Achromatic vision in the honeybee at low light intensities

Summary1.Dark adapted honeybees (Apis mellifera camica) were trained in a T-maze to discriminate spectral light stimuli (λ=533, 430, and 413 nm) from dark or from an achromatic white light stimulus.2.Bees trained to discriminate a spectral stimulus from an equally bright white light respond to the spectral stimulus as if it were the white stimulus within a certain range of intensities. This range lies between the threshold for detection of light (the achromatic threshold) and that for determination of color (the chromatic threshold); it is termed the achromatic interval. Thus the detection of spectral stimuli has two thresholds; a lower one for the absolute detection of the stimulus and a higher one for the perception of color hue. The achromatic interval for wavelengths λ=533, 430, and 413 nm is≃1.5 log10 units of light intensity.3.Trained responses to spectral stimuli disappear at high stimulus intensities. Also, bees can not be trained to spectral lights of high intensity (>2 log10 intensity units above color vision threshold). This bright light effect is attributed to the specific response function of the lamina monopolar cells.4.Achromatic vision is explicable in terms of known characteristics of receptor and neural organization in the bee color vision system. It is concluded that bees pool output of all receptors from a single ommatidium in a neural strategy which produces an achromatic signal. Bees use this neurally derived achromatic signal for orientation at light intensities near visual threshold.