7 things you didn’t know about the swallowtail

Discover fantastic facts about Britain’s largest butterfly. 

Papilio machaon, swallowtail, UK

Swallowtail butterfly © iStock


1. Looks inedible

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


2. Shock tactics

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.


3. Water resistant

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.


4. Eastern delights

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.


5. Visitor attraction

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.

Click here to find out why swallowtails are so restricted in the UK.


6. Big and beautiful

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. 


7. Swallowtail stalker

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.


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