A visually evoked escape response of the housefly

Summary Flies (Musca domestica) avoid danger by initiating a rapid jump followed by flight. To identify the visual cues that trigger the escape response in the housefly, we measured the timing and probability of escapes when the fly was presented with a variety of visual stimuli created by moving targets toward it. Our results show that an escape response is triggered by an approaching dark disk, but not by a receding dark disk. On the other hand, a bright disk elicits escape only when it recedes. A disk with black and white rings is less effective at eliciting escape than is a dark solid disk of the same size. This indicates that the darkening contrast produced by an approaching stimulus is a more crucial parameter than expansion cues contained in the optical flow. Escape is also triggered by a horizontally moving dark edge, but not by a moving bright edge or by a grating. An examination of several visual parameters reveals that the darkening contrast, measured from the onset of stimulation to the start of escape is nearly constant for a variety of stimuli that trigger escape reliably. Thus darkening contrast, coupled with motion may be crucial in eliciting the visually evoked escape response. Other visual parameters such as time-to-contact or target angular velocity seem to be relatively unimportant to the timing of escapes.Abbreviations P_s Probability of successful escape - r_disk radius of disk target - r_arena radius of shielding arena - v_disk linear velocity of disk target - v_edge linear velocity of edge - d_disk angular velocity of disk target boundary - _edge angular velocity of edge - _escape target distance at escape - d_start target distance before onset of target movement - h_edge height of the edge above fly - x_start distance from corner of triangle to start position of edge (0 or 50 mm) - x_escape distance from corner of triangle to the position of the edge when the fly escapes - x_center distance from corner of triangle to point above the center of the pad - x_total distance from the corner of the triangle to the base (height of triangle = base of triangle)