Whether circling garbage cans or cruising through alpine meadows, insects impress us with their agile aerial acrobatics. What external sensory cues and internal physiological processes govern when and where insects change direction? At the center of flight control behavior is a continuous feedback loop by which sensory information from several modalities modify motor patterns that in turn alter wing kinematics and the production of aerodynamic forces. We are currently attempting to dissect this control system in the fruit fly, Drosophila melanogaster, by using an automated free flight tracking arena. The stereo system of the arena operates in the infrared, so that the tracking procedure does not interfere with the visual system of the flies. By tracking the flies within a large arbitrarily-structured arena it is possible to reconstruct what a fly 'sees' as it moves through a structured visual landscape. We are using these data in an attempt to determine how optic flow influences saccade rate, path velocity, altitude, and other aspects of the free flight behavior. The flight paths of fruit flies consist of straight sequences interspersed with rapid saccadic turns, during which the animal changes its heading by approximately 90 degrees. While the temporal and spatial distribution of saccades may appear quite stochastic, after reconstructing the visual input it is possible to link nearly every saccade turn to a point when the rate of expansion within one visual field passed a critical threshold. After each saccade, flies attempt to stabilize sharp contrast edges resulting in the straight flight sequences. As they approach an object, however, the asymmetrically looming object elicits first a decrease in forward velocity and then a rapid collision-avoidance saccade. The net result is that animals fly slowly and saccade frequently within dense clusters of objects, whereas they fly rapidly and saccade less frequently within open landscapes (Figure 1). Attractive chemical odors appear to raise the threshold for collision avoidance responses and lower the threshold for landing responses. The influence of vision and odor combine to produce a quite robust and efficient search behavior.

Figure 1. Birds-eye-view of free flight trajectories of fruit flies in three different visual landscapes. The trajectories each show a single flight sequence from one individual fruit fly. (A) Flies saccade close to the wall when the cylindrical arena is uniformly white with a dark horizon. (B) Flies turn more frequently when the arena has a textured high contrast background. (C) In the presence of four vertical bars (red circles) flies restrict their flight to the center of the arena. The arena is 1 m in diameter and 0.5 m high.

In conjunction with the free flight arena, we use a tethered flight simulator for measuring changes in wing stroke kinematics or flight forces while reconstructing or manipulating the sensory stimuli that the animals experience under free-flight conditions. The device consists of an electronic visual panorama that may be mounted within a rotational gimble and equipped with an odor-laden air stream. This apparatus is used to elucidate the interactions among the vision, olfaction, and gyroscopic (haltere) senses - three critical modalities in flight control. We are currently using data gathered from free flight and tethered flight to construct a comprehensive mathematical control model of the flight system. The goal is to develop a rigorous analytical framework with which we can test and modify increasingly complex models of flight control behavior.

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