Swallowtail butterfly © iStock


Where are swallowtail butterflies found in the UK?

In Britain, swallowtails are confined to the Norfolk Broads. Attempts to reintroduce them to Wicken Fen in Cambridgeshire, from where they disappeared in the early 1950s, have so far proved unsuccessful.


How big does a swallowtail butterfly grow?

With a wingspan of up to 93mm, the swallowtail is Britain’s largest species of butterfly, narrowly beating the purple emperor that has a wingspan of 92mm.


What colour are swallowtail caterpillars?

The black and white patterns of the young caterpillars are believed to mimic bird droppings to deter predators.


How do swallowtail caterpillars deter predators?

When threatened, the more colourful, older caterpillars inflate a fleshy, orange organ, called an osmeterium, from behind their heads. It is not unlike a snake's forked tongue and exudes a pungent pineapple-like odour.


How do swallowtail caterpillars spend winter?

Swallowtails overwinter as pupae attached low down to plant stems. They come in green and brown forms, according to their surroundings and can survive long periods submerged in water.


What do swallowtail caterpillars eat?

The unique British subspecies feeds only on milk parsley. Vagrants from the continent, which feed on a variety of umbellifers, occasionally turn up on the English south coast, but don't breed here.


Do swallowtail caterpillars have any parasites?

An ichneumon wasp named Trogus lapidator preys exclusively on swallowtails. Females lay an egg in a swallowtail caterpillar, but the adult emerges from the pupa, leaving a large tell-tale exit hole.


Why are swallowtails so restricted in the UK? Richard Jones explains:

The British swallowtail butterfly, Papilio machaon britannicus, is limited to the fens of the Norfolk Broads. Its caterpillars feed solely on milk parsley, Peucidanum palustre, which only grows in the East Anglian wetlands.

Yet in France and the Low Countries, the subspecies P. m. gorganus is common and widespread, availing itself to a range of host plants.

Like many organisms at the edge of a range, our swallowtail has a narrow set of very precise habitat requirements. Genetic diversity is the key factor: in southern Europe, intermingling populations keep genetic variability high, resulting in a greater diversity of habits, host plants and ranges.

Outlier populations, such as those in Norfolk, were founded by a small number of individuals – possibly just one – following the last Ice Age. With a shallower gene pool, their members are more sedentary, less experimental when it comes to egg-laying and less capable of colonising new ecological niches.

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There are fears that rising numbers of gorganus migrants into Britain may well establish colonies here and interbreed with the Fenland race, diluting it to the point of non-existence.