Fly Identifier

Flies belong to the order Diptera, meaning "two wings," which is their most distinctive characteristic. With over 150,000 described species, Diptera is one of the largest insect orders. Unlike most flying insects that have four wings, true flies have only one pair of functional flight wings; the hindwings are reduced to small knob-like structures called halteres that act as gyroscopic balance organs.

The single pair of wings is the easiest way to distinguish true flies from other insects that are commonly called flies, such as mayflies, dragonflies, and sawflies, all of which have four wings. Fly wings are typically transparent and membranous, with a pattern of veins that is important for classification.

Head and eye structure are valuable identification features. Many flies have very large compound eyes that occupy most of the head. In some families, the eyes meet on top of the head in males but are separated in females. Hoverflies, robber flies, and blowflies each have characteristic eye sizes and positions.

Mouthpart type varies dramatically and aids identification. House flies have sponging mouthparts for absorbing liquid food. Horse flies have blade-like mandibles for cutting skin. Mosquitoes (see our Mosquito Identifier tool) are a specialized fly family with a long, thin piercing proboscis. Robber flies have a strong stabbing beak for catching prey.

Body shape and size range enormously. Crane flies are large and slender with very long legs, resembling oversized mosquitoes. Fruit flies are tiny at 2-4 mm. Hover flies often mimic bees and wasps with yellow and black patterns. Flesh flies are stout and gray with checkered abdominal patterns.

Coloration ranges from plain gray and brown to metallic blue, green, and copper in blowflies and bottle flies. Some hover flies display remarkably accurate mimicry of bees and wasps, complete with yellow and black banding patterns. Wing markings, when present, can also aid identification, as seen in picture-winged flies.

Behavior provides helpful context. Hover flies are recognized by their ability to hover in place. Robber flies are seen catching prey in mid-air. Crane flies have a characteristic bouncing flight pattern.