Fossilized Insects' Photonic Diamonds Reveal The Evolution Of Structural Color

A reconstruction of one of the fossilized weevils found in 13,000 year old sediments in a Swiss lake. The colors come not from pigments, but tiny structures known as photonic crystals. Univeristy of Cork

Stunning blue-green colored weevils have been found in 13,000-year-old Swiss lake sediments. The colors are more vivid than those from pigments because they are based on photonic diamonds. Living insects have adopted the same technique to produce eye-catching colors, but the newly discovered specimens appear to have used it to hide, rather than to stand out, suggesting something interesting about the evolution of the trait.

For most animals, coloring is a matter of absorbing the majority of the visible light spectrum, while reflecting a narrow band representing a particular shade. A few, however, use structural coloration by reflecting light off features similar in size to the wavelengths of visible light, creating interactions that produce much brighter colors. Some use relatively simple reflectors, while others (including some butterflies, weevils, and longhorn beetles) have developed highly ordered three-dimensional architectures, known as photonic crystals or diamonds.

Two weevils dug from the mud at the bottom of Lake Lobsigensee show the same trait, leading Dr Luke McDonald of University College Cork to have them studied with electron microscopes. “This is only the second time that 3D photonic crystals have been reported from the fossil record,” McDonald said in an emailed statement.

Structurally colored weevil scales imaged using light microscopy (a,b,e,f) and SEM (c,d,g,h) reveaking the colors and three-dimensional photonic nanostructures. (b,d,f,h). McDonald et al/Biology Letters

Although living insects use the same technique, these appear to be for mating displays or to confuse predators. It makes sense that animals hoping to attract partners would want to be as bright as possible, and the iridescence can be disorientating. However, McDonald and co-authors suggest in Biology Letters it appears modern weevils arrived at this technique through a surprising path.

“The fossil record is potentially a treasure trove of information on the evolution of these structures, but it’s largely an untapped resource,” said senior author Dr Maria McNamara. Against the green and brown backgrounds in which these insects would have lived, the colors the authors found would have served as camouflage. Unusually for insects employing structural color, these weevils have little iridescence, having evolved structures that suppressed this, presumably to help them hide.

“Only later do these colors diverge for other uses such as signaling potential mates or as a warning to predators that the insect is not worth eating,” said Dr Vinod Saranathan of Yale-NUS College Singapore, another of the paper's authors.

Simpler examples of fossilized structural color date back many millions of years, but photonic crystals, the most advanced way of making color, are much rarer. The previous example was found in Canada and dates from 735,000 years ago – much older than the Swiss specimens, but still recent in the time span of invertebrate life. The authors are keen to find older examples, perhaps dating back to the Cretaceous, when they think weevil ancestors may have first evolved photonic crystals.

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