On the potential wet self-cleaning and spectral transmittance of snakes' spectacle

The eyes of snakes are protected by transparent scales, commonly called "spectacles" or "brille". As is widely known, snakes have no eyelids or other extremities to clean their eyes. Therefore, we test the hypothesis that their spectacle scales feature surface structures for self-cleaning. Additionally, snakes' spectacle scales might be optimized through evolution to enhance light capturing by the retina. To address these two subjects, we characterize the spectacle scales of 18 snake species belonging to 5 families regarding their wettability, adhesion, spectral transmittance, and micro-ornamentation utilizing optical spectroscopy, contact angle measurements, scanning electron microscopy, and atomic force microscopy. Remarkably, the optical transmittance of spectacle scales is moderate, with values of 70% to 90% above 400 nm. In most cases, the transmittance of spectacle scales is not significantly better than transparent dorsal or ventral scales. Furthermore, the spectacle scales are only slightly hydrophobic, with contact angles between 90° and 105°. In general, the contact angles are even lower than on dorsal and ventral scales. These results can be explained by the fact that the surface geometries of spectacle' scales are flat down to the nanoscale, comprising only polygonal cracks and nanoscale pits. Therefore, we conclude that snakes' spectacle scales do not feature advantageous properties regarding wet self-cleaning and optical transmittance. Additionally, we explain the physical origin of a transmittance peak close to 254 nm observed for scales of various snake species by the fluorescence of keratin.