Printed m Great Britain IS THE LOCUST DCMD A LOOMING DETECTOR?

To a prey, a salient visual characteristic of an approaching predator is 'looming', a figure of average subtended visual angle increasing as some function of time. If the approach is at constant velocity, two important characteristics of the looming figure are an ever increasing velocity (i.e. acceleration) of sideways motion of edges away from the centroid of the figure, and the opposed motion of opposite edges. These characteristics make looming objects distinct from translation of single objects across the visual field, so that the visual looming characteristics may have value in warning of approaching predators. In the locust, a visual neurone, the DCMD (descending contralateral movement detector) in the ventral nerve cord forms an important input to the interneurones triggering the jump response in the locust (Pearson, Heitler & Steeves, 1980; Pearson & Robertson, 1981). Thus it is of considerable interest to determine whether the DCMD responds preferentially to looming visual stimuli. The responses of the DCMD have been characterized quantitatively and in considerable detail (Palka, 1967; Rowell, O'Shea & Williams, 1977; Pinter, 1977, 1979). The DCMD responds vigorously to translation movements across the visua