What do flies look like?

The true flies belong to the Order Diptera and include many common insects such as mosquitoes, midges, sand flies, blowflies and the House Fly. Most of the insects we see flying around do so with four wings (two pairs). However, dipterans (meaning 'two wings') use only one pair. The other pair of wings is reduced to club-like structures known as 'halteres' that they use for balance.

What do flies look like?

Size:

1 mm - 75 mm in length.

Body:

Highly variable from long and skinny to short and stout.
Appears soft or hard.

Antennae:

Generally short.
Variable in appearance for example some are thread-like, antler-like, feather-like, or hair-like with or without a swollen base.

Eyes:

Large and either well separated or touching; rarely small or absent.

Mouthparts:

Often absent, or reduced and non-functional.
Functional mouthparts are tube or spongelike adapted for lapping or sucking as well as abrading or piercing surfaces.
Held downward at rest.

Wings:

One pair of functional wings if present.
Forewings membranous, clear with few cross-veins forming long curving cells.
Hindwings replaced by club-like structures called halteres. The halteres are used for balance.
At rest wings are held in a variety of ways depending on the species.

Limbs:

Six legs, usually long and slender.

Abdomen tip:

Cerci (tails) absent.

Where are flies found?

In most places on land and air, in both marine and freshwater environments.
Inside galls.
In the house often associated with food and waste.

What do flies do?

They are solitary but they often group together around food sources and form swarms associated with mating behaviour.
Some are external parasites.
When disturbed they usually fly away, fly and land soon after, or fly away and return soon after. Some may run particularly parasitic flies.
They are generally strong fliers with a rapid wing beat. Many are capable of hovering and most fly with a sharp weaving flight. Some such as crane flies and mosquitoes are much slower flapping fliers that float from side to side like a falling feather. They also buzz or whine in flight.
They feed on liquid or soluble products such as water, animal and plant secretions, liquids from decomposition, sugars solubilised by salivary secretions, and bodily fluids such as blood.
Some form galls.
They are active any time, but some prefer day or night

What looks similar?

Mayflies maybe confused with flies as they have small hindwings or they may only have one pair of wings. Mayflies maybe distinguished as they have two or three long tail filaments and a wing with numerous cross-veins forming many cells.
Wasps, bees or sawflies, unlike flies, have two pairs of membranous wings. However these wings are usually attached with microscopic hooks so that they look like a single pair.
Male scale insects have only one pair of functional wings, with the other pair reduced to haltere-like structures but not club-like. However forewings of male scale insects have only two or three veins in the wings without any cells, and often have a tail of waxy filaments. Flies never have this combination of characters.
Stylops have a club-like forewing and fully functional membranous hindwing (reverse of Diptera).
Some wood-boring beetles (Order Coleoptera, family Lymexylidae): forewings of these wood-boring beetles are reduced to tiny pads and the hindwings are fully exposed which gives them the appearance of possessing only one pair of wings, however it is the reverse of the pattern seen in flies.