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 Cynipidse, 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 pupse 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 hamadryella) ; 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- 



cularis. 

 cocoon through 



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 byBombyx 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 >ro- 

 methea; v, valve- 

 like arrange- 

 ment for the 

 escape of the 

 adult. 



Fig. 213. Old cocoon of 

 Megalopyge opercttlnris. 



