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.

GENERAL SCIENTIFIC SUMMARY
Introduction and background. Hygrochromic materials have recently raised the attention of part of the scientific community. They consist of a porous structure which can be infiltrated with a liquid or a gas. This infiltration induces a change of colour due to the variation of the refractive index when the liquid replaces air in the medium, or to swelling effects. Such a phenomenon is observed on the cuticle (i.e. the carapace) of the beetle Dynastes hercules due to penetration of water vapor when humidity rises above 80%.

Main results. Scanning electron microscopy gave access to the structure responsible for the greenish colouration of the beetle, changing to black when infiltrated with water. The change of colour has been described in detail from spectrophotometry data. Finally numerical simulation showed that, in contrast to current belief, diffraction is involved in the colouration mechanism. A noticeable fact about this research is that nature has developed such a hygrochromic material through thousands of years of evolution and, in order to engineer the same function, one does not have to repeat the long trial and error sequence followed by natural selection.

Wider implications. X-chromic (meaning changing colour due to X, such as hygrometry hygro, temperature thermo, electric voltage electro...) materials are highly attractive today. A visible change of colour, easily observed by a human eye or a spectrophotometer, gives information about some environmental change. This could be useful for example in food processing to monitor parameters such as the moisture level.

Figure. The Dynastes hercules is greenish under normal humidity conditions (left). It becomes black when the humidity level is above 80% (right).

Received 14 January 2008
Published 11 March 2008