THE METAMORPHOSIS OF INSECTS 



171 



few weeks. The eggs of Bittacus are said to remain unhatched for 

 two years; and a similar statement is made regarding the eggs of 

 our common walking-stick. 



b. THE HATCHING OF YOUNG INSECTS 



Only a few accounts have been published 



■regarding the manner in which a young insect 



frees itself from the embryonic envelopes. In ^'^cockmad?!^^''''^ °^ ^ 



some cases it is evident that the lar\^a cuts its 



way out from the egg-shell by means of its mandibles ; but in others, a 



specialized organ has been developed for this purpose. 



The hatching spines. — 

 An organ for rupturing 

 the embryonic envelopes 

 is probably commonly pre- 

 sent. It has been des- 

 cribed under several 

 names. It was termed an 

 egg-burster by Hagen, the 

 niptor ovi by C. V. Riley 

 an egg-tooth by Heymons, 

 and the hatching spines 

 by Wheeler. 



Fig. 192. — Eggs, larva, cocoon, and adult of 

 Chrysopa. 



C. THE MOLTING OF INSECTS 



The young of insects 

 cast periodically the outer 

 parts of the cuticula; this process is termed molting or ecdysis. 



General features of the molting of insects.— The chitinization of 

 the epidermis or primary cuticula adds to its efficiency as an armor, but 

 it prevents the expansion of the body-v/all rendered necessary by the 

 growth of the insect; consequently as the body grows, its cuticula 

 becomes too small for it. When this occurs a second epidermis is 

 formed by the hypodermis; after which the old epidermis splits open, 

 usually along the back of the head and thorax, and the insect works 

 itself out from it. The new epidermis being elastic, accommodates 

 itself to the increased size of the body ; but in a short time it becomes 

 chitinized; and as the insect grows it in turn is cast off. The cast 

 skin of an insect is termed the exuvice, the plural noun being used as in 

 English is the word clothes. 



