Science Literature

For a long time it has been known that many highly coloured surfaces of living organisms are not the result of pigments or dyes, but are the effects of highly ordered structural mechanisms. These structures are only apparent at nanometre scales (comparable to visible light), and colours are generated by interference. Recent interest in these photonic structures is

"mainly because these media happened to be very interesting examples of optical metamaterials, that draw their optical properties from highly-tunable submicron geometric shapes, rather than from the nature of the materials used to make them. These complex structures, found on many living species: birds, insects, snakes, fish and even mammals, could be a very effective and inspirative track to new visual effects or even new optical devices. The structure described in the present study is very special, because its optical response can be drastically changed by its exposure to humidity. This phenomenon is one of the astonishing characteristics of a tropical beetle, Dynastes hercules."

The Hercules Beetle, one of the largest Coleopterans reaching up to 170 mm in length. It is reputed to be the strongest creature on Earth, carrying loads that are 850 times its own body weight. (The smaller beetle is Eudicella gralli).

The male beetle is normally a greenish colour, but when the humidity rises above 80%, it turns black. Water penetrates the multilayer structure and changes its optical performance. Darwinian adaptationist stories have been proposed. One hypothesis is that the black colour provides camouflage at night when the humidity level is highest and the insect is most active. Then, in the day, the greenish colour returns and the beetle blends in with its environment. Another hypothesis concerns thermoregulation: the insect warms up faster when it is black and then avoids overheating when it is green. Both these proposals are discussed by the researchers and both are rejected. "The reason (if any. . . ) why Dynastes hercules has evolved the ability to change colour with humidity is still a mystery which waits to be unveiled." The authors explain the purpose of their work in this way:

"The aim of the present study is to provide updated data on the morphology of this natural hygrochromic structure, to obtain detailed optical data for various humidity states and to re-examine the colouration mechanism with the help of extensive numerical light-scattering simulations."

These aims have been achieved using a reverse engineering methodology. The hygrochromic structure has been successfully modelled. Theoretical reflectance curves reveal a green colouration for the material under normal humidities and a black colouration when the spaces in the structure are filled with water.

"Hygrochromic behaviour could be an important property of an 'intelligent' material. Such materials could be put to work as humidity sensors, perceptibly changing colour according to the hygrometry level. This could be useful for example in food processing plants to monitor the moisture level. Since optical properties can be transferred to other radiative spectral ranges by scaling the structure lengths up or down, hygrochromic materials could also find thermal uses."

The take-home message for this blog is that design methodologies work and yield outcomes that are practically useful. The authors are explicit on this point: "the present paper addresses the reverse engineering of this complex system in a more complete way and clarifies several aspects of this hygrochromic effect." By contrast, darwinian perspectives appear to have contributed only speculative 'just-so' stories.

Diffractive hygrochromic effect in the cuticle of the hercules beetle Dynastes hercules
M Rassart, J-F Colomer, T Tabarrant and J P Vigneron
New Journal of Physics, 10 (March 2008) 033014 (14pp) | doi:10.1088/1367-2630/10/3/033014

Abstract. 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 μm 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.