Males butterflies in the family Lycaenidae, the so-called Blues, typically have brightly coloured, iridescent colours on the upper (dorsal) surfaces of their wings. Vivid blue iridescence such as this on the Purple Sapphire (Heliophorus epicles) shown here, is usually to do with courtship and mate recognition.

The brightly coloured, iridescent males rely on so-called, structural colouration (described below), which is used both in male-to-male interactions (competition), and in attracting females, via flickering or flashing their bright wings. The females are often dark brown and mostly lacking in these bright structural colours. They may – like female Purple Sapphires – have bright pigmentary colours (orange flashes in this case), but these are probably not secondary sexual characters, i.e. used in courtship and mating. I don’t have a picture of the female, but there are many examples on this website (1).

A variety of different types of microscopic ‘nanostructures’ – extremely small regular structures – have been found to generate blue colours in lycaenid butterflies. Many have so-called multilayers – alternating layers of chitin and air – within the individual scales (2, 3).

Butterfly wings are covered on both sides by rows of tiny overlapping scales, a bit like very thin, flat roof tiles or shingles. Scales can vary markedly in size and shape across the wing of a butterfly, but depending on the species, there are about 200–600 scales per square millimetre of wing. The scales are very delicate, typically one or two microns (i.e. one thousand times smaller than a millimetre) in thickness, and are denuded by wear and tear as butterflies age.

It has been suggested that the fact that scales detach so easily is an adaptation to allow butterflies (and moths) to escape from spider’s webs. (4). Scales that are attached to the sticky threads of the spider’s web can be sacrificed to allow the butterfly to regain its freedom.  

Each scale consists of two layers held together by a series of tiny pillars. The lower layer of the scale is flat and smooth – between 100 to 200 nanometres (one nanometre is a billionth of a metre) in thickness – whilst the upper layer consists of a series of longitudinal ridges or striae – about one or two microns apart – and transverse cross–ribs which create a three dimensional lattice, or honeycomb structure with windows into the interior of the scale (5). It is the elaborate 3-D nanostructures – so-called perforated multilayers – between the lamellae that cause the structural colours and phenomena like iridescence (3).

The reflected iridescence produced by light scattering from the dorsal wing scales of many lycaenids is highly directional, i.e. it is only observable from a narrow angular window. That is why the blue colour is not visible in some photographs (see below), although the scales can also be denuded.

The iridescence produced by male wings of butterflies such Heliophorus epicles, and countless other species, appears to be what is called a secondary sexual character. In other words, female butterflies evaluate these colours when choosing which males to mate with. They have also been called ‘colour badges’ and are thought to be honest signals, or reliable information if you will, of the condition of the males (6).  So the theory is that males with a good pedigree (i.e. genes) and a good upbringing (i.e. favourable environmental conditions) will be bright and showy (!), and females will choose them on the basis that they are more likely to be vigorous and fertile.

Presumably because they are ‘costly’ to produce or difficult to generate, and the scales producing the effect are lost, or worn down as the male butterflies age, then structural colours appear to provide a good indication of male quality and vigour in some species. However, even old and worn males – like the individual shown in the following photograph – still have some iridescent scales with which to attract the ladies!

Although there is, as far as I know, no definitive evidence that female butterflies choose between males on the basis of the quality of the intensity, hue or saturation of their reflective colours, the available evidence supports the idea that brilliant male structural colours evolved as a result of sexual selection (7). It seems that sexual selection in butterflies has homed in on the brightness of these structural colours in the same way that it has in terms of the brightness and ornamentation of the peacock’s tail feathers.

I have focused on the blue patches on the upper sides of the males wings in this blog. The bright yellow and red colours on the undersides also clearly have some function, but it is probably not to do with mating (I’m only guessing!) as the males and females look relatively similar on their undersides. Who knows what really goes on in the minds of these butterflies!

All of these photographs were taken in Thailand.

1. Mazumder, S. 2017. Heliophorus epicles Godart, 1823 – Purple Sapphire. Kunte, K., P. Roy, S. Kalesh and U. Kodandaramaiah (eds.). Butterflies of India, v. 2.24. Indian Foundation for Butterflies.
   http://www.ifoundbutterflies.org/sp/728/Heliophorus-epicles
2. Vértesy, Z., Bálint, Z., Kertész, K., Vigneron, J. P., Lousse, V., & Biró, L. P. (2006). Wing scale microstructures and nanostructures in butterflies − natural photonic crystals. Journal of microscopy, 224(1), 108-110. 
3. Wilts, B. D., Leertouwer, H. L., & Stavenga, D. G. (2008). Imaging scatterometry and microspectrophotometry of lycaenid butterfly wing scales with perforated multilayers. Journal of The Royal Society Interface, rsif-2008. 
4. Eisner, T., Alsop, R., & Ettershank, G. (1964). Adhesiveness of spider silk. Science, 146(3647), 1058-1061.
5. Stavenga, D. G. (2014). Thin film and multilayer optics cause structural colors of many insects and birds. Materials Today: Proceedings, 1, 109-121.
6. Kemp, D. J. (2006). Heightened phenotypic variation and age-based fading of ultraviolet butterfly wing coloration. Evolutionary Ecology Research, 8(3), 515-527.
7. Kemp, D. J., Vukusic, P., & Rutowski, R. L. (2006). Stress‐mediated covariance between nano‐structural architecture and ultraviolet butterfly coloration. Functional Ecology, 20(2), 282-289.