Beetles offer clues to whiteness

Michael J. Lander

Materials that possess and maintain a clean white appearance are highly sought after for many purposes, from lighting to paint. However, in the man-made world — as in nature — whiteness doesn’t come easily.

That’s one reason why researchers led by Peter Vukusic of the University of Exeter in the UK decided to study the Cyphochilus beetle. Structures only 5 μm thick on the exoskeleton of this Southeast Asian insect make it appear stark white to the human eye.

The Cyphochilus beetles that researchers examined have a surprisingly white exoskeleton and stand out from their deeply colored cousins as well as from human skin. Images courtesy of Peter Vukusic, University of Exeter.

This is not to imply that no other materials in nature display this coloration. Human teeth, for instance, have a white appearance, but are quite thick. Milk is white, yet would lose this property if applied in a thin layer. Additionally, the whiteness of both teeth and milk pales in comparison with that of the beetle’s shell, a fact that the scientists verified by comparison of optical measurements from a Datacolor Elrepho photospectrometer with International Organization for Standardization data. They discovered that the same held true for synthetic products — slightly brighter bleached cellulose paper was approximately 25 times thicker than the shell’s coating.

As the researchers explain in the Jan. 19 issue of Science, the main reason for the beetle’s comparatively blinding whiteness lies in its scales. Flat and elongated, they were found with electron microscopy to cover the head, body and legs of the insect. With two-dimensional fast Fourier transforms of microscope images, the scientists ascertained that the scales consist of a highly random three-dimensional network of interconnecting cuticular filaments.

The 3-D filamentous structures randomly reflect incoming light and make the creatures seem bright white against their surroundings.

Analysis of diffraction patterns confirmed that the filaments were, in fact, the cause of the beetle’s whiteness. To obtain the patterns, the researchers focused laser light at the center of individual scales mounted on needle tips and imaged the reflection and transmission diffraction patterns on spherical screens. They observed that when light strikes the structures, all visible wavelengths are scattered randomly from the surfaces — yielding a white reflection.

A view of the beetles from overhead shows white coloration on the legs, head, thorax and abdomen.

Because few organisms produce coverings with this capability, scientists rarely observe bright white color schemes in nature. Furthermore, white coloration generally does not work in an organism’s favor, as it can make the creature stand out from its surroundings. In the case of Cyphochilus species, a white shell might have proven evolutionarily advantageous because it permits camouflaging against the white fungi that are native to the areas that the insects inhabit.

Although the implications of the scientists’ investigation remain unclear, many potential applications exist. Paper and even teeth could appear noticeably brighter if they were coated with substances that have similar properties. Such thin, highly reflective materials also might serve as optical coatings to make organic LEDs and other devices significantly more efficient.