fly

Flies are insects with a pair of functional wings for flight and a pair of vestigial hindwings called halteres for balance. They are classified as an order called Diptera, that name being derived from the Greek δι- di- "two", and πτερόν pteron "wings". The order Diptera is divided into two suborders (although one suborder is non-monophyletic), with about 110 families divided between them; the families contain an estimated 1,000,000 species, including the familiar housefly, horse-fly, crane fly, and hoverfly; although only about 125,000 species have a species description published. The earliest fly fossils found so far are from the Triassic, about 240 million years ago; phylogenetic analysis suggests that flies originated in the Permian, about 260 million years ago.
Many insects, such as the butterfly, contain the word fly in their name, but are not Dipterans. Also, the word "fly" is sometimes used colloquially and non-scientifically as a name for any small flying insect: the term "true fly" is sometimes invoked to make clear the insect being referenced is a Dipteran.
Flies have a mobile head, with a pair of large compound eyes, and mouthparts designed for piercing and sucking (mosquitoes, black flies and robber flies), or for lapping and sucking in the other groups. The suborder Nematocera (from Greek, "thready-horns") have thin, long antennae; while the suborder Brachycera (from Greek "short-horns") have short antennae. Flies have only a single pair of wings to fly; their arrangement gives them great maneuverability in flight. The hindwings (halteres) evolved into advanced mechanosensory organs, which act as high-speed sensors of rotational movement and allow them to perform advanced aerobatics. Claws and pads on their feet enable them to cling to smooth surfaces.
The life cycle of flies consists of the eggs, larva, pupa, and the adult. Flies undergo complete metamorphosis; the eggs are laid on the larval food-source, and the larvae (which lack true limbs) develop in a protected environment, often inside their food source. The pupa in higher dipterans is a tough capsule from which the adult emerges when ready to do so. Flies have short lives: for example, the adult housefly lives about a month; the mayfly about a year. The source of nutrition for adult flies is liquified food, including nectar.
Flies are of considerable ecological and human importance. They are important pollinators, second only to the bees and their Hymenopteran relatives. They may have been responsible for the first plant pollination in the Triassic. Mosquitoes are vectors for malaria, dengue, West Nile fever, yellow fever, encephalitis, and other infectious diseases; and houseflies, commensal with humans all over the world, spread food-borne illnesses. Flies can be annoyances, especially in some parts of the world where they can occur in large numbers, buzzing and settling on the skin or eyes to bite or seek fluids. Larger flies such as tsetse flies and screwworms cause significant economic harm to cattle. Blowfly larvae, known as gentles, and other dipteran larvae, known more generally as maggots, are used as fishing bait, as food for carnivorous animals, and in medicine for debridement to clean wounds. Fruit flies are used as model organisms in research. In culture, the subject of flies appears in religion, literature, cinema, and music.

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