i 



DIPTERA 823 



The lar\^£e are aquatic ; and usually live in swiftly-flowing streams, 

 clinging to the surface of rocks in rapids or on the brinks of falls. 

 They sometimes occur in such large numbers as to form a moss-like 

 coating over the rocks. There is a disk-like sucker fringed with 

 little hooks at the caudal end of the body by means of which the larva 

 clings to the rocks ; and just back of the head there is a fleshy proleg 

 which ends in a similar sucker fringed with hooks (Fig. 1046). By 

 means of these two organs the larva is able to walk with a looping 

 gait similar to that of a measuring-worm. It has also 

 the power of spinning silk from its mouth, which it 

 uses in locomotion. The hooks on the caudal sucker 

 and at the end of the proleg are well adapted to 

 clinging to a thread or to a film of silk spun upon the 

 rock to which the larva is clinging. Respiration is 

 accomplished by means of blood-gills, which appear 

 on the dorsal side of the last abdominal segment, 

 but are evaginations of the ventral wall of the rec- 

 tum, and lie, when retracted completely within the Fig. 10 46.— 

 rectal cavity (Headlee '06). The head bears two Head of larva 



large fan-shaped organs, which aid in procuring 

 food. The food consists chiefly of algae and diatoms. 



When full-grown the larva spins a boot-shaped cocoon within 

 which the pupal state is passed (Fig. 1047). This cocoon is firmly 

 fastened to the rock upon which the larva has lived or to other co- 

 coons, for they occur in dense masses, forming a carpet-like covering 

 on the rocks. The pupa breathes by tracheal gills which are borne 

 on the prothorax. 



The adult fly, on emerging from the pupa-skin, rises to the surface 



of the water and takes flight at once. Soon after this the eggs are 



laid, I have often watched Simillium pictipes 



hovering over the brink of a fall where there 



M was a thin sheet of swiftly-flowing water, and 



m^ have seen the flies dart into the water and out 



WL again. At such times I have always found the 



^Wk surface of the rock more or less thickly coated 



^^L with eggs, and have no doubt that an egg is 



g| fastened to the rock each time a fly darts into 



9 the water. Malloch ('14) states that the eggs 



^ are deposited in many cases on blades of grass, 



Fig. 1047.— Cocoon twigs, or leaves of trees which are dipping in 



and larva. running water. 



Until recently all members of this family 

 were included in the genus Simulium; consequently in nearly all of 

 the published accounts of these insects the various species are placed 

 in this genus. But later writers have divided the old genus Simulium 

 into several genera, Enderlein ('21) now recognizes fifteen genera of 

 which seven are represented in North America. 



Monographic papers on the North American species of this family 

 have been published by Coquillett ('98), Johannsen ('03) and Malloch 

 ('14). 



