Appendix A: Figure A1: Figure A1: The polarization of light. In C and D, the unlabeled block arrows represent the polarizations each photoreceptor absorbs. A) Linearly polarized light has an e-vector along one particular axis. B) In circularly polarized light, the e-vector rotates as the light propagates. C) The photopigments expressed by the rhabdomeric photoreceptors of invertebrates are found in many elongated tubes, known as microvilli. Although the photopigments may arrange themselves randomly within the microvilli, when viewed in the plane perpendicular to the incident light, the photopigments appear to be preferentially oriented parallel to the long axes of the microvilli. As a result, the photoreceptors are more sensitive to a particular polarization plane than to any other. D) The typical ciliary photoreceptors found in vertebrates are polarizationinsensitive. They resemble discs and do not bias the arrangment of photopigments in any particular direction. Therefore typical vertebrate photoreceptors respond equally to all polarizations (shown by the block arrows). A) and B) adapted from Johnsen (2012) Appendix B: Comparisons between the blue and white areas of the H. cydno’s wing show that the polarized reflections are accompanied by large color and luminance differences. Reflectance measurements of both the blue and white regions were taken using a 200 µm fiber coupled with a USB2000 spectroradiometer (Ocean Optics Inc. Dunedin, FL, USA). The incident light was perpendicular to the plane of the butterfly wing and the probe was at an angle of 45°. Using model photoreceptors, both color and polarization Michelson contrast values [(Q1 – Q2)/(Q1+Q2)] of adjacent wing areas were calculated. These ratios were then compared to each other to find the polarization or color distance between the regions. We set the λmax of the model polarization photoreceptor at 450 nm because this value maximized the polarization contrast. Polarized photoreceptors were set at the angles maximally and minimally sensitive to the polarization. Model λmax values for the dichromatic color photoreceptors were 450 nm and 530 nm. The achromatic photoreceptor was set at 450 nm. Photon catch calculations used the Stavenga template (Stavenga et al., 1993) in the light of a terrestrial forest (Endler, 1993).