The diptera (di=two and ptera=wings) are flies bearing one pair of wings. The second set of wings is represented by a structure called the halter which resembles a lollipop. The wings supply motion and the halteres act as stabilizers. Flies, unlike bugs and lice, undergo complete metamorphosis, i.e., there is a pupal stage in which the fly is transformed from a wingless maggot-like stage into the winged adult. The sexes are separate. Most female flies are ovoviviparous, i.e., produce eggs, but some produce larvae, e.g., the blow flies, and some even produce a third-stage larva that is ready to pupate, e.g., the tse-tse. Many of the species of flies are of importance because they require a bloodmeal as adults or because the larval stages are parasitic. The diptera can also serve as important vectors of blood-borne diseases.

There are three major groups of flies, the Nematocera which includes the gnats and mosquitoes, the Brachycera, the deer and horse flies, and the Cyclorrhapha, the bot flies and the house, blow, and flesh flies. In the Nematocera and Brachycera, the fly escapes from the pupal case through a “T”-shaped opening in the back of the pupal case; in the Cyclorrhapha, the fly escapes through a round opening in the front of the pupal case. The important Nematocera and Brachycera are those where the adult female requires a blood meal in order to produce eggs. Some of the Cyclorrhapha are important because the adult flies suck blood and others are important because their larval stages are parasitic.