Celestial Orientation in Dim Light

The polarization pattern of skylight offers many animals a reference for visual compass orientation. In those cases when this optical compass is identified, the receptors involved are often confined to a small part of an eye built predominantly for other visual tasks. This thesis reports the discovery of a unique compass organ in the spider Drassodes cupreus, where a pair of specialised secondary eyes co-operate to analyse skylight polarisation. These eyes have no refracting lens and thus can not form images. Moreover, the eyes use a built-in polariser to precisely determine the direction of polarisation. The spiders become active around sunset and use polarisation cues to find their way back to their nests after foraging trips. Measurements using a model eye indicate that the compass organ is best suited for navigation at twilight. The lack of a lens makes it possible for each receptor of the eye to collect light from a large region of the sky. This makes the compass organ well adapted to polarised light detection at low light levels. A comparative study of the eyes of several spider families suggests that this compass organ may not be an isolated phenomenon.

The dung beetle Scarabaeus zambesianus also starts to forage around sunset. After locating a source of fresh animal droppings it forms a ball of dung and rolls off at high speed along a straight path to escape competition at and around the dung pile. Behavioural experiments in the field and in the laboratory, clearly show that the beetle is able to roll straight by orientating to the polarised light pattern of a twilight sky, as well as to the polarisation of a moon-lit sky. This is the first report of an animal using the polarisation pattern of the moon for orientation. The nocturnal polarised light pattern shows no significant difference in its structure from that of the pattern of polarised light formed around the sun. The two patterns do, however, differ in intensity by about one million times. The nocturnal polarised light detection system of this beetle is also similar to that of day-active insects, but with an increased sensitivity to light. Large rhabdoms, a reflecting tracheal sheet and a lack of screening pigments make the dung beetle eye well adapted for polarised light detection at low light levels.