Chromatic and achromatic stimulus discrimination of long wavelength (red) visual stimuli by the honeybee Apis mellifera

It has long been assumed that bees cannot see red. However, bees visit red flowers, and the visual spectral sensitivity of bees extends into wavelengths to provide sensitivity to such flowers. We thus investigated whether bees can discriminate stimuli reflecting wavelengths above 560 nm, i.e., which appear orange and red to a human observer. Flowers do not reflect monochromatic (single wavelength) light; specifically orange and red flowers have reflectance patterns which are step functions, we thus used colored stimuli with such reflectance patterns. We first conditioned honey bees Apis mellifera to detect six stimuli reflecting light mostly above 560 nm and found that bees learned to detect only stimuli which were perceptually very different from a bee achromatic background. In a second experiment we conditioned bees to discriminate stimuli from a salient, negative (un-rewarded) yellow stimulus. In subsequent unrewarded tests we presented the bees with the trained situation and with five other tests in which the trained stimulus was presented against a novel one. We found that bees learned to discriminate the positive from the negative stimulus, and could unambiguously discriminate eight out of fifteen stimulus pairs. The performance of bees was positively correlated with differences between the trained and the novel stimulus in the receptor contrast for the long-wavelength bee photoreceptor and in the color distance (calculated using two models of the honeybee colors space). We found that the differential conditioning resulted in a concurrent inhibitory conditioning of the negative stimulus, which might have improved discrimination of stimuli which are perceptually similar. These results show that bees can detect long wavelength stimuli which appear reddish to a human observer. The mechanisms underlying discrimination of these stimuli are discussed. Handling Editor: Lars Chittka.