Polarisation-dependent colour vision in Papilio butterflies

Butterflies of the genus Papilio have polarisation-sensitive photoreceptors in all regions of the eye, and different spectral types of receptor are sensitive to different e-vector orientations. We have studied the consequences of this eye design for colour vision in behavioural tests and find that Papilio spp. see false colours due to the polarisation of light. They discriminate between vertically and horizontally polarised light of the same colour in the contexts of oviposition and feeding. The discrimination depends on the spectral composition of the stimuli. In the blue and probably in the green range, discrimination does not depend on intensity. However, colour discrimination is influenced by polarisation. Thus, colour and polarisation processing are not separated in the visual system of Papilio spp. From these results, we propose hypotheses about which photoreceptors contribute to colour vision in Papilio spp. and what adaptational value such a system might have for the butterflies. Finally, we give examples for other eyes that have a similar structure.