190 AN INTRODUCTION TO ENTOMOLOGY 



Far more wonderful than any of the methods of emergence from 

 the cocoon described above are those in which the larva makes pro- 

 vision for the escape of the adult. The most familiar of these is that 

 practiced by the larv^as of Samia cecropia and Callosamia promethea. 

 These larvae when they spin their cocoons construct at one end a coni- 

 cal valve-like arrangement, which allows the adult to emerge without 

 the necessity of making a hole through the cocoon (Fig. 211, v). A 

 less familiar example, but one that is fully as wonderful, is that of 

 a Megalopyge. The larva of this species makes a cocoon of the 

 form shown in Figure 212. After an outer layer of the cocoon has 

 been made, the larva constructs, near one end of it, a hinged partition ; 

 this serves as a trap door, through which the moth emerges. That 

 part of the cocoon that is outside of the partition is quite delicate and 

 is easily destroyed. Hence most specimens of the cocoons in col- 

 lections present the appearance represented in Figure 213. 



The puparium. — The pupal stadiimi of most Diptera is passed 

 within the last larval skin, which is not broken till the adult fly is 

 ready to emerge. In this case the larval skin, which becomes hard 

 and brown, and which serves as a cocoon, is termed a 

 puparium. In some families the puparium retains the 

 form of the larva; in others the body of the larva 

 shortens, assuming a more or less barrel-shaped form, 

 before the change to a pupa takes place (Fig. 214). 



Modes of escape from the puparium. — The pupse of 

 the more generalized Diptera escape from the pupa- 

 rium through a T-shaped opening, which is formed by 

 a lengthwise split on the back near the head end and a 

 crosswise split at the front end of this (Fig. 215), or 

 rarely, through a cross-wise split between the seventh 

 -Pupa- and eighth abdominal segments. In the more special- 

 ■'^''^''ized Diptera there is developed a large bladder-like 

 organ, which is pushed out from the front of the head, 

 through what is known as the frontal suture, and by which the head 

 end of the puparium is forced off. This organ is known as the ptilinum. 

 After the adult escapes, the ptilinum is withdrawn into the head. 



The Different types of pupaB. — Three types 

 of pupce are commonly recognized; these 

 are the following: Fig. 215.— Puparium of a 



Exarate pupcB. — Pupae which, like those ^ ''^ lomyu 

 of the Coleoptera and Hymenoptera, have the legs and. vnngs free, 

 are termed exarate pupag. 



