The spectacular peacock butterfly (Inachis io) is regarded as one of the most beautiful in Britain, it has recently expanded its range in Scotland.
The peacock butterfly occurs in temperate regions of Europe and Asia, with a range that extends from Britain and Ireland eastwards through Russia to eastern Siberia, the Korean peninsula and Japan. It is absent from northern Scandinavia and southern parts of Spain and Portugal. It also occurs in temperate regions of Turkey and northern Iran, and has been recorded at elevations of up to 2,500 metres. It is sometimes known as the European peacock, to distinguish it from another butterfly, from a completely different family, that occurs in North America.
Distribution in Scotland
Prior to the end of the 20th century, the peacock butterfly was only known from some parts of Scotland, having been recorded in less than 30% of the country. However, since then it has spread rapidly, and is now seen in most of the Scottish mainland and many of the islands, including the Outer Hebrides, Orkney and Shetland. While the reasons for this expansion of its range are not fully understood, one possible explanation is the recent warmer weather, due to human-caused global warming.
The peacock butterfly is a large, colourful butterfly in the family Nymphalidae. Adults have a wingspan of up to 6 cm., and the upper surfaces of the wings are predominantly red in colour, with some rusty-brown areas and grey-black margins. There are prominent eye spots, or ocelli, on each of the forewings and hindwings, and on the forewings the central black spot is tinged with red colouration, while on the hindwings the spots contain a pattern of blue-white shapes. The butterfly’s common name is derived from the similarity of these eye spots to those on the tail feathers of a peacock.
By contrast, the undersides of the wings are a plain dark, brown-black colour, and the body is covered with rusty-brown hairs. The colour pattern on the wings is created by pigments contained in numerous tiny scales, which overlap each other, like the slates on a roof. Like all butterflies in the Nymphalidae family, the peacock’s front pair of legs are shortened and are not used for walking, being held folded against the thorax.
The main features on the butterfly’s head are two large compound eyes, twin antennae with club ends, which are used for detecting airborne scents and air currents, and the proboscis, which is used for drinking nectar from plants and is held coiled when not in use. In addition, there are two labial palpi, forward-pointing protuberances that also contain olfactory sensors, and may play a role in protecting the proboscis. Like the body, the butterfly’s head is covered in hairs.
Throughout its lifecycle, the peacock butterfly exhibits a range of defence mechanisms against predators. The dark undersides of the wings provide excellent camouflage, both when the adult rests with its wings closed, and also during hibernation, when it is very hard to detect amongst dead leaves. If an adult is approached by a predator while its wings are closed, it will open them suddenly to reveal the eye spots, accompanied by a hissing sound made by rubbing its wings together. This mimicry can startle or confuse the predator, providing enough time for the butterfly to take flight and escape. A secondary function of the eye spots is that many birds will attack eyes first, so by having the spots towards the edge of its wings, the peacock is deflecting attacks away from its more vulnerable body. Experimental work with blue tits (Parus caeruleus) has shown that peacock butterflies with intact eye spots suffered much lower predation rates than those with missing spots (butterflies such as the peacock are still able to fly with parts of their wings missing, usually caused by unsuccessful predation attempts).
The use of the stinging nettle as the larval food plant discourages some predators, while the communal silk web or tent and the spines on the caterpillar’s body act as further deterrents. If a group of larvae are disturbed, they will often twitch their bodies from side to side in synchrony, thereby possibly giving the appearance of being a much larger organism. The larva can also regurgitate green liquid, and if necessary will curl into a ball and fall to the ground to escape. Camouflage is then used again for the pupation period, with the colour of the chrysalis depending on the site selected for pupation. The sophistication of these various defence mechanisms is considered to be evidence of evolutionary adaptations to substantial pressure from predators.
Adult peacock butterflies emerge from the middle of July onwards and feed on the nectar of summer-flowering plants such as thistles (Cirsium spp.), ragwort (Senecio jacobaea) and devil’s-bit scabious (Succisa pratensis), as well as sap and honeydew. They also visit various garden flowers, and will feed on rotten fruit, through into early autumn. While doing so, they build up reserves of body fats in preparation for hibernation during the winter. Hibernation sites can be hollow trees, piles of dead wood, or buildings such as sheds and barns, and the butterfly will remain there for seven or eight months until the following spring.
The peacock butterfly overwinters as an adult, and in early spring it emerges from hibernation to reproduce. During this time it feeds on the nectar of spring-flowering plants, such as primrose (Primula vulgaris) and bluebells (Hyacinthoides non-scripta). After mating, up to 500 eggs that are green with white stripes on them are laid by the female. She chooses a stinging nettle (Urtica dioica), which is the main food plant for the larvae, for this and lays the eggs on the underside of the leaves of a plant that is growing in full sun. The eggs hatch after about 10 days, during the period from late May until early July, and the larvae spin a communal ‘tent’ or web of silk near the top of the nettle plant, which provides some protection from predators. The larvae feed inside the tent until the food supply is exhausted, and then move to a different part of the plant, where another tent is spun. As the larvae grow, they emerge to feed collectively and then separately, and go through 5 stages of growth or development called instars, which are separated by moults, when the skin they have outgrown is shed. The larvae or caterpillars are dark in colour and covered in spines, and by the fifth or final instar they are up to 4.2 cm long, jet black in colour, with white spots and formidable-looking black spikes.
When it is fully grown, the caterpillar finds a suitable place to pupate, which can be either on the underside of a nettle leaf or on the stem of a plant. The chrysalis or pupa is formed by the caterpillar spinning a protective cocoon around itself, and is either pale yellow-green or grey in colour. The former provides camouflage when the chrysalis is attached to a leaf, while the latter colour serves the same function when the pupation site is a plant stem or tree bark. The chrysalis is suspended head-down, as the caterpillar uses its rear legs to hold on to the leaf or stem.
Inside the chrysalis, the process of complete metamorphosis takes place, whereby the caterpillar body breaks down into a ‘chemical soup’. From this, the adult butterfly begins to take shape, with its structures and organs developing from small groups of special cells known as germinal buds. When this development is complete, after twelve days or two weeks, the adult breaks out of the chrysalis. The newly-emerged butterfly then finds a place to perch, so that it can expand its wings by pumping blood into the veins they contain. After one or two hours the wings are sufficiently stiff and the butterfly is ready for its airborne adult life.
Like other butterflies, the peacock helps to pollinate the flowering plants that it visits for nectar. Whilst feeding, pollen grains become attached to the hairy surface of the butterfly, and are transferred to the next plant it visits.
The peacock butterfly suffers from a number of parasites. These include a parasitoid wasp (Phobocampe confusa) that lays its eggs on the young caterpillars, which subsequently die as the wasp’s own larvae grow within them. Two parasitoid flies (Sturmia bella and Phryxe vulgaris) also cause mortality in the larvae in a similar fashion. Two entomopathogenic (ie insect killing) fungi (Lecanicillium muscarium and Isaria farinosa) have been recorded as affecting peacock butterflies hibernating in underground shelters in Europe.
Predators of the peacock butterfly include various spiders, which attack the caterpillars, undeterred by the spines. Wood mice (Apodemus sylvaticus) will eat hibernating adults if they find them, and birds such as the blue tit, great tit (Parus major) and yellowhammer (Emberiza citrinella) will attack the adults when they are out in the open.
Despite the attentions of parasites and predators, the peacock butterfly is continuing to spread throughout Scotland and is one of the most easily-recognisable butterflies in the country.