This small butterfly has declined throughout much of its range in the UK, but is still common in the Highlands of Scotland.
The pearl-bordered fritillary is widely distributed throughout Europe, occurring from northern Spain to Scandinavia, and as far south as Sicily. Its range also includes much of northern Asia. However, it is in decline in some parts of north western Europe, including Britain, and has disappeared entirely from the Netherlands.
In Scotland, the pearl-bordered fritillary occurs mainly in the Highland and Grampian regions, although it is also found in Perthshire, parts of the central region and in the southwest, in Dumfries and Galloway. Its preferred habitat in Scotland is open birch woodland, and in the area where Trees for Life is working, it has been recorded in both Glen Affric and Grudie Oakwood.
Formerly, the pearl-bordered fritillary was also found in south east Scotland, but its decline there appears to be an extension of the same phenomenon in England, where its range has contracted substantially in the last 100 years. The causes for this reduction include a decrease in coppicing in woodland, a loss of open areas in forests and inappropriate habitat management.
Because of the decline in its range and population, the pearl-bordered fritillary is now classified as Nationally Scarce, and is considered to be threatened in most of the UK. However, despite this, it has no formal protection, although it is a Priority Species, and therefore the subject of a Biodiversity Action Plan (BAP), under the UK government's response to the Convention on Biological Diversity signed at the Earth Summit in 1992.
The pearl-bordered fritillary is a small butterfly, with a wingspan of 4.4 - 4.7 cm. It is pale orange-brown in colour, with black veins on its wings. On the upper sides of the wings, black spots are interspersed between the veins. The under sides of the wings are lighter in colour, with a mixture of orange and pale yellow sections between the veins. Its common name derives from the seven pearly-silver coloured spots along the perimeter of the underside of the hind wings, while two other similar spots are situated closer to the body, in the centre of the wings. These pearls are only visible when the butterfly is at rest with its wings closed. The distribution pattern of the pearls on the hind wing is the main way to distinguish this species from the slightly smaller but closely-related small pearl-bordered fritillary (Boloria selene) - the latter has six or seven pearls near the body.
The pearl-bordered fritillary's body is densely covered in hairs, which vary in colour from brown to beige. The antennae have bulbous knobs at their ends and these are orange-brown at the tips. The legs are also orange-brown in colour. 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. Elaborate patterns, such as those on the pearl-bordered fritillary, are thought to serve either as camouflage, or to warn off predators.
Adults emerge quite early in the year, and they are usually in flight for about six weeks in May and June. Males and females are similar in appearance, and the species usually occurs in colonies, which can contain up to two or three hundred individuals. In Scotland the pearl-bordered fritillary's favoured habitat is open deciduous woodland with a southerly aspect, to gain the benefit of the sun's warmth, and where there are plenty of spring flowers such as bugle (Ajuga reptans), birds-foot trefoil (Lotus corniculatus) or dandelion (Taraxacum officinale), which are used as a source of nectar. Elsewhere in its range, it also occurs in woodland clearings and well-drained grasslands.
After feeding in the early mornings, males fly low over the vegetation, at about knee height, searching for females with whom to mate. Females rest low down on vegetation until a suitable male appears, at which point they will fly up, into the high branches of a tree for example, to copulate. Mating lasts for between 30 and 60 minutes, and the female will then look for a suitable place to lay her eggs. Favoured sites are often in areas of bracken, either amongst bracken litter or on dead leaves, but always close to, or on, the leaves of violets (Viola spp.). The eggs are laid singly and the larvae or caterpillars hatch out after about two weeks.
The larva is black in colour, with several parallel rows of spines down its body length - these can be yellow, white and black in colour. The main food plant for the larva is common dog violet (Viola riviniana), although marsh violet (Viola palustris) and heath dog violet (Viola canina) are sometimes used. The larva eats the leaves of the violet, and alternates between feeding and periods of rest on dead leaves, during which it basks in the sunshine. This is important as the sun warms its body temperature, thereby enabling it to complete its growth and development successfully.
The larva goes through 4 stages of growth, or instars, which are separated by moults, and it becomes progressively larger, reaching a maximum size of about 2.5 cm. Early in the fourth instar, the caterpillar goes into hibernation for the winter, amongst leaf litter on the ground. It resumes feeding in March or April, when its food plants grow again and the temperature rises. The larva then undergoes pupation, which takes place in the leaf litter or amongst low vegetation.
During pupation the larva undergoes complete metamorphosis, in which all its organs dissolve, leaving the pupa filled with fluid. 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 a few weeks in the case of the pearl-bordered fritillary, the butterfly breaks out of the pupal skin. The newly-emerged adult then finds a place to perch, so that it can expand its wings by pumping blood into their veins. After one or two hours the wings become sufficiently stiff and the butterfly is ready for its few weeks of airborne adult life.
Comparatively little is known about the interactions and relationships which the pearl-bordered fritillary has with other organisms or plants. However, it does play a role in pollination of the flowers it visits for nectar - pollen grains get stuck on to its body hairs and then get transferred to other flowers which the butterfly subsequently visits.
Although the feeding by the larva on violets is obviously detrimental to the individual plants which it eats, the overall abundance of violets does not appear to be diminshed by the presence of the butterfly. The larvae themselves are eaten by common insectivorous birds in the forest, while some reports also indicate that they are preyed upon by common lizards (Lacerta vivipara).
In England, wood ants (Formica rufa) were observed to prey on eggs of the pearl-bordered fritillary in a forest near Oxford, although it's not known if this also occurs in Scotland, where two different species of wood ants (Formica aquilonia and F. lugubris) predominate. Similarly, little is known about parasites which affect the pearl-bordered fritillary.
This lack of knowledge, together with the documented decline of the species throughout its range in the UK, indicates how much still needs to be learned about the pearl-bordered fritillary, and its role within the ecosystems where it occurs. The focus on this butterfly provided by its listing as a priority species for biodiversity protection in the UK should help address this knowledge gap. The BAP sets a target of halting the decline of the species by 2005, and, in the longer term, of reintroducing it to parts of its former range - this should ensure that this beautiful butterfly continues to flourish here into the future.
Alan Watson Featherstone
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