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The Journal of Experimental Biology 206, 1535-1543 (2003)
doi: 10.1242/jeb.00289

Twilight orientation to polarised light in the crepuscular dung beetle Scarabaeus zambesianus

Marie Dacke^1,*, Peter Nordström¹ and Clarke H. Scholtz²

¹ Department of Cell and Organism Biology, University of Lund, Helgonavägen 3, S-223 62 Lund, Sweden
² Department of Zoology and Entomology, University of Pretoria, South Africa

^* Author for correspondence (e-mail: marie.dacke{at}cob.lu.se)

Accepted 12 February 2003

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.

Key words: dung beetle, Scarabaeus zambesianus, orientation, ommatidia, receptor, rhabdom, polarisation pattern, skylight.

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