Tetrachromacy in a butterfly that has eight varieties of spectral receptors

Abstract

This paper presents the first evidence of tetrachromacy among invertebrates. The Japanese yellow swallowtail butterfly,Papilio xuthus, uses colour vision when foraging. The retina ofPapiliois furnished with eight varieties of spectral receptors of six classes that are the ultraviolet (UV), violet, blue (narrow-band and wide-band), green (single-peaked and double-peaked), red and broad-band classes. We investigated whether all of the spectral receptors are involved in colour vision by measuring the wavelength discrimination ability of foragingPapilio. We trainedPapilioto take nectar while seeing a certain monochromatic light. We then let the trainedPapiliochoose between two lights of different wavelengths and determined the minimum discriminable wavelength difference Δλ. The Δλfunction ofPapiliohas three minima at approximately 430, 480 and 560 nm, where the Δλvalues approximately 1 nm. This is the smallest value found for wavelength discrimination so far, including that of humans. The profile of the Δλfunction ofPapiliocan be best reproduced by postulating that the UV, blue (narrow-band and wide-band), green (double-peaked) and red classes are involved in foraging.Papiliocolour vision is therefore tetrachromatic.