FLIES, two-winged insects of the order Diptera (q.v.), whose larva are legless, soft and cruciform and are termed "maggots." The group is world-wide in its distribution, and probably quite as numerous as either the beetles or the group of wasps, bees, ants, etc. It is now known to contain about 40,000 species, most of which are incalculably numerous in individuals; so that "swarms" of flies is a proverbial ex pression. Entomologists believe that 8 or 10 times 40,000 species really exist. The vast abun dance is due to the plenitude of their food, and to their fecundity; and it is accompanied by a very high state of organization, so that certain families of flies are held by some students to stand at the head of the insect tribe in special• zation of structure. Intellectually, however, they are far behind the Hymenoptera; and to this fact, to the small size and unattractive ap pearance of most specimens (although great brilliance of color is to be found among some families), and to the comparatively uniform and uninteresting nature of their metamorphoses, are to be charged the relatively small amount of study that has been given to the group. The life-history of the ordinary flies is detailed be low; but many peculiarities exist in other fam ilies of the order. "With some," says Howard, "no eggs are laid, and living larva issue from the body of the female. Such flies then become practically viviparous or With others, although these are few in number, the development within the body of the femalegoes even farther, and when the insect emerges from the body of its mother it is already in the pupal condition. Such forms are called (pupip . . . Many species — comprising, in fact, whole families — are aquatic or subaquatic in their early stages, and some possess the faculty of living under what appear to be most disadvantageous conditions." Some of the flies of the family Ephydrider (whose eggs or larva are eaten by American Indians,— see Koo cHAH-BEE) live in the strongly alkaline lakes of the Far West where little else can exist.
Flies are mainly day-flies, and fond of sun shine, but some appear only at night or in the dusk; a section of the tribe does not fly at all, being wingless and parasitic. They live in the most diverse manner; some attack large ani mals and suck their blood; some prey on smaller insects; some suck honey, and in search of it take part in the cross-fertilization of flowers (see Fwweits AND INSECTS) ; and many find their food in decaying animal and vegetable matter. A large number of dipterous larva eat refuse or carrion,— whence arise serious evils to mankind,— others feed inside growing vege tables; and some maggots prey, or are parasitic upon, other animals.
A type of the group is found in the house fly (Musca domestica), which represents the great family Muscider, in which most of the familiar flies about houses and stables are in cluded, and its life-history represents that of its kind generally. Its eggs are laid preferably on horse-manure, but also on human or other ex crement, decaying vegetables, etc., and hatch in six or eight hours, producing white maggots. These mature in four or five days, when their skins harden and turn brown, forming a pu parium, or case, within which the true pupa forms, and five days later gives birth to a per tected fly. Thus a total life cycle requires in midsummer only about .10 days, and a do;en generations may thus be born in warm climates within a single season. As each fly deposits on the average 120 eggs, and as the maggots of 1,200 house-flies may be sustained by a pound of manure, the possible rapidity of their multi plication is apparent. Most flies live but a few weeks, and toward the end of the season they die with great rapidity, becoming infested with reddish mites, which suck their juices, or with fatal fungous diseases. (See FUNGI). In warm houses a few may survive a winter, but as a rule all adult flies die in the fall, and the species survives and recovers in the spring from the eggs or pupa left over winter in the manure heap or other feeding-place. It is plain that at tempts to mitigate the annoyance and danger resulting from many flies may best be directed toward the destruction of their eggs and young. That such destruction is desirable and the duty of society is plain when one considers the vast amount of injury these insects may do. Many sorts attack vegetables and fruit, for example the hessian-fly, which is perhaps the most de structive insect in the .United States, the apple fly, the gall-flies, fruit-midges, potato-scab gnat and others. Others harm domestic animals, as the bot-flies, sheep-ticks, horn-flies, tsetse and all the horse-flies, bee-killing robber-flies and bee-flies, black-flies, mosquitoes and many more. This catalogue of harmfulness (to which can be opposed only the beneficial work done by the tachina-flies which cause the death, by para sitism, of other kinds of injurious insects) be comes of small importance, however, beside the enormous evil flies do in spreading virulent diseases from man to man and place to place. Were this not so it might be true, as formerly alleged, that their services as scavengers, as parasites, and in the cross-fertilization of plants, balanced the damage caused by some, and left something to their credit.