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Summaries of papers by Dr Andrew R. Parker

Parker, A.R., McKenzie, D.R. & Large, M.C.J. (1998) Multilayer reflectors in animals using green and gold beetles as contrasting examples. The Journal of Experimental Biology 201, 1307-1313.

Three causes of structural colours in animals. A) scattering of lights by small particles (black dots) in a colloidal system. A white or blue diffuse reflection can result under white light. B) a multilayer reflector, here designed to reflect gold light (as found in gold beetles). C) a diffraction grating, here reflecting 1st and 2nd order spectra.

Multilayer reflectors have been the subject of many classical studies in the field of animal structural colours, beginning with Hooke's and Newton's identifications of such a structure in peacock tail feathers (Micrographia, 1665, Optiks, 1704). Multilayer reflectors cause metallic reflections in many beetles. A green beetle, Calloodes grayanus, possesses a multilayer reflector with a fixed spacial periodicity, here termed "regular", which is far removed from the quarterwave, or physically "ideal", condition, but reflects only a narrow range of wavelengths. A structure causing scattering overlies this reflector. As a result, its reflectance is weak and matches the reflectance of background leaves. Hence, camouflage is achieved (this mechanism may be an effective substitute for green pigments, known to be difficult to produce in beetles).

In contrast, the gold beetle, Anoplognathus parvulus, is highly reflective. The gold colour results from a type of multilayer reflector which involves systematically changing optical thicknesses of the component layers with depth in the structure (a "chirped" stack). Hence, a range of wavelengths are reflected in the same direction, which superimpose to form a gold colour. This species is conspicuous, presumably to attract conspecifics.

Transmission electron micrograph of the (chirped) multilayer reflector in the cuticle of the gold beetle Aspidomorpha tecta. Scale = 1 micron.

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