- published: 16 May 2016
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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.
The presence of a single pair of patent, metathoracic flight wings distinguishes true flies from other insects with "fly" in their names, such as mayflies, dragonflies, damselflies, stoneflies, whiteflies, fireflies, alderflies, dobsonflies, snakeflies, sawflies, caddisflies, butterflies or scorpionflies. Some true flies have become secondarily wingless, especially in the superfamily Hippoboscoidea, or among those that are inquilines in social insect colonies.
Some authors draw a distinction in writing the common names of insects. True flies are written as two words, such as crane fly, robber fly, bee fly, moth fly, and fruit fly. In contrast, common names of nondipteran insects that have "fly" in their names are written as one word, e.g., butterfly, stonefly, dragonfly, scorpionfly, sawfly, caddisfly, whitefly. In practice, however, this is a comparatively new convention, and, especially in older books, one commonly might see the likes of "saw fly" and "caddis fly". In any case, non-entomologists cannot in general be expected to tell dipterans, "true flies", from other insects.