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.