Unilateral range finding in diving beetle larvae.

One of the biggest challenges that predators, such as the larvae of the diving beetle Thermonectus marmoratus (Coleoptera: Dytiscidae), are faced with is to accurately assess the distance of their prey. Most animals derive distance information from disparities of images that are viewed from different angles, from information that is obtained from well-controlled translational movements (motion parallax) or from the image size of known objects. Using a behavioral assay we demonstrated that T. marmoratus larvae continue to accurately strike at artificial prey, even if none of these typical distance estimation cues are available to them. Specifically, we excluded bilateral binocular stereopsis by occlusion, confounded possible motion parallax cues with an artificially moving prey, and excluded the possibility that beetle larvae simply approached their targets based on known prey size by presenting different prey sizes. Despite these constraints, larvae consistently struck our artificial targets from a distance of ~4.5 mm. Based on these findings we conclude that T. marmoratus likely employ an unusual mechanism to accurately determine prey distances, possibly mediated by the object-distance-dependent activation of specific subsets of their many-tiered and peculiarly positioned photoreceptors.