188 AN INTRODUCTION TO ENTOMOLOGY 



The cocoon. — The pupal instar is an especially vulnerable one. 

 During the pupal life the insect has no means of offence, and having 

 exceedingly" limited powers of motion, it has almost no means of 

 defense unless an armor has been provided. 



Many larvai merely retreat to some secluded place in which the 

 pupal stadium is passed ; others bury themselves in the ground ; and 

 still others make provision for this helpless period by spinning a silken 

 armor about their bodies. Such an armor is termed a cocoon. 



The cocoon is made by the full-grown larva; and this usually 

 takes place only a short time before the beginning of the pupal stadium. 

 But in some cases several months elapse between the spinning of the 

 cocoon and the change to pupa, the cocoon being made in the autumn 

 and the change to pupa taking place in the spring. Of course a 

 greater or less portion of this period is occupied by the prepupal 

 stadium. 



Cocoons are usually made of silk, which is spun from glands 

 already described. In some cases, as in the cocoons of Bonibyx, the 

 silk can be unwound and utilized by man. 



While silk is the chief material used in the making of cocoons, it is 

 by no means the only material. Many wood-boring 

 larvas make cocoons largely of chips. Many insects that 

 undergo their transformation in the ground incorporate 

 earth in the walls of their cocoons. And hairy cater- 

 pillars use silk merely as a warp to hold together a 

 woof of hair, the hairs of the larva being the most con- 

 spicuous element in the cocoon. 



In those cases in which silk alone is used there is a 

 great variation in the nature of the silk, and in the den- 

 sity of the cocoon. The well-known cocoons of the 

 satumiids illustrate one extreme in density, the cocoons 

 of certain Hymenoptera, the other. 



The fiberous nature of the cocoon is usually obvious ; 



but the cocoons of saw-flies appear parchment-like, and 



Fig. 210.^ the cocoons of the sphecids appear like a delicate foil. 



cocoon^ of While in the more common type of cocoons the 



Trichostibas wall is a closely woven sheet, there are cocoons that 



from ^ which are lace-like in texture (Fig. 210). 



the adult has Modes of escape from the cocoon. — The insect, having 



emerge . walled itself in with a firm layer of silk, is forced to meet 



the problem of a means of escape from this inclosure; a problem 



which is solved in greatly varied ways. 



