The order of insects containing true flies. True flies of the order Diptera (di = two, and ptera = wings) possess a pair of wings on the mesothorax. The presence of a single pair of wings distinguishes true flies from other insects with "fly" in their name (such as mayflies).
Diptera is a large order, containing an estimated 240,000 species of mosquitos, gnats, midges and others, although under half of these (about 120,000 species) have been described.
Definition adapted from URL: http://en.wikipedia.org/wiki/Diptera