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Does reflection polarization by plants influence colour perception in insects? Polarimetric measurements applied to a polarization-sensitive model retina of Papilio butterflies

Gábor Horváth^1,*, József Gál², Thomas Labhart³ and Rüdiger Wehner³

¹ Biooptics Laboratory, Department of Biological Physics, Eötvös University, H-1117 Budapest, Pázmány sétány 1, Hungary
² International University Bremen, School of Engineering and Science, P.O.B. 750561, D-28725 Bremen-Grohn, Campus Ring 1, Germany
³ Institut für Zoologie, Universität Zürich, CH-8057 Zürich, Winterthurerstrasse 190, Switzerland

View larger version (38K): [in a new window]   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 (Emax) and minimum (Emin) electric field vectors (the major and minor axes of the polarization ellipse) and their components that are parallel (Eminpar, Emaxpar) or perpendicular (Eminperp, Emaxperp) 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(rc) (r = blue, green, red) measured by video polarimetry at wavelengths rc. (E) Position of a visual stimulus C with spectral components MR, MG and MB within the equilateral colour triangle of a colour-sensitive visual system with photoreceptor types R, G and B.

View larger version (52K): [in a new window]   Fig. 4. (A) Left, equilateral red-green-blue colour triangle filled with the isoluminant colour shades used; middle, real colours of Campsis radicans in Figs 2, 3A-C, as perceived by a polarization-blind retina with polarization sensitivity PR=PG=PB=1 and microvillar directions ßR, ßG, ßB = arbitrary (number of pixels = 560x736=412160); right, relative frequency distribution of perceived colours (MR, MG and MB) within the colour triangle calculated for the full rectangular picture. Note that the colours used in the white triangles at the right-hand side code the relative frequencies alone and have nothing to do with the perceived colours shown in the rectangular patterns painted by the colours of the colour triangle at the left-hand side in part A. (B-E) Polarization-induced false colours of C. radicans perceived by a polarization-sensitive retina with PR=PG=PB=2, ßR=145°, ßG=35° and ßB=0°, and their relative frequency distribution in the colour triangle as a function of the alignment of the eye's dorso-ventral symmetry plane (indicated by red arrows in the circular insets) measured from the vertical. Note that the isoluminant rectangular images and the isoluminant colour triangle on the left in part A give information on colour alone; intensity information is missing.

View larger version (138K): [in a new window]   Fig. 2. Colour picture (intensity and real colour, number of pixels = 560x736=412160) of red flowers and green leaves of Campsis radicans (trumpet vine; Bigniniaceae), as recorded with a video camera viewing upward with an elevation of 45° at sunset in the open, when the plant was in the shadow of a house and illuminated from above by light from a clear sky, half of which was visible from the site of the plant. Points 1-12, marked with white or black diamonds, have the following typical spectral and polarizational characteristics used for the calculations in Fig. 5 (Table 1): 1 and 2 = bright green, unpolarized light transmitted through a leaf; 3 and 4 = dark green, weakly polarized light reflected from a leaf; 5 and 6 = bright whitish, blue-green, highly polarized light reflected from a leaf; 7 and 8 = bright red, unpolarized light reflected from a petal; 9 and 10 = bright whitish, red, weakly polarized light reflected from a petal; 11 and 12 = bright whitish, red, medium polarized light reflected from a petal. The graphs in Fig. 3D-F represent data measured along the horizontal white line in this picture. The data measured at the pixel marked here with a white vertical bar are used for the calculations in Figs 7, 8 (Table 2).

View larger version (50K): [in a new window]   Fig. 3. (A—C) Patterns of intensity I, degree of linear polarization and angle of polarization (measured from the vertical) of the plant surfaces in Fig. 2 measured by video polarimetry at 650nm (red), 550nm (green) and 450nm (blue). Number of pixels = 560x736=412160. In part C, regions are black where <10%. (D—F) Graphs of I, and in the red, green and blue spectral ranges versus the pixel number along the horizontal line in Fig. 2 and parts A—C. In part F, values are represented by dots where I>20% and >10%. The borders of the section through the red petal are marked with vertical broken lines. The vertical continuous line represents the pixel of a leaf, the optical characteristics of which (Table 2) are used for the calculations in Figs 7, 8. This pixel is marked with a vertical white bar in Fig. 2 and parts A—C.

View larger version (52K): [in a new window]   Fig. 6. (A-C) Colour picture and the patterns of the degree and angle of polarization of Epipremnum aureum (Aracea) - illuminated by light from a full clear sky from above through the glass panes of a greenhouse - measured by video polarimetry at a wavelength of 450 nm (blue). In part C, the regions are represented in black where <10%. Number of pixels = 560x736=412160. (D) Colours (MR, MG and MB) of E. aureum perceived by a polarization-blind retina, with PB=PG=PR=1, and ßR, ßG, ßB = arbitrary (a), and by a polarization-sensitive retina, with PB=PG=PR=2, =0°, ßB=0° as a function of the microvillar directions ßG and ßR of the green and red receptors (b-m). Every microvilli situation is designated by a letter ranging from a to m. The corresponding spectral loci (designated by letters a-m) of two details of the picture, one on a leaf blade (white) and one on the spathe (black) marked by rectangular windows in patterns A-C, are plotted within the equilateral R-G-B colour triangle, the colourless centre of which is represented by +.

View larger version (14K): [in a new window]   Fig. 7. Dependence of the polarization-induced false colour (MR, MG and MB) perceived by a retina with =0°, ßB=0° on the polarization sensitivity PB=PG=PR=P as a function of the microvillar directions ßG and ßR of the green and red receptors (designated by letters b-m) plotted within the equilateral R-G-B colour triangle, the colourless centre of which is represented by +. The colours are calculated for a point on a leaf of Campsis radicans marked by a white vertical bar in Figs 2, 3A-C. The reflection-polarization characteristics of this point are given in Table 2. The arrows start from the spectral locus a of the real colour when PB=PG=PR=1, meaning polarization-blindness, while the arrowheads point to the spectral locus of perceived false colours if PB=PG=PR=P=20. The spectral loci of false colours for P values ranging from 1 to 20 are placed along the straight arrows, on which the loci for P=2, P=5 and P=10 are marked by bars.

View larger version (20K): [in a new window]   Fig. 8. Dependence of the polarization-induced false colour (MR, MG and MB) perceived by a polarization-sensitive retina with PB=PG=PR=2, =0°, ßB=0° on the degree of polarization (R,G,B) of reflected light as a function of the microvillar directions ßG and ßR of the green and red receptors (designated by letters b—m) plotted within the equilateral R—G—B colour triangle, the colourless centre of which is represented by +. The colours are calculated for the point of a leaf of Campsis radicans marked by a white vertical bar in Figs 2, 3A-C. The original reflection-polarization characteristics of this point are given in Table 2. The degrees of polarization of reflected light are calculated as (R,G,B)=n0(R,G,B) and given in Table 2, where n is an arbitrary factor. The arrows start from the spectral locus a of the real colour when n=0 (unpolarized light) and PB=PG=PR=P=1 (polarization blindness), while the arrowheads point to the spectral locus of perceived false colours for n=1.28 (almost totally polarized light in all three spectral ranges). The spectral loci of false colours for n values ranging between 0 and 1.28 are placed approximately equidistant along the straight arrows.

View larger version (21K): [in a new window]   Fig. 10. Spectral loci (designated by A-H, representing the situations A-H in Fig. 9) of the leaf areas marked with a left and a right small rectangular window in Fig. 9 plotted within the equilateral R-G-B colour triangle, the colourless centre of which is represented by +. The arrows start from the spectral locus of real colours perceived by a polarization-blind retina with PB=PG=PR=1 and ßR, ßG and ßB = arbitrary, while the arrowheads point to the spectral locus of false colours perceived by a polarization-sensitive retina with PB=PG=PR=2, =0°, ßR=145°, ßG=35° and ßB=0°.

View larger version (15K): [in a new window]   Fig. 5. Spectral loci (MR, MG and MB) of points 1-12 in Fig. 2 plotted within the equilateral red-green-blue colour triangle, the colourless centre of which is represented by +. The rectangular areas of the colour triangle are enlarged and shown next to the triangle, with arrows starting from the spectral locus of real colours perceived by a polarization-blind retina with PB=PG=PR=1 and ßR, ßG, ßB = arbitrary, while the arrowheads point to the spectral locus of false colours perceived by a polarization-sensitive retina with PB=PG=PR=2, =0°, ßR=145°, ßG=35° and ßB=0°.

View larger version (47K): [in a new window]   Fig. 9. Spectral and reflection-polarization characteristics of a leaf of a Ficus benjamina tree (Ficaceae) as functions of the illumination conditions in the open. The leaf was mounted in front of the camera on a horizontal rod (holder), which rotated in a horizontal plane around a vertical axis together with the camera (insets I1 and I2). The solar elevation was S=55°, and the leaf was illuminated by direct sunlight (parts A, C, E and G) or shaded with a small screen that just occluded the sun and exposed the leaf to the full clear sky (parts B, D, F and H). In the small rectangular left and right window, the leaf blade is approximately horizontal and vertical, respectively. Inset I3 shows the four different horizontal directions of view of the camera with respect to the solar azimuth. ASM, antisolar meridian; SM, solar meridian; EPSM, eastwardly perpendicular to the solar meridian; WPSM, westwardly perpendicular to the solar meridian. Column 1 shows colour video pictures of the leaf. Column 2 shows patterns of the degree of linear polarization of the leaf measured by video polarimetry at a wavelength of 450nm (blue). Column 3 shows patterns of the angle of polarization (measured from the vertical) of the leaf at a wavelength of 450nm, where the dominant (average) electric field vector alignment of the leaf blade is represented by a solid arrow, and the standard deviations are represented by broken arrows.