Color discrimination at the spatial resolution limit in a swallowtail butterfly, Papilio xuthus

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.