Light in biological systems: the white beetles

When propagating in opaque media such as the scales of the Cypochilus white beetle, the wave nature of light is not involved in the appearance of the object, and a bright white is achieved only in presence of multiple scattering, for which thicker, high-refractive-index contrast systems are usually required. Nevertheless, the extremely brilliant whiteness shown by the Cyphochilus beetle is generated by multiple scattering of light inside the ultra-thin scales that cover its body. The intra scale structure is characterized by a dense, nanostructured network of chitin filaments, which seems to be optimised (during millions of years of evolution) to increase the total reflectance, and thus the bright appearance of the beetle, employing as less material as possible. We analysed light transport inside the beetle’s scales, showing that multiple scattering of light occurs, in spite of their thinness (5–9 microns). We proved, with static and time-resolved experiments, that white beetle’s scales show the lowest transport mean free path for low-refractive-index systems (n=1.5) reported until now. We found that the crucial aspect of the optimisation of light scattering is the structural anisotropy of the chitin network. We indeed demonstrated that light transport inside the scales is anisotropic, and it is engineered to increment the scattering strength in the direction orthogonal to the scale surface, at expense of the in plane scattering, which is not relevant for the total reflectance.
Cyphochilus white beetle