312 



The Two-winged Flies 



either whouy submerged or at least constantly dashed over by the breaking 

 waves, I have not been able to determine; but the larva? and pupae cling 



FIG. 424. FIG. 425. 



FIG. 424. A black-fly, Simulium sp. (Four times natural size.) 

 FIG. 425. Diagram of wing of black-fly, Simulium, showing venation. 



tight and secure in their rock basins to small but strong silken nets spun 

 by the larvae. They rest on the under side of these nets, indeed are almost 

 enclosed in them as in a cocoon. This little fly is a most interesting insect 

 because of its ocean-water habitat very few insects live in salt water, and 

 almost no others have so truly an ocean home, except 

 the curious salt-water striders, Halobates (see p. 

 197), which live on the surface of the ocean far out 

 at sea. It is interesting, too, because of its structu- 

 ral modifications, the atrophied wings, rudimentary 

 balancers, etc., which set it off widely from all 

 other flies. Its tide-pool habitat is undoubtedly the 

 result of a slow migration and adaptation in the 

 course of many generations on the part of some 

 shore-inhabiting fly. There are many small flies 

 which frequent ocean beaches and rocks, feeding on 



FIG. 426 Larvae and pupae of Simulium sp. on edge of stream, May-fly on projecting 



twig. (After Felt.) 



