The forewings of jewel scarabs produce gold and silver by having 70 layers of chitin that become progressively thinner with depth resulting in different refractive indices.

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“A team of researchers at the University of Costa Rica has found that the
beetles’ metallic appearance is created by the unique structural
arrangements of many dozens of layers of exo-skeletal chitin in the
elytron, a hardened forewing that protects the delicate hindwings that
are folded underneath… In these beetles, the cuticle, which is just 10 millionths of a meter
deep, has some 70 separate layers of chitin—a nitrogen-containing
complex sugar that creates the hard outer skeletons of insects, crabs,
shrimps, and lobsters. The chitin layers become progressively thinner
with depth, forming a so-called ‘chirped’ structure. ‘Because the
layers have different refractive indices,’ Vargas says, ‘light
propagates through them at different speeds. The light is refracted
through—and reflected by—each interface giving, in particular, phase
differences in the emerging reflected rays. For several wavelengths in
the visible range, there are many reflected rays whose phase
differences allow for constructive interference. This leads to the
metallic appearance of the beetles.’ This is similar to the way in which a prism breaks white light into the
colors of the rainbow by refraction, but in the case of these beetles,
different wavelengths, or colors of light are reflected back more
strongly by different layers of chitin. This creates the initial palette
of colors that enable the beetles to produce their distinctive hues.” (Stark 2011:1)


Beetle bling: Researchers discover optical secrets of 'metallic' beetles

Journal article
Visible light reflection spectra from cuticle layered materialsOptical Materials ExpressApril 22, 2011
Cristian Campos-Fernández, Daniel E. Azofeifa, Marcela Hernández-Jiménez, Adams Ruiz-Ruiz, William E. Vargas

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Living System/s

Organism
Scarab BeetlesScarabaeidaeFamily

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