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As defined by entomologists, a fly (plural flies) is any species of insect of the order Diptera. These typically have one pair of true wings, with the hind wings modified into halteres. Flies are common amongst humans and some can cause the spread of serious diseases such as sleeping sickness. The housefly (Musca domestica) and mosquito are particularly common amongst humans. Other flies, such as the horse fly (Family Tabanidae), can inflict painful bites. The larva of a fly is commonly called a maggot.
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.
The diet of flies varies heavily between species. The horse-fly and mosquitoes feed on blood and nectar, and the house fly eats a semi-digested liquid created by mixing enzyme-rich saliva with its food.
In addition to being an essential part of the food chain, some species of flies spread pollen, hasten the decomposition of plants, animals, and dung, and, in the case of about 5000 species of Tachina flies, eat other insects.
- Further information: Maggot
The fly life cycle is composed of four stages: egg, larva (commonly known as a maggot), pupa, adult. The eggs are laid in decaying flesh, animal dung, manure, or pools of stagnant water - whatever has ample food for the larva. This can include both meat in trash compactors as well as rats that have died between the ceiling and floor of a home.
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. The size of the house fly maggot is 9.5-19.1mm (⅜ to ¾ inch). At the height of the summer season, a generation of flies (egg to adult) may be produced in 12-14 days.
Maggot identification uses a classification called "instar" stages. An instar I is about 2-5 mm long; instar II 6-14 mm; instar III 15-20 mm. These measure about 2-3 days, 3-4 days, and 4-6 days (for average house flies or bottle flies) since the eggs were laid. By use of this data, plus other signs, the approximate time since death can be estimated by forensic scientists.
Various maggots cause damage in agricultural crop production, including root maggots in rapeseed and midge maggots in wheat. Some maggots are leaf miners.
Maggots are bred commercially, as a popular bait in angling, and a food for carnivourous pets such as reptiles or birds. Due to the increasing popularity of maggots, a maggot vending machine has been installed in the English county town of Northampton.
Use in medicine
Through the ages maggots have been used in medicine to clean out necrotic wounds. For more information, see Maggot therapy.
Use in foods
Maggots have been used in food production, particularly cheese. For more information, see Casu marzu.
Transportation of insects
Flies can move from one point to another point by flying, but they can also walk and run (or scurry) around a piece of fruit in search of sugar.
In compound names containing "fly" for members of this order, the name is written as two words as in "crane fly". For insects that are members of other orders the name is written as a single word as in "butterfly".
- mayfly: Ephemeroptera
- dragonfly and damselfly: Odonata
- stonefly: Plecoptera
- whitefly: Hemiptera
- firefly: Coleoptera
- alderfly, dobsonfly, and fishfly: Megaloptera
- snakefly, mantidfly, and owlfly: Neuroptera
- sawfly: Hymenoptera
- caddisfly: Trichoptera
- butterfly: Lepidoptera
- scorpionfly, earwigfly, and hangingfly: Mecoptera
Rarest known flies
The world's rarest known fly families include the Eurychoromyidae, Broad-headed Flies and the Boston Red-Tinted Warbler Flies. While the first family is harmless to human life, the second is known for attacking warm-blooded bodies, especially any exposed skin of humans.
Flies in mythology and religion
- In traditional Navajo religion, Big Fly is an important spirit being.
- In demonology, Beelzebub is referred to as the "Lord of the Flies".
- Myiagros was a Greek god who chased away flies during the sacrifices to Zeus and Athena.
Flies in art and popular culture
- In art, extremely life-like flies have sometimes been depicted in the trompe l'oeil paintings of the 15th century. An example is the painting Portrait of a Carthusian by Petrus Christus, showing a fly sitting on a fake frame. 
- The 1958 science fiction film The Fly, remade in 1986, revolves around the accidental merger of a human and a fly. In reality, the human bot fly lives parasitically in the human body.
- In 2001, Garnet Hertz produced an art project in which a complete web server was implanted into a dead fly.
- In music:
- AC/DC, an Australian hard rock band, released an album called Fly on the Wall in 1985. The cover featured a fly, probably the same fly that is animated in the Fly on the Wall videos.
- Iron Maiden, a heavy metal band, released a song called Lord of the flies as a single for their album The X Factor in 1996 (later released on the live Death on the Road). It was based on the book by the same name.
- Alice in chains released an album called Jar of flies.
- Pictures of Flies and Other Observations
- Picture: Fly on a window
- Fly with implanted web server, by Garnet Hertz.
- How Flies Walk on Ceilings
- How to build a fly-powered toy aeroplane