Air temperature shapes mid-infrared optical properties of butterfly wings in butterflies' adaptations to cimate

Structural origins of thermoregulation and color in nature have long been a topic of research for scientists and engineers. In butterflies, the coloration of wings arises from a combination of pigmentation and structural coloration by periodic microstructures. While butterfly wings have been extensively studied for their structural coloration or optical properties within the visible spectrum, their properties in infrared wavelengths with potential ties to thermoregulation are relatively unknown. The mid-infrared wavelengths of 7.5 to 14 μm are particularly important for radiative heat transfer in the ambient environment, because of the overlap with the atmospheric transmission window. A high mid-infrared emissivity can facilitate surface cooling, whereas a low mid-infrared emissivity can minimize heat loss to the surroundings. Our findings show that the mid-infrared emissivity of butterfly wings from warmer climates is up to 2 times higher than that of butterfly wings from cooler climates, and our computations and experiments explain how periodic microstructures play a critical role in the mid-infrared. The emissivity spectrum governs the temperature of butterfly wings, and our analyses show that butterfly wings in their respective habitats can maintain a moderate temperature range through a balance of solar absorption and infrared emission. Phylogenetic independent contrasts analysis indicates the microstructures of butterfly wings may have evolved in part to regulate mid‐infrared emissivity as an adaptation to climate, rather than as phylogenetic inertia. Our findings offer new insights into the role of microstructures in thermoregulation and suggest both evolutionary and physical constraints to butterflies’ abilities to adapt to climate change.