The Two-winged Flies 
3 2 5 
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 Mycetophilidce; larva, pupa, and adult. 
(Much enlarged.) 
which to transform. Perhaps the most singular habits noted in the family 
are those connected with the strong gregarious instinct which leads the 
larvae 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¬ 
tennae, the March-flies, Bibionidae, serve as a sort of transition family between 
the long-legged, slender-bodied midge type of fly with its thread-like hairy 
antennae, and the compact, heavy-bodied, short-legged type of fly with short 
and club-like three-segmented antennae, characteristic of the many families 
grouped in the section Brachycera. The March-flies (Fig. 454) are from 
one-eighth to one-half inch long, with fairly robust, often hairy, body, black- 
