Weevil hints at more vivid colors

Researchers from the NCCR Bio-Inspired Materials and Yale-NUS College in Singapore have discovered a novel color-generation mechanism in nature that, if it can be translated into artificial materials, has the potential to create cosmetics and paints with purer and more vivid hues, screen displays that project the same true image when viewed from any angle, and even reduce the signal loss in optical fibers.

Using high-energy X-rays, scanning electron microscopy, and optical modelling, NCCR researcher Dr. Bodo Wilts of the Adolphe Merkle Institute and Yale-NUS Assistant Professor Vinodkumar Saranathan examined the rainbow-colored patterns in the elytra (wing casings) of a snout weevil from the Philippines, Pachyrrhynchus congestus pavonius. They discovered that to produce the rainbow palette of colors, the weevil shares a color-generation mechanism that has so far only been found in squid, cuttlefish, and octopuses.

The so-called “rainbow” weevil is distinctive for its rainbow-colored spots on its thorax and elytra. These spots are made up of nearly circular scales that are arranged in concentric rings of different hues, ranging from blue in the center to red on the outside, just like a rainbow. While many insects have the ability to produce one or two colors, it is rare that a single insect can exhibit such a vast spectrum of colors. The AMI and Singapore researchers have been investigating the mechanism behind the natural formation of these color-generating structures, as current technology is unable to synthesize structures of this size.

“The ultimate aim of our research in this field is to figure out how the weevil self-assembles these structures, because so far we are unable to do so,” explains Wilts. “The ability to produce these structures, which are able to provide a high color fidelity regardless of the angle from which they are viewed, could be useful for applications in any industry where vivid and diverse colors are desired. We could use these structures in cosmetics and other pigmentations to ensure high-fidelity hues, or in digital displays in your phone or tablet which would allow you to view it from any angle and see the same true image without any color distortion. We could even use them to make reflective cladding for optical fibers to minimize signal loss during transmission.”

Wilts and Saranathan examined these scales and determined that the scales were composed of a three-dimensional crystalline structure made from chitin, the main component of insect exoskeletons. They discovered that the vibrant rainbow colors on this weevil’s scales are determined by two factors: the size of the crystal structure that makes up each scale, as well as the volume of chitin used to make up the crystal structure. This means that larger scales with larger crystals and more chitin reflect red light, while the smaller scales with smaller crystals and less chitin reflect blue light. 

“The architecture exploited by the weevil is different from the usual strategy employed by nature to produce various different hues on the same animal, where the chitin structures are of fixed size and volume, and different colors are generated by orienting the structure at different angles, which reflects different wavelengths of light,” says Saranathan. Having previously examined over 100 species of insects and spiders and catalogued their color-generation mechanisms, he adds that this ability to simultaneously control both size and volume factors to fine-tune the color produced has never before been shown in insects and, given its complexity, is quite remarkable.

Reference: Wilts, B. D.; Saranathan, V. A Literal elytral rainbow: Tunable structural colors using single diamond biophotonic crystals in Pachyrrhynchus Congestus weevils, Small, 2018, 14, 1802328.