True flies have only one pair of functional wings used for flight. The second pair of wings is reduced to knobs called halteres, which are used for balance. These structures vibrate in time with the wings and act as gyroscopes, enabling the fly to correct its flight attitude if it yaws, rolls, or pitches. Except for male scale insects, only Diptera have hind wings modified into halteres. Many other insects are called flies (e.g., dragonflies, caddisflies, and mayflies), but their wing structures distinguish them from true flies.
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