Flies 



225 



tendency toward the reduction of the size of the head 

 and of the appendages. Many of them are gill-less and 

 many more possess but a single cluster of four tapering 

 retractile anal gill filaments. 



Fig. 133. An adult midge, Tanypus carneus, 



male. 



By far the most important of the aquatic Diptera in 

 the economy of nature are the midges (Chironomid^) . 

 These abound in all fresh waters. The larvae are 

 cylindric and elongate, with distinct free head, and body 

 mostly hairless save for caudal tufts of setae. They are 

 distinguished from other fly larvae by the possession of a 

 double fleshy proleg underneath the prothorax, and a 

 pair of prolegs at the rear end of the body, all armed 

 with numerous minute grappling hooks. Many of 

 them are of a bright red color, and are hence called 

 ''blood worms." 



