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 (Lucilice and Galliphorce), together with the maggot- 

 producing or viviparous flesh-flies (Sarcophagce and Gynomyice), 

 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 larvas 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 

 (Oidicidce), feather-horned gnats (Chironomus, &c), the soldier- 

 flies (Stratiomyadce), 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 (Mycetophilce and various Muscadce) live in mush- 

 rooms, toadstools, and similar excrescences growing on trees : 

 those of others (Sargi, Xylophayidce, Asilidce, Tlierevce, Milesice, 

 Xylotcz, 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, witli 

 others, when transformed to flies, {Asilidce [Plate I. Fig. 4, Asilus 

 asstuans], Rhagionidce, Dolichopidce, and Xylopkagidce,) prey on 

 other insects. Some (Syrphidce), though not predaceous tliem- 

 3 



