Flower-visiting butterflies have color vision, including some sophisticated aspects such as color constancy and simultaneous color contrast. Unlike the trichromatic retinas of humans (blue, green and red cones (plus rods)) and honeybees (UV, blue and green cells), we have found that the compound eyes of butterflies contain six, nine or even fifteen photoreceptor classes in species-specific manner. The eyes of the Japanese yellow swallowtail, Papilio xuthus, contain six classes (UV, violet, blue, green, red and broad-band) of receptors. The six classes of receptors are embedded in small units called ommatidia, each housing nine photoreceptor cells, in three fixed combinations; the Papilio eye is a patchwork of spectrally-heterogeneous ommatidia. For example, one type of ommatidia contains the UV, blue, green and red receptors, together forming a single light-guiding rhabdom. Such organization of rhabdom well explains why Papilio butterflies can discriminate colors of dots whose size correspond to their single pixel. At the molecular level, the Papilio retina expresses five visual pigment opsins. There is therefore no one-to-one relationship between the visual pigments and photoreceptor spectral sensitivities. UV and violet receptors actually share an UV-absorbing visual pigment, while the broad-band receptors coexpress a green-absorbing and a red-absorbing visual pigments, which violates the one receptor-one opsin dogma in vision science. The six classes of receptors are embedded in small units called ommatidia, each housing nine photoreceptor cells, in three fixed combinations; the Papilio eye is a patchwork of spectrally-heterogeneous ommatidia. For example, one type of ommatidia contains the UV, blue, green and red receptors, together forming a single light-guiding rhabdom. Such organization of rhabdom well explains why Papilio butterflies can discriminate colors of dots whose size correspond to their single pixel. At the molecular level, the Papilio retina expresses five visual pigment opsins. There is therefore no one-to-one relationship between the visual pigments and photoreceptor spectral sensitivities. UV and violet receptors actually share an UV-absorbing visual pigment, while the broad-band receptors coexpress a green-absorbing and a red-absorbing visual pigments, which violates the one receptor-one opsin dogma in vision science. Do Papilio use all six for seeing colors, i.e., is Papilio hexachromatic? Trichromatic systems have their best wavelength discrimination ability in two wavelength regions. We found foraging Papilio can discriminate about 1 nm difference in three wavelength regions, which appear even better than in humans. Analysis of the behavioral data using the receptor-noise limited color opponency model indicates that the Papilio color vision is tetrachromatic based on UV, blue, green and red inputs.