Metallic gold and silver colours in some insect cuticles

Broad bandwidth physical gold and silver cuticular interference reflectors were studied with a microspectroscope. Plusiotis optima adults reflect wavelengths throughout the human visible spectrum, appearing silver because of their high reflectivity. In P. gloriosa, the silver stripes are broad bandwidth interference reflectors whereas the green stripes reflect a narrow bandwidth. Pupae of Heliconius erato have regions which are silver and other parts where physical silver is combined with chemical tanning colouration to produce a gold colour. Developmental changes of silver in Antheraea pernyi larvae, of gold in adult Aspidomorpha tecta, and of silver and gold in pupal Heliconius erato are described. Reversible colour changes in living A. tecta adults were mimicked by placing pieces of cuticle in buffer solutions of different pH. During starvation the endocuticle which produces the physical colour is resorbed and the brightness reduces. The diversity of golds and silvers is discussed with reference to location of layers responsible, tanning, pigments, extracellular pH control, cuticular hydration, and possible defence mechanisms.