NO. I BLACK FLIES OF GUATEMALA — ^DALMAT 307 



the Yepocapa onchocerciasis zone — Simulium ochraceum, S. metalli- 

 cum, and S. callidum. 



GENERAL 



The Simuliidae, or black flies, are small nematocerous Diptera, the 

 adults rarely exceeding 5.0 mm. in length. They are often called 

 "buffalo gnats" because of the "hump," or arch, that is characteristic 

 of the thorax. The mouthparts are formed for piercing and sucking, 

 but only the females are known to take blood. 



The larval stage of all known species is passed in running water, 

 varying from waterfalls to minute trickles, seldom in swampy areas 

 or stagnant water. The larvae feed on minute animals and plants, or 

 on particles of these, that are strained from the water with the aid of 

 the mouth brushes. Thus, the current not only helps in supplying the 

 necessary oxygen to the larvae but also transports food to the mouth 

 brushes and maintains the latter open so that they can readily strain 

 the food. 



The larvae are usually found in the shallower parts of the streams, 

 especially where obstructions cause rippling of the water surface. 

 Rocks, stones, leaves, branches, and all sorts of debris serve as sub- 

 strata for them. The larvae attach themselves to the substrata by means 

 of an anal disc which makes it possible for them to retain their position 

 even in very swift water. They seem to prefer for attachment clean 

 substrata rather than ones that are covered by algae or slimy deposits. 

 Their means of locomotion is very interesting and can be observed 

 easily with the dissecting microscope when they are held in a watch 

 glass, or through the glass walls of an aquarium into which air is 

 pumped. My observations coincide with those given by Puri (1925). 

 The larvae cannot swim or move freely through the water, except 

 when they are carried by a current. Their movement appears very 

 similar to that of the geometrid caterpillars. The larva, attached to 

 the substratum by its anal disc, deposits a patch of viscous saliva with 

 its mouthpart, and into it then fixes its pseudopod (thoracic proleg) ; 

 as the proleg is being fixed, the larva simultaneously deposits more 

 saliva to the right or left of the proleg and somewhat in front of it and, 

 with a looping movement, frees the anal sucker, bringing it up to the 

 new patch. The proleg is then released and the process is repeated. 

 Although the resultant path of the larva may be in a straight line, each 

 individual movement is at an angle to it. The larva, by means of its 

 salivary glands, also spins silken threads which it uses as a means of 

 quickly moving downstream, especially when it is disturbed. It can 

 crawl back on the threads to its original place of attachment, using 

 the mouthparts and pseudopod. 



