Fig. 2.

Tough color in the cuticle of T. flammea achieved by damage localization. Top-view (A) SEM and (B) optical images, cross-sectional–view color maps of the multilayer (C) period and (D) orientation across the indentation sites for normal microindentation. (E–H) Corresponding results for inclined microindentation. The maximum load is 250 mN. No radial cracks were observed, and the red photonic color is preserved immediately near the indentation sites. (I) Magnified cross-sectional–view SEM images of the inclined indentation showing the breakage of the vertical micropillars (marked by arrows) due to the tensile stress in the vertical direction. (J) SEM image of a fractured surface in the photonic region shows a microscopically rough surface due to the presence of the vertical micropillars (arrows). (K) FE simulation results of plastic strain, ${\epsilon }_p$, at the maximum load for normal indentations on T. flammea’s photonic structure (Top) and a hypothetical simple multilayer structure (Bottom). Shear bands are formed in the latter case (yellow arrows). (L) Cross-sectional–view high-magnification SEM image of the normal indentation crater, which shows the bending deformation of vertical micropillars (yellow stripes). The inset shows the elastic recovery displacement U(s) of a pillar from simulation. (M) SEM and (N) optical images of a beetle’s cuticle surface directly collected from the field. The arrows mark the cracks in the cuticle. Some scratches (dashed arrow) are seen in the SEM data but hardly observed in the optical image.