The swallowtail (Papilio machaon), a beautiful species confined in the UK to a small area of East Anglia, is perhaps Britain’s most cherished butterfly.
The subspecies Papilio machaon britannicus is unique to the UK, found only in the fens and marshes of the Norfolk Broads. Although its range is restricted, populations of the swallowtail remain stable. Its larval foodplant is milk parsley, a rarer relative of cow parsley. With a wingspan of around 10cm, it is the largest resident butterfly in the UK and is on the wing from late May until early July.
Slightly larger and paler than its British cousin, the European race (Papilio machaon gorganus, shown in these pictures) is broader in its choice of caterpillar food plant. It is relatively common and is found all over Europe, Asia and even North America.
Papilio machaon, the old world swallowtail, is a member of the family Papilionidae. At over 550 species, this is one of the largest butterfly families in the world and includes large, colourful species such as the birdwings.
A distinguishing species of the family is the osmeterium, a fleshy organ on the caterpillar which emits a foul-smelling secretion to deter predators shown in this video. Normally hidden, this forked structure can be everted when the caterpillar is threatened. When British swallowtails are alarmed they emit an acrid smell reminiscent of rotting pineapple!