Dipterans, otherwise known as "flies", are usually vectors of protozoan or helminth parasites. A few species are parasitic.
Adults mate and females produce eggs. A maggot-like larva emerges from the egg and feeds. Eventually the larva pupates and metamorphoses into an adult. In some species the adult is parasitic and development of the other stages takes palce off the host. In other species, the egg is deposited on the host and the larva is parasitic.
Dipteran adults have wings, although the wings may be lost after becoming parasitic. The posterior pair of wings is replaced by a pair of projections called halteres. Although non-functional as wings, the halteres are important sensory structures that aid in balance.