Perceiving motion-in-depth is essential to detecting approaching
or receding objects, predators and prey. This can be achieved
using several cues, including binocular stereoscopic cues such
as changing disparity and interocular velocity differences, and
monocular cues such as looming. Although these have been
studied in detail in humans, only looming responses have been well
characterized in insects and we know nothing about the role of
stereoscopic cues and how they might interact with looming cues. We
used our 3D insect cinema in a series of experiments to investigate
the role of the stereoscopic cues mentioned above, as well as
looming, in the perception of motion-in-depth during predatory strikes
by the praying mantis Sphodromantis lineola. Our results show that
motion-in-depth does increase the probability of mantis strikes but
only for the classic looming stimulus, an expanding luminance edge.
Approach indicated by radial motion of a texture or expansion of a
motion-defined edge, or by stereoscopic cues, all failed to elicit
increased striking. We conclude that mantises use stereopsis to
detect depth but not motion-in-depth, which is detected via looming.