What is the primary characteristic of insects in the order Diptera?

The primary characteristic of insects in the order Diptera is their possession of two pairs of wings. Insects within this order, commonly referred to as flies, are distinctive for having one pair of functional wings, while the second pair has evolved into a set of small structures known as halteres. These halteres play a crucial role in balance and coordination during flight. This unique wing structure is a defining feature of Diptera and sets them apart from other insect orders that typically have three pairs of legs and two pairs of functional wings.

Insects with hard exoskeletons or compound eyes may not be exclusive to Diptera, as these characteristics are common among many different insect groups. The presence of three pairs of legs is characteristic of all insects, making it a trait that does not specifically identify Diptera. Thus, the characteristic of having two pairs of wings is what distinctly categorizes Diptera within the insect classification.