Nature · Insects & other invertebrates

Swallowtail

Chiefly late May to early July, with a partial second brood in hot years · wingspan 76–93 mm

Papilio machaon britannicus — Papilionidae (swallowtails)

Britain's largest resident butterfly and its only papilionid: bold yellow and black with blue lunules, a red-orange eyespot and a tail on each hindwing. The British population is a distinct endemic subspecies, Papilio machaon britannicus, darker and more heavily black-marked than continental swallowtails, and it is confined to the Norfolk Broads because its caterpillars eat only milk-parsley, a fen plant. Continental swallowtails of subspecies gorganus occasionally reach the south coast as migrants and have bred there.

How to spot them

▸Huge yellow butterfly with heavy black veining, a blue-scaled band and a red-orange eyespot on each hindwing
▸A true tail on each hindwing — no other resident British butterfly has one
▸In Britain, effectively restricted to the fens of the Norfolk Broads
▸Caterpillar green with black bands and orange spots; it everts a forked orange organ smelling of rotten pineapple when touched
▸Powerful gliding flight low over open fen; feeds at ragged robin, thistles and buddleia

How to know it

The only British resident butterfly with a true tail on each hindwing, and the largest of them all

Swallowtail — Britain's sole resident member of the Papilionidae.

Its British population is a distinct endemic subspecies confined to the fens of one county

Swallowtail — Papilio machaon britannicus, of the Norfolk Broads.

Its caterpillars in Britain eat one fen umbellifer and nothing else, which pins the butterfly to a single landscape

Swallowtail and milk-parsley, Thyselium palustre (formerly Peucedanum palustre).

Touch its caterpillar and a forked orange organ shoots out from behind the head, smelling of rotten pineapple

Swallowtail — the osmeterium, a defence found throughout the Papilionidae.

A paler, more strongly migratory subspecies from the continent turns up on the south coast and has bred there

Swallowtail — subspecies gorganus, distinct from the resident britannicus.

Its specific epithet is the name of a healer who went to Troy, a son of Asclepius

Swallowtail — Papilio machaon, after Machaon.

Quick-fire facts

•Papilio machaon; the British subspecies is britannicus, an endemic found nowhere else
•Machaon was a surgeon in the Greek army at Troy and a son of Asclepius
•British larvae feed exclusively on milk-parsley, a plant of open, wet, cut fen
•The butterfly's British range is essentially the Norfolk Broads, with Hickling Broad, Ranworth and Strumpshaw Fen among the best-known sites
•britannicus is darker and more heavily marked with black than the continental gorganus, which is a wider-ranging migrant and feeds on wild carrot and fennel
•It is a UK priority species and its habitat requires active fen management; drainage and the abandonment of reed and sedge cutting were behind its historical losses
•It formerly occurred in the Cambridgeshire fens, where it died out in the nineteenth century; reintroduction attempts at Wicken Fen failed
•The osmeterium is a fleshy forked gland behind the head, everted as a defence and releasing a pungent terpene smell

Drill it — 7 cards

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Sources

Butterfly Conservation — Swallowtail
Paul D. Brock — Britain's Insects: A Field Guide to the Insects of Great Britain and Ireland