The ant’s celestial compass system: spectral and polarization channels

Summary

Ants as well as bees derive compass information not only from the direct light of the sun, but also from the scattered light in the sky. In the present account, the latter phenomenon is described for desert ants, genus Cataglyphis. Due to the scattering of sunlight by the air molecules of the earth’s atmosphere, spatial gradients of polarization, spectral composition and radiant intensity extend across the celestial hemisphere. All of these optical phenomena are exploited by the Cataglyphis navigator. Here I concentrate on the use Cataglyphis makes of the polarization and spectral skylight gradients. Either type of information is neurally processed by a separate sensory channel receiving its input from a separate part of the retina. These channels are characterized and their possible interactions are analyzed in a variety of behavioural experiments, in which ants, whose compound eyes are partially occluded by light-tight caps, are presented with spatially restricted and spectrally altered parts of the celestial hemisphere. It is discussed whether skylight patterns are used by the insect navigator simply to read a reference direction (e.g., the azimuthal position of the solar meridian) from the sky, or whether they are used to determine any particular point of the compass. Different approaches to examine these questions - behavioural and neuro-physiological analyses, computer simulations and robotics - are described, and results obtained by these approaches are reported. New ways of portraying the pattern of polarized light in the real sky are presented in Figures 2 (lower part) and 3, and Figure 22 introduces an autonomous agent navigating by polarized skylight. Conceptually, the last paragraph of this chapter provides my most general conclusions drawn from the analyses of the insect’s skylight compass.