The Two-winged Flies 



325 



feed on the decomposing substance in which the eggs are .aid, sometimes 

 spinning silken webs for protection. They pupate in the food-substance or 

 crawl away to some more sheltered spot, often forming a thick cocoon in 



Fig. 452.- 



-A fungus-gnat of the family Mycetophilida; 

 (Much enlarged.) 



larva, pupa, and adult. 



which to transform. P< rhaps the most singular habits noted in the family 

 are those connected with the strong gregarious instinct which leads the 

 larvas of many species to live closely together. Some of the species of Sciara, 

 known as "army-worms," have "the singular propensity of sticking to- 

 gether in dense patches, and will 

 form processions sometimes twelve 

 or fourteen feet in length and two 

 or three inches broad. This phe- 

 nomenon has been observed fre- 

 quently both in Europe and Amer- 

 ica, but the reason therefor is not 

 yet well understood, though the 

 object of the migration seems to be 

 the search for better feeding-grounds." Various species of this genus live 

 in potatoes and other vegetables, while the serious injury to potatoes called 

 "scab" is caused by a fungus-gnat known as Epidapus scabies. 



With larger and more robust bodies and relatively shorter and thicker an- 

 tenna;, the March-flies, Bibionidoe, serve as a sort of transition family between 

 the long-legged, slender -bodied midge type of fly with its thread-like hairy 

 antenna', and the compact, heavy-bodied, short-legged type of fly with .short 

 and club-like three-segmented antenna-, characteristic of the many families 

 grouped in the section Brachycera. The March-flies (Fig. 454) are from 

 one-eighth to one-half inch long, witli fairly robust, often hairy, body, black- 



FlO. 453. — Diagram of wing of fungus-gnat, 

 Mycetophila sp., showing venation. 



