THE DEVELOPMENT OF INSECTS 29 



possible under existing conditions. As the next change will produce an 

 animal practically helpless in most cases, and unable to protect itself 

 from its enemies, its next step is to find as much protection as possible. 

 Accordingly, the full-grown larva usually, though not always, leaves the 

 place where it has been feeding and elsewhere prepares for its next 

 change. Many larvae begin this by spinning around themselves a 

 thread of silk, produced by glands within the body and opening to the 

 surface on the lower lip. This thread is spun backward and forward and 

 around the body until it sometimes forms a complete outer covering, 

 entirely concealing the larva within, from view. This case or cocoon 

 appears to.be protective in its function. 



Some larvae go under ground for this change. Here a cocoon, as such, 

 seems unnecessary, but after digging into the earth a few inches, the 

 insect forms a little earthen chamber or cell in which to lie, and generally 

 lines this more or less densely with silk, probably to keep the earthen walls 

 from falling in and crushing it. A larva transforming in tunnels in wood 

 where it has fed, may make a partial cocoon with more or less of the 

 chewed-wood fragments mixed in. One staying above ground but not 

 in tunnnels or otherwise protected, will spin more or less of a cocoon 

 as already described. 



The completeness of the cocoon, however, varies greatly with differ- 

 ent insects. Instead of being a thick, dense wrapping which entirely 

 conceals the insect, it may be so scanty that the animal within can be 

 seen to some extent. In other cases it is merely a sort of network, in no 

 degree giving concealment; and in still others, a few scattered threads to 

 hold the insect in place are all that represent it. Sometimes hairs from 

 the body of the larva, held together by silk, form most of the cocoon, 

 and in the case of butterflies, only threads enough to attach the hinder 

 end of the body at the place where it is to transform, and to form a 

 supporting loop around its middle, the ends of the loop also being fastened 

 to what it rests on, are produced. In some flies the larva shrinks within 

 its larval skin and transforms, this skin, now called a puparium, function- 

 ing like a cocoon (see Fig. 33c). 



The reason for such variations in a structure presumably formed for 

 the purpose of protection, can only be guessed at. Possibly in the course 

 of generations, some insects found less need of this than others and gradu- 

 ally reduced it, thereby saving the vital energy so much needed for trans- 

 formation, which would otherwise be expended in cocoon making. 



Whether the larva forms a dense or scanty cocoon, or none whatever, 

 the next step in the process is a molt. When the insect escapes from this 

 skin, however, a great change in its appearance is evident, and it is now 

 called a pupa (Fig. 33a and 6). In a general way it may be said that it 

 has at this one molt changed more than half way to its adult condition. 

 This is due in part at least to the unfolding of the imaginal buds already 



