Butterflies that display a rich purple are sought after by enthusiasts, but the color is often an illusion of light rather than a true pigment. These species achieve their violet appearance through the precise physical structure of their wing scales, a phenomenon known as structural coloration. The butterflies that most famously exhibit this purple sheen belong predominantly to two families: the Nymphalidae (brush-footed butterflies) and the Lycaenidae (gossamer-winged butterflies), which includes the blues and hairstreaks.
Identifying Key Purple Species
The most renowned purple-appearing species is the Purple Emperor, Apatura iris, native to the Palearctic region. Its wings are dark brown, but the males possess an iridescent purple-blue sheen that shifts dramatically with the viewing angle. The female lacks this coloration, exhibiting dark brown wings with white bands, a common pattern of sexual dimorphism in structurally colored species.
The smaller Lycaenidae family contains numerous species with purple hues, often on the upper wing surface of males. The Purple Hairstreak, Favonius quercus, is a small European butterfly whose males flash a vibrant purple in direct sunlight. Like the Purple Emperor, this coloration is confined to the males; females show a subdued blue or black with a small purple patch.
Another example is the Great Purple Hairstreak, Atlides halesus, found across North and Central America. This butterfly appears dark brown or black on the underside with patches of dusky purple and metallic blue scales. The Purplish Copper (Lycaena helloides) of North America displays a coppery-orange ground color overlayed with a purple tint, especially in the males. The purple color in these species is often a fleeting phenomenon, designed to appear and disappear as the insect moves through the light.
Understanding Iridescence The Source of Purple
The purple color of these butterflies does not come from chemical pigments like melanin or carotenoids, but from structural coloration, a physical effect. This optical effect occurs because the wing surface is covered in microscopic scales, each containing intricate nanostructures. These structures, often organized in precise, repeating layers of chitin and air, are comparable in size to the wavelength of visible light.
When light strikes these wing scales, it is diffracted and interfered with by the internal architecture of the scale. This process, specifically multilayer interference, selectively reflects a narrow band of wavelengths, corresponding to violet and blue. The exact spacing and thickness of the chitin layers determine the reflected color, and slight variations can cause the color to appear blue, violet, or purple.
The perceived purple color is iridescent, meaning it changes depending on the angle of the light source and the observer’s position. This angular dependency is the definitive sign of structural color, distinguishing it from non-iridescent pigment-based colors. The shifting purple flash likely serves a role in mate signaling or as a flash-and-conceal defense, since the color disappears when the butterfly lands or shifts its wing position.
Geographic Distribution and Preferred Environments
The distribution of purple butterflies is tied directly to the specific host plants required by their larvae. The Purple Emperor, Apatura iris, inhabits mature broadleaved woodlands across Europe and into temperate Asia. The caterpillars feed exclusively on the leaves of sallow and willow species, such as goat willow (Salix caprea), making these wetland-adjacent woodlands their required habitat.
The European Purple Hairstreak, Favonius quercus, is linked to oak trees (Quercus species). The larvae spend the winter as eggs near oak buds and feed on the young leaves in the spring. As adults, both species spend time high in the tree canopy, with males descending rarely to feed on tree sap, honeydew, or mineral-rich substances like animal droppings.
In North America, the Great Purple Hairstreak (Atlides halesus) is found in oak woodlands and areas where its parasitic host plant, mistletoe (Phoradendron), grows. The female lays eggs directly on the mistletoe, and the caterpillars feed on its foliage. The presence of these purple-hued butterflies indicates a healthy ecosystem that supports the specific plant species necessary for their life cycles.