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August 03, 2010

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Reflection is key to jewel beetle colors, scientists say

July 24, 2009
Courtesy Georgia Tech
and World Science staff

“Jewel bee­tles” are widely known for their glossy ex­ter­nal skele­tons that change col­ors as the an­gle of view changes. Now they may be known for some­thing else, sci­en­tists say: pro­vid­ing a blue­print for ma­te­ri­als with new col­or-producing prop­er­ties.

Sci­en­tists have found that jew­el bee­tles change col­or be­cause of the light-re­flecting char­ac­ter­is­tics of the cells that make up their ex­ter­nal skele­tons. 

The struc­ture of jew­el bee­tle cells re­sults in strik­ing col­ors as light hits them from an­gle, sci­en­tists say. (Cred­it: Zi­na De­ret­sky, Nat'l Sci­ence Founda­tion)

“Jewel beetles” are widely known for their glossy external skeletons that change colors as the angle of view changes. Now they may be known for something else, scientists say: providing a blueprint for materials with new color-producing properties. Scientists have found that jewel beetles change color because of the light-reflecting characteristics of the cells that make up their external skeletons. Researchers had been inclined to instead ascribe the unusual shimmering properties to unique, light-absorbing properties in the insects’ pigments. They say the new findings could be important for industries such as car manufacturers that look to reflective light paints for automobiles. The journal Science reported the findings in its July 24 issue. “The Chrysina gloriosa beetle reflects a green light,” said lead researcher Mohan Srinivasarao of the Georgia Institute of Technology. “We’ve determined the colors we see result from the beetle’s physical structure rather than its biology.” When light hits a non-see-through surface, that surface either scatters, absorbs or reflects the light to produce colors. What ultimately determines the color of light is its wavelength—literally the length of its waves, from one peak or trough to the next. In the jewel beetle’s external skeleton, five-, six- and seven-sided cells spontaneously arrange themselves to reflect light at certain wavelengths that produce green, yellow and red, the investigators said. Srinivasarao postulates that beetle cells form patterns similar to a certain kind of liquid crystal, a liquid with a regular arrangement of its molecules. The specific kind of liquid crystal to which Srinivasarao compares the beetle surfaces is called a “cholesteric” one, whose surface has cone-like structures and that has a helical arrangement of molecules. The cones produce colors as light hits them from different angles. The beetle’s structure also forms helices similar to a cholesteric liquid crystal in that its straight cells sit on or are used to form the curved structure of its external skeleton, according to Srinivasarao and colleagues. Research has found that when the width of one complete turn of the helix in cholesteric liquid crystals is close to the wavelength of visible light, the materials reflect light with specific wavelengths, leading to brilliant metallic colors. “It’s stunning how similar the two things are,” said Srinivasarao. Scientists already are studying ways to commercialize and apply materials that have properties similar to jewel beetles. Researchers in New Zealand, for example, are studying beetles to produce a thin, solid mineral that can be ground into flakes and possibly used as an anti-counterfeiting measure in bills. Car companies could use the materials to paint automobiles that change color depending on a person’s angle of view, Srinivasarao said. More uses could include decorative paints and use on any surface requiring light reflection without light absorption. Srinivasarao added that he doesn’t completely understand all the optical properties and behavior of light as it interacts with the beetle’s external skeleton. He said much more work needs to be done, but he and his team are pursuing it.