Flow-field variables trigger landing in flies

The eyes of moving animals experience a continuously changing flow of stimulation. Information contained in this ‘optical flow field’ can be extracted to control movement1–4. I present here evidence that pre-landing deceleration in flies is determined by such information: onset of deceleration is triggered when the ratio of the image expansion of a target on the retina to the image size (‘relative retinal expansion velocity’, RREV) reaches a critical value.