3 12 
The Two-winged Flies 
either whony submerged or at least constantly dashed over by the breaking 
waves, I have not been able to determine; but the larvae and pupa cling 
Fig. 424. 
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 larva. 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.) 
