other classes of insects, preserving, nevertheless, their own especial characteristics.

The mouth, for instance—suited for suction only—is in the form of a trunk, and is composed of a sheath, a sucker, and two palpi. The antennæ are generally composed of only three joints. The eyes—usually two in number—are very large, and sometimes take up nearly the whole of the head. They are both simple and compound. The wings are membranous, delicate, and veined; the limbs long and slight. In giving the history of the principal types of Diptera, we shall explain more fully the formation of these organs.

The Diptera, by their rapid flight, enliven both the earth and the air. The different species abound in every climate, and in every situation, some inhabiting woods, plains, fields, or banks of rivers; others preferring our houses. They like the neighbourhood of vegetation, choosing either the flowers, the leaves, or the stems of the trees of our woods, our gardens, or our plantations. Their food varies very much; and the formation of the sucker is regulated by it. Some imbibe blood, others live on the secretions of animals. Their chief nourishment, however, consists of the juices of flowers, on whose brilliant corollas the Diptera abound, either plundering from every species indiscriminately, or attaching themselves to some particular kind. They display the most wonderful instinct in their maternal care, and employ the most varied and ingenious precautions to preserve their progeny.

The Diptera, besides their variety and the number of their species, are remarkable on account of their profusion. The myriads of flies which rise from our meadows, which fly in crowds around our plants, and around every organised substance from which life has departed, some of which even infest living animals, are Diptera.

The profusion with which they are distributed over the face of the globe, causes them to fulfil two important duties in the economy of Nature. On the one hand, they furnish to insectivorous birds an inexhaustible supply of food; on the other, they contribute to the removal of all decaying animal and vegetable substances, and thus serve to purify the air which we breathe. Their fecundity, the rapidity with which one generation succeeds another, and their great voracity, added to the extraordinary quickness of their reproduction, are such that Linnæus tells us that three flies, with the generations which spring from them, could eat up a dead horse as quickly as a lion could.

These Diptera, which are worthy of so much attention, and deserve so much study with regard to the part they play in the general economy of Nature, are an object of fear and repulsion when one considers their relations to us and other animals. Gnats and mosquitoes suck our blood; the gad-fly and the species of Asilus attack our cattle. The order Diptera is composed of a great number of families, which are again divided into tribes, each comprising several genera. We shall only notice the more remarkable genera of Diptera.

M. Macquart, the learned author of "L'Histoire Naturelle des Diptères,"[11] divides this great class of insects into two principal groups. In one of these groups, the antennæ are formed of at least six joints, and the palpi of four or five: these are called Nemocera. In the other, the antennæ consists only of three joints, and the palpi of one or two: these are the Brachycera.

The Nemocera may generally be distinguished from the other Diptera, independently of the difference in the antennæ and palpi, by the slenderness of the body, the smallness of the head, the shape of the thorax, and the length of the feet and wings. The result of this organisation is a graceful, light, and aerial form.

[Nemocera.]

Abounding everywhere, the Nemocera live, some on the blood of man and animals, some on small insects, and others on the juices of fragrant flowers. From νημα, thread; κερας, horn.

In all climates, in every latitude, in the fields and woods, even in our dwellings, they may be seen fluttering and plundering. The Nemocera are divided into two families, that of the Culicidæ, of which the gnat (Culex), which has a long, thin trunk, and a sucker provided with six bristles, is a member; and that of the Tipulidæ, which have a short thick trunk, and a sucker having two bristles.

Figs. 19 and 20.—The Gnat (Culex pipiens).

We will begin our examination with the Gnat (Culex pipiens), of which Réaumur, in his "Mémoires pour servir à l'Histoire des Insectes," has given such a curious and complete history. "The gnat is our declared enemy," says Réaumur, in the introduction to his memoir, "and a very troublesome enemy it is. However, it is well to make its acquaintance, for if we pay a little attention we shall be forced to admire it, and even to admire the instrument with which it wounds us. Besides which, throughout the whole course of its life it offers most interesting matter of investigation to those who are curious to know the wonders of Nature. During a period in its life the observer, forgetting that it will at some time annoy him, feels the greatest interest in its life-history."

As this is the case, let us explain the history of these insects, which excite so much interest. The illustrious naturalist we have just mentioned will be our guide.

The body of the gnat is long and cylindrical. When in a state of repose one of its wings is crossed over the other. They present a charming appearance when seen through a microscope, their nervures, as well as their edges, being completely covered with scales, shaped like oblong plates and finely striated longitudinally. These scales are also found on all the segments of the body.

The antennæ of the gnat, particularly those of the male, have a fine feathery appearance ([Fig. 21]).

Their eyes, covered with network, are so large that they cover nearly the whole of the head. Some have eyes of a brilliant green colour, but looked at in certain lights they appear red. [Fig. 22] shows the head of the gnat with its two eyes, its antennæ, and trunk.

The instrument which the gnat employs for puncturing the skin, and which is called the trunk ([Fig. 23]), is well worthy of our attention. That which is generally seen is only the case of those instruments which are intended to pierce our skin and suck our blood, and in which they are held, as lancets and other instruments are held in a surgeon's case. The case ([Fig. 24]) is cylindrical, covered with scales, and terminates in a small knob. Split from end to end that it may open, it contains a perfect bundle of stings. Réaumur tried to observe, by allowing himself to be stung by gnats, what took place during the attack. He forgot, in watching the operations of the insect, the slight pain caused by the wound, soliciting it as a favour, his only regret being not to obtain it when he wished.

Réaumur observed that the compound sting, which is about a line in length, enters the skin to the depth of about three-quarters of a line, and that during that time the case bends into a bow, until the two ends meet. He noticed besides, that the trunk-case of certain gnats was even more complicated than that which we have described. But we will not dwell any longer on this point.

Let us now try to give an idea of the construction and composition of this sting, which, after piercing the skin, draws our blood.

According to Réaumur, the sting of the gnat is composed of five parts. He acknowledges, however, that it is very difficult to be certain of the exact number of these parts, on account of the way in which they are united, and of their form. At the present day we know that there are six. Réaumur, as also Leuwenhoek, thought he saw two in the form of a sword blade with three edges. These have the points reversed, and are serrated on the convex side of the bend ([Fig. 25]). To form an idea of the shape of the other points, the reader should look at Figs. [26] and [27]. He will then see that the gnat's sting is a sword in miniature.

The prick made by so fine a point as that of the sting of the gnat ought not to cause any pain. "The point of the finest needle," says Réaumur, "compared to the sting of the gnat, is the same as the point of a sword compared to that of the needle." How is it then that so small a wound does not heal at once? How is it that small bumps arise on the part that is stung? The fact is, that it is not only a wound, but it has been imbued with an irritating liquid.

This liquid may be seen to exude, under different circumstances, from the trunk of the gnat, like a drop of very clear water.

Réaumur sometimes saw this liquid even in the trunk itself. "There is nothing better," he observes, "to prevent the bad effects of gnat bites than at once to dilute the liquid they have left in the wound with water. However small this wound may be, it will not be difficult for water to be introduced. By rubbing, it will be at once enlarged, and there is nothing to do but to wash it. I have sometimes found this remedy answer very well."

Fig. 28.
Larva of the Gnat.

The gnat is not always in the form of a winged insect, greedy for our blood. There is a period during which they leave us in repose. This is the larva period. It is in water, and in stagnant water in particular, that the larva of the insect which occupies our attention is to be found. It resembles a worm, and may be found in ponds from the month of May until the commencement of winter.

If we desire to follow the larva of the gnat from the beginning, we have only to keep a bucket of water in the open air. After a few days this water will be observed to be full of the larvæ of the gnat ([Fig. 28]). They are very small, and come to the surface of the water to breathe; for which purpose they extend the opening of a pipe, A, which is attached to the last segment of the body, a little above the surface. They are, consequently, obliged to hold their heads down. By the side of the breathing-tube is another tube, B, shorter and thicker than the former, nearly perpendicular to the body, its orifice being the exterior termination of the digestive tube. At the anus it is fringed with long hairs, having the appearance, when in the water, of a funnel. At the end of the same tube, and inside the hair funnel, are four thin, oval, transparent, scaly blades, having the appearance of fins. They are placed in pairs, of which one emanates from the right side, the other from the left.

These four blades or fins have the power of separating from each other. Each segment of the abdomen has on both sides a tuft of hair, and the thorax has three. The head is round and flat, and is provided with two simple brown eyes. Round the mouth are several wattles, furnished with hair, of which two of crescent-like form are the most conspicuous. These tufts move with great quickness, causing small currents of liquid to flow into the mouth, by means of which the necessary food, microscopic insects and particles of vegetable and earthy matter, is brought to the larva.

They change their skin several times during their continuance in this state. This latter fact has been remarked by Dom Allou, a learned Carthusian, "whose pleasure," says Réaumur, "consisted in admiring the works of the Almighty, when not occupied in singing his praises." We think it will be interesting to repeat the few lines which accompany the mention made by Réaumur of this worthy Carthusian. They appear to us to be well worth reading, even at the present day.

"If the pious monks who composed so many societies, possessed, like Dom Allou, the love of observing insects, we might hope that the most essential facts in the history of those little creatures would soon be made known to us. What enjoyment more worthy of the calling they have chosen could these pious men pursue than that which would place before their eyes the marvellous creations of an Almighty Power? Even their leisure would then incline them to adore that Power, and would furnish them the means to make others do so who are occupied by too serious or too frivolous employments."

Fig. 29.
Pupa of the Gnat.

After having changed its skin three times in a fortnight or three weeks, the larva of the gnat throws off its covering for a fourth time, and is no longer in the larva state. It is changed both in shape and condition. Instead of being oblong, its body is shortened, rounded, and bent in such a way that the tail is applied to the under part of the head. This is the case when the animal is in repose; but it is able to move and swim, and then, by bending its body and straightening it again, propels itself through the water.

In this new condition, that is to say, in the pupa state ([Fig. 29]), it does not eat. It no longer possesses digestive organs, but it is necessary, even more than before its metamorphosis, that it should breathe atmospheric air. Besides, the organs of respiration are greatly changed. During the time the insect was in the larva state, it was through the long tube fixed to the posterior part that it received or expelled the air; but in casting its skin it loses the tube, two appendages resembling an ass's ears being for the pupa what the tube was for the larva, the opening of these ears being held above the surface of the water. From this pupa the perfect insect will emerge; it is developed little by little, and the principal members may be distinguished under the transparent membranous skin which envelopes it.

When the insect is about to change from the pupa state, it lies on the surface of the water, straightening the hind part of its body, and extending itself on the surface of the water, above which the thorax is raised. Before it has been a moment in this position, its skin splits between the two breathing trumpets, the split increasing very rapidly in length and breadth.

"It leaves uncovered," says Réaumur, "a portion of the thorax of the gnat, easily to be recognised by the freshness of its colour, which is green, and different from the skin in which it was before enveloped.

"As soon is the split is enlarged—and to do so sufficiently is the work of a moment—the fore part of the perfect insect is not long in showing itself; and soon afterwards the head appears, rising above the edges of the opening. But this moment, and those which follow, until the gnat has entirely left its covering, are most critical, and when it is exposed to fearful danger. This insect, which lately lived in the water, is suddenly in a position in which it has nothing to fear so much as water. If it were upset on the water, and the water were to touch its thorax or body, it would be fatal. This is the way in which it acts in this critical position—As soon as it has got out its head and thorax, it lifts them as high as it is able above the opening through which they had emerged, and then draws the posterior part of its body through the same opening; or rather that part pushes itself forward by contracting a little and then lengthening again, the roughness of the covering from which it desires to extricate itself serving as an assistance.

Fig. 30.—Gnats emerging.

"A larger portion of the gnat is thus uncovered, and at the same time the head is advanced farther towards the anterior end of the covering; but as it advances in this direction, it rises more and more, the anterior and posterior ends of the sheath thus becoming quite empty. The sheath then becomes a sort of boat, into which the water does not enter; and it would be fatal if it did. The water could not find a passage to the farther end, and the edges of the anterior end could not be submerged until the other was considerably sunk. The gnat itself is the mast of its little boat. Large boats, which pass under bridges, have masts which can be lowered; as soon as the boat has passed the bridge the mast is hoisted up by degrees, until it is perpendicular. The gnat rises thus until it becomes the mast of its own little boat, and a vertical mast also. It is difficult to imagine how it is able to put itself in such a singular, though for it necessary, position, and also how it can keep it. The fore part of the boat is much more loaded than the other, but it is also much broader. Any one who observes how deep the fore part of the boat is, and how near the edges of its sides are to the water, forgets for the time being that the gnat is an insect that he would willingly destroy at other times. One feels uneasy for its fate; and the more so if the wind happens to rise, particularly if it disturbs the surface of the water. But one sees with pleasure that there is air enough to carry the gnat along quickly; it is carried from side to side; it makes different voyages in the bucket in which it is borne. Though it is only a sort of boat—or rather mast, because its wings and legs are fixed close to its body, it is perhaps, in proportion to the size of its boat, a larger sail than one would dare to put on a real vessel—one cannot help fearing that the little boat will capsize. * * * As soon as the boat is capsized, as soon as the gnat is laid on the surface of the water, there is no chance left for it. I have sometimes seen the water covered with gnats which had perished thus as soon as they were born. It is, however, still more extraordinary that the gnat is able to finish its operations. Happily they do not last long; all dangers may be passed over in a minute.

"The gnat, after raising itself perpendicularly, draws its two front legs from the sheath, and brings them forward. It then draws out the two next. It now no longer tries to maintain its uneasy position, but leans towards the water; gets near it, and places its feet upon it; the water is sufficiently firm and solid support for them, and is able to bear them, although burdened with the insect's body. As soon as the insect is thus on the water it is in safety; its wings are unfolded and dried, which is done sooner than it takes to tell it, at length the gnat is in a position to use them, and it is soon seen to fly away, particularly if one tries to catch it."

Fig. 31.—Eggs of the Gnat, magnified.

One more word about the gnat, whose life is full of such interesting details.