Old World Swallowtail (Papilio machaon)

In this guide

1. Description and identification
2. Life cycle: egg, caterpillar, chrysalis and adult
3. Where the Old World swallowtail lives
4. Host plants and adult diet
5. Behavior: hilltopping, flight and defense
6. Similar butterflies and how to tell them apart
7. Quick facts
8. Did you know?
9. Pictures
10. Frequently asked questions

Description and identification

The Old World swallowtail is the butterfly that gave the whole swallowtail family its English name and the genus Papilio its type species. It is a big, powerful, unmistakably tailed butterfly of open sunny ground, and it is one of the most widely distributed butterflies on the planet — about 37 subspecies are recognised across Europe, North Africa, Asia and northwestern North America, differing mainly in the depth of the yellow, the width of the black bands and the length of the tails.

Adult butterfly

• Wingspan: 2.6–3.4 in (65–86 mm). Alpine and far northern races sit at the small end of that range; southern lowland summer broods at the large end.
• Upperside: the ground colour is yellow — pale straw in some races, deep butter-yellow in others — crossed by black veins and a broad black band along the outer margin of each wing. Inside that black border sits a row of yellow half-moons, and on the hindwing a band of powdery blue crescents. The forewing base and the front of the thorax are heavily dusted with black scales.
• Hindwing: each hindwing is drawn out into a single black tail about 0.4 in (10 mm) long, and just above the tail, at the inner angle, there is a rusty orange-red eyespot capped with black and blue. Tails plus eyespot are the two marks that settle the identification.
• Underside: a softer, paler version of the same design, with the black reduced to grey-brown veining, the blue crescents duller and the orange eyespot still obvious.
• Sexes: there is no colour difference between males and females. Females average slightly larger with a broader, heavier abdomen, but the two cannot be separated reliably by eye in the field.

The description of the wings as white with grey veins, which appears on many websites, is a mistake. Only a few very pale high-altitude and desert forms come close to whitish; the normal Papilio machaon is clearly yellow, and the pale-with-black-stripes butterfly people are usually thinking of is the scarce swallowtail.

How to identify an Old World swallowtail in the field

Look for a very large yellow butterfly with a strong, gliding, purposeful flight over open flowery ground, hilltops, cliff slopes, fens or a garden herb bed. Check three things when it settles: a black tail on each hindwing, a single rust-red eyespot immediately above each tail, and black veins running across a yellow wing rather than solid black bars on a cream wing. In North America the same checks separate it from the anise swallowtail and the black swallowtail, both of which show far more black and a much smaller area of yellow. The British race, flying only over the Norfolk fens, is the same insect with slightly darker markings.

Life cycle: egg, caterpillar, chrysalis and adult

In the north of the range there is a single generation a year, on the wing from late May to early July; across most of Europe and Asia there are two, and in the warm south three, giving a flight season from April to September. In all of them the chrysalis is the stage that passes the winter.

Eggs

The female lays one egg at a time, gluing it to the upper surface of a leaf, a flower bud or the stem of a plant in the carrot family, usually choosing an isolated plant standing clear of the surrounding vegetation where the caterpillar will be warmer. Each egg is a smooth, almost perfect sphere about 1.2 mm across, pale cream or yellowish white when fresh and turning grey-brown with a dark cap shortly before hatching. A female may lay well over a hundred eggs in her life, and they hatch in about five to ten days.

Caterpillar

The caterpillar changes appearance completely as it grows. The first two instars are black with a white patch across the middle of the back and a few short bristles, so that a young larva sitting in the open looks like a bird dropping. From the third instar the pattern switches to the famous one: a smooth, plump, bright green body encircled on every segment by a velvety black band, and set in each black band a row of orange or orange-red spots. The head is green with black marks, and the fully grown larva reaches about 1.6–2 in (40–50 mm) after five instars over roughly three to four weeks.

Behind the head it carries an osmeterium, a fleshy forked organ the colour of a ripe satsuma. A caterpillar that is touched rears up and everts it in an instant, waving the two prongs and releasing a strong smell of rancid butter and pineapple. Ants, wasps and small birds back away from it, and it is harmless to a human handler apart from the odour on the fingers.

Chrysalis

The caterpillar leaves the host plant and pupates on a stem, a fence, a dead stalk or a wall. The chrysalis hangs upright rather than head-down, fixed by a silk pad at the tail and a fine silk girdle around the middle, and it comes in two colour forms: a fresh green one on green stems, and a light brown to dark grey one speckled with whitish flecks on dead vegetation, which is the form that normally overwinters. Summer pupae give an adult in two to three weeks; autumn pupae wait through the cold months and emerge the following spring.

Adult butterfly

Adults live about two to four weeks in the wild. They emerge with the wings limp and folded, pump them out and harden them in an hour or two, and then spend most of their lives feeding, patrolling and, in the case of males, defending a hilltop. The species does not migrate, but strong individuals wander widely, and single vagrants of continental race turn up in southeast England most summers.

Where the Old World swallowtail lives

Few butterflies cover so much ground. Papilio machaon flies throughout continental Europe from Spain and Italy to Scandinavia and Russia, right across Turkey, the Caucasus, Iran and Central Asia, through Siberia, Mongolia, China, Korea and Japan, along the Himalaya into northern India, and across North Africa. It also crossed the Bering land bridge: North American subspecies fly in Alaska, the Yukon, arctic and western Canada, and south through the Rocky Mountain states. It ranges from sea level to well above 10,000 ft in the Alps and the mountains of Central Asia.

The one place it is genuinely scarce is the United Kingdom, where the endemic race britannicus is confined to the reed fens of the Norfolk Broads and a handful of nearby sites. That single population makes it Britain's largest resident butterfly and one of its most strictly protected.

Habitat is anywhere open, sunny and flowery: meadows, rough grassland, chalk downs, steppe, alpine pasture, sand dunes, coastal cliffs, roadsides, orchards, allotments and gardens with a patch of fennel or carrot. Males are strongly drawn to high ground, so the top of a hill, a ridge or even a large rock outcrop is one of the most reliable places to find them.

Host plants and adult diet

Caterpillar host plants. The larvae feed almost entirely on the carrot family, Apiaceae. Across Europe and Asia the usual choices are wild carrot, wild and garden fennel, hogweed, angelica, milk-parsley, wild parsnip, dill, caraway and cultivated parsley and carrot tops. The Norfolk fen race is the exception and is tied almost completely to milk-parsley (Peucedanum palustre), which is why that population cannot spread beyond its wetland. Some races break out of the family altogether: North American and eastern Asian populations use tarragon and other Artemisia species in the daisy family, and southern European ones will use rue (Ruta graveolens) and burning bush in the citrus family. All of these plants contain aromatic oils that the caterpillar tolerates and that help it taste unattractive.

Adult diet. Adults are strong, active nectar feeders and are not fussy. Thistles, knapweeds, scabious, red valerian, buddleia, lavender, marjoram, clover, bramble and cultivated dahlias and zinnias are all worked hard, and a swallowtail usually keeps its wings beating while it feeds, so a photograph of a still one is hard to get. Males also gather at damp mud, animal dung and puddle margins to take up dissolved salts, a habit known as mud-puddling that supplies minerals missing from nectar.

Behavior: hilltopping, flight and defense

Hilltopping. The most striking habit of this butterfly is the way males congregate on the highest point of the landscape. Instead of searching a meadow for females, a male climbs to a summit, a ridge crest or an isolated knoll and patrols a circuit around it, returning again and again to the same prominent perch. Unmated females fly uphill to find them, so the top of a hill can hold a dozen swallowtails while the slopes below hold none. Males chase off rivals in fast spiralling flights and then drop back to the perch.

Flight. The flight is powerful and unhurried, with deep wingbeats broken by long flat glides on wings held in a shallow V. A swallowtail can cross a valley in a few seconds when it wants to, and it is almost impossible to catch up with on foot.

Defense. The species relies on three quite different tricks at three different stages. Young caterpillars imitate bird droppings; older ones threaten with the orange osmeterium and its foul smell; and the adult carries tails and a bright eyespot at the rear of the hindwing, a decoy that draws a bird's strike away from the head and body. Swallowtails with a neatly bitten tail or a triangular beak mark in the hindwing are common, and those are the individuals the trick has saved.

Courtship and egg-laying. Courting pairs fly a fluttering vertical dance, sometimes ten or fifteen feet up, before settling to mate on a tall stem. A mated female then works slowly across open ground with short hops from plant to plant, drumming each leaf with her forelegs to taste it before committing an egg to it. Because she spreads her eggs so thinly, finding one larva on a plant usually means there is no second one.

Similar butterflies and how to tell them apart

Quick facts

Did you know?

• The species name honours Machaon, the surgeon of the Greek army in the Iliad and a son of Asclepius, the god of healing. Linnaeus took most of his early butterfly names from Homer.
• Papilio machaon is the type species of Papilio, the genus that holds around two hundred swallowtails worldwide, so every other Papilio is measured against this one.
• It was the first butterfly to be called a swallowtail in English: the two long black tails reminded early naturalists of the forked tail of a barn swallow, and the name later spread to the whole family, including tailless members such as the apollo.
• The Norfolk Broads population went extinct once already, at Wicken Fen in Cambridgeshire in the mid-twentieth century, when drainage removed the milk-parsley. Repeated reintroductions there have not held.
• The osmeterium is a defensive gland unique to swallowtail caterpillars. Its secretion is a blend of acids and terpenes, and its composition changes with the caterpillar's age.
• A gardener who grows fennel, dill or parsley in a sunny spot in Europe or Asia has a realistic chance of finding the caterpillars, which is one of the few cases where an ornamental herb bed can support a butterfly of this size.

In summary

The Old World swallowtail, Papilio machaon, is a large yellow butterfly with black veins and borders, powdery blue crescents on the hindwing, a single tail on each hindwing and a rust-red eyespot above it, spanning 2.6 to 3.4 inches. It occurs from western Europe and North Africa across Asia to Japan and into Alaska and western Canada, in about 37 subspecies, and in Britain only as a protected fenland race in the Norfolk Broads. The eggs are laid singly on carrot-family plants; the caterpillar begins as a bird-dropping mimic, becomes green with black bands and orange spots, and defends itself with a forked orange osmeterium; the chrysalis, green or brown, is girdled upright to a stem and carries the species through the winter. Adults nectar strongly, puddle at damp mud and gather on hilltops, where males hold territory and wait for females to climb to them.

Old World Swallowtail butterfly pictures

Frequently asked questions about the old world swallowtail butterfly

What does an Old World swallowtail caterpillar look like?

A full-grown caterpillar is smooth, plump and bright green, with a velvety black band around every segment and a row of orange or orange-red spots set into each black band. It reaches about 1.6 to 2 inches. The first two instars look entirely different: black with a white saddle, imitating a bird dropping.

How do you identify an adult Old World swallowtail?

Check for three marks together: a yellow wing crossed by black veins and a broad black outer border, a single black tail on each hindwing, and a rust-red eyespot capped with black and blue just above each tail. A band of powdery blue crescents runs inside the black hindwing border. The wingspan is 2.6 to 3.4 inches.

Is an Old World swallowtail yellow or white?

It is yellow. The ground colour ranges from pale straw to deep butter-yellow depending on the subspecies, and only a few very pale desert and high-altitude forms approach whitish. A large pale swallowtail with bold black wedge-shaped stripes and very long tails is the scarce swallowtail, Iphiclides podalirius, a different species.

Can you tell male and female Old World swallowtails apart?

Not by colour. Both sexes have the same pattern and markings, so the species shows no useful sexual dimorphism. Females average slightly larger with a broader, heavier abdomen, and behaviour is a better clue: a butterfly patrolling a hilltop is almost always a male, while one drumming leaves with its forelegs is a female about to lay.

What colour is the chrysalis of an Old World swallowtail?

It comes in two forms. Pupae formed on green stems are green; those formed on dead stalks, bark or walls are light brown to dark grey, speckled with whitish flecks. The brown form is the one that usually overwinters. Either way the chrysalis stands upright, held by a silk pad at the tail and a girdle around the middle.

Where does the Old World swallowtail live?

It ranges across Europe, North Africa, Turkey, the Caucasus, Central Asia, Siberia, China, Korea and Japan, into the Himalaya, and across the Bering Strait into Alaska, arctic and western Canada and the Rocky Mountain states. It prefers open sunny grassland, meadows, steppe, alpine pasture, dunes, fens and gardens, from sea level to over 10,000 feet.

Is the Old World swallowtail rare?

Globally, no. It is widespread and secure across an enormous range and is not treated as a species of conservation concern. Locally it can be scarce: in the United Kingdom the race britannicus survives only in the reed fens of the Norfolk Broads and is legally protected, and several European countries protect their own populations.

What do Old World swallowtail caterpillars eat?

Mainly plants in the carrot family: wild carrot, fennel, hogweed, angelica, wild parsnip, dill, caraway and garden parsley. The British fen race feeds almost exclusively on milk-parsley. Some populations use plants from other families instead, including tarragon and other Artemisia species in North America and eastern Asia, and rue in southern Europe.

Why does the caterpillar have an orange fork behind its head?

That organ is the osmeterium, found only in swallowtail caterpillars. When the larva is touched or threatened it rears up and everts the fleshy orange fork, releasing a strong smell of rancid butter and pineapple that repels ants, wasps and small birds. It cannot sting or bite, and it is harmless to people.

Why do male Old World swallowtails gather on hilltops?

It is a mating strategy called hilltopping. Rather than search for scattered females across open country, males fly to the highest local point, patrol a circuit around it and defend a perch. Unmated females fly uphill to the same landmark, so both sexes meet reliably. It explains why a summit may hold several swallowtails while the slopes hold none.