The fly, name commonly used for any of a variety of winged insects, but properly restricted to members of the order Diptera, the true flies, which includes the housefly, gnat, midge, mosquito, and tsetse fly. All have sucking or piercing-and-sucking mouthparts and, except for a few wingless species, bear one pair of wings. The hind wings are reduced to knobbed balancing organs called halteres. All flies undergo complete metamorphosis, i.e., a four-stage development. The larvae, which occupy a wide variety of ecological niches, typically require a moist environment such as rotting flesh, decaying fruit, or the internal organs of other animals (see blowfly; botfly; fruit fly; tachinid fly). Adults often feed on nectar and plant sap, but some, such as the female horsefly and female mosquito, feed on blood; the adults of some species do not feed at all. A few species are found worldwide, often dispersed by humans; more than 16,000 species are found in North America. Many flies are harmful either as carriers of disease or as destroyers of crops. Some parasitize harmful insects. Some, such as the fruit fly, are important in laboratory studies.
Flies have a mobile head with eyes, and, in most cases, have large compound eyes on the sides of the head, with five small ocelli on the top. The antennae take a variety of forms, but are often short, to reduce drag while flying.
Because no species of fly have teeth or any other organ or limb that allows them to eat solid foods, flies consume only liquid food, and their mouthparts and digestive tract show various modifications for this diet. Female Tabanidae use knife-like mandibles and maxillae to make a cross-shaped incision and then lap up the blood. The gut includes large diverticulae, allowing the insect to store small quantities of liquid after a meal.