Why ‘false’ colours are seen by butterflies

A combination of colour and polarized reflections helps them to choose oviposition sites. Light can be described by its intensity, spectral distribution and polarization, and normally a visual system analyses these independently to extract the maximum amount of information1. Here I present behavioural evidence that this does not happen in butterflies, whose choice of oviposition substrate on the basis of its colour2 appears to be strongly influenced by the direction of polarization of the light reflected from the substrate. To my knowledge, this is the first record of ‘false’ colours being perceived as a result of light polarization. This detection of false colours may help butterflies to find optimal oviposition sites.