MULTILAYER REFLECTORS IN ANIMALS USING GREEN AND GOLD BEETLES AS CONTRASTING EXAMPLES

The light reflectors in the beetles Calloodes grayanus and Anoplognath us parvulus are examined. Contrasting multilayer reflectors are reveal ed, Calloodes grayanus appears a weak green colour, matching its backg round leaves, while A. parvulus is strongly metallic-gold coloured, Th e former reflection is diffuse, as the result of a structure causing s cattering that overlies the multilayer reflector, whereas the latter r eflection is strongly directional, The green colour of C, grayanus is achieved by a multilayer reflector with a fixed spatial periodicity, h ere termed 'regular', which is far removed from the quarterwave, or ph ysically 'ideal', condition. The gold colour of A. parvulus is achieve d by a type of reflector which involves systematically changing optica l thicknesses of the component layers with depth in the structure. A l ayer of melanin underlies the reflector of C, grayanus to absorb the t ransmitted portion of light and prevent its back-reflectance, which wo uld otherwise alter the green colour. The resultant structural reflect ance from C, grayanus effectively matches green pigments, which are ra re in beetles.