Butterfly wings can serve a variety of different functions, enabling them to fly, hide, startle and fool predators, warm up, identify each other, and last but not least, choose mates. They do this in a myriad different ways but certain general patterns emerge. For example, in butterflies which can rest with wings folded upright, the undersides are often the means by which they blend into the background. The subtle colours and patterns are a form of camouflage which enables them to effectively disappear – from the eyes of a predator – at the flick of a muscle!
Uppersides on the other hand can be a lot more showy (below), flashes of bright colour and iridescence, which can be shown off when the coast is clear. Often it is the males who sport the most showy adornments, hoping to impress a female enough that she will choose to mate with him. Scientists call this ‘partitioning the function’, which is a fancy way of saying that butterflies divide the purpose of their wings between the two surfaces, enabling them to switch from one behaviour to another, in an instance. The little South East Asian butterfly known as the Common Gem (below) is a good example of this dichotomy of function.
It is also worth remembering that even though the colours and patterns of butterfly wings amaze and enthrall us, we only see a limited selection of the palette observable to the butterflies themselves; for example, the splendour of UV colours eludes us as we lack the ultraviolet receptors they possess.
Although much is known about what I call the ‘language of spots’ on butterfly wings, it has often been worked out for a small number of species, and it is not at all clear whether this knowledge translates from one species to another. For example, much work has been done on the Squinting bushbrown, Bicyclus anynana, a small brown African butterfly that is widely used in laboratory research. It has been shown, that in this species, females choose males on the basis of their dorsal forewing eyespots. The UV-reflectivity of these so-called ‘sexual ornaments’ – in particular small, white, UV-reflecting wing scales at the centre of the eye-spots are what the females are interested in, and how they choose which males to mate with during courtship. The flickering spots, together with male pheromones, act to turn the female on to the male!
Knowing that features such as this – there are many other examples – on butterfly wings are used by one sex to make choices about the other, perhaps we should be on the look out for potential signs and signals on unstudied species? The means by which partners choose each other as mates, is to the best of my knowledge, unknown for many, if not most, of our common species. They have all these wonderful colours and patterns, but we don’t really know how the butterflies use them in terms of charming and winning over each other! Assuming that they don’t just rely on pheromones? We can only observe and speculate.
The upper wings of Fritillary butterflies (below) are beautifully complex patterns which vary subtly from species to species but probably all function as some sort of disruptive pattern which breaks up the outline of the butterfly and allows it to merge in with the background? The pattern – the Fritillary theme as it were – is so widespread it must have a useful function? And butterflies must also know their own pattern and be able to distinguish members of their own species. Are there any particular patterns or markings which might be used in courtship? And would they have any of the features – such as special scales – which have been shown to be used in other butterflies like the the Squinting bushbrown?
It was whilst looking at images I had taken of Small pearl-bordered fritillary butterflies (Boloria selene) in Spain, that I noticed how prominent the round black spot was on the underside of the hindwings; particularly when viewed from below (below). It almost looks like an eye!
Such a prominent spot! A white bordered black dot with brown scales in the middle. But it is quite a variable feature (try Googling Boloria selene and checking the numerous images!). There does not appear to be any consistent difference between the sexes (as can be seen by comparing mating pairs). Are there special scales within the dot? Could it has some function or reflectance we cannot see? The white and brownish scales within the black spot can be seen here and here and in my own photographs (below).
The prominence of these spots is particularly striking when viewed from below, as on this photo here, and on this pinned specimen showing the underside (below).
The idea that this spot may play some role in the courtship and mating of this species, is pure speculation on my part, but as we have seen, the idea has precedents in other species and it is the only spot on the underside with such elaborate features. Namely a white ring and coloured scales within. Butterflies have remarkable eyesight and could certainly distinguish such features at close range, in my opinion.
Perhaps some experts out there might like to comment, or shoot down my suggestion! Has anyone ever witnessed courtship in this species? Male butterflies often flicker their wings during courtship; but it would be rather difficult to decide if the females were watching the spots or some other compelling features! Maybe there are scent scales within the spots?
So much to learn in nature and so little time to study all the wonderful things out there!
Robertson, K. A. & Monteiro, A. (2005). Female Bicyclus anynana butterflies choose males on the basis of their dorsal UV-reflective eyespot pupils. Proc. R. Soc. Lond. [Biol] 272, 1541–1546.
San Martin, G., Bacquet, P. & Nieberding, C. M. (2011). Mate choice and sexual selection in a model butterfly species, Bicyclus anynana: state of the art. Proc Neth Entomol Soc Meet, 22, 9-22.