312 ENTOMOLOGICAL NEWS. 



[Jan 



the embryo. These groups of indifferentiated cells are called 

 imaginal discs or histoblasts. The legs and wings of the irnago 

 develop each from a single histoblast, which is simply orig- 

 inally an invaginated part of the embryonic skin. Among 

 those tissues and organs of the larva which break down are 

 most of the muscles, most of the alimentary canal, the salivary 

 glands, much of the tracheary tissue, the fat body, etc. The 

 reproductive organs, the nervous system and probably the 

 heart of the larva do not break down, but pass over with sim- 

 ple modifications into the imaginal body. One of the most 

 interesting things about these remarkable phenomena is the 

 fact that in many insects in Calliphora, for example the 

 breaking down of the larval tissues is accomplished by phago- 

 cytosis. That is, the tissues are attacked by certain cells of 

 the blood tissue, which destroy the tissues by eating them ; 

 hence the name of the cells, phagocytes. But the disintegra- 

 tion can occur, and does in the case of certain insects, without 

 the intervention of the phagocytes. 



Now because the study among the Dipt era of these phenom- 

 ena of post embryonic development have been confined almost 

 wholly to the specialized flies, the study of the post embryonic 

 development of some Nematocerous form ought to be of special 

 interest. But Blepharocera, although belonging to the gen- 

 eralized Nematocerous flies, and with an imago in really very 

 generalized condition (see the account later of the mouth parts 

 of the female), has such a highly modified larva that it shows 

 a very specialized condition of post embryonic development. 

 It is believed by some morphologists that phagocytes aid in 

 the disintegration of the larval tissues only where the post 

 embryonal development takes place in a very short time, as in 

 the Muscidte. But my preparations show that phagocytosis 

 occurs conspicuoiisly and on a large scale in the development 

 of Blepharocera, and this development is not specially hurried, 

 the pupal stage lasting probably about two weeks. I am in- 

 clined to believe, therefore, that a high specialization of the 

 larva, with the more radical differences between the larval 

 and imaginal organs necessitating a greater destruction of 

 larval tissues and organs, is quite a sufficient cause for phago- 

 cytosis, as rapid development. 



