We are in the middle of The big butterfly count which is a nationwide survey run by Butterfly Conservation, Friday 14 July to Sunday 6 August, to assess the status of the nation’s butterflies (and moths). The Small tortoiseshell (Aglais urticae) is one of the species the Great British Public is being asked to look out for, partly because sightings were significantly down across the UK last year (2016), according to Butterfly conservation. So today, I devoted some time to photographing this species in and around my home patch in Scarborough. Most of these photographs were taken of the butterflies on thistles in the grounds of Scarborough Castle, north Yorkshire. There were good numbers of individuals present on the meadows in the castle grounds and on the slopes around the promontory.
Butterfly wings are covered on both sides by rows of tiny – e.g. 50 to 100 µm wide and 150 to 200 µm long – overlapping scales, a bit like very thin, flat roof tiles or shingles. The colours seen on a butterfly wing are produced by the combined layers of scales on either side of the wing, together with the wing membrane itself. The overall effect of these individually coloured scales has been described as being similar to that of a pointillist painting.
The upper (dorsal) wing surfaces of many nymphalid species are strikingly colourful, with highly reflective spots and vivid bands formed by scales of different pigment-based colours which also utilise structural colours. Structural colours – like the blue patches of scales along the margins of the wings on the Small tortoiseshell butterfly wing – are produced by interactions between incident light and interference and reflections caused by the surface structures on the scales. These bright colours are the result of an optical phenomena – predominantly light interference – produced by nanoscale structures on and in the scales themselves, together with selective absorption by pigments.
The wings of common nymphaline butterflies like the Peacock, Painted lady, Red admiral and the Small tortoiseshell shown here, have patterns based on individual scales of many different colours (blue, black, red, orange, white and yellow). The colours of most of these scales – blue and white are the exceptions – are largely determined by internal pigments. For example, the yellow, orange and red scales contain pigments such as xanthommatin and the black scales contain high concentrations of melanin. The blue scales, are virtually unpigmented and the diffuse colour is caused by nanostructures (microscopic repeating shapes on the surface of the scales and within the lumen of the scale).
I don’t know for sure whether it is the case in the Small tortoiseshell, but iridescence scales on the wings of butterflies are often secondary sexual characters which are used by the butterflies to evaluate the fitness of a mate. Iridescence scales are thought to be highly sensitive to factors such as the quality of food and other conditions, whist the butterflies were caterpillars, and as such they are good indicators of the condition of the butterfly and the environment in which it has developed. This is because these bright scales are complex in structure and energetically ‘costly’ to produce, so there are therefore, thought to be honest signals, or reliable information if you will, of the phenotypic condition of the butterflies. In other words, bright shiny butterflies are genuinely healthy and good to mate with!
The underside of the wing (above) is a distinct contrast to the bright and colourful upperside. This is because it has a different function: namely camouflage. Butterflies have cleverly evolved different ways of separating out divergent and sometimes conflicting functions – by putting them on either side of the wing! The upper wing surface in this species is bright and colourful with conspicuous signals aimed at mates and rivals, whilst on the underside it has cryptic colouration, which has evolved to help it escape detection by predators. The butterfly has cleverly allocated these different functions (or needs) to different wing surfaces, and it can therefore switch functions at the drop of a hat – or in this case the flapping shut of the wings.
Insect compound eyes are composed of numerous individual light gathering elements called ommatidia. In the case of this species, the eyes appear to be covered in tiny hairs; presumably to keep them clean and free from dust? Below the lens, the ommatidia each contain nine photoreceptor cells fused together to form a long, cylindrical, optical-sensing structure called a rhabdom. The process of vision in butterflies begins with the absorption of light by visual pigments called rhodopsin. All butterfly eyes contain ultraviolet-, blue-, and green-sensitive rhodopsins, with peak sensitivities in the following regions of the spectrum: 300–400nm (UV light), 400–500 nm (blue light) and 500–600nm (long wavelength light). We humans use a red-green-blue trichromatic system and are able to discriminate colours across a range of the electromagnetic spectrum from 400 to 700 nm. In other words, we cannot see UV light like butterflies can! Butterflies use their longer wavelength red receptors for recognising host plants and flower colours. Butterflies have a remarkable array of photoreceptors which give them a spectral sensitivity that is almost unrivalled in the animal kingdom. They use their superb colour vision to find the right kind of flowers for their needs, and also to observe and understand the beautiful and colourful patterns on their wings. We can only imagine what the world must look like to them as they flit through their three-dimensional habitat, interacting spontaneously with other butterflies and other insects, skipping rapidly from flower to flower. They did not evolve all this beauty for our benefit, but we can enjoy and marvel at it none the less.