DEVELOPMENT 



149 



cytcs, which attack some tissues and absorb their material, but later are 

 themselves food for the developing imaginal tissues. The construction 

 of tissues is termed histogencsis. 



In Coleoptera, however, the degeneration of the larval muscles is entire- 

 ly chemical, there being no evidence of phagocytosis, according to Dr. R. 

 S. Breed. Berlese, indeed, goes so far as to deny in general the destruc- 

 tive action of leucocytes on larval tissues. 



Imaginal Buds. The wings and legs 

 of a fly originate in the larva in the form 

 of cellular masses, or imaginal buds, as 

 Weismann discovered. Thus in the larva 

 of Corethra, there are in each thoracic seg- 

 ment a pair of dorsal buds and a pair of 

 ventral buds (Fig. 220), each bud being 

 clearly an evagination of the hypodermis 

 at the bottom of a previous imagination. 

 The six ventral buds form the legs eventu- 

 ally; of the dorsal buds, the middle and 

 posterior pairs form, respectively, the 

 wings and the halteres, and the anterior 

 pair form the pupal respiratory processes. 

 Each imaginal bud is situated in a peri- 

 podal cavity, the wall of which (peripodal 

 membrane) is continuous 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 direct- 

 ly into the corresponding structures of the 

 adult. Corethra, indeed, is in many re- 

 spects intermediate between heterome- 

 tabolous 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 stalk 

 (Fig. 221), and the processes of histolysis and histogenesis become ex- 

 tremely complicated. The hypodermis, muscles, alimentary canal and 

 fat-body are gradually broken down and remodeled, and part of the 

 respiratory system is reorganized, though the dorsal vessel and the central 



FIG. 222. Imaginal buds of full 

 grown larva of Picris, dorsal aspect. 

 b, brain; ?, mid' intestine; s 1 , pro- 

 thoracic spiracle; s 4 , first abdominal 

 spiracle; sg, silk gland; /, pro- 

 thoracic bud; II, bud of fore wing; 

 ///, bud of hind wing. After 

 GONIN. 



