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 se nsitive to different e-vector orientations. We have studied the consequence s of this eye design for colour vision in behavioural tests and find that P apilio spp. see false colours due to the polarisation of light. They discri minate between vertically and horizontally polarised light of the same colo ur in the contexts of oviposition and feeding. The discrimination depends o n the spectral composition of the stimuli. In the blue and probably in the green range, discrimination does not depend on intensity. However, colour d iscrimination is influenced by polarisation. Thus, colour and polarisation processing are not separated in the visual system of Papilio spp. From thes e results, we propose hypotheses about which photoreceptors contribute to c olour vision in Papilio spp. and what adaptational value such a system migh t have for the butterflies. Finally, we give examples for other eyes that h ave a similar structure.