Large Blue (Phengaris arion)

In this guide

1. Description and identification
2. Life cycle: egg, caterpillar, chrysalis and adult
3. Where the large blue lives
4. Host plants, ants and diet
5. Behaviour, conservation and the British reintroduction
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 large blue belongs to the blues, coppers and hairstreaks of the gossamer-winged butterfly family, and within it to the genus Phengaris, the small group of ant-parasitic blues that was until recently called Maculinea. Both names are still in print, and older books also use the spelling Lycaena arion. It is often said that the species is named "large" only in contrast to the small blue; in fact it is genuinely the largest blue butterfly in Europe, noticeably bigger and heavier than the common or the Adonis blue.

Adult butterfly

• Wingspan: 1.5–2.0 in (38–52 mm), the biggest European blue. Colonies on poor, hot ground produce smaller individuals than those on richer turf.
• Upperside: a deep, slightly silvery blue, bordered by a broad smoky grey-brown margin that is widest at the wing tip. Across the middle of each forewing runs a row of heavy black spots that are elongated and seed-shaped rather than round, and there are usually a few smaller black marks towards the wing base. The hindwing is more lightly marked, with dark veins reaching the border.
• Underside: pale grey-beige to fawn, with no blue except for a dusting of blue-green scales over the base of the hindwing. Both wings carry rows of black spots neatly ringed in white, larger and better spaced than in the other grassland blues.
• Sexes: the two are similar enough to be described as lacking strong dimorphism, but there is a consistent difference. Males are a brighter, cleaner blue with narrower dark borders; females are duller, with broader grey margins, heavier black forewing spots and rather more rounded wings.

How to identify a large blue in the field

Begin with the habitat: a hot, south-facing, closely grazed slope with wild thyme and no fertiliser, in the last week of June or the first three weeks of July. Then look at size and spotting. The large blue is visibly bigger than the common blue, and its forewing spots are big, black and oblong, whereas the common blue has orange lunules along the margins that the large blue never shows. The Adonis blue is a far more brilliant sky-blue with black bars crossing the white wing fringes, the mazarine blue is smaller and much more thinly spotted, and the holly blue is a pale silvery blue with an almost unspotted underside and is usually seen around shrubs and hedges rather than open turf.

Life cycle: egg, caterpillar, chrysalis and adult

There is one generation a year, and only about three weeks of it are spent as a butterfly. Almost the whole cycle happens underground, in the nest of a red ant, which is why the large blue is among the most intensively studied insects in Europe.

Eggs

The female lays her eggs singly, pushing each one into an unopened flower bud of wild thyme (Thymus) or, in warmer parts of the range, wild marjoram (Origanum vulgare). The egg is a flattened, semi-globular disc about 0.5 mm across, pale bluish-white with a finely pitted surface, and it is well hidden between the bud scales. She selects buds that are at exactly the right stage and avoids plants that already carry an egg. Hatching takes five to ten days.

Caterpillar

The newly hatched caterpillar is about 0.8 mm long, pale ochre-yellow tinged with green, with rows of backward-curved, serrated hairs. For its first three instars it lives inside the flowerhead, eating florets and developing seeds, and it will eat any smaller sibling it meets. After the third moult it leaves the plant in the late afternoon, drops to the ground and simply waits.

What happens next is the reason for the species' fame. A foraging red ant of the genus Myrmica finds the larva and begins to milk a gland on its back that secretes a sugary, amino-acid-rich fluid. After an hour or more of this, the caterpillar rears up, the ant seizes it, and carries it back to the nest as though it were one of the colony's own grubs. The larva's surface chemistry closely imitates the ants' own recognition scent, and it also stridulates, producing sounds like those of the ant brood, so it is accepted and protected. Once inside it turns predator: for the next ten months it feeds on the ant grubs themselves, putting on roughly ninety-eight per cent of its final weight underground and passing the winter there. One nest can usually support only a single large blue larva.

Chrysalis

In late May or early June the full-grown larva pupates in an upper chamber of the ant nest. The chrysalis is around 0.5 in (13 mm) long, apricot-yellow at first and darkening to amber except over the wing cases, and it too stridulates, which keeps the ants attentive. Three weeks later the adult butterfly emerges, crawls up the ant galleries in the early morning and climbs a grass stem to expand its wings before the ants recognise it as an intruder.

Adult

Adults are on the wing from about the third week of June to the middle of July, a flight period of roughly three weeks. Individual butterflies are short-lived even by the standards of the family: the average is about five days, and two weeks is exceptional. Everything therefore happens quickly, with mating usually on the first or second day and egg laying immediately afterwards. Our guide to how long butterflies live puts that figure in context.

Where the large blue lives

The large blue is a Palaearctic species rather than a purely European one. It is found from northern Spain, southern France and Italy through the Alps and Central Europe, across Poland, the Baltic states and southern Scandinavia, through European Russia, the Urals and southern Siberia to the Altai, Mongolia, north-western China and the Amur region. It is missing from the far north, from most of southern Spain and from Portugal, and everywhere it is patchy, because the sites that suit it are scattered. In Britain the last native colony died out in 1979 and the species now survives only where it has been reintroduced, so its status among the butterflies of the United Kingdom is entirely the result of conservation work.

The habitat requirement is unusually precise. The butterfly needs unfertilised, well-drained grassland — usually limestone downland, but also acid coastal grassland and, in Europe, dry meadows and forest steppe — on a warm south-facing slope, with wild thyme or marjoram present and, critically, with a very short sward. Turf taller than about an inch cools the soil surface just enough for the red ant Myrmica sabuleti to be replaced by other Myrmica species in which the caterpillar cannot survive. A slope that stops being grazed by sheep, cattle or rabbits therefore loses the butterfly within a few years, even though it still looks like perfect meadow and the thyme is still flowering. Our page on butterfly habitat explains why so many species depend on this kind of fine structural detail.

Host plants, ants and diet

Caterpillar food. The large blue needs two hosts in succession, and losing either ends the colony. The plant host is wild thyme (Thymus praecox, T. pulegioides) or, in southern and central Europe, wild marjoram, and the caterpillar eats only the flowers and seeds for its first three instars. The animal host is the red ant Myrmica sabuleti, whose brood provides the rest of its food. Myrmica scabrinodis and one or two other species will adopt the larva, but far fewer survive to pupation in their nests, so a colony's real limit is the density of M. sabuleti nests within a couple of yards of a flowering thyme plant.

Adult diet. Adults drink flower nectar and take relatively little of it, since they live only a few days. Wild thyme and marjoram are the main sources, with bird's-foot trefoil, knapweed and other low downland flowers used as well. They also sip moisture from damp ground on hot afternoons, and because they rarely stray far from the turf where they emerged, colonies are compact and easy to count but also easy to lose.

Behaviour, conservation and the British reintroduction

Flight. The large blue flies with a steady, even, rather heavy beat, low over the turf, and settles frequently on thyme heads and bare stones. It is less restless than the smaller blues and is easy to approach when feeding or basking with the wings half open.

Courtship. Males patrol short circuits over the thyme patches through the middle of the day, investigating anything blue. Mating is brief and usually happens within a day of the female's emergence; she then begins laying almost at once, distributing perhaps sixty to a hundred eggs over many separate plants. Our page on how butterflies mate and reproduce covers the general sequence.

Why it collapsed. The British extinction in 1979 was for decades blamed on collectors. The real cause turned out to be the height of the grass. Myxomatosis removed the rabbits, sheep grazing was withdrawn, the sward grew an inch or two taller, soil temperatures dropped, and Myrmica sabuleti was quietly replaced by cooler-loving ants that could not raise the caterpillar. Colony after colony faded out on sites that looked, to the eye, completely unchanged.

The comeback. Once that mechanism was understood, sites in south-west England were restored to hard grazing and stock from the Swedish island of Öland was released from 1983 onwards. The large blue now breeds again at several dozen managed sites in Somerset, Gloucestershire and Devon, in some years in greater numbers than anywhere else in Europe. It remains listed as Near Threatened on the global Red List and as Endangered within the European Union, and it is protected under the Bern Convention and the EU Habitats Directive. The lesson has been applied to other short-turf specialists, among them the green hairstreak and the small blue, which share the same warm unimproved grassland.

Similar butterflies and how to tell them apart

Quick facts

Did you know?

• The caterpillar spends about ninety-eight per cent of its growth underground. It leaves the thyme weighing a few milligrams and comes out of the ant nest ten months later as a full-sized larva, having eaten several hundred ant grubs.
• It does not just smell like an ant, it sounds like one. The larva and the pupa both stridulate, producing calls close to those made by the ants themselves, which keeps the workers attending and defending them.
• An inch of grass drove it extinct in Britain. When grazing stopped the turf grew slightly taller, the soil cooled, and the one ant species that could raise the caterpillar was replaced — a change invisible to anyone looking at the flowers.
• The species was brought back from Sweden. Stock from the island of Öland was released in south-west England from 1983, and the reintroduced population is now one of the strongest in Europe.
• Its relatives took a different path. Several other Phengaris blues are fed mouth-to-mouth by the ants like oversized grubs, whereas the large blue is a straightforward predator that eats the brood.

In summary

The large blue, Phengaris arion, is Europe's biggest blue butterfly, 1.5 to 2.0 inches across, deep blue above with a row of heavy oblong black forewing spots and a grey-beige, white-ringed underside with no orange. It flies in one short brood from late June to mid-July on hot, closely grazed, unfertilised grassland from Spain and Britain east to China. Its eggs go into thyme or marjoram buds, the young caterpillar eats flowers and seeds, and the fourth instar is carried into a Myrmica sabuleti nest, where it spends ten months eating ant grubs before pupating underground. Lose the grazing, and the ant, the caterpillar and the butterfly all go with it.

Large Blue butterfly pictures

Frequently asked questions about the large blue butterfly

Why is the large blue butterfly called large?

Because it really is the largest blue butterfly in Europe, with a wingspan of 1.5 to 2 inches, clearly bigger and heavier-bodied than the common blue or the Adonis blue. The name is often explained as a contrast with the small blue, Britain's tiniest butterfly, but the size difference is genuine rather than merely comparative.

How does the large blue butterfly use ants in its life cycle?

After its third moult the caterpillar drops from the thyme to the ground and waits. A red ant of the species Myrmica sabuleti milks a sugary gland on its back, then carries it into the nest because its scent and its stridulation imitate ant brood. Inside, the caterpillar eats ant grubs for about ten months before pupating.

What does an adult large blue butterfly look like?

The upperside is deep, slightly silvery blue with a broad smoky grey border and a row of heavy, elongated black spots across each forewing. The underside is pale grey-beige with rows of black spots neatly ringed in white and a dusting of blue-green scales at the base of the hindwing. There are no orange markings.

What is the conservation status of the large blue butterfly?

It is listed as Near Threatened on the global IUCN Red List and as Endangered within the European Union, and it is protected under the Bern Convention and the EU Habitats Directive. Its narrow habitat requirements and its dependence on one ant species make it highly vulnerable to any change in grazing or land management.

What habitat does the large blue butterfly need?

It needs warm, south-facing, unfertilised grassland kept in very short turf by grazing, with wild thyme or marjoram in flower and abundant nests of the red ant Myrmica sabuleti. If the sward grows more than about an inch tall the soil cools, that ant is replaced by others, and the butterfly disappears within a few years.

How can you identify a large blue caterpillar?

The newly hatched larva is only about 0.8 mm long, pale ochre-yellow with a greenish tinge and rows of backward-curved serrated hairs, and it lives hidden inside a thyme flowerhead. The mature larva is white to pinkish, fat and grub-like, and is found only inside a red ant nest, so it is almost never seen in the open.

Are male and female large blue butterflies different?

Only slightly. Males are a brighter, cleaner blue with narrower dark borders, while females are duller, with broader grey-brown margins, heavier black forewing spots and more rounded wings. The species shows no strong sexual dimorphism, so the two sexes are easily confused in a quick field view or a photograph.

Why did the large blue go extinct in Britain?

Its last native colony died out in 1979 because grazing declined. Myxomatosis removed the rabbits and sheep were withdrawn, so the turf grew slightly taller and the soil cooled, and Myrmica sabuleti was replaced by other red ants that cannot raise the caterpillar. Collecting, long blamed, was a minor factor at most.

Has the large blue been reintroduced successfully?

Yes. Once the grazing mechanism was understood, English sites were restored to close grazing and butterflies from the Swedish island of Öland were released from 1983 onwards. The large blue now breeds at several dozen managed sites in Somerset, Gloucestershire and Devon, in some years in greater numbers than anywhere else in Europe.

How long does a large blue butterfly live?

An adult lives about five days on average, and two weeks is exceptional, so the whole colony's flight period lasts only about three weeks in late June and July. The insect itself, however, takes a full year from egg to adult, and roughly ten months of that are spent underground inside an ant nest.

References

1. Thomas, J. A., Simcox, D. J. & Clarke, R. T. (2009). Successful conservation of a threatened Maculinea butterfly. Science 325(5936), 80–83. doi.org/10.1126/science.1175726
2. Als, T. D. et al. (2004). The evolution of alternative parasitic life histories in large blue butterflies. Nature 432, 386–390. doi.org/10.1038/nature03020