Unilateral range finding in diving beetle larvae

Abstract

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.

Multitasking in an eye: How the unusual organization of the principal larval eyes of Thermonectus marmoratus allows for far and near vision and might aid in depth perception

“…Specifically, we found that E1 is well adapted for far vision while E2 better serves near vision, and that, during prey approach, objects tend to be in their near field, allowing for images to be focused onto distinct retinal layers. The latter organization provides a good optical basis for an unusual range-finding mechanism, and may explain recent behavioral findings (Bland et al, 2014) that T. marmoratus larvae can successfully gauge distances even when conventional rangefinding mechanisms are unavailable.…”

“…The eye organization of T. marmoratus may support unilateral range finding Because T. marmoratus strike prey from a relatively constant distance (Bland et al, 2014), they must have a suitable mechanism to gauge prey distance. In insects, only a few such mechanisms are well described (Collett and Harkness, 1982).…”

“…The Journal of Experimental Biology (2014) discovered that T. marmoratus larvae are able to correctly estimate distances, even when none of these mechanisms were available to them (Bland et al, 2014), raising the question of what alternative cues they could be using.…”

“…While tracking, the larvae perform scanning movements, which extend the vertically narrow visual fields of these eyes (Buschbeck et al, 2007). Immediately prior to striking, larvae are able to maintain a constant distance to their prey (which occasionally even requires swimming backwards), even if the latter continues to move (Bland et al, 2014;Buschbeck et al, 2007). This behavior, in combination with their ability to maintain typical striking distances when commonly known range-finding mechanisms (Howard, 2012) are severely limited or excluded, suggests that larvae have an unusual mechanism to gauge prey distance (Bland et al, 2014).…”

“…Immediately prior to striking, larvae are able to maintain a constant distance to their prey (which occasionally even requires swimming backwards), even if the latter continues to move (Bland et al, 2014;Buschbeck et al, 2007). This behavior, in combination with their ability to maintain typical striking distances when commonly known range-finding mechanisms (Howard, 2012) are severely limited or excluded, suggests that larvae have an unusual mechanism to gauge prey distance (Bland et al, 2014). Specifically, in these experiments on second instars, horizontally moving artificial prey against a homogeneous background confounded potential motion parallax cues.…”