Color sensors of butterflies

Abstract

Colorful butterflies are strongly visual animals, and have sophisticated color vision. The compound eyes of the Japanese yellow swallowtail butterfly, Papilio xuthus, contain ultraviolet, violet, blue, green, red and a broad-band class of receptors, which are embedded in the ommatidia in three fixed combinations. The eyes are, therefore, a mesh of three types of spectrally-heterogeneous ommatidia. Given that the eyes of Papilio are equipped with six classes of spectral receptors, their color vision may be hexachromatic. The foraging Papilio can only discriminate a one nanometer difference at three wavelength regions around 430, 480 and 560 nm, indicating that their color vision is actually tetrachromatic. The noise-limited color opponency model has predicted that the tetrachromacy is based on the ultraviolet, blue, green and red receptors. These receptors are contained together in at least one type of ommatidia, where they form a single rhabdom. In principle, such an organization makes the butterfly able to discriminate colors at the single-pixel level. In fact, a foraging butterfly can discriminate between a colored disk and a gray one at their visual subtense angle of around 0.7-1.0 degree, which is close to the limit of the spatial resolution predicted for their eye optics.