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CAS Colors of Chemistry
Metallic-green is most frequently displayed by beetles, but other metallic colors are also observed - blue, red, gold, silver, and purple. Why do bugs have the ability to display iridescent colors while mammals have only shades of brown or black? A beetles body is a thick cuticle comprised of multiple laminated layers of thin-film chitin with sometimes a crystal lattice microstructure that refract and polarize light-waves. If "beauty is as beauty does" what use do beetles have for iridescence? To answer, one must look beyond mammalian-centric vision to see what beetles see. Unlike insects and their avian predators, mammals cannot see colors (primates excepted) or short ultraviolet light-waves. Suppose you are a male rose chafer sitting on a green leaf. Green-on-green, you are invisible to mammals. A female chafer alights; you shift the angle of your body to refract a "come-hither" color. A male rival interrupts. You re-orient yourself and signal "get lost!" A pair of beady bird-eyes startles you. You reposition, take aim, and blind the predator with glare as you fly away.
Related Records from CAplus22: 14770 Structural colors in insects. III. Mason, C. W. Journal of Physical Chemistry 1927, 31 1856-72. cf. C. A. 21, 2217. "Metallic" iridescent insect integuments owe their color phenomena to a thin laminated layer at or just beneath the surface, which acts as a multiple thin film. The color-producing structure may be relatively thick with many more laminae, as in Plusiotis gloriosa where it is embossed rather than smooth. "Enameled" iridescent integuments owe their color phenomena to a thick multiple film layer whose properties are modified by minute rods which perforate it. Diffraction iridescence does not occur in "metallic" or "enameled" integuments but the former has been identified in Serica. Selective reflection ("surface color") is not exhibited by iridescent insect integuments. A detailed discussion is given of the properties and criteria for identification of the several types of iridescence. 88: 101973 Metallic gold and silver colors in some insect cuticles. Neville, A. C. Dep. Zool., Univ. Bristol, Bristol, UK. Journal of Insect Physiology 1977, 23(10), 1267-74 (Eng). A microspectroscope was used to show that the cuticle of Plusiotis optima reflected wavelengths throughout the human visible spectrum, appearing silver because of their high reflectivity. In P. gloriosa, the silver stripes were broad bandwidth interference reflectors whereas the green stripes reflected a narrow bandwidth. Pupae of Heliconius erato had silver regions and also silver combined with chem. tanning to produce a gold color. Developmental changes of silver in Antheraea pernyi larvae, of gold in adult Apsidomorpha tecta, and of silver and gold in pupal H. erato occurred. Reversible color changes in living Aspidomorpha tecta were mimicked by placing the cuticle in different pH solns. During starvation, the endocuticle was resorbed and the brightness reduced. 149: 195798 Discovery of a diamond-based photonic crystal structure in beetle scales. Galusha, Jeremy W.; Richey, Lauren R.; Gardner, John S.; Cha, Jennifer N.; Bartl, Michael H. Department of Chemistry, University of Utah, Salt Lake City, UT, USA. Physical Review E: Statistical, Nonlinear, and Soft Matter Physics 2008, 77(5-1), 050904/1-050904/4 (Eng). We investigated the photonic crystal structure inside iridescent scales of the weevil Lamprocyphus augustus. By combining a high-resoln. structure anal. technique based on sequential focused ion beam milling and SEM imaging with theor. modeling and photonic band-structure calcns., we discovered a natural 3-dimensional photonic structure with a diamond-based crystal lattice operating at visible wavelengths. Within individual scales, the diamond-based structure is assembled in the form of differently oriented single-cryst. micrometer-sized pixels with only selected lattice planes facing the scales top surface. A comparison of results obtained from optical microreflectance measurements with photonic band-structure calcns. reveals that it is this sophisticated microassembly of the diamond-based crystal lattice that lends L. augustus its macroscopically near angle-independent green coloration. Additional Information
Updated: 1/23/2009 3:50:38 PM
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