DICKINSON, M.H.: The structure and function of flight trajectories in freely-flying fruit flies.

Without buoyancy or a solid horizontal plane for support, stability is of crucial importance to flying animals. Further, for animals that rely on spatially complex sensory cues for food search, mating, or predator avoidance, stability is required not only to keep the locomotor machinery in an appropriate orientation, but also to keep body oscillations from contaminating sensory signals. My laboratory has been studying the flight behavior of fruit flies, highly maneuverable organisms that rely on rapid sensory-motor feedback, and not passive mechanisms, for their robust stability. The flight paths of fruit flies consist of straight sequences interspersed with rapid saccadic turns, during which the animal quickly changes its heading by roughly 90 degrees. While flies are thought to rely heavily on visual feedback in flight control, evidence suggests that during the straight flight sequences they fly under the exclusive control of their halteres, mechanosensory structures that act as an inertial guidance system. Since mechanosensory cells are capable of faster response dynamics than photoreceptors, feedback from the halteres can tune the stability reflexes much more sharply than could feedback from the eyes. The saccades appear to represent fixed action patterns, the magnitude of which does not depend upon prior visual experience. The visual system does, however, determine the direction and timing of saccades. During straight flight, the visual system appears to integrate optic expansion, triggering a collision avoidance saccade when the expansion reaches a critical threshold. The role of the straight flight sequences may be to stabilize gaze during this integration process in order to minimize the errors introduced by body oscillations. Thus, the involuntary inertial stability system may play an important role in voluntary, visually-guided behaviors.