DEVELOPMENT 



161 



Imaginal Buds. The wings and legs of a fly originate in the larva 

 in the form of^cellular masses, termed imagined buds, or histoblasts, as 

 Weismann discovered.. Thus in the larva of Corethra, there are in 

 each thoracic segment a pair of dorsal buds and a pair of ventral buds 

 (Fig. 222), each bud being clearly an evagination of the hypodermis 

 at the bottom of a previous invagination. The six ventral buds form 

 the legs eventually; of the dorsal buds, the middle and posterior pairs 

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



FIG. 222. Diagram- 

 matic transverse section of 

 Corethra larva, to show 

 imaginal buds of wings (w) 

 and legs (I); h, hypoder- 

 mis; *, integument. Modi- 

 fied from Lang's Lehrbuch. 



C D 



FIG. 223. Diagrammatic transverse sections of.muscid 

 larvae, to show imaginal buds, h, larval hypodermis; ', larval 

 integument; ih, imaginal hypodermis; /, imaginal bud of leg; 

 w, imaginal bud of wing. Modified from Lang's Lehrbuch. 



form the pupal respiratory processes. Each imaginal bud is situated in 

 a peripodal cavity, the wall of which (peripodal membrane) is continu- 

 ous with the general hypodermis; as the legs and wings develop, they 

 emerge from their peripodal sacs and become free. 



In Corethra but little histolysis occurs, most of the larval structures 

 passing directly into the corresponding structures of the adult. Core- 

 thra is, indeed, in many respects intermediate between heterometabo- 

 lous and holometabolous insects as regards its internal changes. 



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

 buds are more deeply situated, the peripodal membrane forming a 



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