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

Luke T. McDonald,Suresh Narayanan,Maria E. McNamara,Vinodkumar Saranathan,Alec Sandy

doi:10.1098/rsbl.2020.0063

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.

Highlights

Colour is a critical component of inter- and intraspecific visual communication in extant animals, including signalling strategies such as aposematism, mating displays and crypsis [1,2,3,4]
Using scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS), we show that the bright colours of these scales are produced by a single-diamond three-dimensional photonic crystal (PC) structure and have optical properties consistent with substrate matching
Each subfossil specimen comprises a partial elytron of Phyllobius/ Polydrusus exhibiting sparse bright blue to yellow-green scales that each show 8–10 parallel ridges oriented along the scale 2 axis

Summary

Introduction

Colour is a critical component of inter- and intraspecific visual communication in extant animals, including signalling strategies such as aposematism, mating displays and crypsis [1,2,3,4]. Biological photonic nanostructures––integumentary structures that scatter incident light––are a key evolutionary innovation [4,5,6,7,8,9,10,11,12]; they produce the most vibrant, highly saturated colours known in biological systems and can manipulate the directionality [9,13] and polarization properties [14,15] of scattered light Such nanostructures are distributed broadly in extant insects and vary in their complexity, ranging from relatively simple multilayer reflectors to more complex three-dimensional architectures that include amorphous networks and highly ordered crystals [10,16,17,18,19,20,21,22,23]. The fossil nanostructures, represent intermediate stages in models for the evolution of three-dimensional photonic nanostructures and confirm the importance of the Pleistocene fossil insect record as a key source of data on the evolution of three-dimensional photonic nanostructures

Methods

Results

Conclusion