Bug alchemy: Gold and silver beetles shine with structural color
All that glitters is not gold. That metallic glint might be sunlight bouncing off a beetle's shell. The Chrysina aurigans [left] and Chrysina limbata [right] specimens shown here bear such an uncanny resemblance to polished nuggets of gold and silver it may be hard to believe that their exoskeletons are made of the same stuff—chitin—that covers drab cockroaches and crayfish.
These beetles shine not because of chemical pigmentation or the incorporation of actual metals. Instead, a closer look at their elytra—the hard forewings that conceal the beetles' more delicate hindwings—reveals a multilayer nanostructure that tricks the light in just the right way to create metallic effects. In a study published April 22 in Optical Materials Express, researchers from the University of Costa Rica provide new details of this structural color.
The beetles' elytra has a so-called "chirped structure" consisting of some 70 layers of chitin stacked from top to bottom in decreasing thicknesses. The layers have different refractive indices, and incoming light waves are bent and reflected at each interface. Constructive interference of reflected rays intensifies their brightness and color. Using a special spectrometer designed to measure the light reflecting from the curved surface of the elytra, researchers found that the silver beetle reflects light across the entire visible spectrum whereas the golden beetle reflects light of wavelengths larger than 515 nanometers—similar to the reflection spectra for the actual metals.
Unlike other examples of structural color in nature, such as butterfly wings and peacock feathers, the beetles do not iridesce—instead they appear a steady gold or silver from any angle. Their dewy appearance would make the beetles easy to miss in the rain-drenched forests of Costa Rica, the researchers hypothesize.
Despite the resemblance, the beetles' sheen does not result from the same process that makes metal shine. "Actual gold and silver optical properties are determined by the contributions of free and bound electrons to the absorption of light," explains study co-author William Vargas. Replicating the chirped nanostructure using technology currently used to manufacture 3D photonic crystals might be possible, says Vargas. And if the beetle specimens shimmering in museum cases are any indication, these faux metallic coatings could last untarnished for hundreds of years.