viii AKTHKOPODA 279 



pouch lying behind and above the opening of the mouth, there are 

 two pairs of " flat " imaginal discs, the most posterior pair being the 

 rudiments of the compound eyes, the more anterior the rudiments of 

 the antennae. Just before the close of larval life a median imaginal 

 disc is formed in the head pouch, in front and below the mouth, 

 which is the rudiment of all mouth-parts and of the proboscis of 

 the fly. From the bottoms of the imaginal "pouches," wings, the 

 anterior functional and the posterior vestigial, forming the balancers 

 or halteres, and the legs grow up (Fig. 223, A). As they grow the 

 pouches enlarge and the strings of cells connecting them with the 

 ectoderm shorten. At the same time the head pouch also shortens, 

 its outer portion becoming evaginated so that the opening of the 

 mouth becomes external, and the imaginal disc for the proboscis lies 

 posterior. 



Then comes the pupal stage. The imaginal pouches now open to 

 the exterior exposing wings and legs (Fig. 223, C) ; the head pouch 

 becomes completely evaginated so that the head of the fly is extruded. 

 The dorsal imaginal pouches of the prothorax give rise to a stigma with 

 a bunch of tracheae. The portions of the evaginated pouches surround- 

 ing the bases of the wings and legs form the adult ectoderm of the 

 thorax. They spread out till they meet each other. The abdominal 

 imaginal discs also extend till dorsal and ventral ones meet each 

 other and right and left discs are united across the middle line. In 

 this way the larval ectoderm is completely replaced. At the same 

 time the alimentary canal undergoes profound changes. There is an 

 imaginal ring of active proliferating cells situated at the inner ends 

 of both stomodaeum and proctodaeum. The surface of the mid-gut 

 has " islands " of embryonic tissue dotted over it. By the activity 

 of these imaginal areas a completely new mid -gut epithelium is 

 formed, as well as a new lining for stomodaeum and proctodaeum. 

 The salivary glands have " imaginal rings " situated on their ducts 

 where these begin to branch. New salivary glands are formed 

 from the rings on the ducts of the larval glands. 



The metamorphosis of most of the higher insects, so far as it has 

 been studied, is intermediate in character between the types 

 exemplified by the Chrysonielidae and the Muscid Diptera, and its 

 real nature is brought out clearly by the researches of Verson (1905) 

 on the transformations undergone by the alimentary canal of the 

 silkworm moth, Bombyx tnori. In this case the imaginal discs are 

 disposed as in Musca viz. there is a ring at the inner end of the 

 stomodaeum, a similar ring at the inner end of the proctodaeum, and 

 a number of islands scattered over the surface of the mid-gut. 



Now what Verson clearly shows is that a normal cell which forms 

 part of the wall of the stomodaeum or proctodaeum, has a limited 

 life, and, after fulfilling its function for a time and developing its 

 physiological peculiarities, dies and is replaced by a new cell formed 

 by the imaginal ring, which is therefore a zone of proliferation. He 

 points out further that the formation of both stomodaeum and 



