Summer Woodland Fritillaries
Our five orange and brown-winged fritiliary butterflies are insects of spring and high summer. It’s easy to recognise a fritiliary, but another matter to tell which is which.
All fritiliary butterflies share a characteristic wing pattern: the upper surface is an intricate mixture of dark markings on a lighter back-ground. The pattern, but not the colour, is similar to the snake’s head fritiliary flower after which the butterflies were named. Fritiliary butterflies are insects of high summer and frequent sunlit woodlands. It is easy enough to recognise a butterfly as a fritiliary, but telling one species from another is more difficult. The best distinguishing features are size and the details of the underwing markings.
The five most widespread and common fritillaries in the British Isles feed, in their caterpillar stage, on the leaves of violets. The violet-feeding species are all members of one sub-family, Argynninae, and can be distinguished from other fritillaries by the warm, orange-brown ground colour of their wings. (The other species are darker brown with lighter brown markings.) You can also tell that a butterfly is one of the violet-feeding fritillaries by the silver marks on the underside of its wings.
Sight and scent
The five violet-feeding species never interbreed with each other, so the butterflies must have ways of telling each other apart that do not involve looking at the underside of the wings. (When the butterflies are active the wings are open.) Sight, including colour vision, plays some part in this recognition process. Fritillaries always divert from their flight path to investigate another fritiliary sunning itself on a flower, so experimenters in an Oxfordshire wood cut out paper models of pearl-bordered fritillaries on. All the models had the correct wing pattern but only some of them had the right colour. Only those with the right colour (or a colour close to the natural one) got many visits from passing butterflies. These experiments showed that sight enables one butterfly to spot another from a distance and to identify it as a fritillary.
The males have special scent glands on their wings and it seems that at closer quarters scent is more important than sight. The females can identify the scent of the males of their own species and so find a suitable partner.
Only one generation of fritillaries is produced each year, and most species overwinter in the caterpillar stage in a shrivelled violet leaf. They wake up and begin feeding as soon as the violet leaves start growing in the spring.
The smaller fritillaries fly in spring and summer-the pearl-bordered in May and June and the small pearl-bordered in June and July. Both species frequent woodland with open rides where flowers such as bugle and thistles grow. The butterflies visit these for nectar.
You can often find the small pearl-bordered fritillary in the wetter parts of woods, and also in marshy areas away from woodlands. It does not occur in Ireland, while the pearl-bordered is found there only in County Clare, on the limestone pavements of the Burren. This is the only area where this species lives away from woodland.
Both these species of butterfly lay their eggs on the leaves of violet plants. The eggs hatch in 10-14 days, and the caterpillars start feeding and growing immediately. They grow and moult their skins three times within five or six weeks. Each caterpillar then finds a shrivelled leaf at the base of the plant in which to pass the coming winter. Most pearl-bordered caterpillars begin hibernation at the end of July. In the following March the caterpillars, which have shrunk to half their size during hibernation, start feeding and growing again. They moult one more time as a caterpillar, then reach full size and pupate. The chrysalis stage lasts only 10-14 days.
The larger fritillaries fly in July and August, much later than the smaller ones. They are attracted especially to the flowers of bramble and thistles. The silver-washed and high brown are strictly woodland and wood-edge butterflies, and are most common in the south and west of England. The high brown does not occur in Ireland, and neither species breeds in Scotland. The dark green fritillary sometimes flies in woodland, but is just as much at home in open areas such as cliff-tops or sand dunes where violets grow among the grass. It is much more widespread than the other large fritillaries.
All these three larger species of fritillary have life cycles that ensure that the young caterpillars, which do not feed at all before winter, are ready to start feeding as early as possible in spring. But the way they survive the winter is different in each species.
The high brown lays its eggs low down on the stems of violets, but although the young caterpillar develops quickly within the egg, it does not eat its way out of the eggshell until spring. The dark green fritillary lays its eggs a little higher up on the plant. On the leaves or stems. The young caterpillars hatch in just over two weeks, but they do not start to feed; they crawl down to the bottom of the plant and hibernate in a shrivelled leaf.
Strangest of all is the silver-washed fritillary. It is one of the few species of butterfly that does not lay its eggs on its caterpillar’s foodplant. The female butterfly does find a good clump of violets-but then she lays her eggs on a nearby tree-trunk, sometimes as much as 60-90cm (2-3ft) above ground. The caterpillars hatch in early autumn but then hibernate in a crevice in the bark. In spring they have to make the long walk to the ground in search of violet plants. This is a long journey for a caterpillar only 2-5mm (1/10in) long which has lived for nine months with nothing to eat apart from the top of its egg-shell which it eats when it hatches.
All five species of violet-feeding fritillary are quite common in some years in the south and west of England. They often decline in numbers, and may then increase again. Even before World War I, entomologists were writing gloomily about the declining numbers of fritillaries, and the situation has worsened since then. In part, these declines may be natural-the fritillaries are at the edge of their range in Britain (they are more common further south in Europe), and a hard winter may kill many caterpillars, or a cold summer may prevent the butterflies laying all their eggs. Also parasitic wasps attack the caterpillars.
The usual pattern is that in good years the butterflies spread out and colonize the woods from which they had previously disappeared. Unfortunately, this does not always happen today because of the fragmentation of habitats by man. In dairy farming areas there are still flower filled fields and hedgerows along which fritillaries can fly when dispersing to new woods; but in the cereal-growing east of England fritillaries have almost disappeared. The few remaining woods in eastern England often have a very sharp boundary at a ploughed field, with little in the way of brambles and thistles at the edge, and the next wood may be a long way off across a flowerless barley field. Inside the woods, lack of coppicing and overgrown rides reduce the flowers for the butterflies.
One hopeful sign is that fritillaries seem to thrive in Forestry Commission conifer plantations. The wide, flower-filled rides are just what the butterflies require and. If the plantation was previously a deciduous wood, there are usually plenty of violets.