title: Color discrimination at the spatial resolution limit in a swallowtail butterfly, Papilio xuthus.
abstract: 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 degrees whereas that of diurnal insects is typically about 1.0 degrees . 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 degrees for blue, 1.53 degrees for green or 0.96 degrees for red: they appeared to see colors in single pixels to some extent. This ability may compensate their rather low spatial resolution.
authors.affiliation: School of Advanced Sciences, The Graduate University for Advanced Studies, Hayama 240-0193, Japan.
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Takeuchi
authors.forename: Kentaro
authors.initials: K
authors.lastname: Arikawa
authors.forename: Michiyo
authors.initials: M
authors.lastname: Kinoshita
mesh_terms: D000818:Animals; D002080:Butterflies / Q000502:physiology*; D003118:Color Perception / Q000502:physiology*; D008297:Male