True flies are insects of the order Diptera, the name being derived from the Greek di = two, and ptera = wings. Insects of this order use only a single pair of wings to fly, the hind wings being reduced to club-like balancing organs known as halteres.

Flies have a mobile head, with a pair of large compound eyes, and mouthparts designed for piercing and sucking (mosquitoes, black flies, and robber flies), or for lapping and sucking in the other groups. Their wing arrangement gives them great maneuverability in flight, and claws and pads on their feet enable them to cling to smooth surfaces. Flies undergo complete metamorphosis; the eggs are laid on the larval food-source and the larvae, which lack true limbs, develop in a protected environment, often inside their source of their food. The pupa is a tough capsule from which the adult emerges when ready to do so; flies mostly have short lives as adults.