True flies are insects of the Order Diptera (Greek: di = two, and pteron = wing), possessing a single pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax.
The presence of a single pair of wings distinguishes true flies from other insects with "fly" in their name, such as mayflies, dragonflies, damselflies, stoneflies, whiteflies, fireflies, alderflies, dobsonflies, snakeflies, sawflies, caddisflies, butterflies or scorpionflies. Some true flies have become secondarily wingless, especially in the superfamily Hippoboscoidea, or among those that are inquilines in social insect colonies.
It is a large order, containing an estimated 240,000 species of mosquitos, gnats, midges and others, although under half of these (about 120,000 species) have been described . It is one of the major insect orders both in terms of ecological and human (medical and economic) importance. The Diptera, in particular the mosquitoes (Culicidae), are of great importance as disease transmitters, acting as vectors for malaria, dengue, West Nile virus, yellow fever, encephalitis and other infectious diseases.
Diptera are a diverse order with an enormous range of ecological roles. Every type of trophic level pattern can be seen in the Diptera. Dipteran predators include the robber flies (Asilidae). A variety of herbivores can be found in the Diptera, such as the economically important fruit flies (Tephritidae). Flies are often parasites, including internal parasites such as the bot fly and external parasites such as the mosquito, black fly, sand fly or louse fly. Myiasis is the special term for diseases cause by flies (such as the screw worm fly) infecting living tissue. Many flies eat dead organic matter (detritovores), plant or animal remains. This is especially common in the larval stage, seen in the filter-feeding mosquitoes and black flies, the dung-feeding blow flies (Calliphoridae), or the organic deposit feeding rat-tailed maggot. A number of taxa feed on blood, including horse flies and mosquitoes.
Some flies can be important pollinators for many species of plant (many such fly-specialized plants, such as Stapelia, Rafflesia, and Aristolochia, produce carrion odors), and many flies feed on pollen and nectar of common plants, and can perform incidental pollination. Similar relationships occur between flies and various fungi, with flies dispersing the fungal spores.
The basic life cycle of a fly is egg, larvae (maggots — see below), pupa and adult (winged stage), called holometabolism. There is often a difference in food sources for larvae versus adult dipterans of the same species. For example, mosquito larvae live in standing water and feed on detritus while the adults feed on nectar as their energy source while females utilize blood as their energy source for egg production. Various maggots cause damage in agricultural crop production, including root maggots in rapeseed, midge maggots in wheat, and numerous species of leaf miners (note that since fly maggots have no legs, they almost exclusively feed internally on plants).
Flies rely heavily on sight for survival. The compound eyes of flies are composed of thousands of individual lenses and are very sensitive to movement. Some flies have very accurate 3D vision. A few, like Ormia ochracea, have very advanced hearing organs.
There are two generally accepted suborders of Diptera. The Nematocera are usually recognized by their elongated bodies and feathery antennae as represented by mosquitoes and crane flies. The Brachycera tend to have a more roundly proportioned body and very short antennae. A more recent classification has been proposed in which the Nematocera is split into two suborders, the Archidiptera and the Eudiptera, but this has not yet gained widespread acceptance among dipterists.
Beyond that, considerable revision in the taxonomy of the flies has taken place since the introduction of modern cladistic techniques, and much remains uncertain. The secondary ranks between the suborders and the families are more out of practical or historical considerations than out of any strict respect for phylogenetic classifications (some modern cladists tend to spurn the use of Linnaean rank names). Nearly all classifications in use now, including this article, contain some paraphyletic groupings; this is emphasized where the numerous alternative systems are most greatly at odds. See list of families of Diptera.
Dipterans belong to the group Mecopterida, that also contains Mecoptera, Siphonaptera, Lepidoptera (butterflies and moths) and Trichoptera. Inside it, they are sometimes classified closely together with Mecoptera and Siphonaptera in the superorder Antliophora .
Not surprisingly, in art and entertainment, flies are also used primarily to introduce elements of horror or the simply mundane; an example of the former is the 1958 science fiction film The Fly (remade in 1986), in which a scientist accidentally exchanges parts of his body with those of a fly. Examples of the latter include trompe l'oeil paintings of the 15th century such as Portrait of a Carthusian by Petrus Christus, showing a fly sitting on a fake frame , a 2001 art project by Garnet Hertz in which a complete web server was implanted into a dead fly , and various musical works (such as Yoko Ono's album Fly, U2's song "The Fly," Dave Matthews' song "The Fly" and Béla Bartók's "From the Diary of a Fly"). The ability of flies to cling to almost any surface has also inspired the title of Human Fly for stunt performers whose stunts involve climbing buildings, including both real life and fictional individuals.
Aside from the fictional and conceptual role flies play in culture, there are practical roles that flies can play (e.g., flies are reared in large numbers in Japan to serve as pollinators of sunflowers in greenhouses), especially the maggots of various species.
Some types of maggots found on corpses can be of great use to forensic scientists. By their stage of development, these maggots can be used to give an indication of the time elapsed since death, as well as the place the organism died. Maggot species can be identified through the Use of DNA in forensic entomology. The size of the house fly maggot is 10–20 mm (⅜–¾ in). At the height of the summer season, a generation of flies (egg to adult) may be produced in 12–14 days.