Shrinkage of Larval Prothorax 125 



embryo ^ This may mean that the prothoracic tracheal 

 gills are of comparatively recent origin, and that they 

 are not truly in series with the dorsal appendages of the 

 two hinder thoracic segments. 



We feel no great confidence in any such explanations 

 of the origin of the dorsal prothoracic rudiments as we or 

 any others may have entertained. The possibility that 

 they were once wing-like is not to be lost sight of till 

 it is disproved, but it is at least possible that they have 

 never existed in any other form than the launch of 

 filaments, the tube 

 ojDen or closed, or 

 some other pupal 

 respiratory organ. 



In the Chirono- 

 mus-pupa the wings 

 are of simple out- 

 line, bnt they are 

 too large to expand 

 within the larval 

 skin, and are there- 

 fore for a time 

 much folded. The 

 imaginal wings form within the j)^^pal vvings, and are 

 also much folded. 



The prothorax shrinks greatly during the last days of shrinkage 

 the larA^a. The head and the tracheal gill, which were prothorax. 

 widely separated, come gradually nearer together, and in 

 the pupa the gill lies just behind the head. Fig. 95 

 shows a dorsal j^rotrusion filled with disintegrated larval 

 tissues, which represents the way in which a great part 

 of the larval prothorax is made to disappear. 



In the late larva and pupa the body-cavity, especially Phago- 

 in the thorax, may contain clusters of cells which very 

 ^ Weismann, Entw. der Bipteren, p. 145. 



Fig. 94. — Sagittal section of larva, passing through 

 base of piipal tracheal gill. The compound eye of 

 the fly is seen within the prothorax ; the muscles of 

 the larval head are undergoing liistolysis. s, an- 

 terior thoracic spiracle, r, tracheal gill, wi, vertical 

 mesothoracic muscles. 



