Fly, term commonly applied to many actively flying INSECTS; restricted by scientists to 2-winged flies of order Diptera. This order includes BLACK FLIES, blow flies, bot flies, crane flies, deer flies, face flies, flesh flies, gnats, horn flies, horse flies, house flies, MIDGES, MOSQUITOES, no-see-ums, stable flies and warble flies. Four-winged "flies," eg, ALDERFLIES, BUTTERFLIES, CADDISFLIES, DAMSELFLIES, DRAGONFLIES, LACEWINGS, MAYFLIES, SAWFLIES, scorpionflies, snakeflies, STONEFLIES and whiteflies, are not true flies.


 About 100 000 species of Diptera have been described worldwide; the actual number may be double that. Over 7000 species have been reported in Canada, a figure also estimated to represent only half the species present. Their relative abundance in the insect fauna increases northwards; in the most extreme arctic localities, they outnumber all other winged insects, both as species and as individuals. Flies range in size from 1 mm to 5.5 cm.


 The most obvious characteristic distinguishing true flies from other insects is the modification of the hind wings into knob-shaped gyroscopic organs (halteres), essential for maintaining balance in flight. These structures allow flies to perform outstanding feats of aerobatics (eg, landing upside-down on ceilings).

Adult flies have sucking or piercing mouth parts and lack the mandibles with which other insects bite food. Many so called "biting flies" (eg, horseflies, mosquitoes, no-see-ums, black flies, stable flies, tse-tse flies) feed on VERTEBRATE blood. They pierce skin with a stabbing action using various of their mouth parts (depending on the group of flies to which they belong).


It is thought that adult flies were, primitively, predators of other insects, using their mouth parts to pierce prey's integument (body covering) and that the habit of sucking vertebrate blood evolved from this. Many modern flies, including voracious predators such as robber flies and dance flies, still attack only other insects. Most predatory flies feed on carbohydrates, eg, honeydew and nectar, as well as on animal prey. Some flies have abandoned predation and use pollen (hover flies) or decaying material (house flies) as their protein source. Adult warble and bot flies do not feed, relying on fat reserves accumulated in the larval stage.

Reproduction and Development

Dipteran larvae are legless. Most are maggots living concealed within a moist substrate, but some hoverflies have free-living larvae, which prey on APHIDS. Specialized swimming larvae have evolved in mosquitoes and related groups of nonbiting midges. Most fly larvae feed on micro-organisms or decaying matter. A few are predatory or parasitic: larvae of marsh flies (sciomyzids) kill snails; those of horse flies (tabanids), robber flies (asilids) and dance flies (empidids) prey on other insect larvae; those of tachinids (parasitic flies) are specialized internal parasites, important in biological control of INSECT PESTS.

Although few groups of flies have evolved exclusively plant-eating larvae (notably fruit flies, gall midges and leaf-mining flies), those that have are unusually diverse and actively evolving. Among aquatic larvae, those of nonbiting midges form a major part of the INVERTEBRATE biomass in Canadian waters.

Flies are holometabolous insects, ie, they pass through a nonfeeding pupal stage before becoming adult. In the more highly evolved flies, the pupa is enclosed in an outer shell (puparium) which gives added protection against drying, drowning, etc. The earliest described fossil Diptera date from the Upper Triassic (227.4-205.7 million years ago). More advanced Diptera probably did not appear until the late Cretaceous, but dating is uncertain. Western Canadian amber from the Cretaceous (144.2-65 million years ago) is a promising source of FOSSIL Diptera.

Interaction with Humans

The enlarged salivary glands of many Diptera contain giant chromosomes convenient for genetic studies. This trait, and their willingness to breed in bottles, make Drosophila a popular experimental animal. In many parts of the world, including Canada, "biting flies" are among the most annoying insects; many species (especially mosquitoes) are vectors of serious diseases of humans and livestock such as malaria, sleeping sickness, yellow fever, encephalitis, onchocerciasis and dengue.