DEVELOPMENT 



181 



nation. The six ventral buds form the legs eventually; of 

 the dorsal buds, the middle and posterior pairs form, respec- 

 tively, the wings and the halteres, and the anterior pair form 

 the pupal respiratory processes. Each imaginal bud is situ- 

 ated in a peripodal cavity, the wall of which (peripodal mem- 

 brane) is continuous with the general hypodermis; as the legs 

 and wings develop, they emerge from their peripodal sacs and 

 become free. 



In Corcthra but little histolysis occurs, most of the larval 

 structures passing directly into the corresponding structures 

 of the adult. Corethra, indeed, is in many respects interme- 

 diate between heterometabolous and holometabolous insects as 

 regards its internal changes. 



FIG. 222. 



Section through left hind wing in larva of Pieris rapes, the section being a frontal 

 one of the caterpillar; the base of the wing is anterior in position, and the apex 

 posterior, c, cuticula; h, hypodermis; t, trachea; w, developing wing. After MAYER. 



Muscidae. In Muscidse,, as compared with Corethra, the 

 imaginal buds are more deeply situated, the peripodal mem- 

 brane forming a stalk (Fig. 220), and the processes of his- 

 tolysis and histogenesis become extremely complicated. The 

 hypodermis, muscles, alimentary canal and fat-body are grad- 

 ually broken down and remodeled, and part of the respiratory 

 system is reorganized, though the dorsal vessel and the central 

 nervous system, uninterrupted in their functions, undergo 

 comparatively little alteration. 



The imaginal hypodermis of the thorax arises from thick- 



