Fig. 4. Hexagon colour space showing the excitations (E) of photoreceptors and the different colour classes for the honeybee. The relative number of flowers occurring in each hue category is plotted in the centre of the hexagon. Hue, in colour space, corresponds to angle, as measured from the centre. Therefore we counted the number of floral colour loci in narrow hue sectors each 10° wide, with the highest frequency normalised to 1.0 (for details, see Chittka et al., 1994). Blue-green flowers are most common, pure ultraviolet flowers are rare.