Fly, the common name for many flying insects. True flies, however, include only insects of the order Diptera. Such insects as dragonflies, fireflies, and mayflies are not true flies. This article describes true flies only. .)

HousefliesHouseflies have compound eyes, a single pair of wings, and six legs.

The world's 85,000 or more species of true flies include, in addition to houseflies and mosquitoes, horseflies, botflies, robber flies, tachina flies, testse flies, and fruit flies. Midges, gnats, and the sheep tick are also classified as true flies. Flies are found on all continents and most islands, but not in the coldest parts of the polar regions. Flies have been caught in traps carried to high altitudes by airplanes.

Horse fliesHorse flies are swift, large bloodsuckers.
Facts in brief about flies
Names: Male, none; female, none; young, maggots or wrigglers; group, swarm.
Number of newborn: 1 to 250 at a time, depending on species. As many as 1,000 a year for each female.
Length of life: Average 21 days in summer for house flies.
Where found: Throughout most of the world.
Scientific classification: Flies belong to the class Insecta, and they make up the order Diptera.

Flies range in size from the almost invisible punkies, or no-see-ums, to crane flies and robber flies that may be two inches (5 cm) long. All flies, except for a few kinds that are wingless, have a single pair of wings. The wings are usually transparent, and are often iridescent. They lie flat on the fly's back when it is at rest. Behind their wings, almost all flies have two small, club-shaped sense organs (not visible in the drawing) that serve as balancers to aid them in flight. These are called halteres (hăl-tē'rēz).

The fairy flyThe fairy fly is one of the smallest insects.

The mouth parts of adult flies are adapted for lapping or sucking, and sometimes for piercing, but never for chewing. Flies' heads may be large or small compared with the body size. Most flies have very large eyes that allow them to see in all directions. The antennae of flies are probably sense organs for hearing or smelling. They vary greatly in length and structure according to species and the sex of the individual fly. All flies have six legs. The legs of almost all species end in claws. Under each claw, most flies have at least one hairy pad called a pulvillus. It secretes a sticky substance that permits the fly to walk upside-down on smooth surfaces.


The life cycle of flies is divided into four stages: egg, larva, pupa, and adult. This kind of life cycle is called complete metamorphosis. Flies lay eggs, usually in large numbers, in all types of places where food will be ready for their larvae as soon as they hatch. Fly larvae that feed on plant sap, decaying matter, or living flesh, are called maggots. They are legless, and look like tiny worms. Larvae that develop in water are called wigglers.

After a feeding period that varies in length with the species and circumstances, most larvae enter the pupal stage. During this stage they are enclosed in a protective cover called the pupal case. They emerge as adult flies.

Importance of Flies
Harmful Flies

Some kinds of flies are among man's deadliest enemies. Various species of mosquitoes, for example, harbor inside their bodies the germs that cause malaria, yellow fever, elephantiasis, dengue fever, and other diseases. When they bite, they transmit the germs from one person to another. Many kinds of flies, notably the common housefly, often carry disease germs on the hairs of their bodies and legs, and leave them on human food. Typhoid fever, cholera, diarrhea, dysentery, and other diseases are spread in this way.

HousefliesHouseflies cannot bite, but they carry disease causing microorganisms on their bodies.

Other flies, and their maggots, are serious agricultural pests. Stable flies, horn flies, botflies, and other species torment cattle and sheep, spoil their hides, and cause the animals to lose weight or produce less milk or wool. Certain flies transmit fungus diseases to rye and other grains. The seed-corn maggot infects plants with soft-rot bacteria. The larvae of the Hessian fly destroy wheat and barley. Fruit flies damage apples, cherries, melons, oranges, and other fruits.

Warble fliesWarble flies resemble bumble bees but are related to bot flies.

Flies are controlled by eliminating or destroying their breeding places and feeding areas. Other means of keeping flies in check range from fly swatters and electrified screens to power sprayers operated from trucks and airplanes. Many kinds of flies and their larvae are killed by chemical insecticides.

Beneficial Flies

Flies and their maggots serve as food for birds and fish. Many species of flies keep injurious insects under control. The robber fly, for example, catches flies and other insects in flight, and sucks out their body juices. Tachina flies lay their eggs on other insects, particularly caterpillars, and the maggots eat their hosts. One kind of tachina fly is especially useful in destroying the army worm. The syrphid flies and others, when feeding on nectar, pollinate flowers. The maggots of many species of flies are scavengers that help rid the soil of dead vegetation or animal matter.

True flies make up the order Diptera (from Greek words meaning “two” and “wing”).

Some flies that spread disease
Apple maggots Bacterial rotApples
Black flies Onchocerciasis (River blindness)Human beings
Deer flies Tularemia (Rabbit fever)Human beings, rodents
Fly maggots Bacterial soft rotPotato, cabbage, other vegetables
Horse flies AnthraxHuman beings, animals
House flies Amebic dysenteryHuman beings, animals
Typhoid feverHuman beings
Bacillary dysenteryHuman beings
CholeraHuman beings
Mosquitoes FilariasisHuman beings
MalariaHuman beings
Yellow feverHuman beings, monkeys, rodents
DengueHuman beings
EncephalitisHuman beings, horses
Olive fruit fly Olive knotOlives
Sand flies Kala azarHuman beings
Tsetse flies African sleeping sicknessHuman beings