These caterpillars continue to feed in this manner for about a week, remaining side by side when feeding, and marching in processions from one leaf to another as the food supply is exhausted. Wherever they go, each spins a silken thread on the surface traversed, so that the combination of all the threads makes a sort of carpet that serves as a foothold for the caterpillars. At the end of the week they moult or cast their skins, a process in which the skin of each larva splits open along the back, and the larva crawls out covered with a new skin that had been formed beneath the old one. This new skin stretches somewhat after the caterpillar emerges, so that the insect is able to increase considerably in size. At the period of moulting, the caterpillars remain quiet for a short time, but they soon become active again and begin feeding with increased voracity. [Illustration: Eggs of Mourning-Cloak, laid in a cluster on a twig. (From Holland).] During the next three weeks, this moulting process is repeated three times, the caterpillars becoming larger each time, and leaving their cast skins upon the denuded twigs. They soon scatter more or less over neighboring leaves, but remain in closely associated colonies. As they increase in size, they eat more and more of the leaf substance; when half grown, they devour all but the midrib and the side veins; but when they get larger, only the midribs are left. The carpet web that they form becomes more conspicuous as the caterpillars become full grown. They then leave the tree or shrub on which they have been feeding, and scatter about, seeking some sheltered situation. Having found this--perhaps beneath a stump or along the under side of a fence--each caterpillar spins a web of silk along the surface. It then entangles the hooked claws of its hind legs in the silken web, and lets its body hang vertically with the head end curved upward. It remains in this position some hours before the skin along the back just behind the head splits apart and is gradually wriggled upward, until finally it is all removed and there hangs in place of the caterpillar a peculiar object having no definite form. But it rapidly assumes a definite form--that of the chrysalis--which is grayish brown, different specimens varying somewhat in shade. In this quiet chrysalis, the insect is apparently almost as inert as a mummy. If you touch it it will wriggle a little, but otherwise it hangs there mute and helpless. On the inside, however, the tissues are being made over in such a wonderful way that, in about two weeks, from the mummy case into which the caterpillar entered there comes a beautiful butterfly.