A drab brown butterfly rests on a tree trunk, its jagged outline providing a perfect camouflage against the moss, twigs and broken branches of the tree. The underside of this butterfly, a Camberwell beauty (Nymphalis antiopa), looks like the bark itself, dark brown and wrinkled, almost like a piece of charred wood (below).

An alien from outer space, who had never seen a butterfly before, would not be expecting the bright red and blue colours on the upperside (below).

We know that the upper (dorsal) wing surfaces of butterflies are often brightly coloured like this, it’s what attracts them to us. But it is worth thinking for a moment about the contrast with the dull and inconspicuous undersides. Clearly, not all butterflies are like this – some have black upper wings surfaces – but for many there is a marked difference between the two wing surfaces. The Small tortoiseshell (Aglais urticae) is another example (below).

This difference between the top and bottom sides – open and closed wings – reflects their contrasting functions of course. The upper surfaces are used to signal messages – warnings or startling flashes aimed at predators, or sexual signals between males and females – whereas the under surfaces, visible when the butterfly is at rest with its wings held vertically, are a form of camouflage.

Simple but clever. The butterfly can ‘disappear’ by closing its wings and moving to a sheltered site. Or it can become wonderfully apparent, flashing its colours in the sun to advertise itself to its fellows. Exactly what is it saying with these bright, iridescent scales, positioned so prominently along the margins of both wings? Remarkably similar patches of blue, iridescent scales are displayed on both of these nymphalid species (below).

Surprisingly, we don’t really know exactly how these butterflies use these bright iridescent patches. Or at least I am not aware of any studies on these species. Bright and colourful, light-reflecting scales like this are usually composed of photonic crystals, which manipulate light to produce high levels of reflectance. These scales are energetically expensive to produce so they must have an important function. It is probably to do with sex, or mating, but both sexes – males and females – have them. It is surprising – to me at least – that although we know all about the genes and DNA of butterflies, we don’t always know very much about their behaviour!

This property of being able to hide in some situations, whilst being conspicuous and highly apparent in others, is at the heart of butterfly biology, and is I think, why we find them so interesting. Butterflies have evolved remarkable ways of achieving these conflicting objectives. Signal partitioning, as it is called, is a solution to conflicting evolutionary pressures. Namely, forewing traits (markings and colours) are concerned with communication between the sexes – which are sexually selected and rapidly evolving – whilst hindwing patterns providing camouflage, are more fixed, or stabilised as a result of natural selection (see Oliver et al., 2009).

In conclusion, there are a spectacular variety of mechanisms by which butterflies have evolved ways of achieving these contrasting functional ends – i.e. avoiding or evading predators;  communicating with others of the same species – including light absorbing pigments, light-reflecting scales and so-called photonic crystals, which are housed in the beautiful blue scales.

Oliver, J. C., Robertson, K. A., & Monteiro, A. (2009). Accommodating natural and sexual selection in butterfly wing pattern evolution. Proceedings of the Royal Society of London B: Biological Sciences, 276(1666), 2369-2375.