Brilliant angle-independent structural colours preserved in weevil scales from the Swiss Pleistocene

Abstract

Extant weevils exhibit a remarkable colour palette that ranges from muted monochromatic tones to rainbow-like iridescence, with the most vibrant colours produced by three-dimensional photonic nanostructures housed within cuticular scales. Although the optical properties of these nanostructures are well understood, their evolutionary history is not fully resolved, in part due to a poor knowledge of their fossil record. Here, we report three-dimensional photonic nanostructures preserved in brightly coloured scales of two weevils, belonging to the genus Phyllobius or Polydrusus , from the Pleistocene (16–10 ka) of Switzerland. The scales display vibrant blue, green and yellow hues that resemble those of extant Phyllobius/Polydrusus . Scanning electron microscopy and small-angle X-ray scattering analyses reveal that the subfossil scales possess a single-diamond photonic crystal nanostructure. In extant Phyllobius/Polydrusus , the near-angle-independent blue and green hues function primarily in crypsis. The preservation of far-field, angle-independent structural colours in the Swiss subfossil weevils and their likely function in substrate matching confirm the importance of investigating fossil and subfossil photonic nanostructures to understand the evolutionary origins and diversification of colours and associated behaviours (e.g. crypsis) in insects.

Optical costs and benefits of disorder in biological photonic crystals

“…The bodies and elytra of many weevils and longhorn beetles are adorned with scales, showcasing a multifarious colour palette adapted for diverse environmental pressures. Numerous weevils, including Pachyrrhynchus congestus pavonius, 99,100 Lamprocyphus augustus, 101,102 Entimus imperialis, 75,[103][104][105][106][107] Eupholus cuvieri, 108 Eupholus magnicus 51 and Phyllobius/Polydrusus sp., 109 exhibit ordered photonic crystal structures (Fig. 2).…”

Large‐Scale Artificial Production of Coleoptera Cuticle Iridescence and Its Use in Conformal Biodegradable Coatings

“…[17] This last approach predominates among dark-colored beetles living in concealed (i.e., dark) and damp habitats and produces less conspicuous results than the striking iridescence created by the multilayer reflectors of other beetles (e.g., leaf beetles; Figure 1b and S1, Supporting Information). [17,19] The fact that gratings on the beetles' cuticles significantly impact their wettability and surface friction in moist environments [20] suggests that iridescence, when produced by cuticular gratings in dark habitats, could be simply a byproduct of selection for nonoptical properties. [19] In this study, we use recent advances in micro-and nanostructuring of chitinous polymers to reproduce, in a controlled manner, the iridescence of a beetle's chitinous diffraction grating.…”

“…[17,19] The fact that gratings on the beetles' cuticles significantly impact their wettability and surface friction in moist environments [20] suggests that iridescence, when produced by cuticular gratings in dark habitats, could be simply a byproduct of selection for nonoptical properties. [19] In this study, we use recent advances in micro-and nanostructuring of chitinous polymers to reproduce, in a controlled manner, the iridescence of a beetle's chitinous diffraction grating. However, we also seek to move beyond the animal kingdom's achievements and attempt to produce vibrant iridescent colors by optimizing those gratings-a route to achieving such optical effects that is underexplored by evolution compared to the optimization of the multilayer reflectors typically observed in leaf beetles (Chrysomelidae).…”

“…[ 17,19 ] The fact that gratings on the beetles’ cuticles significantly impact their wettability and surface friction in moist environments [ 20 ] suggests that iridescence, when produced by cuticular gratings in dark habitats, could be simply a byproduct of selection for nonoptical properties. [ 19 ]…”