British Butterflies: Swallowtail (Papilio machaon britannicus)

Posted on February 13, 2011


Taxonomic classification

Kingdom           Animalia

Phylum             Arthropoda

Class                Insecta

Order                Lepidoptera

Family               Papilionidae

Genus               Papilio

Species             machaon

Subspecies:      britannicus


Of the twelve European representatives of the family Papilionidae, Papilio machaon subsp. britannicus (Seitz, 1907; The British Swallowtail) is the only U.K. endemic1,2.

Papilio machaon britannicus © Julian Dowding

P. machaon subsp. britannicus is not only the largest butterfly to be found in the U.K. but it is also among the rarest with the population having plummeted in recent history due to habitat destruction. Presently, P. machaon subsp. britannicus is exclusively found on the Norfolk Broads where suitable habitat and healthy populations of its larval host plant Peucedanum palustre (Milk-parsley) remain2,3.

Papilio machaon subsp. gorganus (Fruhstorfer, 1922; the Swallowtail found throughout the rest of Europe) has also been recorded in the U.K. but as this subspecies is not resident, all subsequent information will relate to P. machaon subsp. britannicus1,2.


Historically, P. machaon extended throughout the fens of East Anglia to many other parts of England including the mashes along the Thames, the Somerset Levels and up to Yorkshire4. The last population outside of the Norfolk Broads inhabited Wicken Fen in Cambridgeshire where it first became extinct c. 19525. Subsequent reintroductions to the site from 1955 to 1969 were ultimately unsuccessful; drying of the fen and further drought in 1976 compromised the P. palustre population, the sole larval food plant of P. machaon5. Failing to recover the Wicken Fen population once again became extinct at the site. Presently P. machaon is found only in the Norfolk Broads inhabiting a mere 1% of its former range where populations are considered to be fragmented4

Distribution map available at


P. machaon inhabits open mixed fen systems containing healthy populations of P. palustre dominated by Carex spp. (sedges) or Phragmites spp. (reeds) 2,6.

Annual cycle

In years that yield one generation imagines (adults) fly from May to July. Flight coincides with the laying of ova (eggs) from which larvae (caterpillars) emerge after a duration of approximately one week. Larval development takes one month at the end of which larvae achieve their final instar and pupate. Individuals overwinter as pupae to emerge as imagines (adults) the following May to July7.

In years where conditions are favourable enough to produce a second generation, individuals emerge from their pupae in August following a three week metamorphosis8. The imagines proceed to mate and lay ova with their offspring then overwintering as pupae to emerge in May to July of the following year7.8.

Life Cycle

Papilio machaon britannicus ovum © Pete Eeles


P. machaon is among the many species of butterfly to lay ova singularly on the leaves of their larval host plant. This is a strategy that aims to increase the probability of survivorship for offspring produced by any given female through maximising food resources per larva and decreasing the likelihood that all offspring succumb to predation by parasitoids, birds, insectivorous mammals and even other larvae9.

Upon being laid on a P. palustre leaf, the spherical ovum is a greenish yellow. The colour of the ovum rapidly changes to green and as time progresses turns purple-black and finally to a light pearly transparency a few hours prior to larval emergence. The ovum is eaten by the larva upon emergence as it is a valuable source of nutrition8.


Papilio machaon britannicus caterpillar © Pete Eeles

The larval stage of P. machaon lasts approximately one month during which time individuals can reach 41mm in length by feeding on the leaves, buds and flowers of P. palustre  8.  Newly emerged larvae are black with a white patch near the middle of their back; a camouflage colouration that resembles bird guano2. With each consecutive instar moult larvae develop the characteristic and conspicuous “warning” colouration of a green head with black markings and a green body with transverse black bands and red spots7,8.  In addition to “warning”colouration, P. machaon larvae possess an osmeterium which develops after the third instar. The osmeterium is a bifurcated orange scent gland hidden within the first thoratic segment that produces a pungent odour when erect3,7,8. The odour produced coupled with the visual display of the osmeterium itself is considered an effective defence mechanism against birds3.


Papilio machaon britannicus pupa (green) © Pete Eeles

Larvae pupate within 10m of their larval host plant and most frequently do so low down on the stems of Phragmites spp. but have been known to pupate on woody plants and other individuals of their larval host plant2,8. The pupa is secured to the pupation site in an upright position by both a cremaster (anal hooks) attached to a silken pad and a silken girdle2,8.

Pupae of P. machaon are either greenish-yellow or brown with black markings in order to blend in with the surrounding environment2,8. Pupae are able to survive submersion in water for long periods, an adaptation to an environment where later levels may fluctuate greatly over the year7.


As an imago, P. machaon is quite striking. The upper wing pattern is more vibrant than the underside consisting of a mingling of yellow and black creating a stunning visual effect. The characteristic red spot capped with blue can be found on the hind wings10. P. machaon subsp. gorganus is extremely similar in appearance but can be distinguished by its lighter colouration and larger size2. Sexual dimorphism not well characterised in wing pattern but can be distinguished by abdominal characteristics i.e., larger abdomen in females3.

P. machaon imagines are dependent on nectar as a source of nutrition feeding on food plants such as Hyacinthoides non-scriptus (Bluebell), Succisa pratensis (Devil’s-bit Scabious), Lychnis flos-cuculi (Ragged Robin), Trifolium patense (Red Clover), Dipsacus fullonum (Teasel) and Cirsium spp. and Carduus spp. (Thistles). Typically wings continue to beat during feeding2.

Conservation status

P. machaon is listed as a species of conservation concern in the U.K. Biodiversity Action Plan and in Schedule 5 of the Wildlife and Countryside Act, 1981.


3. Tolman, T and Lewington, R (2008) Collins Butterfly Guide. HarperCollins Publishers, Hong Kong.
4. Asher, J., Warren, F., Fox, R. Harding, P., Jeffcoate, G. & Jeffcoate, S. (2001) The Millennium Atlas of Butterflies in Britain and Ireland. Oxford University Press, Oxford.
5. Dempster and Hall (1980) An attempt at re-establishing the swallowtail butterfly at Wicken Fen. Ecological Entomology, 5, 327-334
7. A Field guide to Caterpillars of butterflies and moths in Britain and Europe
8. The Caterpillars of the British butterflies including the eggs chrysalids and food-plants
9. Ecology of Butterflies in Europe, eds. J. Settele, T. Shreeve, M. Konvicka and H. Van Dyck (2009) Cambridge University Press, Cambridge. pp. 29-42.
10. Still, J. (1996) Collins Wild Guide Butterflies and Moths of Britain and Europe. HarperCollins Publishers, London.

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