TWO-WINGED INSECTS. 
17 
apparently harmless in the winged state, deposit their eggs on 
plants, on the juices of which their young subsist, and are often¬ 
times productive of immense injury to vegetation ; among these 
the most notorious for their depredations are the gall-gnats 
{Cecidomyice), including the wheat-fly and Hessian fly, the root¬ 
eating maggots of some of the long-legged gnats {Tipulce), those 
of the flower-flies {Anthomyice), and the two-winged gall-flies and 
fruit-flies ( Ortalides). To this list of noxious flies are to be added 
the common house-flies {Miiscce)^ which pass through the maggot 
state in dung and other filth, the blue-bottle or blow-flies, and 
meat-flies {Lucilice and Calliphorce), together with the maggot- 
producing or viviparous flesh-flies {Sarcophagce and Cynomyice), 
whose maggots live in flesh, the cheese-fly {Plophila)^ the parent 
of the well-known skippers, and a few others that in the larva state 
attack our household stores. 
Some flies are harmless in all their states, and many are emi¬ 
nently useful in various ways. Even the common house-flies, and 
flesh-flies, together with others for which no names exist in our 
language, render important services by feeding while larvae upon 
dung, carrion, and all kinds of filth, by which means, and by 
similar services rendered by various tribes of scavenger-beetles, 
these offensive matters speedily disappear, instead of remaining 
to decay slowly, thereby tainting the air and rendering it unwhole¬ 
some. Those whose larvae live in stagnant water, such as gnats 
{CulicidcB), feather-horned gnats {Chironomus^ &c.), the soldier- 
flies {Siratiomyadce), the rat-tailed flies {Helophilus)^ &c., &c., tend 
to prevent the water from becoming putrid, by devouring the de¬ 
cayed animal and vegetable matter it contains. The maggots of 
some flies {Mycetophilae and various Muscadce) live in mush¬ 
rooms, toadstocds, and similar excrescences growing on trees ; 
those of others (Sargi, Xylopkagidce, Asilidcs, Therevce, MilesicB^ 
Xylot(B, Borbori, &c., &c.), in rotten wood and bark, thereby join¬ 
ing with the grubs of certain beetles to hasten the removal of 
these dead and useless substances, and make room for new and 
more vigorous vegetation. Some of these wood-eating insects, with 
others, when transformed to flies, {Asilidce [Plate I. Fig. 4, Asilus 
aestuans], Rhagionidce, Dolichopid€e, and Xylophagidce^ prey on 
other insects. Some {SyrphidcB)^ though not predaceous them- 
3 
