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True flies are insects of the order Diptera (from the Greek di = two, and ptera = wings). They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax. Apart from secondarily flightless insects (including some flies), the only other order of insects with any form of halteres are the Strepsiptera, and theirs are on the mesothorax, with the flight wings on the metathorax.