Predation via motion parallax in one of two gleaning insects.

A predator's survival is highly dependent on correctly deciding whether to attack potential prey. Pursuit predators, for example, can estimate the size of a moving target from the ratio between its angular speed and size. Such heuristic rules are not available, however, when ambushing stationary prey. Here, we investigated how pixie robber flies (Psilonyx annulatus) and damselflies (Ischnura posita) hunt stationary prey using different sensory strategies, relating to their marked differences in eye morphology. We show that pixie robber flies assess prey using whole-body translational movements. During this assessment, the prey is outside the pixie robber fly's stereopsis range, yet attacks are launched from a distance dictated by absolute, not angular, prey size. These findings suggest that pixie robber flies use motion parallax to infer three-dimensional cues, such as prey distance and/or size, before attacking. Motion parallax may be particularly suitable for pixie robber flies as they hunt in cluttered, low-lighting conditions and have a small size, making it difficult for potential prey to detect their movement, even in close proximity. Damselflies probably rely on alternative processes to assess prey, as translational movements are absent in the assessment phase.