Metallic Beetles

May 2, 2011

There was an interesting catch phrase on Monty Python's Flying Circus. John Cleese, sitting behind a desk, would say, "...And now, for something completely different," followed by a ridiculous phrase, such as, "A man with a tape recorder up his nose." There was also a Monty Python film by the name, "And Now for Something Completely Different."

Humans disdain the regular and crave novelty. This explains the instant popularity of The Beatles and the British Invasion of the 1960s. Sure, Rockabilly was entertaining, and very American, but all things have their season. Not content with their original popular style, The Beatles reinvented themselves with each subsequent album release.

The Beatles dissolved as a group just as Heavy Metal hit the scene. It would have been interesting to have heard their response to this musical genre. Since this article is about metallic beetles (as in Animalia Arthropoda Insecta Pterygota Neoptera Endopterygota Coleoptera), how could I resist such a contrived lead-in?

An interesting article was just published in the inaugural issue of Optical Materials Express, an open-access journal published by The Optical Society (OSA, also known as The Optical Society of America). The paper, "Visible light reflection spectra from cuticle layered materials," looks at the reason behind the gold and silver coloration of the two beetles, C. limbata (silver) and C. aurigans (gold).[1-3] A photograph of these two appear below, courtesy of OSA.

The beetles, Chrysina aurigans (gold) and Chrysina limbata (silver). Photograph by Eduardo M. Libby via OSA.

In order to understand why the beetles are colored gold and silver, we need to know what it means for something to be "silver" or "gold." There's no better reference than the actual metals themselves. The spectra below tell the whole story. These metals essentially reflect all wavelengths above certain values, and absorb all wavelength below. The critical wavelength for gold is about 500 nm. For silver it's about 300 nm, although any value sufficiently below the visible spectrum would work.

Reflectance spectra of silver and gold in the visible and near-IR. (Spectra by Bob Mellish via Wikimedia Commons, modified.)

How would you simulate such spectra without recourse to any silver or gold? Spectral features like these are quite common in optics, since they are found in dielectric mirrors. These mirrors are formed from alternating stacks of low and high refractive index materials. Not only can dielectric mirrors be made wavelength selective, forming notch and bandpass filters, they are also much more reflective than a metal mirror.

One material available to beetles is chitin, a complex sugar containing nitrogen. Chitin is responsible for the hard shells of insects, and those of sea creatures such as lobster and shrimp. Chitin has a refractive index of between 1.7 and 2.0 in the visible wavelengths.

An available low refractive index material is air, which has a refractive index of about 1.0. A multilayer stack of chitin and air of appropriate thicknesses will produce the observed spectra. The cuticle of these beetles is 10 micrometers thick, and it contains about seventy chitin layers.

Close examination of the beetle spectra shows that the beetle mirrors are chirped; that is, the layers are not of uniform thickness. Chirped mirrors allow a wider bandpass response. This mirror structure on the beetle shell has been confirmed by electron microscopy. It's presumed that the silver and gold coloration has an evolutionary advantage in camouflaging the beetles in their tropical rainforest environment.

This fundamental study may have applications. William Vargas, an author of the study, thinks there may be an application in providing decorative metallic coatings that contain no metal. There might be an environmental benefit, since everything ends up in the trash, eventually.

About a decade ago, I worked with dielectric mirrors. The figure below is an example spectrum of a passband mirror centered around the red line of a helium-neon laser. In this case, tantalum oxide (Ta₂O₅) was used as the high refractive index material (n ≈ 1.80), and silica (SiO₂) was used as the low refractive index material (n ≈ 1.52)

Calculated reflectance of a dielectric mirror formed from twenty alternating layers of tantalum oxide and silica.

References:

1. Angela Stark, "Beetle Bling: Researchers Discover Optical Secrets of 'Metallic' Beetles," Press Release of The Optical Society, April 25, 2011
2. Spotlight on Optics -Highlighted Articles From OSA Journals: Visible light reflection spectra from cuticle layered materials
3. Cristian Campos-Fernández, Daniel E. Azofeifa, Marcela Hernández-Jiménez, Adams Ruiz-Ruiz, and William E. Vargas, "Visible light reflection spectra from cuticle layered materials," Opt. Mater. Express, vol. 1, no. 1 (April 22, 2011), pp. 85-100.