3 J 4 
The Two-winged Flies 
larval life (under favorable conditions of temperature and food-supply), the 
larva spins a little silken cornucopia-like cocoon (Fig. 426) fastened to the 
rock by the little end, and often fastened by the sides to adjacent cocoons. 
The large free end is left open. In this cocoon it pupates, and after about 
three weeks the winged fly issues. The eggs are laid in patches on the rocks 
Fig. 429. —Longitudinal section of head of old larva of black-fly, Simulium sp., showing 
adult mouth-parts developing inside of or corresponding with the larval mouth- 
parts. l.md., larval mandible; l.mx., larval maxilla; l.li., larval labium; /.c., larval 
cuticle; l.a., larval antenna; i.md., adult mandible; i.mx., adult maxilla; adult 
labium; i.d., adult hypoderm (cell-layer of skin); i.a. } adult antennae; i.e., adult 
eye. (Much enlarged.) 
just below the surface of the water, or on the spray-dashed sides of boulders 
in the stream or on its margin. 
In the same places where the Simulium larvae live, that is, on the smooth 
rock faces of stream bed and lip of fall under the thin apron of swift silver 
water of mountain streams, live also the curious flattened larvae (Fig. 430) of 
the net-winged midges or Blepharoceridae. This small family of interesting 
flies, comprising only eighteen species in the whole world, of which seven 
belong to this country, is one with which the general collector will hardly 
become acquainted unless he takes particular pains to do so. But the pains 
are well worth while, for they are not pains at all, but pleasures. In the first 
place, the larvae—and they must be looked for first, the winged flies being very 
rare, very retiring, and hardly distinguishable, until captured, from a number 
of other common and less interesting kinds—live only in the most attractive 
parts of the most attractive mountain brooks. I have found them in a tiny 
swift stream near Quebec, in two or three hillside brooks near Ithaca, 
N. Y., in roaring mountain torrents in the Rocky Mountains, and in similar 
plunging streams in the Sierra Nevada and Coast Range. Clinging by a 
ventral series of six suckers to the smooth shining rock bed, the short broad 
