304 Mr. J. A. Dell on the Structure 



rise to the clear circular spots previously mentioned. 

 Except at these places the tracheal extension is separated 

 from the wall of the respiratory appendage by a space, in 

 which sections show the presence of a tissue, affected by 

 ordinary staining re-agents (probably the epithelium which 

 secreted the tracheal extension). A transverse section 

 shows scattered setae projecting inwards from the wall of 

 the tracheal extension (Fig. 10). Traced backwards, the 

 tracheal extension passes through the space between the 

 body of the fly and the pupal skin, and finally enters the 

 body in the fore part of the mesothorax. There are two 

 large, longitudinal tracheae (Fig. 8) communicating directly 

 with the tracheal extensions just described. They are 

 connected by a series of nine commissures, one in the 

 mesothorax, two in the metathorax, and one in each ab- 

 dominal segment, except the seventh and eighth. There 

 is also a series of lateral loops exactly like those of the 

 larva. The first, however, which arises in the metathorax, 

 runs through two segments ; all the others run from one 

 segment to the next. The longitudinal tracheae end in 

 tapering extremities in the last segment. 



The tracheal system of the pupa is formed around that 

 of the larva, and consequently in a larva about to pupate 

 the walls of the tracheae appear double. The longitudinal 

 tracheae of a late larva communicate with the pupal respi- 

 ratory appendages, which are formed during this stage. 

 The respiratory appendages are not modelled on any pre- 

 existing larval organ, hence it is not surprising to find that 

 the larval trachea, just behind the respiratory appendage, 

 comes out through the wall of the pupal tracheae, and 

 makes its way to the prothoracic spiracle of the larva. It 

 is also continuous with the base of the respiratory append- 

 age. This makes a sharp angle with the larval trachea, 

 and can be seen through the larval cuticle just in front of 

 the larval prothoracic spiracle. Its distal end turns down- 

 wards, and almost touches its fellow of the opposite side 

 in the mid-ventral line. In the pupa which has just shed 

 the larval skin, the respiratory appendages lie close to the 

 body, much as they do in the larva ; but when the pupa 

 makes its way to the surface of the mud, the respiratory 

 appendages stiffen, and stand out clear of the surface. 

 Pupae can be easily detected in a sample of sewage, by 

 their projecting respiratory appendages. A pupa, when 

 kept in a bottle of water, is unable to rise to the surface, 



