What makes a locust jump when a shadow—say, of a hand—passes over it? One of the most thoroughly studied of all insect neurones is the descending contralateral movement detector (DCMD) of the locust. The DCMD runs from the "brain" to the metathoracic ganglion, and responds to movement in the visual field. These experiments use a female locust with legs and wings removed which is secured by the dorsal surface of the thorax to the shaft of the electrodes by means of beeswax. The locust’s field of vision is restricted, visual stimulus provided by passing a circular spot on a strip of white paper in front of the locust’s eye, and the locust’s response measured. The experiments show the effect of different speeds of movement of a spot across the locust’s visual field. The resulting data provides a useful index of the behavior of locusts as demonstrated in the capacity to respond to different sizes of spots and different speeds. (15 minutes)