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What Gives Chrysina Beetles their Metallic Appearance?

This morning, I stumbled across this fantastic photograph from Thomas Shahan (@ThomasShahan) that was produced for the Oregon Department of Agriculture.  It shows the same Chrysina sp. beetle illuminated under different lighting conditions.

The beetles are famed for their characteristic “jeweled” appearance that easily catches the eye with its brilliant shades of gold or silver.  But what is it that imparts all this glitz onto these curious little Coleopterans?  Furthermore, why would evolution select for such a seemingly conspicuous appearance?

Chrysina strasseni.  Image: Mauricio Gigli
Chrysina strasseni. Image: Cosmin O. Manci

In 2011, a study published by researchers at the University of Costa Rica provided an answer.  The researchers postulated that the layers of chitin making-up the beetles’ cuticle likely had different refractive indices which split the light into various wavelengths that subsequently recombined to create the metallic appearance through some form of collective interference.  In other words, the layered material within the cuticle was separating the light like a prism and then progressively combining and reflecting those ‘parcels’ of light to both amplify the their brightness and create the glimmering colors.

The device used to measure the reflectance of light as it contacted the Chrysina beetles.
Device used to carry out the direct reflectance measurements under normal incidence of non-polarized light on the elytron of a beetle. Image: Cristian Campos-Fernández, Daniel E. Azofeifa, Marcela Hernández-Jiménez, Adams Ruiz-Ruiz, William E. Vargas, Giovanni Piredda, Opt. Mater. Express 1, 85-100 (2011)

Using a device specifically designed to measure the reflection of light as it struck the beetles’ elytra, the researchers demonstrated that when light strikes the interface between each successive layer of chitin, one portion of that light is reflected and another portion is transmitted down to the next interface.  As this occurs successively down the layers of chitin, the brilliant gold or silver metallic effects are produced.

According to the Optical Society of America, which operates the open access journal in which the study was published, “In the two beetle species, interference patterns are produced by slightly different wavelengths of light, thus producing either silver or gold colors.  For the golden-like beetle, the constructive interference is found for wavelengths larger than 515 nm, the red part of the visible wavelength range, while for the silver-like beetle it happens for wavelengths larger than 400 nm — that is, for the entire visible wavelength range.

Reflection spectrum of C. aurigans.
Caption from Article: Fig. 3. Reflection spectra of C. aurigans beetle when illuminating an area of its head or a rear section with non-polarized radiation under near normal illumination. The reflectivity spectrum of gold, evaluated from its optical constants taken from the literature [ 8 ], and the eye sensitivity curve are included in the figure. Image: Cristian Campos-Fernández, Daniel E. Azofeifa, Marcela Hernández-Jiménez, Adams Ruiz-Ruiz, William E. Vargas, Giovanni Piredda, Opt. Mater. Express 1, 85-100 (2011).
Reflection spectra from the University of Costa Rica Study.  Image sourced from Article.
Caption from Article: Smoothed reflection spectra of chirped multilayer structures with linearly decreasing values in the thickness of the sequence of layers, and low contrast between the refractive indices of successive layers. The number of layers is 68 for both cases: (a) C. aurigans , and (b) C. limbata . The larger and lower thicknesses values were set as indicated in the text. The blue thin solid lines correspond to reflection measurements. Image: Cristian Campos-Fernández, Daniel E. Azofeifa, Marcela Hernández-Jiménez, Adams Ruiz-Ruiz, William E. Vargas, Giovanni Piredda, Opt. Mater. Express 1, 85-100 (2011).

All that is fantastic, but shouldn’t a brilliantly gold or silver beetle stand out in the crowd a bit and attract the attention of predators?  Recalling a previous post on how harlequin beetles can be camouflaged using a bright orange coloration, conspicuousness is really a subjective observation and depends on the conditions of both the observer and the environment.  While the beetles may be very conspicuous in a laboratory or when viewed under artificial light, the researchers from the University of Costa Rica suggest that they may blend in quite well with the under the conditions of their native rainforest habitat.

Chrysina sp. by Thomas Shahan for Oregon Dept of Agriculture.
Photograph of the same Chrysina sp. under different lighting conditions. Image by Thomas Shahan for Oregon Dept of Agriculture.

According to William Vargas, the lead researcher on the study, “The metallic appearance of these beetles may allow them to be unnoticed, something that helps them against potential predators.”  Their elytra “reflects light in a way that they look as bright spots seen from any direction.  In a tropical rainforest, there are many drops of water suspended from the leaves of trees at ground level, along with wet leaves, and these drops and wet leaves redirect light by refraction and reflection respectively, in different directions. Thus, metallic beetles manage to blend with the environment.

Fascinating!…

Further Reading:
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