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Most flying insects have four wings, but flies have only two. The rear pair of wings appears to have evolved into (5) special movement sensors called halteres. Each haltere is a rod with a weight at the end and hundreds of sensory organs at the base.

During flight, the halteres vibrate (6) up and down. If the fly turns left or right, up or down, or tilts either way (7), the halteres immediately sense the change and complex nerve circuits (8) adjust many tiny wing muscles.