Simuliidm, or Black Flies 



105 



white slime, which becomes darker, until finallyit becomes black just 

 before the emerging of the larvae. The egg stage lasts a week. 



The larvae (fig. 75) are black, soft skinned, somewhat cylindrical 

 in shape, enlarged at both ends and attenuated in the middle. The 

 posterior half is much stouter than the anterior part and almost club- 

 shaped. The head bears two large fan-shaped organs which aid in 

 procuring food. Respiration is accomplished by means of three so- 

 called blood gills which are pushed out from the dorsal part of the 

 recttim. The larvae occur in enormous numbers, in moss-like patches. 

 If removed from their natural habitat and placed in quiet water they 

 die within three or four hours. Fastened to the rock by means of a 

 disk-like sucker at the caudal end of 

 the body, they ordinarily assume an 

 erect position. They move about on 

 the surface of the rocks, to a limited 

 extent, with a looping gait similar to 

 that of a measuring worm, and a web 

 is secreted which prevents their being 

 washed away by the swiftly flowing 

 water. They feed chiefly upon algae 

 and diatoms. 



The complete larval stage during 

 the summer months occupies about 

 four weeks, varying somewhat with the 

 temperature and velocity of the water. 

 At the end of this period they spin 

 from cephalic glands, boot-shaped 

 silken cocoons within which they pupate. The cocoon when spun 

 is firmly attached to the rock and also to adjacent cocoons. 

 Clustered continuously over a large area and sometimes one above 

 another, they form a compact, carpet-like covering on the rocks, 

 the reddish-brown color of which is easily distinguishable from the 

 jet-black appearance of the larvae. The pupal stage lasts about 

 three weeks. The adult fly, surrounded by a bubble of air, quickly 

 rises to the surface of the water and escapes. The adults (fig. 76) 

 are apparently short lived and thus the entire life cycle, from egg 

 to egg is completed in approximately eight weeks. 



In the case of Simulium pictipes at Ithaca, N. Y., the first brood 

 of adults emerges early in May and successive generations are produced 

 throughout the summer and early autumn. This species winters in 



76. 



Simulium venustum, Cx8). 

 After Garman. 



