Native to Southeast Asia, the Cyphochilus beetle has ultra-thin scales that cover its body and make it appear much whiter than even paper. Researchers studying its optical properties have finally figured out how the beetle became so brilliantly bright: Its scales scatter light more efficiently than any other biological tissue known. The study was published in Scientific Reports last week.
Bright colors in the wild are produced for a variety of purposes, from camouflage to mate choice. For our eyes to perceive color, pigments like melanin in penguins and carotene in flamingos absorb certain wavelengths of light while reflecting others. To appear white however, all wavelengths of light must be reflected equally. “Current technology is not able to produce a coating as white as these beetles can in such a thin layer,” Silvia Vignolini from University of Cambridge explains in a news release.
A team led by Vignolini and Matteo Burresi from the European Laboratory for Non-Linear Spectroscopy in Florence studied how light propagates in the scales of the ultra-white Cyphochilus and Lepidiota stigma beetles, quantitatively measuring their scattering strength for the first time.
Their coloration is achieved by exploiting the geometry of a dense and complex network of chitin -- the same protein found in mollusk shells, insect exoskeletons, and the cell walls of fungi. These chitin filaments are just a few billionths of a meter thick, much thinner than paper. Over years and years of evolution, the beetles developed a compressed chitin network that’s “directionally-dependent” (or anisotropic), allowing high intensities of reflected light for all colors at the same time in one direction, while using very little material. This is what makes them appear so vividly white.
Furthermore, this efficiency keeps the material incredibly lightweight, which is particularly important for flying insects. “In order to survive, these beetles need to optimize their optical response but this comes with the strong constraint of using as little material as possible in order to save energy and to keep the scales light enough in order to fly,” Vignolini says.
By itself, chitin isn’t especially good at reflecting light because of its relatively low refractive index. The complex structure of the scales gives rise to something that’s more than the sum of its parts, Burresi explains. “Our simulations show that a randomly packed collection of its constituent elements by itself is not sufficient to achieve the degree of brightness that we observe.”
The physical properties of these ultra-white, ultra-thin scales could inspire whiter paper, plastics, and paints that use far less material.
Images: Lorenzo Cortese and Silvia Vignolini CC BY-NC-SA 3.0