Origin of Retroreflection from a Wing of the Morpho Butterfly

We have performed detailed angular and spectroscopic measurements on a typical species of Morpho butterflies, Morpho rhetenor , and have found that the butterfly wing shows characteristic retroreflection in a blue region, when light is incident around 10--30 \circ inclined from the normal to the scales on the wing. Various angular and spectroscopic measurements such as detector and sample rotations, and $\theta$--$2\theta$ scan are tested, and apart from an inclusive method of $\theta$--$\phi$ scan, the sample rotation method is found to be the most appropriate to characterize its optical properties. In order to investigate the origin of the retroreflection, non-standard finite-difference time-domain method is applied and alternate shelf structure is found to be responsible for it. Its optical characteristics are more intuitively understood by two simple models, destructive interference between two units of one-sided shelf structure and two inversely inclined multilayers, although such models are not complete in a strict sense. The presence of retroreflection in this type of animal will be extremely beneficial to show their colors clearly without specular reflection that has no color selectivity.