When the caterpillar has selected the under-side of the leaf or other object to which it purposes suspending itself, its first process is to spin upon it a little hillock of silk consisting of numerous loosely interwoven threads; it then bends its body so as to insinuate the anal pair of prolegs amongst these threads, in which, by a slight exertion, the little crochets which surround them[494] become so strongly entangled as to support its weight with ease. It now suffers the anterior part of the body to fall down, and it hangs perpendicularly from its silken support with its head downwards. In this position it remains often for twenty-four hours, at intervals alternately contracting and dilating itself. At length the skin is seen to split on the back near the head, and a portion of the pupa appears, which by repeated swellings acts like a wedge, and rapidly extends the slit towards the tail. By the continuance of these alternate contractions and dilatations of the conical pupa, the skin of the caterpillar is at last collected in folds near the tail, like a stocking which we roll upon the ancle before withdrawing it from the foot. But now comes the important operation. The pupa, being much shorter than the caterpillar, is as yet at some distance from the silken hillock on which it is to be fastened; it is supported merely by the unsplit terminal portion of the latter's skin. How shall it disengage itself from this remnant of its case, and be suspended in the air while it climbs up to take its place? Without arms or legs to support itself, the anxious spectator expects to see it fall to the earth. His fears, however, are vain; the supple segments of the pupa's abdomen serve in the place of arms. Between two of these, as with a pair of pincers, it seizes on a portion of the skin; and bending its body once more, entirely extricates its tail from it. It is now wholly out of the skin, against one side of which it is supported, but yet at some distance from the leaf. The next step it must take is to climb up to the required height. For this purpose it repeats the same ingenious manœuvre, making its cast-off skin serve as a sort of ladder, it successively with different segments seizes a higher and a higher portion, until in the end it reaches the summit, where with its tail it feels for the silken threads that are to support it. But how can the tail be fastened to them? you ask. This difficulty has been provided against by Creative Wisdom. The tail of the pupa is furnished with numerous little hooks pointing in different directions[495], as well adapted to the end in view as the crochets of the larva's prolegs, and some of these hooks are sure to fasten themselves upon the silk the moment the tail is thrust amongst it. Our pupa has now nearly completed its labours; it has withdrawn its tail from the slough, climbed up it, and suspended itself to the silken hillock—manœuvres so delicate and perilous, that we cannot but admire that an insect which executes them but once in its life, should execute them so well: nor could it, as Reaumur has well and piously observed, had it not been instructed by a Great Master. One more exertion remains: it seems to have as great an antipathy to its cast-off skin, as one of us should, when newly clothed after a long imprisonment, to the filthy prison garments we had put off. It will not suffer this memento of its former state to remain near it, and is no sooner suspended in security than it endeavours to make it fall. For this end—it seizes, as it were with its tail, the threads to which the skin is fastened, and then very rapidly whirls itself round, often not fewer than twenty times. By this manœuvre it generally succeeds in breaking them, and the skin falls down. Sometimes, however, the first attempt fails: in that case, after a moment's rest, it makes a second, twirling itself in an opposite direction; and this is rarely unsuccessful. Yet now and then it is forced to repeat its whirling, not less than four or five times: and Reaumur has seen instances where the feet of the skin were so firmly hooked, that after many fruitless efforts the pupa, as if in despair, gave up the task and suffered it to remain[496]. After these exertions, it hangs the remainder of its existence in this state until the butterfly is disclosed.
We are now to consider the second mode of suspension, in which larvæ by means of a silken girth round their middle, fix themselves horizontally under leaves, &c. These follow the same process with that of those last described, in spinning a small hillock of silk to which they fasten their hind legs; and if the operation concerned the larva state alone, this would be sufficient, as by means of this support, and of their prolegs, they could easily retain themselves in a horizontal position. But these larvæ act as if they foresaw the assumption of a state in which they will be deprived of legs. It is the suspension of the forthcoming pupa that is the object in view; and though this can be hung by the tail in the same way with those of the first class, yet it is plain that it cannot be retained in a horizontal position, which for some unknown reason is essential to it, without some support to its anterior extremity. It is necessary for the larva, therefore, not only to fix its posterior legs amongst a collection of silken fibres, but to spin a girth of the same material round its body. This girth, though apparently of a single thread, will be found on examination to be composed of several, often as many as fifty or sixty; and is fastened on each side of the body of the larva about the middle, to the surface under which it is placed. Three different modes of fixing these girths are adopted by the caterpillars of different butterflies. Some, as those of the common cabbage-butterfly (Pieris Brassicæ), which have remarkably pliable bodies, bend them almost double on one side, then fix the thread and carry it over to the other in the same position, repeating this operation as often as is necessary. Others, as that of Lycæna Argus and many more of the Papiliones Rurales and Urbicolæ L., which have a short and more rigid body, after having bent the head on one side so as to fix one end of the thread, bring themselves into a straight position, and, by a manœuvre not easily described, contrive to introduce the head under the thread, which they then bend themselves to fasten on the other side, pushing it to its proper situation by the successive tension and contraction of their segments. But the most curious mode, though indeed that which seems most natural, is adopted by the caterpillar of the beautiful swallow-tail butterfly (Papilio Machaon) and others of the same family. This first forms the loop which is to serve for its girth, and then creeps under it. But the difficulty it has to surmount is, to keep itself from being entangled in the fifty or sixty fine distinct threads of which the girth is composed, and to preserve them all extended so as to be able to introduce its body beneath them. For this purpose it makes use of the two first pair of its fore-legs, employing them as a woman does her hands in winding a skein of cotton, to collect and keep all the threads of its card unentangled and properly stretched; and it is often with great difficulty, towards the end of the process, that it prevents them from slipping off. When a sufficient number of threads is completed, the animal bends its head between its legs, and insinuates it under the collected loop, which by its annular contraction it easily pushes to the middle of the body.
In about thirty hours after the larvæ which girth themselves have finished their operations, the skin splits, and the pupa disengages itself from it by those contractions and dilatations of its segments which have been before described, pushing the exuviæ in folds to the tail, by different motions of which it generally succeeds in detaching them. One would have thought there would be considerable difficulty in slipping the skin past the girth; but this, according to Reaumur, seems to be easily effected[497].
If you are desirous of witnessing for yourself the manœuvres by which these curious modes of suspension are effected, you may be readily gratified. It is only necessary to collect and feed until their metamorphosis the black spinous caterpillars of the common peacock-butterfly (Vanessa Io), which in most places may be found upon nettles, or those of the Pieris Brassicæ, which swarm in cabbages or brocoli in every garden. The former will exhibit to you a specimen of vertical, the latter of horizontal suspension. It should be observed, however, that to hit the precise moment when these processes are going on, it is necessary to feed a considerable number of the larvæ of each kind; some one of which, if you watch them narrowly when they have attained their full growth, you will scarcely fail to surprise in the act.
I must observe here, that although the vertical and horizontal are the two principal positions in which caterpillars suspend themselves, yet that others are inclined at various angles; and some are attached with less art, appearing only to be fastened by some part of their abdomen to the body upon which they are fixed[498].
2. The larvæ whose procedures I am in the next place to describe, are those which, previously to assuming the pupa state, inclose themselves in cases or cocoons of different materials. For the sake of method, I shall divide these into two great classes: First, those which form their cocoons entirely or principally of silk; and secondly, those which form them chiefly of other substances.
To begin with the first. The larvæ which inclose themselves in silken cocoons are chiefly of the Lepidopterous tribes of Bombycidæ and Noctuidæ; but a few Geometræ (G. papilioniaria, lactearia, &c.); most of the Hymenoptera; some Coleoptera, as certain of the weevil tribe (Hypera Arator, Rumicis Germ.), and those brilliant beetles frequenting aquatic plants constituting the genus Donacia F.; the Neuropterous genera Hemerobius and Myrmeleon; Mycetophila and a few others in the Diptera; and Pulex in the Aphaniptera fabricate coverings of the same material. In all, with the exception of Myrmeleon and Hemerobius (and perhaps Hypera Rumicis, &c.?) which have their spinning apparatus at the extremity of the abdomen, the silken thread employed in forming these coverings proceeds from the middle part of the under-lip, as before explained; and is in fact composed of two threads gummed together as they issue from the two adjoining orifices of the spinner.
Of the larvæ which inclose themselves in silk, the most familiarly known is the silk-worm: the cocoon of this consists exteriorly of a thin, transparent, gauze-like coating, through the interstices of which can be seen an inner, smaller, oval ball of a more close and compact texture. The whole is in fact composed of one single thread, but arranged in two distinct modes. To form the exterior envelope, which is merely the scaffolding by means of which the inner and more solid covering is constructed, the caterpillar, after fixing upon a space between two leaves or twigs or angles suitable for its purpose, begins by glueing one end of its thread to one of the adjoining surfaces. This thread it next conducts to another part and then fastens, repeating this process and interlacing it in various directions, until it has surrounded itself with a slight and loosely spun netting. In the centre of this, when contracted into a space sufficiently small, it lays the foundation of the interior cocoon. Fixing itself by its prolegs to some of the surrounding threads, it bends its body, and by successive motions of its head from side to side spins a layer of silk on the side opposite to it: when this is of the requisite thickness, the larva shifts its position, and repeats the same process in another quarter, covering each layer in turn with a new one until the interior cavity is reduced to the size desired. Thus, the silken thread which forms this new cocoon is not, as might have been supposed, wound circularly as we wind the thread of a ball of cotton; but backwards and forwards in a series of zigzags, so as to compose a number of distinct layers. Malpighi could distinguish six of these layers[499], and Reaumur suspects there is often a greater number[500]. The former found the length of the thread of silk composing them when wound off, without including the exterior case, to be not less than 930 feet[501]; but others have computed it at more than a thousand[502]: consequently the threads of five cocoons united would be a mile in length. Estimating by the weight,—the thread of a pound of cocoons, each of which weighs about two grains and a half, would extend more than 600 miles[503], and such is its tenuity, that the threads of five or six cocoons require to be joined to form one of the thickness requisite in the silk manufacture. It is the continuous thread of the inner cocoon which is most valuable; the outer loose coating from its irregularity cannot be wound off, and is known in commerce by the name of floss silk.
Manœuvres in their general principle similar to those of the silk-worm are followed by most of those larvæ which inclose themselves in silken cocoons. Many species, however, adopt variations in the mode of procedure all of which it would be tedious to particularize, but some of them are worth mentioning. The larvæ of Tortrix prasinana, and other species of moths which form cocoons resembling a reversed boat, arrange their threads in layers, so as to construct two parallel walls gradually inclining towards the top and ends, where they finally force them to approach each other by means of an apparatus of silken cables[504]. And the larva of Saturnia Pavonia, though it forms the base of its flask-shaped cocoon by spinning like the silk-worm a number of interwoven zigzags, places the threads which compose the interior funnel-like opening of the apex nearly straight, parallel to each other, and converging towards the same point in the centre[505].
These last, as well as almost all larvæ, constantly remain in the inside of the cocoon during its construction. But De Geer has given us the history of a minute caterpillar of a species of moth (Tinea L.) which feeds on the under side of the leaves of the Rhamnus Frangula, or Black Alder, that actually weaves half of its cocoon on the outside. This cocoon, which is very small, is beautifully fluted, consisting of several longitudinal cords, with the intervals filled by fine net-work, and shaped like a reversed boat[506]. The animal begins by laying the foundations of one of the ends of her cocoon, she adds new threads to this small beginning, and so proceeds. As the work advances she retreats backwards, and her body is situated nearly in the same line with the cocoon she has begun, and quite out of it; she only touches with her head and legs its anterior margin. When half the cocoon, or rather of its exterior layer, is finished, she suspends her operations for some moments. She then for the first time introduces her head into this demi-cocoon, and turns herself in it by doubling her supple body, and passing one part over the other, so that at last she manages to bring her tail into the pointed end of the cocoon, the head and the anterior half of her body remaining without. Thus situated, she commences her operations afresh. At a distance from the margin of the demi-cocoon, equal to its length, she begins to spin the pointed end of the other moiety, the length of her body serving her as a measure that enables her to begin at the proper distance from it. This new portion she spins in the same manner as the other; but as she is prevented by the demi-cocoon in which the posterior part of her body is lodged from retreating backwards, she contracts her body more, which answers the same purpose. When the new work is so advanced that she can no longer contract her body, she bends the anterior part of it considerably, and reverses her head. When the distance between the margin of the two halves of the cocoon is very small, so as no longer to admit the head between them, in order to unite them she is obliged to have recourse to another manœuvre. Withdrawing her head, she extends silken longitudinal threads between the two margins, and thus unites them. This part is more clumsy, and not so regular as the rest of the cocoon, so that the point of union is always discoverable. These caterpillars do not always divide the cocoon into two equal portions, for often they will finish three quarters of the cocoon before they enter it, and begin at the other end[507].