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 exuvke 
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 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* 
