Yellow structurally modified fluorescence in the longhorn beetles Celosterna pollinosa sulfurea and Phosphorus virescens (Cerambycidae)

Celosterna pollinosa sulfurea and Phosphorus virescens are longhorn beetles which both have a sulphur-yellow pigmentary coloration and, more surprisingly, a yellow-green fluorescence coloration. In this study, we used a refined experimental examination to characterize the angular distribution of light and unveiled a different behavior between these reflection and emission processes. A key feature of these observations is the peculiar architecture of the scales (a three-dimensional photonic structure), which acts differently on each phenomenon. For reflection, this architecture can be viewed as a scattering device, while, for emission, it can be viewed as a waveguide. These effects were investigated by scanning electron microscopy (SEM), Bidirectional Reflectance Distribution Function (BRDF) diagrams and spectrophotometric measurements. Collection and analysis of data at every emergence direction was found to be crucial when studying optical properties in materials with spatial changes in composition at the scale of the light wavelength.