Temporal range:
Middle Triassic – Recent, 245–0 mya
A hoverfly, showing dipteran features: large eyes, small antennae, sucking mouthparts, single pair of flying wings, hindwings reduced to clublike halteres
Scientific classification
Kingdom: Animalia
Class: Insecta
Infraclass: Endopterygota
Order: Diptera
A Fly by Matthias Zimmermann
A fly

A fly (plural: flies) is an insect of the order Diptera. The Diptera is a large order of advanced flying insects.

Their most obvious distinction from other insects is in their flight. A typical fly has two flight wings on its thorax and a pair of halteres. The halteres, which evolved from the hind wings, act as flight sensors: they are balance organs. Also flies have large eyes with excellent wide-angle vision.

With the help of their eyes and halteres, flies are exceptional fliers. They can avoid most predators, and are the most difficult insects to capture by hand. Their jinks, dives and turns to avoid their predators is their main adaptation. "These flies do a precise and fast calculation to avoid a specific threat and they are doing it using a brain that is as small as a grain of salt".... "And they can fly like an ace at birth. It's like putting a newborn baby in the cockpit of a fighter aircraft and it knowing what to do".

The only other order of insects bearing two true, functional wings plus any form of halteres are the Strepsiptera, a small order of insects. In contrast to the flies, the Strepsiptera evolved their halteres from their front wings and their flight wings are their rear wings.

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, sawflies, caddisflies, butterflies or scorpionflies.

Some true flies have become secondarily wingless, including some that live in social insect colonies.

Flies are also holometabolous, with complete metamorphosis.

Kinds of flies

A poster with sixteen different species of flies

There are an estimated 1,000,000 species, although only about 150,000 species have been described.

There are many different kinds of flies. Scientists have named 85,000 kinds (species). Houseflies are grey or black and can be found wherever people are. Horse flies and deer flies can bite people and animals. Fruit flies can be found near fruit that is too ripe. Hoverflies look like small wasps, but they have no sting.

Some flies do not have the word fly in their name, so people may not know they are flies. Mosquitoes are flies that bite people and can carry diseases, such as malaria. Midges are small flie, a bit like like mosquitoes, but not all midges bite. Gnats are small flies that sometimes are in big groups called swarms.

Some insects have the word fly in their names, but they are not flies at all.

Fireflies are a kind of beetle that can make light.

Dragonflies and damselflies are from the order Odonata and are not real flies.

Butterflies are not flies. They belong in the order Lepidoptera, along with moths.

Anatomy and biology

Musca domestica Portrait
Portrait of a flesh fly (Sarcophagidae)
An image of a house fly eye surface by using scanning electron microscope at 450× magnification

Flies are adapted for aerial movement, and typically have short and streamlined bodies. The first tagma of the fly, the head, consists of ocelli, antennae, compound eyes, and the mouthparts (the labrum, labium, mandible and maxilla make up the mouthparts). The second tagma, the thorax, bears the wings and contains the flight muscles on the second segment, which is greatly enlarged; the first and third segments have been reduced to collar-like structures. The third segment of the thorax bears the halteres, which help to balance the insect during flight. A further adaptation for flight is the reduction in number of the neural ganglia, and concentration of nerve tissue in the thorax, a feature that is most extreme in the highly derived Muscomorpha infraorder.

Fly Eye 30wd 3x3
A scan of a house fly taken at 40 magnifications under a scanning electron microscope

Flies have a mobile head with eyes and in most cases have large compound eyes on the sides of the head, with three small ocelli on the top. The antennae take a variety of forms, but are often short, which reduces drag while flying.

Because no species of fly has teeth or any other organ or limb that allows them to eat solid foods, flies consume only liquid food, and their mouthparts and digestive tracts show various modifications for this diet. Female Tabanidae use knife-like mandibles and maxillae to make a cross-shaped incision in the hosts' skin and then lap up the blood. The gut includes large diverticulae, allowing the insect to store small quantities of liquid after a meal.

Reproduction and development

Anthomyiidae sp. 1 (aka)
Mating anthomyiid flies

The female lays her eggs as close to the food source as possible, and development is rapid, allowing the larvae to consume as much food as possible in a short period of time before transforming into adults. The eggs hatch immediately after being laid, or the flies are ovoviviparous, with the larvae hatching inside the mother.

Larve de moustique
Mosquito larva. A typical Nematoceran. Note the head capsule and clearly defined three segments of the thorax
Syrphid larva. Maggot-like anatomy, typical of a Brachycerid fly, lacking both a conspicuously demarcated thorax and head capsule

Larval flies have no true legs. Some Dipteran larvae, such as species of Simuliidae, Tabanidae, and Vermileonidae, have prolegs adapted to such functions as holding onto a substrate in flowing water, holding onto host tissues, or holding prey. Roughly speaking, there is some anatomical distinction between the larvae of the Nematocera and the Brachycera (see Classification section, below); especially in the Brachycera, there is little demarcation between the thorax and abdomen, though the demarcation may be very visible in many Nematocera, such as mosquitoes (see image, both here and in the mosquitoes article); in the Brachycera, the head of the larva is not clearly distinguishable from the rest of the body, and there are few, if any, sclerites. Informally, such Brachyceran larvae are called maggots, but the term is nontechnical and often applied indifferently to fly larvae or insect larvae in general. The eyes and antennae of Brachyceran larvae are reduced or absent, and the abdomen also lacks appendages such as cerci. This lack of features is an adaptation to food such as carrion, decaying detritus, or host tissues surrounding endoparasites. Nematoceran larvae generally have visible eyes and antennae, though usually small and of limited function.

The pupae take various forms, and in some cases develop inside a silk cocoon. After emerging from the pupa, the adult fly rarely lives more than a few days, and serves mainly to reproduce and to disperse in search of new food sources.


Maggots, London Zoo, London
Maggots used as animal feed at London Zoo

Maggots found on corpses are useful to forensic scientists; specifically forensic entomology. By their stage of development, these maggots indicate the time elapsed since death, as well as the place of death.

Maggot species can be identified using their DNA. The housefly maggot measures 10–20 mm (⅜–¾ in) in size. At the height of the summer season, a generation of flies (egg to adult) may be produced in 12 to 14 days. Other insect families, such as Histeridae, feed on maggots. Thus the lack of maggots would increase the estimated time of death.

Maggots are bred commercially, as a popular bait in angling, and as a food for carnivorous pets such as reptiles or birds.

Maggots are used in medicine to clean out necrotic wounds and in food production, particularly of cheeses such as casu marzu designed to rot as part of their aging process.

Flies are reared in large numbers in Japan to serve as pollinators of sunflowers in greenhouses, especially the maggots.


Fly for Kids. Kiddle Encyclopedia.