Fig. 2. Wing and leg responses elicited by an expanding object (recorded as V). In response to a square expanding at a rate of 500° s^-1, the fly generates both wing and leg responses. The time course of stimulus expansion is shown in the bottom traces. If the object is displaced laterally, the inside wing (that on the side of the stimulus) shows a transient increase in wing-beat amplitude, while the outside wing decreases in stroke amplitude. (A) If the object is to the left of the fly, the left wing-beat amplitude (blue) increases while the right wing-stroke amplitude (red) decreases, causing the square to move to the rear of the fly's field of view. In contrast, expansion of centrally positioned objects elicits smaller changes in wing motion, causing little change in the position of the object (B). Image expansion in the frontal field of view elicits leg extension as well as an increase in wing-beat frequency, both indicative of a landing response. When the stimulus is to the right of fly, the sign of the change in both wing-beat responses is reversed, again causing the object to move to the rear of the fly's field of view (C). Laterally positioned image expansion elicits a transient increase in wing-beat frequency but does not evoke a leg response.

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