Beetle shells provide beauty and brawn

The microstructures that give flower beetles’ exoskeletons their dazzling iridescence also enhance structural strength, according to a research team led by mechanical engineer Ling Li of Virginia Tech (Proc. Natl. Acad. Sci. U.S.A. 2021, DOI: 10.1073/pnas.2101017118). The team of multidisciplinary experts from six universities came together to investigate the relationship between the optical and mechanical properties in the colorful shell of the flower beetle—namely, how does the shell provide visual cues and protection, and is there a function that’s prioritized? Using a scanning electron microscope to analyze the shell, the researchers found vertical micropillars supporting alternating nanoscale layers of exoskeleton material in a plywood-like structure. Experimental and theoretical analyses revealed that the micropillars scatter light in a way that maximizes the viewing angle of the reflected color. The micropillars also give the exoskeleton flexibility and damage tolerance, like holding a seal around a puncture site to prevent further damage. Optical and mechanical models show that more micropillars would increase strength but decrease structural color. After analyzing a range of beetle color morphs, the researchers found that the micropillars seem to be distributed for maximum optical performance rather than mechanical properties. These results provide a window into how unique multifunctional materials evolve in nature to cope with evolutionary pressures, and they could inspire mechanically robust photonic materials.