DISTINGUISHING CRITERIA OF INSECTS.

[57]As the author’s idea of this substance being of the nature of talc, does not appear correct, and I cannot find that entomologists are agreed in the definition of it, I shall just give the following extract on the subject from the Cyclopœdia by Rees, and submit the decision to the reader.“The substance which connects and fills up the spaces between these ribs, is of so peculiar a nature, that it is not easy to find any name to design it by, at least there is no substance that enters the composition of the bodies of the larger animals, that is at all analogous to it. It is a white substance, transparent and friable, and seems indeed to differ in nothing from that of the large and thick ribs, but in that it is extended into thin plates; but this is saying little toward the determining what it really is, since we are as much at a loss to know by what name to call the substance they are composed of. Malpighi indeed calls them bones; but though they do serve in the place of bones, rendering the wing firm and strong, &c. yet, when strictly examined, they do not appear to have any thing of the structure of bones, but appear rather of the substance of scales, or of that sort of imperfect scales, of which, the covering of crustaceous insects is composed.”Edit.

“The substance which connects and fills up the spaces between these ribs, is of so peculiar a nature, that it is not easy to find any name to design it by, at least there is no substance that enters the composition of the bodies of the larger animals, that is at all analogous to it. It is a white substance, transparent and friable, and seems indeed to differ in nothing from that of the large and thick ribs, but in that it is extended into thin plates; but this is saying little toward the determining what it really is, since we are as much at a loss to know by what name to call the substance they are composed of. Malpighi indeed calls them bones; but though they do serve in the place of bones, rendering the wing firm and strong, &c. yet, when strictly examined, they do not appear to have any thing of the structure of bones, but appear rather of the substance of scales, or of that sort of imperfect scales, of which, the covering of crustaceous insects is composed.”Edit.

Ever since the microscope was invented, the dust that covers these wings has engaged the attention of microscopic observers; as by this instrument it is found to be a regular collection of organized scales of various shapes, and in whose construction there is as much symmetry, as there is beauty in their colours. A view of some of these scales, as they appear in the microscope, is exhibited at F E H I, in Fig. 7.Plate XVI.and in Fig. 8. of the natural size. Their shapes are not only very different in moths of various species, but those on the same moth are also found to differ. Of the scales, some are so long and slender that they resemble hairs, except that they are a little flattened and divided at the ends; some are short and broad; some are notched at the edges, others smooth; some are nearly oval, while others are triangular: they are mostly furnished with a short stalk or stem to fix them to the wing. With the microscope, a variety of large stripes or ribs are to be discovered; between these larger lines, minuter ones may be seen with a deep magnifier. The larger stripes rise in general from the exterior notches; some have a rib running down the middle, through their whole length. The upper and under parts of the wing are equally supplied with them.

The regular arrangement of these plates, one beside and partly covering the other, as in the tiling of an house, is best seen by examining a wing in the opake microscope. The prodigious number of small scales which cover the wings of these beautifulinsects, is a sure proof of their utility to them, because they are given byHIMwho makes nothing in vain.

That the lively and variegated colours, which adorn the wings of the moth and butterfly, arise from the small scales or plates that are planted therein, is very evident from this, that if they be brushed off from it, the wing is perfectly transparent: but whence this profusion and difference of colour on the same wing? is a question as difficult to resolve, as that of Prior, when he asks.

As the wings of the moths and butterflies are very light, they can support themselves for a long time in the air; their manner of flying is ungraceful, generally moving in a zigzag line, to the right and to the left, alternately ascending and descending; this undulating motion however has its uses, as it disappoints the birds who chase them in taking aim; by which means they frequently elude their pursuit, though continued for a considerable time.

Dr. Hooke[58]endeavoured to investigate the nature of the motions of the wings of insects; and, although he was not able, from the experiments he made, to give a satisfactory account of them, yet as they may be useful to some future inquirer, and lead him more readily into the path of truth, I hope an extract therefrom will not prove unacceptable to the reader. To investigate the mode or manner of moving their wings, he considered with attention those spinning insects that suspend, or as it were poisethemselves in one place in the air, without rising or falling, or even moving backwards or forwards; by looking down on these, he could, by a kind of faint shadow, perceive the utmost extremes of the vibratory motion of their wings; the shadow, while they were thus suspended, was not very long, but was lengthened when they endeavoured to fly forwards. He next tried by fixing the legs of a fly upon the top of the stalk of a feather with glue, wax, &c. and then making it endeavour to fly away; he was thereby able to view it in any posture. From hence he collected, that the extreme limits of the vibrations were usually somewhat about the length of the body distant from each other, often shorter, and sometimes longer. The foremost limit was generally a little above the back, and the hinder one somewhat beneath the belly; between these, to judge by the sound, they seemed to move with an equal velocity. The manner of their moving them, if a just idea can be formed by the shadow of the wing, and a consideration of its nature and structure, seemed to be this: the wing being supposed to be in the extreme limit, it is then nearly horizontal, the forepart only being a little depressed; in this situation the wing moves to the lower limit; before it arrives at this, the hinder part begins to move fastest; the area of the wing begins to dip behind, and in that posture it seems to be moved to the upper limit back again. These vibrations, judging by the sound, and comparing them with a string tuned in unison thereto, consist of many hundreds, if not thousands, in a second of time. The powers of the governing faculty of the insect, and the vivacity of its sensations, whereby every organ is stimulated to act with so much velocity and regularity, surpass our present comprehension.

[58]Hooke’s Micrographia, p. 172.

These are admirably adapted for their intended service, to give the most convenient and proper motion, and, from the variety intheir construction, their various articulations, &c. furnish the microscopic observer with an abundance of curious and interesting objects: the most general number is six; many of the class aptera have eight, as the spider; the crab has ten; the oniscus fourteen; the julus has from seventy to one-hundred and twenty on each side. The legs of those insects that have not more than ten, are affixed to the trunk; while those that exceed that number, have part fixed to the trunk, the rest to the abdomen.

The legs of insects are generally divided into four parts. The first, which is usually the largest, is called the femur; the second, or tibia, is joined to the former, and is commonly of the same size throughout, and longer than the femur; this is followed by the third part, which is distinguished by the name of tarsus, or foot; it is composed of several joints, the one articulated to the other, the number of rings varying in different insects; the tarsus is terminated by the unguis, or claw.

The writers on natural history, in order to render their descriptions clear and accurate, have given several names to the legs of insects, from the nature of the motions produced by them. Thus cursorii, from that of running; these are the most numerous. The saltatorii, those that are used for leaping; the thighs of these are remarkably large, by which means they possess considerable strength and power to leap to great distances. The natatorii, those that serve as oars for swimming; the feet of these are flat and edged with hairs, possessing a proper surface to strike against the water, as in the dytiscus, notonecta, &c. Such feet as have no claws are termed mutici. The chelæ, or claws, are an enlargement of the extremity of the fore feet, each of which is furnished with two lesser claws, which act like a thumb and finger, as in the crab. The under part of the feet in some insects is covered with a kind of brush or sponge, by which they areenabled to walk with ease, on the most polished substances, and in situations from which it would seem they must necessarily fall.

Motion is one of the principal phenomena of nature; it is as it were the soul of our system, and is as admirable in the smallest animal, as in the universe at large. It is the principal agent in producing all that diversity and change which perpetually affect every object in the creation. The motions of animals are proportioned to their weight and structure, a flea can leap to the distance of at least two hundred times its own length; were an elephant, a camel, or an horse to leap in the same proportion, their weight would crush them to atoms. The same remark is applicable to spiders, worms, and other insects; the softness of their texture, and the comparative smallness of their specific gravity, enable them to fall without injury from heights that would prove fatal to larger and heavier animals.[59]

[59]The parts of some of the larger animals are, however, so admirably constructed for swiftness, as to enable them to perform surprizing acts of agility; for instance, the Siberian jerboa, mus saliens, Pennant; this animal springs forward by successive leaping so very nimbly, that it is said to be very difficult for a man well mounted to overtake it; it is about the size of a large rat. The kanguroo, opossum of Pennant, macropus giganteus, Shaw, leaps to so uncommon a height, and to so great a distance, as to outstrip the swiftest greyhound; its size is that of a full-grown sheep. Accurate coloured figures of both these extraordinary animals are given in that elegant work, the Naturalist’s Miscellany.Edit.

Many insects can only move the thigh in a vertical direction, while others can move it in a variety of ways. The progressive motion of insects, and the various methods employed to effect it, will be found a very curious and important subject, and well worthy the attention of the naturalist. The intelligent mechanic will not find it lost labour if he bestow some time on the same subject. Very little has been done on this head, and that principallyby Reaumur, in his excellent Memoires; and by M. Weiss, in a Memoir published in the Journal de Physique for 1771. The reader may also consult Borelli de Motu Animalium.

Cauda, the tail, terminates the abdomen, and is constructed in a wonderful manner for answering the purposes for which it is formed, namely, to direct the motion of the insect, to serve as an instrument of defence, or for depositing the eggs; the figure and size thereof varying in each genus and its families. In most insects it is simple, simplex, and yet capable of being extended or drawn back at pleasure; in others, elongata, elongated, as in the crab and scorpion; setacea, shaped like a bristle, as in the raphidia; triseta, with three appendages like bristles, as in the ephemera; in some it is forked, furcata, as in the podura; and in others it is furnished with a pair of forceps, forcipata, as in the forficula; in the blatta, grylli, and others, it is foliosa, or like a leaf; in the scorpion and panorpa it is telifera, furnished with a dart or sting. Further particulars may be obtained from the Philosophia Entomologica of Fabricius.

Aculeus, or the sting, is an instrument with which insects wound and instil a poison; the sting generally proceeds from the under part of the last ring of the belly: in some it is sharp and pointed, in others serrated or formed like a saw. It is used by many insects both as an offensive and defensive weapon; by others it is only used to pierce the substances where they mean to deposit their eggs. This instrument cannot be properly seen or known, but with the assistance of a microscope.

Of bees, it is only the labourers and the queen that have stings. The apparatus is of a very curious construction, fitted for inflicting a wound, and at the same time conveying poison into that wound.

The apparatus consists of two piercers conducted in a sheath, groove, or director.

This groove is rather large at the base, but terminates in a point; it is affixed to the last scale of the upper side of the abdomen by thirteen thin scales, six on each side, and one behind the rectum. These scales inclose the rectum all round, and are attached to each other by thin membranes which allow of a variety of motions; three of them are however attached more closely to a round and curved process, which comes from the basis of the groove in which the sting lies, as also to the curved arms of the sting which spread out externally. The two stings may be said to begin by those two curved processes at their union with the scales, and converging towards the groove at its base, which they enter, and then pass along to its point.

The two stings are serrated or notched towards the points; they can be thrust out a little way, and drawn within it. These parts are all moved by very strong muscles, which give motions in almost all directions, but most particularly outwards. It is wonderful how deep they will pierce solid bodies with this sting.

To perform this by mere force, two things are necessary, power of muscles, and strength of sting; neither of which they seem to possess in a sufficient degree. Mr. J. Hunter thinks that it cannotbe by simple force, because the least pressure bends the sting in any direction. It is probable that the serrated edges may assist, by cutting their way like a saw.

The apparatus for the poison consists of two small ducts, which are the glands that secrete the poison; these lie in the abdomen among the air cells, they soon however unite into one oblong bag; at the opposite end of which a duct passes out, which runs towards the angle where the two stings meet, and, entering between them, forms a canal by the union of the two stings at this point. From the serrated construction of the stings the bee can seldom disengage them, and hence, when they pass into materials of too strong a nature, the bee generally leaves them behind, and often a part of the bowels therewith.[60]

[60]Phil. Trans. for 1792, page 189.

It has already been observed, that the bodies of insects are covered with a hard skin, answering the purpose of an internal skeleton, and forming one of the characters by which they are distinguished from other animals. This external covering is very strong in those insects which, from their manner of life, are particularly liable to great friction, or violent compression; but is more tender and delicate in such as are not so exposed. The skin of insects, like that of larger animals, is porous; the pores in some species are very large; many insects often change or cast off their skin; this exuvia forms an excellent object for the microscope.

Another distinguishing criterion of insects is the colour of their circulating fluid or blood, which is never red; this, at first sight, seems liable to some objections, on account of the drop of redliquor which is often procured from small insects when squeezed or pressed to pieces. It does not appear, however, that this is the blood of the little animal; when it existed as a worm there was no such appearance, and when transformed to the perfect, or fly state, it is only found in the eye, and not in the body, which would be the case if it circulated in the veins of the insect. It is probable there is a circulation of some fluid analogous to the blood in most insects: with the assistance of the microscope this circulation may be perceived in many; but the circulating liquor is not red.

To these discriminating characteristics we may also add the following particulars:

1. That the body of insects is divided by incisuræ, or transversal divisions, from whence they take their name.

2. That they are furnished with antennæ, which are placed upon the fore part of the head; these are jointed and moveable in various directions.

3. That no insect in its perfect state, or after it has gone through all its transformations, has less than six legs, though many have more. There are some moths, whose two fore feet are so small, as scarcely to entitle them to that name.

4. That insects have neither the organs of smell nor hearing; at least they have not as yet been discovered, though it is reported that Fabricius has lately found and described the organs of hearing in a lobster.[61]

[61]That many insects are susceptible of a shrill or loud noise, is a fact so well ascertained, as to be indisputable; but in what manner, or by what organs the sensation is conveyed, is not so evident; Barbut, however, supposes them to possess the sense of hearing in a very distinct manner. Many insects, he observes, are well known to be endued with the power of uttering sounds, viz. large beetles, bees, wasps, common flies, gnats, &c. The sphinx atropos squeaks, when hurt, nearly as loud as a mouse: this faculty certainly must be intended for some purpose, and as they vary their cry occasionally, it appears designed to give notice of pleasure or pain, or some affection in the creature which possesses it. “The knowledge of their sounds,” says he, “is undoubtedly confined to their tribe, and is a language intelligible to them only; saving when violence obliges the animal to exert the voice of nature in distress, craving compassion; then all animals understand the doleful cry; for instance, attack a bee or wasp near the hive or nest, or a few of them; the consequence will be, the animal or animals, by a different tone of voice will express his or their disapprobation or pain; that sound is known to the hive to be plaintive, and that their brother or brethren require their assistance, and the offending party seldom escapes with impunity. Now, if they had not the sense of hearing, they could not have known the danger their brother or brethren were in, by the alteration of their tone.” Another proof, which he reckons still more decisive, was taken from his observation on a spider, which had made a very large web on a wooden railing, and was at the time in a cavity behind one of the rails, at a considerable distance from the part where a fly had entangled himself; the spider became immediately sensible of it, though, from the situation of the rail, he could not possibly have seen the fly. This observation, however, cannot be considered as conclusive, as it is very probable that the spider was alarmed by the tremulous motion of the threads of the web occasioned by the fluttering of the fly, which he might well know how to distinguish from their vibration by the wind. It is this author’s opinion, that the organ of hearing is situated in the antennæ; he likewise supposes that the organs of smell reside in the palpi or feelers. For his reasoning on these subjects, see the Genera Insectorum, Preface, p. vii. & seq.Edit.

5. That they do not respire air by the mouth, but that they inspire and exhale it by means of organs which are placed on the body.

6. That they move the jaws from right to left, not up and down.

7. That they have neither eye-lid nor pupil.

To these we may also add, that the mechanism resulting from theLIFEof insects is not of so compound a nature as in animals of a larger size. They have less variety of organs, though some of them are more multiplied; and it is by the number and situation of these that their rank in the great scale of beings is to be determined.

These characters are often united in the same insect; there are, however, some species in which one or two of them are wanting.

The student in entomology, who wishes to attain a proper knowledge of the science, and indeed every microscopic observer,desirous of availing himself of the discoveries of others, and of communicating intelligibly his own, will find it necessary to make himself conversant with the various classes, genera, &c. into which insects have been divided by Linnæus. Every system has its defects, and probably some may be found in that of this truly celebrated naturalist, but the purpose of science is answered by using those discriminations which are generally adopted.

The following general idea of the Linnæan classes may serve as a foundation for this knowledge: a more particular account may be obtained by consulting the under-mentioned works.

Institutions of Entomology, a translation of Linnæus’s Ordines et Genera Insectorum, or systematic arrangement of insects, &c. by Thomas Pattinson Yeats.

Fundamenta Entomologiæ, or an Introduction to the Knowledge of Insects, translated from Linnæus by W. Curtis, the ingenious author of Flora Londinensis, the Botanical Magazine, &c.

The Genera Insectorum of Linnæus, exemplified by variousSpecimens of English Insects, drawn from Nature, by James Barbut.[62]

[62]This work contains two excellent plates, illustrative of the Distinctions of the Ordines and Genera Insectorum, by their antennæ, tarsi of the feet, &c.Edit.

Class the first.Coleoptera. The insects of this class have four wings; the upper ones, called the elytra, are crustaceous, being of a hard horny substance; these, when shut, form a longitudinal suture down the back, as in the scarabæus, melolontha, or cockchaffer, &c.

2.Hemiptera.These have also four wings; but the elytra are different, being half crustaceous, half membranaceous: the wings do not form a longitudinal suture, but extend the one over the other, as in the gryllus, grasshopper, &c.

3.Lepidoptera.Those which have four membranaceous wings covered with fine scales, appearing to the naked eye like powder or meal, as in the butterfly and moth.

4.Neuroptera.These have four membranaceous transparent wings, which are generally reticulated, the tail without a sting, as in the libellula, or dragon fly.

5.Hymenoptera.These, like the preceding class, have four membranaceous naked wings; but the abdomen is furnished with a sting, as in the bee, wasp, ichneumon, &c.

6.Diptera.These have only two wings, and are furnished with halteres, or poisers, instead of under wings, as in the common house fly, gnat, &c.

7.Aptera.These are distinguished by having no wings, as in the spider, louse, acarus, &c.

Insects are farther distinguished from other animals by the wonderful changes that all those of the winged species without exception, and some which are destitute of wings, must pass through, before they arrive at the perfection of their nature. Most animals retain, during their whole life, the same form which they receive at their birth; but insects go through wonderful exterior and interior changes, insomuch that the same individual, at its birth and middle state, differs essentially from that under which it appears when arrived at a state of maturity; and this difference is not confined to marks, colour, or texture, but is extended to their form, proportion, motion, organs, and habits of life.

The ancient writers on natural history were not unacquainted with these transformations, but the ideas they entertained of them were very imperfect and often erroneous. The changes are produced in so sudden a manner, that they seem like the metamorphoses recorded in the fables of the ancients, and it is not improbable that those fables owe their origin to the transformation of insects. It was not till towards the latter end of the last century that any just conception of this subject was formed; the mystery was then unveiled by those two great anatomists Malpighi and Swammerdam, who observed these insects under every appearance, and traced them through all their forms; by dissecting them at the time just preceding their changes, they were enabled to prove that the moth and butterfly grow and strengthen themselves, that their members are formed and unfolded underthe figure of the insect we call a caterpillar, and that the growth was effected by a developement of parts; they also shewed that it is not difficult to exhibit in these all the parts of the future moth, as its wings, legs, antennæ, &c. and consequently that the changes which are apparently sudden to our eyes, are gradually formed under the skin of the animal, and only appear sudden to us, because the insect then gets rid of a case which had before concealed its real members. By this case it is preserved from injuries, till its wings, and every other part of its delicate frame are in a condition to bear the impulse of the sun, and the action of the air naked; when all the parts are grown firm, and ready to perform their several offices, the perfect animal appears in the form of its parent. Though these discoveries dissipated the false wonders of the metamorphoses that the world before believed, they created a fund of real admiration by the discovery of the truth. These transformations clearly prove, that without experience every thing in nature would appear a mystery; so much so, that a person unacquainted with the transformation of the caterpillar to the chrysalis, and of this to the fly, would consider them as three distinct species; for who, by the mere light of nature, or the powers of reason unaided by experience, could believe that a butterfly, adorned with four beautiful wings, furnished with a long spiral proboscis or tongue, instead of a mouth, and with six legs, proceeded from a disgusting hairy caterpillar, provided with jaws and teeth, and fourteen feet? Without experience, who could imagine that a long white smooth soft worm hid under the earth, should be transformed into a black crustaceous beetle? Nor could any one, from considering them in their perfect state, have discovered the relation which they bear to the several changes of state, and their corresponding forms, through which they have passed, and which are to appearance as distinct as difference can make them.

The life of those insects which pass through these various changes, may be divided into four principal parts, each of which will be found truly worthy of the utmost attention of the microscopic observer.

The first change is from theEGGinto theLARVA, or, as it is more generally called, into the worm or caterpillar. From theLARVA, it passes into thePUPA, or chrysalis state. From thePUPA, into theIMAGO, or fly state.

Few subjects can be found that are more expressive of the extensive goodness of Divine Providence, than these transformations, in which we find the occasional and temporary parts and organs of these little animals suited and adapted with the most minute exactness to the immediate manner and convenience of their existence; which again are shifted and changed, upon the insects commencing a new scene and state of action. In its larva state the insect appears groveling, heavy, and voracious, in the form of a worm, with a long body composed of successive rings; crawling along by the assistance of these, or small little hooks, which are placed on the side of the body. Its head is armed with strong jaws, its eyes smooth, entirely deprived of sex, the blood circulating from the hind part towards the head. It breathes through small apertures, which are situated on each side of the body, or through one or more tubes placed in the hinder part thereof. While it is in the larva state, the insect is as it were masked, and its true appearance concealed; for under this mask the more perfect form is hidden from the human eye. In the pupa, or chrysalis state, the insect may be compared to a child in swaddling cloathes; its members are all folded together under the breast, and inclosed within one or more coverings, remaining there without motion. While in this state, no insects but those ofthe hemiptera class, take any nourishment. The change is effected various ways; in some insects the skin of the larva opens, and leaves a passage, with all its integuments; in others, the skin hardens and becomes a species of cone, which entirely conceals the insect; others form or spin cones for themselves, and in this state they remain till the parts have acquired sufficient firmness, and are ready to perform their several offices.

The insect then casts off the spoils of its former state, wakes from a death-like inactivity, breaks as it were the inclosures of the tomb, throws off the dusky shroud, and appears in its imago or perfect form; for it has now attained the state of organical perfection, which answers to the rank it is to hold in the corporeal world: the structure of the body, the alimentary organs, and those of motion, are materially changed. It is now furnished with wings magnificently adorned, soars above and despises its former pursuits, wafts the soft air, chooses its mate, and transmits its nature to a succeeding race. Those members, which in the preceding state were wrapped up, soft, and motionless, now display themselves, grow strong, and are put in exercise. The interior changes are as considerable as those of the exterior form, and that in proportion as the first state differs from the last; some organs acquire greater strength and firmness, others are rendered more delicate; some are suppressed, and some unfolded, which did not seem to exist in the former stages of its life.

As the larvæ or caterpillars of the moth and butterfly[63]form the most numerous family among the tribe of insects, I shall firstdescribe them, and their various changes from this state to their last and perfect form, and then proceed to those insects which differ most from the caterpillar in one or all of their various changes.

[63]Butterflies are distinguished from moths by the time of their flying abroad, and by their antennæ; the butterflies appear by day, their antennæ are generally terminated by a little knob; the moths fly mostly in the evening, and their antennæ are either setaceous or pectinated.

The greater part of those insects which come forth in spring or summer, perish or disappear at the approach of winter; there are very few, the period of whose life exceeds that of a year; some survive the rigours of winter, being concealed and buried under ground; many are hid in the bark of trees, and others in the chinks of old walls; some, like the caterpillar of the brown-tailed moth,[64]at the approach of winter not only secure and strengthen the web in which the society inhabit, and thus protect themselves from impertinent intruders, but each individual also spins a case for itself, where it rests in torpid security, notwithstanding the inclemency of the season, till the spring animates it afresh, and informs it, that the all-bountiful Author of nature has provided food convenient for it. Many that are hatched in the autumn retire and live under the earth during the winter months, but in the spring come out, feed, and proceed onward to their several changes; while no small part pass the colder months in their chrysalis or pupa state: but the greater number of the caterpillar race remain in the egg, being carefully deposited by the parent fly in those places where they will be hatched with the greatest safety and success; in this state the latent principle of life is preserved till the genial influences of the spring call it into action, and bring forth the young insect to share the banquet that nature has provided.

[64]This moth was uncommonly numerous and destructive near London in the year 1782, and, aided by the predictions of an empirical imposter, occasioned a considerable alarm in the minds of the ignorant and superstitious. The judicious publication of a short history of the insect, by Mr. Curtis, in some measure contributed to dissipate their fears.Edit.

All caterpillars are hatched from the egg, and when they first proceed from it are generally small and feeble, but grow in strength as they increase in size. The body is divided into twelve rings; the head is connected with the first, and is hard and crustaceous. No caterpillar of the moth or butterfly has less than eight, or more than sixteen feet; the six first are crustaceous, pointed, and fixed to the three first rings of the body; these feet are the covering to the six future feet of the moth; the other six feet are soft and flexible or membranaceous; they vary both in figure and number, and are proper only to the larva state; with respect to their external figure, they are either smooth or hairy, soft to the touch, or hard like shagreen, beautifully adorned with a great variety of the most lively teints; on each side of the body nine little oval holes are placed, which are generally considered as the organs of respiration. There are on each side of the head of the caterpillar five or six little black spots, which are supposed to be its eyes. These creatures vary in size, from half an inch long to four and five inches.

The caterpillar, whose life is one continued succession of changes, often moults its skin before it attains its full growth; not one of them arrives at perfection, without having cast its skin at least once or twice. These changes are the more remarkable, because when the caterpillar moults, it is not simply the skin that is changed; for we find in the exuvia, the skull, the jaws, and all the exterior parts, both scaly and membranaceous, which compose its upper and under lip, its antennæ, palpi, and even those crustaceous pieces within the head, which serve as a fixed basis to a number of muscles; we further find in the exuvia, the spiracula, the claws, and sheaths of the anterior limbs, and in general all that is visible of the caterpillar.

The new organs were under the old ones as in a sheath, so that the caterpillar effects the changes by withdrawing itself from the old skin, when it finds itself lodged in too narrow a compass. But to produce this change, to push off the old covering, and bring forward the new, is a work of labour and time. Those caterpillars who live in society, and have a kind of nest or habitation, retire there to change their skins, fixing the hooks of the feet, during the operation, firmly in the web of their nest. Some of the solitary species spin at this time a slender web, to which they affix themselves. A day or two before the critical moment approaches, the insect ceases to eat, and loses its usual activity; in proportion as the time of change advances, the colour of the caterpillar becomes more feeble, the skin hardens and withers, and is soon incapable of receiving those juices by which it was heretofore nourished and supported. The insect may now be seen, at distant intervals, to elevate its back, and stretch itself to its utmost extent; sometimes to lift up the head, move it a little from side to side, and then let it fall again; near the change, the second and third rings are seen to swell considerably; by these internal efforts the old parts are stretched and distended as much as possible, an operation which is attended with great difficulty, as the new parts are all weak and tender. However, by repeated exertions, all the vessels which conveyed the nourishment to the exterior skin are disengaged, and cease to act, and a slit is made on the back, generally beginning at the second or third ring; the new skin may now be just perceived, being distinguished by the freshness and brightness of its colour; the caterpillar then presses the body like a wedge into this slit, by which means it is soon opened from the first down to the fourth ring; this renders it large enough to afford the insect a passage, which it soon effects in a very curious manner. The caterpillar generally fasts a whole day after each moulting, for it is necessarythat the parts should acquire a certain degree of consistency, before it can live and act in its usual manner; many also perish under the operation. The body having grown under the old skin, till the insect was become too large for it, it always appears much larger after it has quitted the exuvia: now as the growth was gradual, and the parts soft, the skin pressed them together, so that they lay in a small space; but as soon as the skin is cast off, they are as it were liberated from their bonds, and distend themselves considerably. Some caterpillars, in changing their skin, from smooth, become covered with fine hair; while others, that were covered with this fine hair, have the last skin smooth.[65]The silk-worm, previous to its chrysalis or pupa state, casts its skin four times; the first is cast on the tenth, eleventh, or twelfth day, according to the nature of the season; the second, in five or six days after; the third in five or six days more, and the fourth and last in six or seven days after the third.

[65]Valmont de Bomare Dictionnaire Universel d’Histoire Naturelle, vol. ii. 2d edit. 12mo. p. 394.

Before we describe the change of the larva into the pupa state, it will be necessary to give the reader an account of those names by which entomologists distinguish the different appearances of the insect in its pupa state. It is called Coarctata, when it is straitened or confined to a case of a globular form, without the smallest resemblance to the structure of the insect it contains, as in the diptera. It is called Obtecta, disguised or shrouded, when the insect is inveloped in a crustaceous covering, consisting of two parts, one of which surrounds the head and thorax, the other the abdomen. It is termed Incompleta, when the pupa has perceptible wings and feet, but cannot move them, as in most of the hymenoptera. Semicompleta; these can walk or run, but have only the rudimentsof wings. The difference between the pupa and the larva of this class is very inconsiderable, as they eat, walk, and act, just as they did in their primitive state; the only remarkable difference is a kind of case which contains the wings that are to be developed in their fly state. Completa; those designed by this name take their perfect form at their birth, and do not pass, like other insects, through a variety of states, though they often change their skin.

It is a general rule, that all winged insects pass through the larva and pupa state, before they assume their perfect form: there are also insects which have no wings, and yet undergo similar transformations, as the bed bug, the flea, &c. Other insects, which have no wings, and which always remain without them, never pass through the pupa state, but are subject to considerable changes, as well with respect to the number, as the figure of their parts; thus mites have four pair of feet, and two smaller ones at the fore part of the body, near the head; yet some of these are born with only three pair of feet, the fourth is not perceived till some time after their birth.[66]The figure of the monoculus quadricornis of Linnæus (Fauna Suecica, edit. Stockholm, 1761, No. 2049) changes considerably after its birth.[67]The julus is an insect with a great number of feet, some species having an hundred pair and upwards. M. De Geer has given a description of one with more than two-hundred pair,[68]and yet these at their birth have only three pair, the rest are not perceived till some time after.

[66]De Geer Memoires pour servir a l’Histoire des Insectes, tom. 1. p. 154.[67]Ibid.[68]Memoires des Scavans etrangers, tom. 3, p. 61.

[66]De Geer Memoires pour servir a l’Histoire des Insectes, tom. 1. p. 154.

[67]Ibid.

[68]Memoires des Scavans etrangers, tom. 3, p. 61.

I shall now return to the caterpillar, and take notice of the care and provision it makes to pass from the larva state into that of the pupa or chrysalis; which is, in general, a state of imperfection, inactivity, and weakness, through which the insect, when it has obtained a proper size, must pass; and in which it remains often for months, sometimes for a whole year, exposed, without any means of escaping, to every event; and in which it receives the necessary preparations for its perfect state, and is enabled once more to appear upon the transitory scene of time. During its passage from one state to the other, as well as when it is in the pupa form, the microscopical observer will find many opportunities of exercising his instrument.

The transitions of the caterpillar from one state to another, are to it a subject of the most interesting nature; for in passing through them, it often runs the risk of losing its life, that precious boon of heaven, which is ever accompanied with a degree of delight, proportioned to the state in which the creature exists, and the use it makes of the gift it has received. If the caterpillar could therefore foresee the efforts and exertions it must make to put off its present form, and the state of weakness and impotence under which it must exist while in the pupa state, it would undoubtedly choose the most convenient place, and the most advantageous situation, for the performance of this arduous operation; one where it would be the least exposed to danger, at a time when it had neither strength to resist, nor swiftness to avoid the attack of an enemy. All these necessary instructions the caterpillar receives from the influence of an all-regulating Providence, which conveys the proper information to it by its own sensations: hence, when the critical period approaches, itproceeds as if it knew what would be the result of its operations. Different species prepare themselves for the change different ways, suited to their nature and the length of time they are to remain in this state.

When the caterpillar has attained to its full growth, and the parts of the future butterfly are sufficiently formed beneath its skin, it prepares for its change into the pupa state; it seeks for a proper place in which to perform the important business: the different methods employed by these little animals to secure this state of rest, may be reduced to four: 1. Some spin webs or cones, in which they inclose themselves. 2. Others conceal themselves in little cells, which they form under ground. 3. Some suspend themselves by their posterior extremity; 4. While others are suspended by a girdle that goes round their body. I shall describe the variety in these, as well as the industry used in constructing them, after we have gone through the manner in which the caterpillar prepares itself for, and passes through the pupa state.

Preparatory to the change, it ceases to take any food, empties itself of all the excrementitious matter that is contained in the intestines, voiding at the same time the membrane which served as a lining to these and the stomach. The intestinal canal is composed of two principal tubes, the one inserted into the other; the external tube is compact and fleshy, the internal one is thin and transparent; it is the inner tube, which lines the stomach and intestines, that is voided with the excrement before the change. It generally perseveres in a state of rest and inactivity for several days, which affords the external and internal organs that are under the skin an opportunity of gradually unfolding themselves. In proportion as the change into the pupa form approaches, the body is observed often to extend and contract itself;the hinder part is that which is first disengaged from the caterpillar skin; when this part of the body is free, the animal contracts and draws it up towards the head; it then liberates itself in the same manner from the two succeeding rings, consequently the insect is now lodged in the fore part of its caterpillar covering; the half which is abandoned remains flaccid and empty, while the fore part is swoln and distended. The animal, by strong efforts, still forcing itself against the fore part of the skin, bursts the skull into three pieces, and forms a longitudinal opening in the three first rings of the body; through this it proceeds, drawing one part after the other, by alternately lengthening and shortening, swelling and contracting the body and different rings; or else, by pushing back the exuvia, gets rid of its odious reptile form.

The caterpillar, thus stripped from its skin, is what we call the pupa, chrysalis, or aurelia, in which the parts of the future moth are inclosed in a crustaceous covering, but are so soft, that the slightest touch will discompose them. The exterior part of the chrysalis is at first exceedingly tender, soft, and partly transparent, being covered with a viscous fluid; this soon dries up, thickens, and forms a new covering for the animal, capable of resisting external injuries; a case, which is at the same time the sepulchre of the caterpillar, and the cradle of the moth; where, as under a veil, this wonderful transformation is carried on.

The pupa has been called a chrysalis, or creature made of gold, from the resplendent yellow colour with which some kinds are adorned. Reaumur has shewn us whence they derive this rich colour; that it proceeds from two skins, the upper one a beautiful brown, which lies upon or covers a highly polished and smooth white skin: the light reflected from the last, in passing through, gives it the golden yellow, in the same manner as this colour isoften given to leather; so that the whole appears gilt, although no gold enters into the tincture. The chrysalis of the common white butterfly furnishes a most beautiful object for the lucernal opake microscope.

Those who are desirous to discover distinctly the various members of the moth in the pupa, should examine it before the fore-mentioned fluid is dried up, when it will be found to be only the moth with the members glued together; these, by degrees, acquire sufficient force to break their covering, and disengage themselves from the bands which confine them. While in this state, all the parts of the moth may be traced out, though so folded and laid together, that it cannot make any use of them; nor is it expedient that it should, as they are too soft and tender to be used, and pass through this state merely to be hardened and strengthened.

To examine the moth concealed under the skin of a caterpillar, one of them should be taken at the last change; when the skin begins to open, it should be drowned in spirit of wine, or some strong liquor, and be left therein for some days, that it may take more consistency and harden itself; the skin of the caterpillar may then be easily removed: the chrysalis, or feeble moth, will be first discovered, after which the tender moth may be traced out, and its wings, legs, antennæ, &c. may be opened and displayed by an accurate observer.

The parts of the moth or butterfly are not disposed exactly in the same manner in the body of the caterpillar, as when left naked in the chrysalis. The wings are longer and narrower, being wound up into the form of a cord, and the antennæ are rolled up on the head; the tongue is also twisted up and laid upon the head, but in a very different manner from what it is inthe perfect animal, and different from that which it lies in within the chrysalis; so that it is by a progressive and gradual change, that the interior parts are prepared for the pupa and moth state. The eggs, hereafter to be deposited by the moth, are also to be found, not only in the chrysalis, but in the caterpillar itself, arranged in their natural and regular order.

While in this state, the creature generally remains immoveable, and seems to have no other business but patiently to attend the time of its change, which depends on the parts becoming hard and firm, and the transpiration of that humidity which keeps them soft; the powers of life are as it were absorbed in a deep sleep; the organs of sensation seem obliterated, being imprisoned by coverings more or less strong, the greater part remains fixed in those situations which the caterpillar had selected for them till their final metamorphosis; some, however, are capable of changing place, but their movements are slow and painful.

The time, therefore, which the moth or butterfly remains in the pupa state is not always the same, varying in different species, and depending also upon the warmth of the weather, and other adventitious circumstances; some remain in that situation for a few weeks; others do not attain their perfect form for eight, nine, or eleven months: this often depends on the season in which they assume the pupa form, or rather on the time of their birth. Some irregularities are also occasioned by the different temperature of the air, by which they are retarded or accelerated, so as to be brought forward in the season best suited to their nature and the ends of their existence. I have heard of an instance, where the pupa, produced from caterpillars of the same eggs, nourished in the same manner, and which all spun up within a few days of each other in the autumn, came into the fly state at three different and distant periods; viz. one-third of them thespring following their change, one-third more the succeeding spring, and the remainder the spring after, making three years from their first hatching; a further and manifest proof of the beauty and wisdom of the laws of Divine order, which are continually operating for the best interests of all created beings. As the transformation of insects is retarded by cold, and accelerated by heat, the ordinary period of these changes may sometimes be altered, by placing them in different degrees of heat or cold; by these they may be awakened sooner to a new state of existence, or kept in one of profound sleep.[69]

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