One of a pair of modified wings in flies and other insects, used for balance during flight.
From Greek 'altera' (dumbbell) or Latin 'alteres', referring to the weight-like appearance. The modern spelling uses the h- from confusion with halter or influence from 'halt'.
Flies literally have gyroscopes in their wings—these tiny halteres vibrate millions of times per second to keep the insect stable in flight. Without them, flies would be as clumsy as helicopters.
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