The insect curves the part which is below its tail in such a manner that that part can embrace and seize the packet to which it holds on. It then gives to its body a violent shock, which makes it spin round many times on its tail, and that with great rapidity. During all these pirouettes the chrysalis acts against the skin; the hooks of its legs fray the threads, and break them or disentangle themselves from them. Sometimes the threads do not break at once. Then the animal recommences its revolutions in an opposite direction, and this time it is almost certain to succeed. Réaumur, however, saw a chrysalis which, after having tired itself in vain in its endeavours to get entirely free of its old skin, despairing of ever being able to manage it, abandoned it where it was so solidly fixed. We represent ([Fig. 107]), rather magnified, the chrysalis arrived at its final state, and suspended to a branch of a tree by a network of silk. [38]
We come now to the mode of suspension employed by those caterpillars, which, after having fixed themselves by the tail, strengthen the support by means of a small silk cord passed round their body.
It is again to Réaumur, that indefatigable observer of the habits of insects, that we go for the details of this manner of suspension. According to Réaumur, these caterpillars make and put on this belt in three different ways. But of these three ways the simplest, and the least liable to meet with accident, is that employed by the larva of the Cabbage Butterfly (Pieris brassicæ). When the time for its metamorphosis is only a few days distant, one may observe this caterpillar engaged in stretching threads on different parts of the case in which it is confined. It then chooses a spot, which it covers entirely with threads, some more compact than the others, and disposed in layers, which cross each other in different directions. These threads form a thin white cloth, against which the belly of the caterpillar and later that of the chrysalis are applied. Very soon we see a small hillock of silk rising. The caterpillar hooks itself on to this by the claws of its hinder feet, and sets to work to secure itself.
To understand this process, it suffices to know that after having lengthened its body to a certain point, this caterpillar can turn back its head on to its back, and reach to the fifth ring, having its three pairs of true legs in the air. But without putting the caterpillar into such an unnatural position, let us take it in a position in which it is simply bent sideways in such a manner that its head, with the thread-spinning apparatus, which is below, can be applied opposite and pretty near to one of the legs belonging to the first pair of membranous legs. Our caterpillar begins by fixing on this point a thread, which is the first of those that are intended to tie it up securely.
Fig. 108.—Caterpillars of the Cabbage Butterfly (Pieris brassicæ).
"This thread," says the illustrious author of the "Mémoires pour servir à l'Histoire des Insectes, "must pass over the caterpillar's body, and be attached by its other end near the leg corresponding to that near which the first end was fastened. To spin the thread the proper length, and at the same time to fix it in its proper place, the caterpillar has only to bring round its head to the fifth segment. The thread will be drawn from the spinning apparatus as the head advances over half the circumference of the circle which it has to describe; and when it has described this, there will only remain for it to secure the second end of the thread against the support. Thus the head, which was at first placed against one of the legs, advances little by little on the outline of the fifth ring as far as to its middle ([Fig. 108]). It is the facility the caterpillar has of reversing its body that enables it to make its head perform this journey, in proportion as it moves it over the circumference of the ring, it twists its body. And at last, when it has brought it over the top of the segment, its body is exactly folded in two; it draws it little by little from this situation by bending towards the other side, and by causing its head to pass gently over the last quarter of the circle. At last the caterpillar finds itself bound on the second side; the head rests on the thread-covered plane, and the insect fixes the second end of the thread."
