176 NATURAL HISTORY OF AQUATIC INSECTS CH. 



single larva will be seen. They are clustered for the 

 most part on the under side, and only become visible 

 when the leaf is plucked or turned over. They are 

 most numerous where the current is brisk. In York- 

 shire I find them plentiful in weedy streams which 

 come down from the moors, and especially in the 

 rapids above waterfalls Hagen describes the larvae 

 and pupae of Simulium pictipes as abounding in the 



FIG. 59. Group of larvae of Simulium attached to a stone. 



rapids of the Ausable River, in the Adonirack Moun- 

 tains. Here the pupa-cases are fixed to the rocks in 

 clusters, which resemble small wasps' nests. 1 Though 

 the larvae require well-aerated water, it need not be 

 pure. Streams contaminated with sewage often con- 

 tain them in great numbers. 2 



1 Proc. Boston Soc. Nat. Hist. Vol. XX. pp. 305-7 (1880). 



2 The species which I have chiefly studied appear to be S. 

 latipes (stony streams) and S. reptans (on water- weeds in rapid 

 streams). 



