Praying mantids determine prey-capture distances with the aid of binocular mechanisms and estimate distances to stationary objects such as jump targets with the aid of retinal image velocity. The latter mechanism makes use of the fact that the retinal image velocity resulting from translational self-motion (peering movements) is inversely proportional to the distance of the target object. The present study shows that for South African mantids Polyspilota sp., the interocular distance, an aspect of binocular vision, and the amplitude and velocity of peering movements, increase over the course of postembryonic development from 2^nd instar to adult. In both cases there is a close positive correlation with body size. Calculated convergence angles and retinal image velocities indicate an improvement in absolute depth perception with increased body size. This is adaptive, because it allows the range of visual depth perception to increase with body size, strike distance and jumping capability, throughout the growth of the insect, from 2^nd instar to adult.