Binocularity in insects is generally assumed to have the same function as in many vertebrates—the perception of depth. Evidence for this hypothesis stems from the observation that one-eyed dragonfly larvae, tiger beetles, praying mantids and water scorpions rarely catch prey1–5 but no definitive evidence is available. Depth perception and catching behaviour depend on visual attention and visual behaviour and it is difficult to assess what is impaired when one eye is occluded6. A more promising approach to studying the importance of binocular disparity is one that does not interfere with normal binocular vision, and allows potential monocular depth cues to be controlled carefully. These criteria were met in the present study by the use of prismatic lenses placed in front of the compound eyes of the praying mantis, thus creating a conflict between binocular disparity and monocular cues. The results demonstrate that mantids do indeed rely on binocular triangulation when estimating the distance of prey, and thus provide the first unequivocal evidence for stereoscopic vision in an invertebrate.