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

Figs. 102, 103.—Caterpillars of the small Tortoise-shell

Figs. 104, 105.—Chrysalides of the small Tortoise-shell Butterfly freeing themselves from the Caterpillar skin.

To understand the difficulty which the first of these operations presents, we must consider the problem which the caterpillar has to solve. In this problem there are two unknown quantities to be discovered. Firstly, the caterpillar must suspend itself firmly; and secondly, the pupa, having no communication with the object which supports it, must be suspended in the same manner. This problem is difficult, apparently impossible, to solve. It is only by watching these insects at work that one can discover the wonderful mysteries of their lives. Swammerdam, Valisnieri, and other observers who have studied insects, had not, however, observed the manœuvres of caterpillars in this curious phase of their existence. It is to Réaumur again that science is indebted for the most charming and valuable observations on this point. He got together a great number of caterpillars of the small Tortoise-shell Butterfly (Vanessa urticæ), black prickly caterpillars which are common on the stinging-nettle, where they live in companies, and suspend themselves by the tail. When the time approaches at which the caterpillars of this species ought to undergo their transformations, they usually leave the plant which had up to that time served them as food. After having wandered about a little, they select some convenient spot, where they hang themselves up head downwards (Figs. [102], [103]).

In order to hang itself in this way, the caterpillar begins by covering, with threads drawn in different directions, a pretty large extent of the surface of the body against which it wishes to fix itself. After having covered it thus with a kind of thin cobweb, it adds different layers of threads on a small portion of this surface, in such a manner that the upper one is always smaller than that upon which it is laid. In this manner a small hillock of silk is formed, the tissue of which is not at all compact. It resembles an assemblage of loose or badly interwoven threads. The membranous feet of the caterpillar are armed with hooks of different lengths, with the aid of which it suspends itself. By alternately contracting and elongating its body, it pushes its hindermost legs against the hillock of silk, presses against it the hooks of its feet, so as to get them better entangled, and lets its body fall in a vertical position.

It remains hanging thus, often for twenty-four hours, during which time it is occupied in a difficult task, that of splitting its skin. In order to effect this, it incessantly curves and recurves its body ([Fig. 102]), until at last a split appears on the skin of the back, and through this split emerges a part of the body of the chrysalis. This acts as a wedge, and little by little the split widens from the head to the last of the true legs, and beyond them. Then the opening is sufficient to allow of the chrysalis drawing out its anterior portion from the envelope, which it immediately does. To set itself entirely free, the chrysalis lengthens and shortens itself alternately ([Fig. 105]). Each time that it shortens itself, and when it consequently distends the part of its body which is outside the old skin, that part acts against the edges of the slit, and gradually pushes the old skin upwards. Thus the caterpillar skin ascends, its plaits are pushed nearer and nearer together, and it is soon reduced to a packet so small that it covers only the end of the tail of the chrysalis ([Fig. 106]).

But here comes the culminating point, the most difficult part of the operation. The chrysalis, which is shorter than the caterpillar, is at some distance from the silky network to which it must fix itself; it is only supported by that extremity of the caterpillar's skin which had not been split open. It has neither legs nor arms, and yet it must free itself from this remaining part of the skin, and reach the threads to which it is to suspend itself.

Fig. 106.—Chrysalis of the small Tortoise-shell Butterfly completing the operation of casting its larval skin.		Fig. 107.—Chrysalis divested of the larva skin.

The supple and contractile segments of the chrysalis serve for the limbs which are wanting to it. Between two of these segments, as with a pair of pincers, the insect seizes a portion of the folded skin, and with such a firm hold that it is able to support the whole of its body on it. It now curves its hinder parts slightly, and draws its tail entirely out of the sheath in which it was enclosed. It then reposes for an instant only, for it has not yet finished the laborious operation of its deliverance. It must free itself entirely from the dry skin which surrounds the extremity of its body.