Pupal Tracheal System 143 



stem, slightly flattened, supports the base of the gill, and 

 gives passage to a multitude of fine tracheae. 



When the pupal skin is cast, two sets of branching 

 tracheal tubes are found attached, one to its superficial and 

 one to its deep surface, in the region of the tracheal gill. 

 Those on the superficial face ]3ass through the stem into 

 the gill ; those on the deep face are withdrawn from the 

 tracheal system of the fly, which forms outside that of 

 the jDupa in the same way that this formed outside that 

 of the larva. While one set of tracheae is withdrawn 

 from the anterior spiracle of the fly, another much smaller 

 set, further back, is withdrawn from the posterior or 

 metathoracic spiracle. 



The cast pupal skin in the prothorax and fore part of 

 the mesothorax is marked by three scars, nearly in a line. 

 The uppermost scar, which is also in front of the others, 

 is oval and has a sieve-like apjiearance, which we are 

 unable to explain. Next comes the base of the tracheal 

 gill, fringed by innumerable broken tubes. Last and 

 lowest is a pit-like depression of the pupal skin, which 

 looks rather like a pupal spiracle, though we believe that 

 it is impervious ; it is in close relation to the imaginal 

 spiracle beneath (fig. iii), and the pupal tracheae are 

 withdrawn at this point. 



In the abdomen of the pupa a pair of narrow longi- 

 tudinal tracheae can be traced, which are placed in com- 

 munication with impervious spiracles by minute initial 

 branches. One spiracle lies near the middle of the second 

 abdominal segment ; the rest in the fore part of each of 

 the segments from the third to the seventh inclusive. 



The tracheal system of the pupa is larger and more 

 continuous than that of the larva. Sections through the 

 thorax now show a double wall in all the larger tracheae, 

 the outer being the imaginal structure, while the inner 

 is the comparatively narrow pupal trachea. 



