Optical Metamaterials and the Hercules Beetle

You may recall that objects have particular colors because, at the molecular level, light rays at certain frequencies (corresponding to certain colors) are reflected while at other frequencies the light is absorbed. In other words, an object’s color has to do with its chemistry. But, as David Tyler points out here, coloration can also arise from repeating, detailed submicron geometrical structures at the object’s surface. These surface structures are finely tuned to interact with and control the incoming light, including controlling the frequencies, and hence color, of the reflected light. In these cases, the object’s color has to do with its repeating surface geometry, rather than its chemistry.

That’s interesting because not only can surface structures be designed and fabricated to have a certain color, the structure can also be designed to be sensitive to environmental factors, such as temperature and humidity. Therefore one can construct a panel whose color indicates an environmental reading. Scientists and engineers are busy researching and developing these and other applications for these so-called metamaterials.

But those scientists and engineers were not the first to create metamaterials. Once again, the biological world has “been there, done that,” and has many of its own such metamaterials. One study analyzed the hercules beetle which changes color in high humidity due to an incredibly complex three-dimensional carapace surface structure. As the study explains:

The elytra from dry specimens of the hercules beetle, Dynastes hercules appear khaki-green in a dry atmosphere and turn black passively under high humidity levels. New scanning electron images, spectrophotometric measurements and physical modelling are used to unveil the mechanism of this colouration switch. The visible dry-state greenish colouration originates from a widely open porous layer located 3 micro-meters below the cuticle surface. The structure of this layer is three-dimensional, with a network of filamentary strings, arranged in layers parallel to the cuticle surface and stiffening an array of strong cylindrical pillars oriented normal to the surface. Unexpectedly, diffraction plays a significant role in the broadband colouration of the cuticle in the dry state. The backscattering caused by this layer disappears when water infiltrates the structure and weakens the refractive index differences.

According to evolution all of biology arose via random causes such as mutations. No, natural selection didn’t help—it just killed off the bad designs. The hercules beetle and its fantastic optical metamaterial (and everything else in biology) must have arisen by a long, long series of random mutations, which just happened to lead up to marvelous designs.

Religion drives science, and it matters.