Structural Colors of Tiger Beetles and Their Role in Heat Transfer through the Integument

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Abstract
The thermal significance of interference (metallic) colors in insect cuticles was investigated by comparing the reflectivities and heat gain of cuticles from brightly metallic and achromatic morphs of two polymorphic tiger beetle species (genus Cicindela). The cuticular basis of color differences was determined using transmission electron microscopy. The reflectivities of metallic blue-green cuticle from Cicindela horni were 6.9% higher than black cuticle from the same species at wavelengths between 400 and 700 nm, but differences between 250 and 2,000 nm were only 1.2%. In contrast, the white elytra of C. formosa were 21% more reflective in the visible and 35% more reflective in the near-infrared than metallic red elytra from the same species. When elytra were heated in pairs within a closed chamber by visible light from a tungstenhalogen lamp, no differences were observed in the temperature excesses of iridescent green and black cuticles (elytra) of C. horni (mean difference = 0.01 C). In comparisons of white and metallic red elytra of C formosa, the metallic red elytra attained significantly higher temperature excesses (mean difference = 2.2 C). Our results confirm that metallic colorations in the elytra of tiger beetles are not effective in reducing heat gain from solar radiation, but that beetles with white elytra likely experience significantly lower body temperatures than either black or iridescent beetles under comparable environmental conditions.