This paper presents the first evidence of tetrachromacy among invertebrates. The Japanese yellow swallowtail butterfly, Papilio xuthus, uses colour vision when foraging. The retina of Papilio is furnished with eight varieties of spectral receptors of six classes that are the ultraviolet (UV), violet, blue (narrow-band and wide-band), green (single-peaked and double-peaked), red and broad-band classes. We investigated whether all of the spectral receptors are involved in colour vision by measuring the wavelength discrimination ability of foraging Papilio. We trained Papilio to take nectar while seeing a certain monochromatic light. We then let the trained Papilio choose between two lights of different wavelengths and determined the minimum discriminable wavelength difference Dl. The Dl function of Papilio has three minima at approximately 430, 480 and 560 nm, where the Dl values approximately 1 nm. This is the smallest value found for wavelength discrimination so far, including that of humans. The profile of the Dl function of Papilio can be best reproduced by postulating that the UV, blue (narrow-band and wide-band), green (double-peaked) and red classes are involved in foraging. Papilio colour vision is therefore tetrachromatic.

BackgroundWe have found that the spectral sensitivity of the compound eye in the summer fruit tortrix moth (Adoxophyes orana) differs in laboratory strains originating from different regions of Japan. We have investigated the mechanisms underlying this anomalous spectral sensitivity.MethodsWe applied electrophysiology, light and electron microscopy, opsin gene cloning, mathematical modeling, and behavioral analysis.ResultsThe ERG-determined spectral sensitivity of dark-adapted individuals of all strains peaks around 520 nm. When light-adapted, the spectral sensitivity of the Nagano strain narrows and its peak shifts to 580 nm, while that in other strains remains unchanged. All tested strains appear to be identical in terms of the basic structure of the eye, the pigment migration in response to light- and dark-adaptation, and the molecular structure of long-wavelength absorbing visual pigments. However, the color of the perirhabdomal pigment clearly differs; it is orange in the Nagano strain and purple in the others. The action spectrum of phototaxis appears to be shifted towards longer wavelengths in the Nagano individuals.ConclusionsThe spectral sensitivities of light-adapted eyes can be modeled under the assumption that this screening pigment plays a crucial role in determining the spectral sensitivity. The action spectrum of phototaxis indicates that the change in the eye spectral sensitivity is behaviorally relevant.

Papilio butterflies use a tetrachromatic color vision to discriminate a rewarding flower, approach, land and take nectar from the flower. In the course of further analyzing their foraging behavior in a laboratory condition, we found that some butterflies could not land on the target flower even they discriminated and tried to land on it, especially when the target was dark. This phenomenon, which we call "landing suppression", indicates that the cue for landing differs from the cue for visually locating a flower. We hypothesized that a possible cue for landing was intensity contrast between the target and background, and have initiated to test this hypothesis. We tested the butterflies' landing behavior to targets of various colors and intensities presented on background of black or various densities of gray. As a result, the landing was most strongly suppressed when the intensity contrast was close to zero irrespective of the target colors, suggesting that the butterflies used the target-background intensity contrast when landing.

SUMMARYUltraviolet (UV) vision is widespread in a variety of animals, playing important roles in behaviours such as foraging and reproduction. Despite accumulated information about UV vision and UV-dependent behaviours of animals, little is known about the effect of temporal changes and local variations in UV light on UV-dependent behaviour. Here we report the mating behaviour of male cabbage butterfly, Pieris rapae crucivora, in environments with varying content of UV light. We first confirmed that the relative UV content is higher in shaded places than in sunny places. We furthermore arranged experimental areas with varying UV contents in the field, where we compared three aspects of male mating behaviour: visual localization of females, female-searching flight and copulation success rate. In all aspects males performed more actively in UV-rich environments: males searched females for longer, approached females preferentially in the shade and copulated there more frequently. Apparently, female-searching males detect females more easily in a UV-rich environment. The present findings should be taken into consideration when UVdependent behaviours, visual mate choice in particular, are studied.