Nowhere in nature is there so much beautiful colour as on the wings of butterflies. Scientists, however, are still baffled about exactly how these colours are created. Marco Giraldo has been examining the structure of the surface of the wings of the cabbage white and other butterflies. Among the things he has discovered is why European cabbage whites are rebuffed more often than Japanese ones. Giraldo will be awarded a PhD by the University of Groningen on 25 January 2008.
The colours on butterfly wings are used as an advertisement. The patterns on the wings enable butterflies to recognize their own species at a distance and differentiate between males and females - rather handy when you're hunting for a partner. Just like a pointillist painting, the surface of the wing is constructed of a huge collection of coloured dots, called scales, each about 50 x 250 micrometers in size.
However, scientists don't yet know very much about how the colour on the wings is formed. What they do know is that the colours are created in two different ways: via pigments and via nanostructures on the scales, which ensure that light is distributed in ways that are sometimes spectacular. These so-called structure colours can clearly be seen on the morpho butterflies of the South American rainforests.
Marco Giraldo examined the structure and the pigments of the wings of the cabbage white and other Whites from the Pieridae family. The physicist chose the Whites because they have relatively simple pigmentation. By comparing the scales of various sorts under an electron microscope, he discovered how the colouration of Whites is caused. Giraldo is the first to clarify how the colour of these butterflies is influenced by the nanostructural characteristics.
Although the spatial structure of a scale depends on the type of butterfly, there are a number of general characteristics: A scale consists of two layers, linked by pillars. The undersurface is virtually smooth and without structure, but the upper surface is formed by a large number of elongated, parallel ridges, about one to two micrometers from each other. The colour is determined by the dispersal of light by the scale structures and by the absorption of light by any pigments present. The pigments of the cabbage white, for example, absorb ultraviolet light and the brimstone blue light. At the same time they also scatter white or yellow light respectively.
Giraldo also discovered that the wings of Whites are constructed in a surprisingly effective way. Both sides of the wings have two layers of overlapping scales that reflect light. The more scales there are, the more light is reflected. This light reflection is very important as butterflies want to be seen. Giraldo discovered that these two layers form an optimal construction: with more than two layers the reflection may be improved, but the wing would become disproportionately heavy.
Giraldo has also discovered why Japanese male cabbage whites are better at recognizing females than European cabbage whites, who still make mistakes in this area. This is because the wings of Japanese male and female cabbage whites differ subtly, unlike those of their European relatives: the scales on the wings of Japanese female cabbage whites lack specific pigment grains, those that ensure that UV light is absorbed. Males do have these pigment grains, as do both sexes of the European cabbage whites. This difference makes it easier for Japanese male cabbage whites, who unlike humans can see UV light, to differentiate between males and females.
New colour methods can be developed using the knowledge derived from Giraldo's research. It may be possible to apply the nanostructures observed in butterflies to create impressive optic effects in paint, varnish, cosmetics, packaging materials and clothes. Industry is thus following butterfly wing research with great interest.
Nanostructure is an object of intermediate size between molecular and microscopic (micrometer-sized) structures.
In describing nanostructures we need to differentiate between the number of dimensions on the nanoscale. Nanotextured surfaces have one dimension on the nanoscale, i.e., only the thickness of the surface of an object is between 0.1 and 100 nm. Nanotubes have two dimensions on the nanoscale, i.e., the diameter of the tube is between 0.1 and 100 nm; its length could be much greater. Finally, spherical nanoparticles have three dimensions on the nanoscale, i.e., the particle is between 0.1 and 100 nm in each spatial dimension. The terms nanoparticles and ultrafine particles (UFP) often are used synonymously although UFP can reach into the micrometre range.