@article{oai:ir.soken.ac.jp:00004020,
 author = {越髙, 久晴 and KOSHITAKA, Hisaharu and KINOSHITA, Michiyo and VOROBYEV, Misha and ARIKAWA, Kentaro},
 issue = {1637},
 journal = {Proceedings of the Royal Society B: Biological Sciences, Proceedings of the Royal Society B: Biological Sciences},
 month = {Apr},
 note = {This paper presents the first evidence of tetrachromacy among invertebrates. The Japanese yellow swallowtail butterfly, Papilio xuthus, uses colour vision when foraging. The retina of Papilio is 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 foraging Papilio. We trained Papilio to take nectar while seeing a certain monochromatic light. We then let the trained Papilio choose between two lights of different wavelengths and determined the minimum discriminable wavelength difference Δλ. The Δλ function of Papilio has three minima at approximately 430, 480 and 560 nm, where the Δλ values approximately 1 nm. This is the smallest value found for wavelength discrimination so far, including that of humans. The profile of the Δλ function of Papilio can be best reproduced by postulating that the UV, blue (narrow-band and wide-band), green (double-peaked) and red classes are involved in foraging. Papilio colour vision is therefore tetrachromatic.},
 pages = {947--954},
 title = {Tetrachromacy in a butterfly that has eight varieties of spectral receptors},
 volume = {275},
 year = {2008}
}