Foraging butterflies have been believed to have color vision. But there have been no convincing proof of their color vision, until we demonstrated it in the Japanese yellow swallowtail butterflies, Papilio xuthus, in 1999. Naive Papilio butterflies can be trained to take sucrose solution on a paper disk of certain color or on a monochromatic light projected on a screen. By using this beahavioral response, we first tested two points; color vision and color onstancy. The Papilio butterflies that learened a certain color easily discriminated the training color not only from other colors, but also from differnt shades of grays. Color constancy was tested by changing the color of illumination: Pailio butterflies could discriminate the correct color in color Mondrian pattern under illuminations of various colors, beacuse of their color constancy.
Next we tried to correlate the color vision performance with the structure of the compound eye. Here we tested two more points; the action spectrum of foraging behavior and the size limit for color detection. The action spentrum of foraging behavior exihibits high sensitivity at wavelength regions around 380, 500, and 600 nm. As indicated by the variety of spectral receptors contained in the Papilio retina, which are UV, violet, blue, green, red and broad-band receptors, it was first convincingly demonstrated that their visible wavelength range covered at least from 360 to 680 nm: this is much wider than that of humans. The size limit of color detection was measured by using a Y-maze apparatus. We trained Papilio to visit a disk of certain color, and then subjected them to the tests where they have to choose one of the two targets, colored or gray, each presented in one of the arms of the Y-maze. It turned out that Papilio could discriminate the color of the target until the size was decreased down to about 1 degree. One degree of the visual angle corresponds to the interommtidial angle that determines spatial resolution of the compound eye. This means that Pailio butterlfies can detect colors of the targets whose size is close to their spatial resolution, which is not the case in humans. This is probably due to the fact that one ommatidium contains at least two classes of spectral receptors.

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