MOULTING OF THE LARVA.
We are now to enter upon a very interesting part of the history of a larva: this is called moulting, and consists in the larva casting off its old skin, and appearing clothed in a new garment, often more brilliant than before. The change is well and clearly described in the following passage from the Introduction to Entomology, of Messrs. Kirby and Spence:—"A day or two previously to each change of skin, the larva ceases eating altogether; it becomes languid and feeble; its beautiful colours fade; and it seeks a retreat in which it can undergo this important and sometimes dangerous and even fatal operation in security. Here, either fixing itself by its legs to the surface on which it rests, or, as is the case with many caterpillars, by its pro-legs, to a slight web spun for this purpose, it turns and twists its body in various directions, and alternately swells and contracts its different segments. The object of these motions and contortions seems to be, to separate the exterior skin, now become dry and rigid, from the new one just below it. After continuing these operations for some hours, resting at intervals, without motion, as if exhausted by their violence, the critical moment arrives: the skin splits in the back, in consequence of the still more violent swelling of the second and third segments. The opening thus made is speedily increased by a succession of swellings and contractions of the remaining segments; even the head itself often divides into three triangular pieces, and the enclosed larva by degrees withdraws itself wholly from its old skin." The engraving is intended to show the larva just escaping out of its old skin.
Sometimes the larva comes out of its skin at the side; sometimes it has been seen to bite off portions of the skin. The most common way is that above described. The skin, when cast in this manner, resembles a sort of universal coat, which has fitted into every crack and joint of the insect's body; and just as a lady's glove, if we could suppose it tinted of a flesh-colour, and marked for the various markings on the hand, might be mistaken, if cast down after inflating it with air, for a hand cut off, so, only far more closely, does the cast skin represent, in the minutest particular, the larva which has emerged from it. It is a perfect mould of all its parts, even to the very antennæ, eyes, jaws, &c. "Thus," say the authors last quoted, "if you saved the skins cast by the larva of the insect called Callimorpha, or Arctia Caja, you would appear to have ten different specimens of caterpillars, furnished with every external necessary part, and differing only in size and in the colour, perhaps, of the hairs, and all representing the same individual."
In order to show how completely this is the case, some singular experiments have been made by various observers, in the following manner:—Just before the larva was about to cast off its skin, they have, by means of a sharp instrument, cut off one or two of its feet. The larva was then allowed to moult, and was carefully examined, and it was invariably found that the feet cut off when in its old skin were also wanting when it appeared in its new robes; thus plainly proving that the feet were really sheathed in the old skin completely. It was just as if we had gloves on our hands and were to put one finger or more between a pair of sharp and powerful shears, we should, of course, find that when we took off the glove we should be short of one or two fingers!
But the hairs are not thus sheathed. The old skin, if the larva was a hairy one, such as many of those with which we are familiar under the more popular title "caterpillars," is cast off with the hairs attached to it—a circumstance which makes the cast skin look still more like the real larva. How, then, it may be asked, does the larva acquire new hairs to take the place of the old ones? Were we to take a larva just before its moulting, and by a sharp and delicate instrument, to slit open and raise its old skin, we should soon perceive how this has been contrived. We should there perceive sundry little tufts of very delicate hairs, lying down smoothly on the surface of the larva's body, and arranged in certain directions, with great regularity. When the old skin is cast, in the course of a little time these hairs stand upright, and assume precisely the same appearance as those on the former skin.
The keeper of silk-worms can tell us well, and perhaps with many a sigh, that the period of moulting is one of great peril to his tender charge. The larvæ are sick and feeble for some time after each period of moulting; and large numbers die at this time, apparently unable to bear the exhaustion attending the loss of their old skin. Larvæ generally are much debilitated by each moult that takes place; for a variable period, sometimes for some hours, sometimes for a whole day, they will refuse food, and lie without motion. All their parts are very soft and tender, and require hardening by exposure to the air. After a certain period, however, these effects pass off; the larva recovers its original strength, its body becomes firm, its colour brightens, and appears more brilliant than ever; and, above all, like convalescents among ourselves, its appetite is twice as keen as before, which, when we call to mind what has been already said about the voracity of larvæ, it will be confessed, appears scarcely possible.