Metallic Black Color Obtained by Two Microstructures in Elytron Scales of Sagra Femorata Purpurea Lichtenstein

Brilliant structural color has been widely explored in nature to obtain new technologies and methods conducive to the engineering. The metallic black appearance is a special existence compared to other purple, purple-red and green S. fp. Lichtenstein formed by microstructures. Here, the color formation mechanism of black elytra was investigated through microstructural observation, optical experiments and FDTD analysis. The reflectance of light gradually increases from 4% to 8% as the incidence angle adjusted from 0° to 60° with no peaks at the wavelength between 440 nm and 800 nm. According to the optical analysis of optical coupled bionic models, the circular convex closures have influence on angle dependent color changing light caused by irregular reflection and refraction while the incident angle less than 30°. The excessive thick layer in EPI can reflect and refract incident light absolutely irregular at different angles. It implies that the metallic black color of S. fp. Lichtenstein is achieved with the key factor of thick layer under the alternately distributed multilayers in EPI and the synergistic action of circular convex closures.