The apparent suddenness of this change is really surprising, but in reality the transformation is not nearly so sudden as it appears. Dissection of a caterpillar a few days before the final moult is due will show that the changes are already going on. In fact, a simple removal of the skin will prove that the organs of the future butterfly are developing. Still, in proportion to the short time occupied, the change is extremely great; and it may reasonably be inquired, Why so great a change within so short a space of time?--why is not the change continued steadily and equally through the larval existence? The reason has already been hinted at. Caterpillars are living eating machines, whose office is to remove excess of vegetable matter. Consequently they must have their jaws and bulky digestive apparatus in full development to the end. If these organs were to _gradually_ disappear as the caterpillar reaches its non-eating stages, it would simply be starved to death. So the change from the larval to the pupal state, which we may regard as the final moult of the caterpillar, is a far greater change than any of the preceding ones, and occupies a proportionately longer time, although it is principally confined to the last few days of the caterpillar life. A number of caterpillars, and especially those of the butterflies, suspend themselves when about to change; and the peculiarities of the modes adopted must be left for our descriptions of species in a future chapter; but we will find room here for one more interesting example, taking this time the larva of one of the commonest of the Vanessas (page 166)--the Small Tortoiseshell Butterfly. The caterpillars of this insect are gregarious when young; and if ever you meet with one, you are almost sure to be able to obtain a hundred or so without much searching. But as they grow older they feed singly, yet generally without straying very far from their birthplace. When full grown they sometimes stray to a neighbouring plant or fence to undergo the change to a chrysalis, but more commonly they are perfectly satisfied with the protection afforded by the leaves of their food plant. We will now watch one of these as we did the larva of the Large White Butterfly. Of course the under side of the leaf is chosen. Here a silken carpet is spun as before described; but the caterpillar, instead of clinging with _all_ its claspers, suspends itself in a vertical position by its hindermost pair only. Here it hangs, head downwards, awaiting the coming events. The splitting and casting of the skin goes on just as in the case of the Large White, but there is this puzzle to be solved: how can the insect shuffle itself out of its old coat without falling to the ground, leaving the cast-off garments still hanging by the hooks of the claspers? This really seems a matter of impossibility, since the little hooks which alone suspend the insect are thrown off with the skin of the claspers.