Thermoregulation and spectral selectivity of the tropical butterfly Prepona meander:: a remarkable example of temperature auto-regulation

In butterflies favouring direct absorption of solar energy through their wings, the absorption by the black areas situated at the wing bases provides the body with the most part of the exogenous energy. For the studied butterfly (Archeoprepona meander), absorption is very high over the whole solar spectrum ( alpha similar to 0.95). This absorption is due to both the dendritic structure of the scales and the melanin pigments. In the infrared range, the absorptivity reveals two strong peaks at 3 mu m and 6 mu m and then remains more or less constant at a relatively high value (similar to 0.4) up to 25 mu m. This situation contains the outline for a remarkable self-regulation phenomenon of the radiative balance, allowing the stabilization of temperature in the survival area of the butterfly. The first absorption peak, centred on 3 mu m, i.e. between the solar irradiance and the thermal emission spectra, is not involved in the radiative balance at these temperatures. However, the second, on the edge of the emission spectrum, plays a key role as a thermal regulator. Its marginal situation is such that, according to the wing temperature, it overlaps or not the emission spectrum, modifying the collector efficiency, and thus the temperature.