Pupal Tracheal System 



141 



with long setae. In some species the pupa has claw- like 

 projections on the sides of the abdominal segments, a pair 

 to each segment. In pupae of the plamosus section the 

 flanges of the last abdominal segment are furnished with 

 very long setae, and constitute a tail-fin (Plate, figs. 5, 6). 

 The laterally expanded abdomen acts powerfully on the 

 water in the incessant movements of flexion and exten- 

 sion, and drives it in a continual stream through the tube, 

 in which, under normal circumstances, the pupal stage is 

 passed. 



The pupa aerates its tissues by means of tracheal gills Pupai 

 and a system of air- tubes. system. 



The tracheal gill of the pupa forms in the hinder part 



Fig. 107. — Wing and haltere of fly, enclDsed in their papal sheaths. 



of the larval prothorax. This might not of itself prove 

 that it is truly prothoracic, since pupal and imaginal 

 structures characteristic of a particular segment may 

 form within a larval segment which does not occupy 

 the same place in the series. The best proof that the 

 tracheal gill is really prothoracic seems to be furnished 

 b}^ the following observations : — The larval skin was 

 removed from a larva nearly ready to pupate (fig. 108). 

 A well-marked transverse line was then seen, which 

 apparently marked the junction of two segments. By 

 a simple dissection with needles the longitudinal meso- 

 thoracic muscles were traced precisely to this line, and 

 no further. It thus became clear that the line marks the 



