Blacker than black

Abstract

The colour black is found widely amongst the array of others used in a variety of biological species. In fact for those that use colours for sexual display or for other reasons of attracting attention, a black border or contrast heightens the visual impact of the coloured regions. The appearance of blackness has typically occurred as a feature of surfaces of low reflectivity and attributed to pigments strongly able to absorb incoming light of all wavelengths. So traditional studies of black coloration have focussed on the pigments involved.But new work suggests there may be more to blackness in animals than light-absorbing pigments. In some butterflies, at least, P. Vukusic and J. Sambles at the University of Exeter and C. Lawrence at DERA in Farnborough, UK, report in the latest edition of the Proceedings of the Royal Society of London, Series B (published online), that the minute details of the structure of the scales displaying the black colour may have additional light-absorbing properties to enhance the black effect. The researchers studied individual scales from the wing of a preserved male specimen of the butterfly species Papilio ulysses to test its light-absorbing properties. This butterfly displays two sorts of black on its wings; one a matt black, and the other a lustrous black. They found that both types of scales were able to absorb between 90 and 95 per cent of all normally incident light at each wavelength tested. They then tested them in a liquid which eliminated any structural involvement in light absorption so that light reaching the scale would only be subject to the absorption within it, and not to any interference, scattering or diffraction effects associated with the scale structure.Under these conditions, the researchers found that there was a 40 per cent decrease in the optical absorption of the scale from the matt black part of the wing but only 20 per cent from the lustrous parts.The researchers show that the detailed scale structure differences between the two types is responsible for the difference in visible blackness so that, in this case at least, pigments are not the whole story.View Large Image | View Hi-Res Image | Download PowerPoint Slide