Fig. 1. (A) Relative absorption functions of the blue-, green- and red-sensitive receptors of the butterfly Papilio xuthus (Kelber et al., 2001). (B) Microvilli orientations (ß) measured clockwise from the eye's dorso-ventral meridian in the photoreceptors of different spectral types (red, green and blue) in P. xuthus (Kelber et al., 2001). (C) Definition of the different parameters of partially linearly polarized light and a polarization-sensitive photoreceptor. The hatched area indicates the microvilli orientation ß. The angle of the eye's dorso-ventral meridian is clockwise from the vertical. is the angle of polarization of light measured clockwise from the vertical. The arrows represent the maximum (E_max) and minimum (E_min) electric field vectors (the major and minor axes of the polarization ellipse) and their components that are parallel (E_min^par, E_max^par) or perpendicular (E_min^perp, E_max^perp) to the microvilli. (D) Replacement of the blue (400-500 nm), green (500-600 nm) and red (600-700 nm) parts of function f() [f=I (intensity) or f= (degree of linear polarization) or f= (angle of polarization)] by discrete constant values f(_r^c) (r = blue, green, red) measured by video polarimetry at wavelengths _r^c. (E) Position of a visual stimulus C with spectral components M_R, M_G and M_B within the equilateral colour triangle of a colour-sensitive visual system with photoreceptor types R, G and B.