Fig. 1. Experimental apparatus. (A) A digital flight simulator comprises a wrap-around cylinder of light emitting diodes (LED). An infrared (IR) beam casts a shadow of the tethered fly's two wings on an optoelectronic sensor that measures instantaneous changes in right and left wing beat amplitude and frequency in response to image motion. (B) Panoramic patterns of vertical stripes move horizontally to elicit compensatory optomotor steering responses. Visual expansion (bottom) differs from visual rotation (top) only in the direction of motion across the rear field of view.