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 (Muscce), which pass through the maggot 
state in dung and other filth, the blue-bottle or blow-flies, and 
meat-flies ( Liicilice and Galliphorce), together with the maggot- 
producing or viviparous flesh-flies (Sarcophagi and Cynomyice ), 
whose maggots live in flesh, the cheese-fly (Piophila), 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 larva 1 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 larva? live in stagnant water, such as gnats 
( Culicidi ), feather-horned gnats ( Chironomiis , &c.), the soldier- 
flies (Strati omyadce), the rat-tailed flies (Helophilus), &c., &c., tend 
to pt event the water from becoming putrid, by devouring the de- 
cayed animal and vegetable matter it contains. The maggots of 
some flies (Mycetophilce and various Muscudce) live in mush- 
1001ns, toadstools, and similar excrescences growing on trees ; 
those of others (Sargi, Xylophagidce, Asilidce , Therevce, Milesice, 
Xyloti, 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 
te-tuans] , Phagwnidce, Dolicliopidce, and Xylophagidce,) prey on 
othet insects. Some (Syrp/uda), though not predaceous tliem- 
3 
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