Abstract
Beetles appear to be iridescent under broad-spectrum light. For many species, this iridescence originates in microsculptures on the dorsal wing covers (elytra) and lower body segments (tergites). These microscale features act as diffraction gratings, but beetles are unlikely to see the iridescent spectra, leading to the hypothesis that they serve a physical function of mediating the beetle interaction with its environment. Namely, these small features could lead to reduced friction as the beetle burrows or digs through mud or leaf litter. To test this hypothesis we identified 4 phylogenetically contrasting pairs, or species that are nearly identical but do or do not exhibit diffraction grating-based iridescence. We tested each specimen sliding against fibrous and smooth countersurfaces and measured the friction response. In general the beetles which exhibit iridescence have highly-developed, high-aspect ratio features on their bodies with characteristic spacing of 0.5-3 µm. Those without have underdeveloped features, or features of characteristic size > 5 µm. The friction response was slightly altered by the presence of the diffraction gratings. This work introduces multifunctional aspects of insect cuticles.