74 INTRODUCTION. 



The history of insects, . fruitful as it is in all its 

 branches, in instances of nice adaptations and in- 

 genious mechanism, presents few topics more de- 

 serving of consideration than are afforded by the 

 proceedings of these caterpillars when they change 

 their skin, and when they prepare to enter upon that 

 dormant state which precedes the development of the 

 perfect fly. The proximate cause of the moulting 

 or change of skin, is the internal growth of the body, 

 which thus becomes too bulky for the envelope in 

 which it is enclosed. The latter can scarcely be 

 Baid to grow, but is merely capable of being dilated 

 to a certain point, after which it offers so much re- 

 sistance to the expansion of the enclosed animal, 

 that it becomes necessary to throw it off entirely. 

 It must be evident, however, that it is no easy task 

 to withdraw the body from a shell in which every 

 separate limb and articulation is closely encased, 

 without any assistance being derived from without. 

 This, however, the animal accomplishes so adroitly, 

 that the cast-off skin appears almost entire, and even 

 retains all the spines and other appendages with 

 which it may have been furnished. The operation, 

 which seems to be a painful one, and even at times 

 attended with fatal consequences, is thus described 

 by Reaumur : 



A day or two before the critical moment arrives, 

 the caterpillars cease to eat, and become very inac- 

 tive, usually remaining stationary on a single spot. 

 They select some place where they may be in greater 



