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Fig. 3. Chasing a realistically moving target by the virtual blowfly. The gain factor for retinal velocity input relative to the fixation controller is varied. The data shown in A–D are from a virtual blowfly using the `position-only servo', i.e. the virtual blowfly steers its flight direction only by minimising the error angle. (A) Trajectory of the virtual fly chasing the target (plotted as in Fig. 2A). After sharp turns of the target, the virtual fly makes saccade-like turns, but tends to overshoot the target and then makes a correctional movement. This behaviour leads to a curved path and fluctuations of the error angle (B) and yaw velocity (D). The yaw orientation (C) of the virtual fly changes in a step-like manner similar way to that seen in real flies (Fig. 1A). If the virtual blowfly uses a `position-plus-velocity servo', i.e. it uses both position and velocity information, (G_v=0.0015), flight performance is stabilised by reducing the overshooting of the target (E–H). Saccade-like turns are characterised by brief yaw velocity peaks (arrows in D,H). Increasing the gain of the velocity signal to higher values (G_v=0.025) leads to rather smooth flight trajectories and an elimination of saccade-like turns. This chasing performance differs greatly from the flight trajectories of real flies (I–L).

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