The Swallowtail Butterfly is rare in the UK and only found in Norfolk but is common in continental Europe

The Swallowtail (Papilio machaon) is one of the rarest and largest butterflies found in the UK, and in fact can only be seen flying in the fenlands in the Norfolk Broads in the east of England. This is because its caterpillar only feeds on Milk Parsley (Peucidanum palustre) in Britain. The Swallowtail has decreased in numbers due to habitat destruction and loss of its food-plant. Efforts have been made to reverse this situation and Swallowtails still breed and fly in the Norfolk Broads.

However, the Swallowtail is far more widely distributed in Europe and extends throughout the Northern Hemisphere through Asia as far as Japan. It is also found in North Africa and in North America. In these countries the Swallowtail caterpillar will accept a far more varied selection of food-plants and will eat various species in the Apiaceae or Parsley family, such as Fennel (Foeniculum vulgare), and Rue (Ruta graveolens) in the Rutaceae.

The Swallowtail has yellow wings veined with black and with projections on its rear wings that give it its name. It also has a row of blue markings on the hind wings and a red spot on each. The Swallowtail is a very pretty butterfly indeed and is very hard to mistake for any other in the UK.

Swallowtail butterfly caterpillars are black and marked with white when young. This gives them the appearance of bird-droppings. As they grow bigger though they take on a much more attractive appearance with black and orange stripes on a green body. There is a forked organ known as an osmeterium that can be protruded from the caterpillar’s head. It is a form of defence because it gives off an unpleasant odour. The caterpillar will display this organ if it is disturbed.

The chrysalis of the Swallowtail is either brown or green and is held attached to a plant stem by a silken girdle. In the UK the butterfly overwinters in the chrysalis stage and emerges in May but on the continent in warmer countries the butterfly is double brooded. British Swallowtail butterflies can be seen on the wing from May to July.

Swallowtails are often seen in gardens in Spain, Portugal and other European countries where they may lay their eggs on plants such as Rue.

The British Swallowtail is a subspecies known as P. machaon ssp. britannicus. The continental variety P. machaon ssp. gorganus has been reported in the south of England and it is thought that the species has migrated from France or other parts of Europe. Eggs laid by females of these butterflies are deposited on plants such as the Wild Carrot (Daucus carota).

Throughout the world there are very many other species in the Papilionidae or Swallowtail family.