THE METAMORPHOSIS OF INSECTS 



189 



In many insects in which the adult has biting mouth parts, the 

 adult merely gnaws its way out by means of its mandibles In some 

 cases, as the Cynipidae, it is said that this is the only use made of 

 its mandibles by the adult. 



In some cases the mandibles with which the cocoon is pierced per- 

 tain to the pupal instar, this is true of Chrysopa and Hemerobius; 

 and the Trichoptera break out from their cases, by means of their 

 mandibles, while yet in the pupal state. 



For those insects in which the adult has sucking mouth parts, the 

 problem is even more difficult. Here it has been met in several quite 

 distinct ways. The pupae of many Lepidop- 

 tera possess a specialized organ for breaking 

 through the cocoon; in some the anterior 

 end of the pupa is furnished with a toothed 



crest (Lithocolletes hamadryelld); in certain satur- 



niids there is a pair of large, stout, black spines, 



one on each side 



of the thorax, at 



the base of the 



fore wings with 



which the pupa 



cuts a slit in the Fig. 212. Cocoon of Megalopyge oper- 



cocoon through cularis ' 



which the adult emerges, this was observed by 



Packard in Tropaa luna; but as these spines are 



present in other saturniids, where the cocoon is too 



dense to be cut by them, and where an opening is 



made in some other way,. 



it is probable that, as a 



rule, their function is loco- 

 motive, aiding the pupa to 



work its way out from the 



cocoon, by a wriggling 



motion. 



One of the ways in 



which saturniids pierce 



their cocoons is that practiced by Bombyx and Telea. 



These insects soften one end of the cocoon by a 



liquid, which issues from the mouth; and then, by 

 forcing the threads apart or by breaking them, make an opening. 



Fig. 211. Longi- 

 tudinal section 

 of a cocoon of 

 Callosamia pro- 

 methca;v, valve- 

 like arrange- 

 ment for the 

 escape of the 

 adult. 



Fig. 213. Old cocoon of 

 Megalopyge opercularis. 



