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 horsefly (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 horsefly 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.
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.
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. Lucilia cuprina is a species of fly responsible for most flystrike in sheep in Australia, a condition where eggs are laid in the flesh of living animals and maggots eat either dead or living tissue. Control of fly strike is a major activity of sheep husbandry in Australia, impacting on timing of shearing, leading to the development of mulesing, and requiring crutching of sheep which will have longer wool during the fly season.
Use in medicine
Through the ages maggots have been used in medicine to clean out necrotic wounds.
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. Flies also adhere to surfaces in two ways: The first is with microscopic bristles on the ends of their feet, which wrap onto the rough surfaces. The other way is with the arolia or pulvilli on the tarsi, which contains sticky hairs so that it can adhere to smooth surfaces.
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.
- In Greek mythology, Myiagros was a god who chased away flies during the sacrifices to Zeus and Athena. Also, Zeus sent a fly to sting the horse Pegasus causing Bellerophon to fall back to Earth when he attempted to ride to Mount Olympus.
The plague of flies is described in Exodus chapter 8.
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 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.
- Jules Dorrian (3 June 2006). Battling the blowfly – plan for the future (pdf). Australian Wool Innovation. Retrieved on 27 September 2013.