Meeting Abstract
Iridescence is found throughout the natural world, including the cuticle of many beetles (Coleoptera). Iridescence can arise through multiple mechanisms, such as parallel nanoscale ridges, slits, or fringes of the cuticle that diffract light into ordered spectra – called diffraction gratings. This iridescence has been found in many polyphagan families; however, its function in an evolutionary context is still unknown. Some iridescent beetles are known to burrow through different substrates, such as sand, leaf litter, and fungus. For these beetles, it is unlikely that iridescence has an adaptive role in visual cues. We hypothesized that diffraction gratings reduce friction for beetles traveling through various media. The friction coefficients of five pairs of closely related carabid, scarabaeid, and staphylinid, beetles (one species with diffraction grating-induced iridescence and another species without iridescence) sliding against a fibrous countersurface were measured using microtribometry in both wet and dry configurations. Iridescence was quantitatively confirmed using broad light spectroscopy. Coefficients of friction for all species increased by a factor of at least two in the wet versus the dry configuration. Goniometry (hydrophobicity) and cuticle geometry measurements were used as inputs to a friction model. The morphology of the beetle surfaces rather than the presence of a physical diffraction grating determines their wettability and friction behavior when sliding against a wet, fibrous surface. The roughness and orientation of features controls the area in contact, and dynamic changes in the contact give rise to friction.
