II FLIES WITH AQUATIC LARV^ 175 



is renewed without effort through the thin-walled 

 bladders.^ 



The head and thorax of the pupa are defended and 

 roughened by many minute projections. The abdomen 

 is softer, but armed with five circles of fine spines. 

 Thus, as in Dicranota and many other Insects, the 

 pupa is enabled to creep to the surface of the ground 

 before the extrication of the fly. Like many other 

 Dipterous pupjE, it has a fair degree of mobility, and 

 can travel about in the ground, so as to choose a 

 situation as convenient as possible in respect of 

 dampness. 



SiMULIUM. 



In brisk and lively streams the little, blackish 

 larvae of Simulium can sometimes be found in count- 

 less numbers. They attach themselves by choice to 

 water-weeds, such as float-grass, water-cress, water- 

 crowfoot, and the like. They may also be found on 

 stones, but I believe that the larvae found in stony 

 streams belong to a different species. In a stream 

 where all the necessary conditions are fully satisfied, 

 where there is a never-failing supply of well-aerated 

 water, plenty of submerged foliage, and plenty of 

 microscopic organisms, the larvae sometimes abound, 

 looking like small black worms, five-eighths of an 

 inch or less in length. When the leaves are examined 

 by the eye alone, without handling, perhaps hardly a 



^ Grobben describes minute orifices in the outer wall of 

 these bladders which I have not myself seen. It is often a 

 matter of the greatest difficulty to decide whether a minute and 

 transparent membrane is completely closed or perforated. 



