The fly (any two-winged insect belonging to the order Diptera) affects human welfare more extensively than any other insect. Flies may transmit disease directly, as when they contaminate food; the larvae of certain flies invade the bodies of men and animals; and other larvae feed on, and thus damage, plants.
It is while feeding that flies inflict their damage. Larvae which live inside growing plant material and causing damage and loss of crops include those of gall midges and fruit flies. Other fly larvae live in carrion and in the flesh of animals, including that of man. For example, the larvae of various bot flies and screwworms are parasitic within domestic animals. The adult flies feed upon fluids, which they draw up through a tubular proboscis. The basic food of most flies is carbohydrates and water. To get these substances, flies frequent flowers, pools of water, fresh dung, and most damp, rotting or fermenting matter. It is during these activities that flies may mechanically transmit disease organisms through the contamination of food. All the adults in some species and only the females in some other species supplement the basic carbohydrate diet with protein. These flies have mouth parts that can pierce the skin of another insect or of a vertebrate animal in order to suck blood. Bloodsucking flies include mosquitoes, black flies, buffalo gnats, sand flies, biting midges, horseflies, tsetse flies, and stable flies. Such flies are a nuisance and often cause physical pain and severe irritation. But their greatest concern to man is their ability to transmit disease-producing organisms found in the blood of man and other animals. Some flies live as external parasites in the hair of mammals or in the feathers of birds. They feed upon the blood of their hosts and in birds are known to transmit diseases.