The butterfly that came back from the dead – thanks to the red ant

Michael McCarthy celebrates the 25th anniversary of large blue's reappearance

It flutters across a hillside in Somerset – the lovely blue butterfly that came back from the dead. And the man who brought it back is watching, celebrating 25 years of its resurrection.

The return of the large blue, which went extinct in Britain in 1979, is one of the great conservation success stories, and the 25th anniversary of its reintroduction is being celebrated today at Montacute House, the Somerset stately home in the heart of large blue country, by Britain's leading conservationists, headed by Sir David Attenborough.

What they will be celebrating, in fact, is the work of Jeremy Thomas, Britain's leading butterfly expert, who discovered the way to bring back a creature which has the most astonishing life cycle of any British insect – it spends most of its life as a caterpillar inside an ants' nest, and without the ants, it cannot turn into a butterfly.

As a young researcher, Professor Thomas, as he now is, discovered which species of red ant has this relationship with the large blue (and there is only one) and then worked out how to keep the ant colonies flourishing. It was a question of having the grassland where they lived grazed very short, so it would remain warmed by the sun. If the grass was only a few centimetres too long, the ants would disappear, and with it, the butterflies.

Historically, rabbits had kept the grass short, but in the mid-1950s rabbit populations were suddenly devastated by the plague-like disease myxomatosis. On many large blue sites, the turf grew too long and the butterfly began rapidly to die out – although no one had any idea why.

It took Jeremy Thomas six summers of work on the very last of the large blue sites, on the edge of Dartmoor, from 1972 to 1977, to unravel the chain of events which was leading to the rapid demise of one of Britain's most beautiful wildlife species.

He was just too late to save it. But the knowledge to bring it back and keep it was now in place; and in 1983, working with another butterfly conservationist, David Simcox, he brought large blue caterpillars from Sweden and released them on the Dartmoor site; they emerged as adult insects the following summer.

Today marks the 25th anniversary of the first reappearance of free-flying large blues in June 1984. Yesterday, as we sat on the slopes of the Polden Hills in Somerset, Jeremy Thomas recalled how he solved the mystery of the missing butterfly which has become the centre of the large blue reintroduction effort, with no fewer than 28 colonies now established (the area is better than Dartmoor).

Green Down, the Somerset Wildlife Trust nature reserve where we we sat, is now thought to be the densest colony of large blues in the world, with 5,000 butterflies in a mere five hectares. They fluttered about us in twos and threes. "I never get tired of them, with their inky-blue wings and their jinking flight," he said. "They are such lovely things." I asked him if he remembered seeing the first reintoduced adult, 25 years ago. He smiled and said he did: "It flew out from behind a gorse bush, and I cried yes!"

*We didn't only see the large blue yesterday – we added two more species to our tally in The Independent's Great British Butterfly Hunt. They were two butterflies of high summer which have just emerged – the meadow brown and the marbled white. These three take our total to 30 species, with 28 to go.

The Great British Butterfly Hunt Species 24-26 (of 58)

24. Large blue

Maculinea arion

This medium-sized butterfly is the largest and rarest of our blues, recognisable by the row of black spots on its upper forewing. It is doubly remarkable, in that it has been successfully reintroduced after going extinct, and spends much of its life as a caterpillar inside ants' nests.

Larval food plants Caterpillars initially feed on the flower-heads of wild thyme, but later they trick red ants ( Myrmica sabuleti) into taking them into their nests, where they feed on the ant grubs, until they pupate and turn into a butterfly.

Where seen Now restored to 33 grassland sites in south-west England, principally Somerset.

Current conservation status Still very rare but steadily expanding.

25. Common blue

Polyommatus icarus

Britain's most widespread blue butterfly, found right across the UK. Similar to (but not as bright as) the adonis blue. Told from the holly blue by having brown underwings, rather than sky blue. Females are generally brown on upper wings, too.

Larval food plants The main one is common bird's-foot trefoil, but others are utilised, including restharrow, black medick and white clover.

Where seen Widespread in grassy places where food plants grow, but also in waste land and urban habitats such as cemeteries.

Current conservation status Officially a 16 per cent decline since 1976, but not thought to be threatened.

26. Adonis blue

Lysandra bellargus

One of the loveliest of Britain's butterflies, of a blue so brilliant it seems to be lit from within, like the blue of a kingfisher, this is a species found exclusively on the chalk downs of southern England.

Larval food plants Horseshoe vetch, a bright yellow downland flower.

Where seen On downland in the south. The adonis blue needs short, herb-rich sward, and sunny south-facing slopes.

Current conservation status Decline in the mid-20th century because of changes in farming but conservation work is bringing it back well in some areas.

In the ninth of our status reports, we look at the blues, including the reintroduced large blue, the widespread common blue and the striking adonis blue