Reflector Causes Color and Surface Change

a beetle with a shiny gold exoskeleton rests on a green leaf

Biological Strategy

Tortoise beetle

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Sense Light (Visible Spectrum) From the Environment

Living systems constantly receive signals from their environment that help them survive. Light (in the visible spectrum) can come from other living systems (such as fireflies) or from non-living sources (such as the sun). Survival often depends on sensing and responding to challenges like low light conditions or light that has been altered in some way. Because basic survival is at stake, living systems must excel at meeting those challenges. A well-known phenomenon is how water bends light. A stork trying to catch a fish underwater can compensate for this bending effect so that when it strikes at the fish, it has a good chance of catching it.

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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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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 tortoise beetle changes color and reflective properties due to a chirped multilayer reflector filled with grooves that fill and empty of fluid to cover and reveal, respectively, the bottommost layer.

When gazing upon the golden tortoise beetle one may think they are observing a dew drop on the surface of a leaf, for its metallic sheen gives off a reflective glare. One glance away, however, and one may think the beetle has disappeared to be replaced by a red lady beetle. Not to be fooled, this insect is the same one as before! Under the hard, transparent armor of the beetle is an intricate multilayer filled with a pattern of grooves. The layers become thicker farther down the layered column (a structure referred to as a “chirped” multilayer).

Moisture causes humidity to fill these grooves. When the beetle is disturbed, in virtually any manner, the fluid in these grooves is displaced in the top-most parts of the multilayer thus revealing a deep, less-reflective red-color in the bottommost layer. This layer manifests a wide-angle diffusion, lacking the metallic properties that the gold coloring displayed. This type of morphism is explained using the “switchable mirror theory” where random porous patches provide a scattered pattern of space in which moisture may be displaced. This contradicts many well known theories where a “hydraulic mechanism” is used to explain color change when liquid is injected into an area (as opposed to displaced out of an area). The remarkable thing about the golden tortoise beetle is that it is able to toggle between these two very different colors and shading. The full mechanism is not entirely understood, but it is certain that if it could be understood, applications in the textile and sensory areas of development could benefit greatly.

Last Updated October 25, 2016

References

“The tortoise beetle Charidotella egregia is able to modify the structural color of its cuticle reversibly, when disturbed by stressful external events. After ?eld observations, measurements of the optical properties in the two main stable color states and scanning electron microscope and transmission electron microscope investigations, a physical mechanism is proposed to explain the color switching of this insect. It is shown that the gold coloration displayed by animals at rest arises from a chirped multilayer re?ector maintained in a perfect coherent state by the presence of humidity in the porous patches within each layer, while the red color displayed by disturbed animals results from the destruction of this re?ector by the expulsion of the liquid from the porous patches, turning the multilayer into a translucent slab that leaves an unobstructed view of the deeper-lying, pigmented red substrate. This mechanism not only explains the change of hue but also the change of scattering mode from specular to diffuse…We can refer to this behavior as ‘hygro-chrome,’ underlining the change of color with varying hygrometry. Tunable materials like electrochrome ?lms change color with varying applied electric ?elds or thermo-chrome ?lms that change color with varying temperatures all have a strong potential for applications in sensing or switching devices” (Vigneron et al. 2007: 1, 10).

Journal article

Switchable reflector in the Panamanian tortoise beetle Charidotella egregia (Chrysomelidae: Cassidinae)

Physical Review E | 12/09/2007 | Jean Pol Vigneron, Jacques M. Pasteels, Donald M. Windsor, Zofia Vértesy, Marie Rassart, Thomas Seldrum, Jacques Dumont, Olivier Deparis, Virginie Lousse, László P. Biró, Damien Ertz, Victoria Welch

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