Cuticle network and orientation preference of photonic crystals in the scales of the weevil <i>Lamprocyphus augustus</i>.

Abstract

This paper reports the structural and optical investigations of the structural colour of the weevil <i>Lamprocyphus</i><i>augustus</i> The photonic crystal structure within the weevil's scales was investigated using sequential focused ion-beam milling and scanning electron microscopy imaging. We carefully analysed the reconstructed three-dimensional structure to determine the unit cell of the photonic crystal. It was found that the cuticle network of the cubic unit cell perfectly matches the previously reported diamond-based network. However, different results were obtained for the crystal orientations of the small crystal domains that comprise the entire photonic crystal structure in the scales: <111> directions are highly preferred along the surface normal of the scale. This finding explains the fact that the scale is almost uniformly coloured despite the multi-domain structure. It is confirmed experimentally and theoretically that the wavelength range of the reflection band corresponds to the gap of the photonic band.