Structural fluorescence in the butterfly Morpho sulkowskyi (Nymphalidae)
Résumé
Evolution and natural selection have generated complexity and efficiency in all living families. Morpho sulkowskyi - a butterfly from Neotropic ecozone (South America) and belonging to the Nymphalidae family - concentrates on its wings distinct but complementary features contributing to its exceptional visual attraction: i) the wings are predominantly white but ii) present a bright blue metallic flash due to an iridescence process; iii) the presence of fluorescent molecules producing a violet-blue coloration when irradiated by ultraviolet light and finally iv) the particular ultrastructure of the scales presenting a three-dimensions natural photonic crystal. Due to the confinement of the fluorescent sources in a photonic crystal, the emission is preferentially directed in space and its efficiency is enhanced for particular detection angles. Furthermore, a clear correlation is observed between the reflection and the fluorescent processes that control the surface optical response. So, collecting and analyzing data over every emerging direction is shown to be crucial. To quantify these observations and characterize these optical effects, three types of measurements were carried out. First of all, the morphology of the butterfly was examined by means of scanning electron microscopes. In addition, the angular distribution of the reflected light was measured with a high performance viewing angle instrument, providing BRDF data (Bidirectional Reflectance Distribution Function). Finally, an automatic method coupling an ultraviolet source to a gonio-spectrophotometer allows fluorescent emission characterization. This set-up, developed on purpose, is composed of various excitation and analysis modules and provides angular emission maps. Tentative explanation for the measured correlation are presented.