True flies are insects of the order Diptera (from the Greek di = two, and ptera = wings). The most obvious distinction from other orders of insects is that a typical fly possesses a pair of flight wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax. (Some species of flies are exceptional in that they are secondarily flightless.) The only other order of insects bearing two true, functional wings plus any form of halteres are the Strepsiptera, and in contrast to the flies, the Strepsiptera bear their halteres on the mesothorax and their flight wings on the metathorax.
Fruit flies get their common name from their small size and fondness of some fruits. Small fruit flies are nuisance pests, but may act as disease vectors.
Horse flies likely received their common name because they are notorious pests of horses and other mammals. They are commonly found in both suburban and rural areas near bodies of water, which serve as breeding sites, and where mammal hosts are most abundant.
House flies get their name from being the most common fly found around homes. Adult house flies can grow to one-quarter of an inch long and usually live between 15 and 25 days.