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 are classified in the phylum Arthropoda , class Insecta, order Diptera.
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