ENTOMOLOGY. 85 



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 exuviae 

 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 aeriferous 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 larvae do not change their skin at 

 all, not even when about to become pupae. These 

 belong chiefly to the order Diptera, whose membran- 

 ous 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 prece4ed, 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* 



