The mechanical implementations in three kinds of color-changing beetle elytra

With superior adaptability, some elytra as modified forewings can provide mechanical protection for the underlying hind wings and soft body while changing in color in a dynamically natural environment. There are mainly three color-changing styles for elytra: angle-dependent, irreversible and reversible color changes. Here, Popillia indgigonacea Motsch, Sagra femorata purpurea Lichtenstein and Dynastes tityus were chosen as specimens to investigate the relationship between the color formation of the three color-changing styles and the mechanical properties of light weight and high strength. The optical and nanomechanical properties of purplish red S. femorata purpurea Lichtenstein were studied by using an angle-resolved spectroscopy system and a nanoindenter, respectively. In finite-element analysis, three geometrical structural models – namely, color-change-relevant, color-change-irrelevant and whole-elytron models – were designed, and then the mechanical properties of six coupled bionic models of each beetle were investigated. The multilayer, convex closures and photonic crystal in the epicuticle can complete the dynamic presentation of structural colors by interference, scattering and photonic bandgap as they when resourcefully resist the external forces in low/high stress and strain with high/small elastic modulus and hardness. Meanwhile, other microstructures of multilayer and trabecular hollow were reasonably constructed to display co-operatively the mechanical properties of light weight and high strength.