one beneath it. By continuing these, a rent is soon made, usually on the back behind the head, through which the creature forces its way, while the exuviæ are held back by the hinder extremity, which the larva was careful previously to attach to some object with this very design. The divestment is so complete and adroitly managed that the cast integument may easily be mistaken for a living larva, as it exhibits all the parts which characterised it, apparently without having undergone any material change. This process of renovation, however, is not wholly confined to the external parts; several of the viscera also are said to cast a fine skin, and even the aëriferous tubes, though so numerous and delicate, undergo the same process, and the rejected membranes are gradually expelled from the body through the stigmata.
Several tribes of larvæ do not change their skin at all, not even when about to become pupæ. These belong chiefly to the order Diptera, whose membranous contractile heads and expansible skin removes the necessity of subjecting them to the same law as those differently circumstanced in these respects.
The last change of skin, which occurs just before the assumption of the pupa state, is essentially similar to the others, but it is preceded, in many instances, by certain precautions and preparations, with a view to security and comfort during the quiescent condition about to ensue. The larva leaves its wonted haunts, and seeks some retreat where it will be less exposed to the weather and its numerous living enemies; the crevices of trees and walls, the shelter of dead leaves,
