The true flies are one of the largest of the insect orders, and also the most ecologically diverse.

The main characteristic of the true flies are the hind wings, which have been modified into two small, club-like halteres, which serve as gyroscopes when the insect flies. The fore wings are functional, and are usually membraneous and transparent with an often rather simple venation. The head is well developed, and most flies have large compound eyes which give excellent vision. Mandibles are lacking, and the mouth parts are modified for either sucking or piercing. The larvae are very diverse and can be found in most habitats. A common trait for all fly larvae is that the thoracic legs are lacking.