There are plenty of blue organisms in the world, but blue is nevertheless the rarest pigment found in nature. Most blue colours are produced by physical effects and are called structural colours. There are no true blue pigment in plants and thus no true blue flowers! There are blue flowers of course, but they produce this colour using modified red (anthocyanin) pigments.
Many blue colours in nature – from blue eyes to the blue sky – are caused by Tyndall scattering. Blue occurs because of the presence of tiny particles (or voids) about the same size as the wavelength of blue light (c. 500 nm). The smaller wavelengths (blue and violet colors) of the incident white light are scattered, and the longer wavelengths (red and yellow) pass undisturbed through the medium.
In butterflies like the beautiful blue Morpho (below), structural blue colours are produced by interactions between incident light and reflections caused by surface structures on the wing scales. All structural colours produced by butterfly scales are as a result of what is termed coherent scattering (Prum et al., 2006). That is to say, ‘constructive interference’ between light waves scattered coherently by the nano-structures on the surfaces of the scales. Thin-film interference effects from the wing scales enhance the blue color and produces extra reflection peaks in the infrared and ultraviolet regions.
The scales on the wings are blue Morphos – and many other shiny blue butterflies – are covered with microscopic ridges running longitudinally down the scale (about 1,300 thin, parallel ridges per mm). This patterning creates a periodic or repeating structure – called a diffraction grating – in the horizontal scale plane. The ridges are constructed of a series of overlapping lamellae – called a multilayer system – each about 90-100 nm thick (depending on species), which in cross-section, appear as Christmas (or pine) tree-like geometric shapes running perpendicular to the longitudinal plane of the scale (see below).
These tiny stacked layers (e.g. 10-12 multi-layers) of cuticle stick out on either side of the thousands of ridges running the length of the scales, and produce interference effects in the blue part of the visible spectra. It is the interference in light between these layers of air and cuticle (with their different refractive indices) which produces the iridescent colours of the blue butterflies.
It is thought that these nano-structures are physiologically costly for an organism to produce, and as such they are good, honest indications of the quality of a butterfly. Not surprising therefore, some male butterflies display highly reflective blue UV patches to females, during courtship dances, and to other males, in territorial disputes. The brightness of these iridescent UV patches in the common eggfly, Hypolimnas bolina (below) appears to be a crucial factor in attracting females and achieving successful mating (Kemp, 2007). Males of this species have oval-shaped areas of bright blue/UV iridescence on the dorsal (upper) sides of both fore- and hind-wings.
The female Hypolimnas bolina butterflies lack these bright blue patches (below) but they do have some blue wing scales arranged in tiny blue spots.
Some butterflies do produce bluish colours – not true blue – using pigments. For example, the Common bluebottle, Graphium sarpedon (below) has wings in which the central bands have high concentrations of a bile pigment, called sarpedobilin. This molecule is combined with a strongly blue-absorbing carotenoid, lutein, resulting in blue-green or green coloured wing patches (Stavenga, 2010). These colored patches lack the usual wing scales found on the rest of the wing.
The pigment pterobilin also provides green for patches on the wings of the Tailed jay, Graphium agamemnon (below). Another butterfly which manages to get a blue colour from the same pigment, pterobilin, is the obrina olivewing, Nessaea obrinus.
So, whether it is by physics or chemistry, butterflies have evolved many ways of producing blues and greens, but true blue is – I think – always produced via the wonders of nanoarchitechtrure, microscopic patterns on the surfaces of the wing scales which do magic with the light simply by using the different light refracting properties of chitin and air.
Kemp, D.J. (2007a) Female butterflies prefer males bearing bright iridescent ornamentation. Proceedings of the Royal Society of London B: Biological Sciences, 274(1613), 1043-1047.
Prum, R. O., Quinn, T., & Torres, R. H. (2006). Anatomically diverse butterfly scales all produce structural colours by coherent scattering. Journal of Experimental Biology, 209(4), 748-765.
Stavenga, D. G., Giraldo, M. A., & Leertouwer, H. L. (2010). Butterfly wing colors: glass scales of Graphium sarpedon cause polarized iridescence and enhance blue/green pigment coloration of the wing membrane. Journal of Experimental Biology, 213(10), 1731-1739.
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