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First published online July 20, 2006
Journal of Experimental Biology 209, 2873-2879 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02311
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Color discrimination at the spatial resolution limit in a swallowtail butterfly, Papilio xuthus

Yuichi Takeuchi*, Kentaro Arikawa and Michiyo Kinoshita{dagger}

School of Advanced Sciences, The Graduate University for Advanced Studies, Hayama 240-0193, Japan

{dagger} Author for correspondence (e-mail: kinoshita_michiyo{at}soken.ac.jp)

Accepted 4 May 2006

Spatial resolution of insect compound eyes is much coarser than that of humans: a single pixel of the human visual system covers about 0.008° whereas that of diurnal insects is typically about 1.0°. Anatomically, the pixels correspond to single cone outer segments in humans and to single rhabdoms in insects. Although an outer segment and a rhabdom are equivalent organelles containing visual pigment molecules, they are strikingly different in spectral terms. The cone outer segment is the photoreceptor cell part that expresses a single type of visual pigment, and is therefore monochromatic. On the other hand, a rhabdom is composed of several photoreceptor cells with different spectral sensitivities and is therefore polychromatic. The polychromatic organization of the rhabdom suggests that insects can resolve wavelength information in a single pixel, which is an ability that humans do not have. We first trained the Japanese yellow swallowtail butterfly Papilio xuthus to feed on sucrose solution at a paper disk of certain color. We then let the trained butterflies discriminate disks of the training color and grey disks each presented in a Y-maze apparatus. Papilio correctly selected the colored disk when the visual angle was greater than 1.18° for blue, 1.53° for green or 0.96° for red: they appeared to see colors in single pixels to some extent. This ability may compensate their rather low spatial resolution.

Key words: color vision, detection, rhabdom, compound eye, Papilio xuthus






© The Company of Biologists Ltd 2006