Twilight orientation to polarised light in the crepuscular dung beetleScarabaeus zambesianus

Abstract

The polarisation pattern of skylight offers many arthropods a reference for visual compass orientation. The dung beetle Scarabaeus zambesianus starts foraging at around sunset. After locating a source of fresh droppings, it forms a ball of dung and rolls it off at high speed to escape competition at and around the dung pile. Using behavioural experiments in the field and in the laboratory, we show that the beetle is able to roll along a straight path by using the polarised light pattern of evening skylight. The receptors used to detect this skylight cue can be found in the ommatidia of the dorsal rim area of the eye, whose structures differ from the regular ommatidia in the rest of the eye. The dorsal rim ommatidia are characterised by rhabdoms with microvilli oriented at only two orthogonal orientations. Together with the finding that the receptors do not twist along the length of the rhabdom, this indicates that the photoreceptors of the dorsal rim area are polarisation sensitive. Large rhabdoms, a reflecting tracheal sheath and a lack of screening pigments make this area of the eye well adapted for polarised light detection at low light levels. The fan-shaped arrangement of receptors over the dorsal rim area was previously believed to be an adaptation to polarised light analysis, but here we argue that it is simply a consequence of the way that the eye is built.

Polarization-Sensitive Descending Neurons in the Locust: Connecting the Brain to Thoracic Ganglia

“…showed that polarization-sensitive (POL-) neurons connect the central complex to the lateral accessory lobes and the posterior protocerebrum, brain areas that are innervated by intersegmental neurons (Williams, 1975;Strausfeld and Seyan, 1985;Okada et al, 2003). Although polarization-mediated behavior in insects has been studied extensively (bees and ants: Wehner and Rossel, 1985;Wehner, 2003;crickets: Brunner and Labhart, 1987;Henze and Labhart, 2007;flies: von Philipsborn and Labhart, 1990; beetles: Dacke et al, 2003;locusts: Mappes and Homberg, 2004;butterflies: Reppert et al, 2004), the anatomical and functional connections from POL-neurons in the brain to thoracic motor centers have not been explored in any species.…”

Night vision by cuttlefish enables changeable camouflage

“…Despite the fact that many animals sacrifice colour vision for increased sensitivity to low light, colour vision has been found in nocturnal hawkmoths and geckoes, as well as in some deep-sea fish and cephalopods, which opens the possibility that colour vision under low-light conditions might not be as unusual as previously thought (Denton and Locket, 1989;Kelber et al, 2002;Partridge et al, 1988;Roth and Kelber, 2004). Other remarkable adaptations include the use of polarized moon and night-time sky light for navigation in some insects (Dacke et al, 2003a;Dacke et al, 2003b) and the body curling behaviour of mesopelagic animals in response to threatening stimuli in dim light (Robison, 1999). Considering how successful these and many other nocturnal and deep-sea animals are, the physiological and morphological adaptations that go alongside their low-light active life styles are perhaps not surprising (although certainly impressive).…”

It's Not a Bug, It's a Feature: Functional Materials in Insects

“…[369,374] Crickets have developed a prominent dorsal area, recognizable by smooth facet lenses and ommatidia that are devoid of screening pigment that feature exclusively blue-sensitive photoreceptors. [379] African dung beetles [380,381] and bees [382,383] navigate using the polarization pattern created by of the sun, the moon or the milky way on cloudless nights. [2] …”