Humidity Changes Exoskeleton Color

photograph of green beetle with black spots standing on a rock

Biological Strategy

Hercules beetle

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Sense Atmospheric Conditions

For some living systems, the ability to detect changes in atmospheric conditions can be very valuable. By predicting changes in regional weather or in very localized conditions, living systems can avoid or take advantage of those changes. Since such adjustments can be very subtle, living systems must be able to detect miniscule variations in moisture, barometric pressure, ions in the air, and other environmental cues. Many insects and birds, for example, can predict oncoming storms and take cover before their lives are put at risk.

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Modify Light/Color

Color in living systems comes from pigments and the interaction of light with surfaces. Color serves many purposes for living systems, such as attracting prey or mates, providing warnings, or protecting through camouflage. To create the effects needed for each purpose, living systems must control the expression or visibility of pigments and the interactions (such as reflectance and refraction) of light. To do so, they have strategies that modify color or light to increase or decrease the color’s position, intensity, opacity, and more. Male hummingbirds, for example, have brightly colored feather patches on their throats; the coloring comes from pigments, structures that refract light, or a combination of the two. When a male hummingbird hovers near a potential mate, it modifies the angle of these feathers to create a bright, colorful display. However, when it needs to mute the colors to reduce conspicuousness, such as to avoid a predator, it modifies the angle again.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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The exoskeleton of the Hercules beetle changes from green to black with increasing humidity using thin film interference by reversible modification of layer thickness.

Image: Jessie Kiesow /

Image: Marie Rassart /

Provided with permission from Marie Rassart, Laboratoire de Physique du Solide, University of Namur, Belgium.

Image: Marie Rassart /

Provided with permission from Marie Rassart, Laboratoire de Physique du Solide, University of Namur, Belgium.

Image: Sogang University, Korea /

Photos provided for use on AskNature by Seung-Yop Lee of Sogang University.

“The Hercules beetle, Dynastes Hercules [sic] L., can change the colour of its elytra—horny fore-wings—from black to greenish yellow and back again to black all within a few minutes. It does this in a way previously unknown among insects. Apart from the reversible migrations of granules in the iris cells, physiological or rapidly reversible colour changes are very rare in insects. Among beetles, Coptocyclia, Aspidomorpha, and many other Cassidinae can change the colour of their elytra by varying the amount of water in the cuticle and thereby the thickness of the thin films responsible for the interference colours.” (Hinton and Jarman 1972:160)

“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 3μm 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.” (Rassart et al. 2008:1)

“The visible dry-state greenish coloration originates from a open porous layer located at 3 μm below the cuticle surface. This layer has three-dimensional photonic crystal structures, which are a network of filamentary strings, arranged in layers parallel to the cuticle surface [Fig. 1d]. In dry state, nanosized holes in the layer are occupied with air (refractive index 1) but the empty holes are filled with water (refractive index 1.33) under high humidity. The change in refractive index with respect to the humidity level induces the variation in the visible color.” (Kim et al. 2010:103701-1)

Physiological Colour Change in the Hercules Beetle
H. E. HINTON & G. M. JARMAN
Nature 238, 160-161 (21 July 1972) | doi:10.1038/238160a0; Received 24 December 1971

Last Updated October 16, 2016

References

Journal article

Physiological colour change in the elytra of the hercules beetle, Dynastes hercules

Journal of Insect Physiology | 25/10/2003 | H.E. Hinton, G.M. Jarman

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Reference

Journal article

Diffractive hygrochromic effect in the cuticle of the hercules beetle Dynastes hercules

New Journal of Physics | 12/03/2008 | M Rassart, J-F Colomer, T Tabarrant, J P Vigneron

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Reference

Journal article

Biologically inspired humidity sensor based on three-dimensional photonic crystals

Appl. Phys. Lett. | 07/09/2010 | Jae Hyun Kim, Jun Hyuk Moon, Seung-Yop Lee, Jungyul Park

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Reference

“The Hercules beetle, Dynastes Hercules [sic] L., can change the colour of its elytra—horny fore-wings—from black to greenish yellow and back again to black all within a few minutes. It does this in a way previously unknown among insects. Apart from the reversible migrations of pigment granules in the iris cells, physiological or rapidly reversible colour changes are very rare in insects. Among beetles, Coptocyclia, Aspidomorpha, and many other Cassidinae can change the colour of their elytra by varying the amount of water in the cuticle and thereby the thickness of the thin films responsible for the interference colours.” (Hinton and Jarman 1972:160)

Journal article

Physiological color change in the Hercules beetle

Hinton, HE; Jarman, GM

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