The Morphology of Broad-Spectrum Reflectance in the Painted Lady Butterflies, Vanessa cardui

Abstract Animal coloration influences both ecological interactions and physiological processes through the absorption, reflection, and scattering of solar radiation. Although studies on coloration often focus on wavelengths visible to humans and the role of color in visual signals, animals interact with ultraviolet (UV), visible (VIS), and near-infrared (NIR) radiation, each of which offers distinct ecological and physiological functions. For instance, NIR wavelengths include the majority of incoming solar radiation, and can strongly impact heat balance in ectotherms. We quantified UV, VIS, and NIR reflectance across eight areas of the wing anatomy including dorsal and ventral surfaces, proximal and distal wing regions, and both fore- and hindwings in 48 museum specimens of the Painted Lady Butterfly, Vanessa cardui. Reflectance varied significantly across both anatomy and wavelengths, but the magnitude of anatomical variation in reflectance was strongly wavelength dependent. For instance, distal regions reflected 19.6% more NIR radiation compared to proximal regions, compared to only 4.8% and 2.4% differences in visible and UV reflectance, respectively. Across all wavelengths, ventral wing surfaces consistently reflected more radiation than dorsal wing surfaces. Broadly, our results show that butterfly wings have spatial heterogeneity in reflectance, particularly in thermally important NIR wavelengths, emphasize the importance of integrating fine-scale anatomical sampling with broad spectrum measurements when studying the evolution and function of animal coloration.