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The British Swallowtail Butterfly

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An illustration of a swallowtail

Nature provides us with some truly wondrous sights. In the insect world of ugly bugs and nightmarish creepy crawlies, there is a creature which is a living work of art - the butterfly. With their brightly-coloured and intricately-patterned gossamer wings, they resemble a child's image of what a fairy must look like. Swallowtails are quite common throughout Europe, Africa, North America, and Asia. Other swallowtail cousins live in tropical regions, and are especially abundant in citrus orchards, where the caterpillars1 are actively discouraged due to their destructive nature.


The majestic British swallowtail Papilio machaon britannicus is unique in the world. Although it cannot be found in the wild anywhere else, it is the sub-species of Papilio machaon (Old World Swallowtail), which is a little larger and lighter in colour. The swallowtail is the largest native British butterfly, having a wingspan of up to 3¾" (95mm). Its double set of wings are scalloped-edged from the tip to the base. Each wing is dark yellow with black engraving2 and there is a splash of royal blue before the final yellow piping of the wing edge. Two protruding 'tails' (wing extensions) and a red 'eye' on the base of each wing mimic the head of the creature when it is feeding. This is meant to confuse predators about which end is the head, giving the butterfly a chance to escape an attack. These two 'tails' form a shape not unlike that of a swallow's tail, giving rise to the butterfly's name.

Life Cycle

Adults emerge from the chrysalis (cocoon) from late May and live until early July. They find another swallowtail butterfly to mate with as soon as possible. Males have claspers in their abdomen area which grip the female's body to enable successful mating. Once the males have done their bit, that's the end of their parenting duties and they can fly off and inseminate another mate. The female lays pearl-coloured sticky eggs individually high up on the stalks of the milk parsley plant, as that is the required food of the caterpillars3. The egg darkens as the embryo inside grows.

The caterpillars hatch after about ten days' gestation. Their first meal is the nourishing eggshell they have been developing in. The caterpillars are distinctively coloured, having striped segments of yellowish green with a black-and-orange-spotted inner part. A fully grown caterpillar has a pair of menacing-looking orange organs called osmeteria (similar to the horns of snails) which it can raise from its head to warn off predators and retract when not required. The caterpillars are also known to emit a smell akin to that of an overripe pineapple, which is (apparently) offensive to other insects like wasps, although it doesn't deter birds seeking tasty morsels to feed their own growing young.

The caterpillars eat the leaves of the milk parsley until they are ready to transform themselves into a chrysalis, shedding several skins in the meantime. The resulting cocoon is camouflaged to match the colour of the stalk it is attached to by strong silk. There it remains until the start of the following summer when the adult swallowtail butterfly emerges to stretch its folded wings, dries then pumps them full of blood, and takes off for its maiden flight.

Although the caterpillars eat only the milk parsley plant, the adults feed on nectar from a variety of flowering plants. They are especially partial to yellow flag iris and thistle plants. Another popular flower colour is pink, against which the supping adult, already a visual feast, becomes particularly outstanding.

The Fate of the British Swallowtail Butterfly

The British swallowtail used to be abundant in wildflower meadows and wetland like fens and marshes, but their numbers dwindled virtually to extinction. Swallowtails had not been seen in Cambridgeshire since the late 1940s, and an attempt was made to reintroduce them during the 1970s. Over 3,000 milk parsley4 (Marsh Hog's Fennel), were planted around the Adventurers' Fen in the Wicken Fen in 1974, and a few hundred captivity-bred swallowtail butterflies were released there in 1975. A later study showed that those butterflies laid approximately 20,000 eggs and around 2,000 caterpillars survived to adulthood. Although a lot of the young milk parsley plants were gobbled up by ducks, it seems enough survived to provide a home and nourishment for the swallowtail offspring. Unfortunately that successful year was followed by the hottest British summer ever recorded, and the milk parsley did not reach its full potential. The swallowtails dwindled in number and by 1980 none were to be found.

A further effort at reintroduction was attempted but the milk parsley did not thrive due to the long summer droughts. Captive-bred larvae had been introduced and some did survive a full life-cycle, but swallowtails have not been seen on the Wicken Fen since 1996. Now the only place in the UK where anyone can hope to see a swallowtail butterfly in the wild is on the Norfolk Broads: this man-made habitat was excavated for peat in the Middle Ages and is now a popular tourist destination thanks to the diversity of the wildlife and the protection offered by a special act of Parliament in 1989. The British Swallowtail butterfly itself is protected by the Wildlife and Countryside Act of 1981, which means they cannot be deliberately disturbed, caught or killed without risk of prosecution leading to a fine or imprisonment.

A Swallowtail Day in conjunction with Norfolk Butterfly Conservation took place on 6 June, 2010, at Wheatfen Nature Reserve, Surlingham, near Norwich.

Strumpshaw Fen [on the Norfolk Broads] is excellent for them [swallowtail butterflies]. In fact it is a tremendous reserve full stop. Excellent butterflies, dragonflies, flowers and birds. It positively oozes with life.
– Lepidopterist ChrisJB of Lancashire, a contributor to the Wild About Britain website message board

Conservationists at the Norfolk Broads are encouraged by the late second batch of swallowtail butterfly offspring in August 2010, and the record number of adults spotted by observers is the best total in a century. However, the Norfolk Broads area itself is under threat from rising sea levels, so if you want to see a British swallowtail butterfly in its natural habitat, it's probably a good idea to plan your annual holiday there sooner rather than later. These magnificent, delicate creatures personify the saying 'poetry in motion', and should anyone be lucky enough to spot one in the wild then they can consider themselves truly blessed.

Artwork created by Insecteen

1The larva of the Citrus swallowtail is called 'Orange dog'.2Nature enthusiast Bill Oddie compares their wings to stained glass windows.3A common name for the butterfly larvae.4Staple diet of the caterpillars.

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