Bejeweled Weevil May Inspire Synthetic Gem

John Roach
for National Geographic News
January 12, 2004
The study of a weevil with an opal-like shell from the dimly-lit tropical forests of northeastern Queensland, Australia, may enable humans to more easily manufacture synthetic versions of the gem.

Understanding how the beetle manufactures the tiny structures in its scales may benefit jewelers seeking a less expensive opal and the computer and telecommunications industries seeking to manufacture tiny electronics.

Synthetic opal is notoriously difficult to make in the solid form and humans have yet to perfect the process. One of the problems is that technology cannot keep up with the design theories, said Andrew Parker, an evolutionary biologist at the University of Oxford in England.

"What we have here is an animal making perfect opal," he said. "If we can copy the manufacturing process, we hope to [also] make perfect opal."

Opal gemstones consist of a collection of disordered spheres that form a layered reflector that reflects a specific wavelength of light in a given direction. Since they are spheres, they reflect this wavelength light in most directions and so the structure appears the same color from every direction. The interplay of these colors gives the gemstones their visual luster.

The weevil, Pachyrhynchus argus, has patches of scales on the top and sides of its body, some of which contain structures that resemble the spheres in opals, according to Andrew Parker, an evolutionary biologist at the University of Oxford in England.

The weevil's scales consist of two parts—an outer shell and an inner structure. The inner structure is made up of closely packed, tiny, hexagonal spheres.

"As you move around the weevil, the color stays the same, which is quite intriguing. It sounds simple, but actually it isn't," said Parker, who together with his colleagues describes the weevil's opal-like structures in the December 18 issue of the science journal Nature.

The light reflected off the opal-like spheres appears yellowish-green. Scales that lack the inner structure appear transparent and the light that passes through them is purple, though it is absorbed by the weevil's black body, said Parker. The effect is a black weevil covered in metallic, yellowish-green speckles.

Brett Ratcliffe, a scarab beetle (weevil cousins) expert at the University of Nebraska in Lincoln, said he is not surprised by P. argus's metallic color, as a lot of scarabs have translucent, deep metallic color.

"It's not surprising they are finding this kind of color, but they are the first to describe how the color is working by getting into the physics and structure of it," said Ratcliffe.

Making Opal

Parker and his colleagues discovered P. argus in a collection of dead and damaged weevils and beetles stored in a museum collection at Oxford. The insects serve educational purposes but are of no use to the museum because they lack sufficient data.

After noticing the weevil's color, the researchers cut into it and examined it under an electron microscope, which revealed the opal-like inner structure of the scales.

The team is now in the process of acquiring live specimens from Southeast Asia so that they can study how the cells build the structures, looking for little tricks they perform with molecular motors, rulers, and templates, said Parker.

"Are they made within the cells and extruded out or does [the structure] form during the extrusion process?" he said. "We should be able to answer that in the next project."

The researchers will also study the weevil's genetics, to see if they can detect the genes that control for patterns and the size of the tiny opal-like spheres.

The Opal Advantage

Why P. argus evolved the opal-like structures remains an open question, said Parker, but it may be related to helping the weevils recognize each other, as each has a unique pattern.

He has identified several other species of beetles with the opal structure, ranging from bright red to violet, a sign that the color may be important for species recognition.

Ratcliffe said many insects, including the beetles he studies, rely on chemical signals and touch for mate recognition.

"I think it acts as a camouflage to protect it from animals," he said. Many scarab beetles are metallic in color, which allows them to blend into wet forests and avoid predation by birds. Ratcliffe thinks the weevil does this as well.

© 1996-2008 National Geographic Society. All rights reserved.