11 FLIES WITH AQUATIC LARV^ 173 



as in the Eristalis larva, be adapted to a varying 

 depth of water. 



At the extremity of the tail are two openings ^ by 

 means of which air is passed into the two tracheal 

 tubes, which traverse the length of the tail, and are 

 continued through the whole length of the body. In the 

 centre of most of the segments the tracheae become 

 greatly enlarged, the intervening portions, from which 

 many branches arc given off, being comparatively 

 narrow. Each tube, therefore, resembles a row of 

 bladders connected by small necks. A cross-section 

 shows that the tubes are not cylindrical, but flattened, 

 and that while the lower surface is stiffened by the 

 usual parallel thickenings, the upper surface is thrown 

 into two deep, longitudinal furrows, so that it is 

 readily inflated into a cylindrical shape, and readily 

 collapses again when the air is expelled. It seems 

 likely that the buoyancy of the larva can thus be 

 regulated, and a larger or smaller quantity of air 

 taken in as desired.^ 



The larva, thus fortified against any interruption of 

 its supply of air^ lies at its ease on or in the water, 

 stretching out its long tail to the surface, and break- 

 ing the surface-film, like the Eristalis larva, by the 

 extremity of the appendage. 



The pupa has a pair of respiratory tubes, which 

 are carried on the thorax, close behind the head. 

 One of these tubes is very long, the other very short 

 and altogether functionless. The long tube is twice 



^ Grobben, loc. cit. 



2 Grobben observes that in very young larvte the main 

 tracheae are cylindrical, and of uniform diameter. 



