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The Large Blue butterfly larva mimics the behaviour of the red ant to survive, but human intervention has also played a part. Sanjida O'Connell reports
Once there was a butterfly, a Large Blue butterfly, which depended on one type of ant for its survival. It earned itself the name "cuckoo butterfly", since its caterpillars duped red ants into believing that they were the ants' larvae. The ants carried the caterpillars tenderly into their colony; once established in the nursery, though, the Large Blue caterpillars ate the ants' grubs. The ants began to die out, but the Large Blue butterfly died out completely.

It reads like a bleak Aesop fable, perhaps with the moral "Never trust anyone". But far from being fiction, the story is real - and one of the most perplexing in biology. For the Large Blue butterfly with its predatory instincts did become extinct in Britain. Now, it is being reintroduced from Sweden, the last chapter in a curious tale of the impact of human beings on nature.

The Large Blue is a product of a manufactured Britain. Many people lament the destruction of our wilderness and want nature to be truly "natural". In reality, though, there is precious little land in Britain that can be described as such. In some cases, this may be no bad thing. The life cycle of the Large Blue butterfly has been maintained precisely because of human intervention over the past 5,000 years. Now that land is no longer managed in the same way, the butterfly has become extinct.

This summer, Dr Jeremy Thomas from Furzebrook Research Station in Dorset - who has studied the Large Blue for 20 years - will reintroduce it to an area of Gloucestershire. Despite being its saviour, he uncovered the strange and bizarre life cycle of the Large Blue butterfly too late; it disappeared from Britain just as he was slotting together the final pieces in the puzzle.

It was over 70 years ago that Dr T A Chapman found some Large Blue cater- pillars in a red ant's nest and guessed there was some relationship between them. Quite what it was he did not know, but he believed it could explain its slow decline from as long ago as 1884. Some conservationists attributed the diminishing numbers to over-zealous butterfly collectors; others blamed air pollution or a change in the climate. A project was launched to save the Large Blue, yet numbers continued to fall from 100,000 adults in the mid 1950s to a single small colony in the early 1970s.

In 1978, five years after Dr Thomas began studying the Large Blue, entomologists still believed there was plenty of land suitable for it. Some factor other than disappearing habitat had to hold the key. Dr Thomas resolved to return to a traditional approach, observing animal behaviour rather than using the techniques of pure conservation. Two decades later he is able to show that it is not only changing patterns of land use that are to blame, but the quirky nature and exacting requirements of the Large Blue itself that have proved its downfall.

What Dr Thomas discovered was that Large Blue larvae will feed for several months on thyme. But unlike other caterpillars that can increase their body weight up to 50-fold, the larvae remain thin. After their final moult they develop a panel of sensory organs which, Dr Thomas says, "attract, excite and appease the ants." The caterpillars fall off the plant on which they feed, and wait to be found by foraging ants. Their smell and behaviour exactly mimic those of a large ant grub that has escaped - so the ants "return" the wanderer to its home.

However, the thyme on which the caterpillars graze has to grow within 2-3m of the foraging ants, or the caterpillar will die; a minimum number of host ants also has to be present. There are further complications. Up to eight species of red ant may forage under the caterpillar's food plant, and all will pick up the caterpillar and take it back to the colony - but the caterpillar can only survive in the nest of one particular species.

"Red ants are quite tolerant of things that smell and sound similar to their own grubs," explains Dr Thomas. "But as soon as there is any stress - such as hot, dry weather or a food shortage - they become less benign and kill anything that doesn't act quite right. They will even kill their own kind in extreme situations. The Large Blue caterpillar is a perfect mimic of only one species of red ant, and so by definition it is not a perfect mimic of other species."

Once successfully inside the colony, the caterpillars eat the ants' grubs and hibernate for 11 months. Because of their appetite, feeding on the creatures that have sheltered them, very few caterpillars can be reared per nest and the ant colony itself is totally destroyed. As a result, the next generation of caterpillars has to hatch near a new ant colony in order to survive.

Red ant queens can also cause the Large Blue's downfall. The caterpillars feed on large ant larvae and become contaminated by their smell. Any large grubs are seen as a threat by a queen ant, because they too could grow into a queen. If there is a queen in the colony, she therefore produces chemicals that make the workers attack these "rival" grubs. Any caterpillars that also smell "big" will be systematically attacked, so those in a colony with a queen are three times more likely to be killed.

As if that weren't enough, the butterfly - somewhat ironically - has a parasite of its own which can wipe out up to a quarter of the butterfly population. This type of parasite, an ichneumon, has an eerie ability: it can determine from above ground which nests contain the specific host ants the Large Blue requires, and even whether that colony contains any caterpillars. Ichneumon have become specially adapted to nip into the ant nest, find and sting the caterpillar and escape unscathed.

Dr Thomas believes there are 40 sites in Britain rich enough in thyme to support 1,000 butterflies, but there aren't enough host ants at most of them. The ants are only abundant when the grass is less than 3cm tall, but thyme is most abundant when the grass is 3-5cm high. The length of the grass affects temperature, and these red ants can only survive when their mini-climate is sufficiently warm. Moreover, because they love heat more than any other species of red ant, they will only live on heavily grazed, south-facing, southern sites.

Given the exacting habits of this unusual pair of insects, there are fewer than 100 acres of land in Britain are suitable for them. Both species need land with no fertiliser added to it, which has been grazed intensively. Fertiliser encourages weed species which shade out the delicate meadow plants that the Large Blue needs for survival. Unfortunately, less than three per cent of chalk grassland in Britain is free of fertilisers. Grazing also keeps these rank grasses at bay and the meadow turf very short, which is necessary to keep the ground warm enough for the red ants.

These steep, unfertilised slopes have been progressively abandoned by farmers over the last century. Cattle, sheep and ponies were gradually removed down to the lowlands as it became increasingly uneconomical to farm fields with thin soil and steep sides. Rabbits, which kept the grass short until the 1950s, were wiped out in their thousands by the disease myxomatosis. "99 per cent of rabbits were killed by the virus," says Dr Diana Bell, a biologist from the University of East Anglia.

After the death of the rabbits, conservationists failed to keep up the heavy grazing. Neither did rabbits reappear in sufficient numbers to mow the meadows. "It is a myth that myxamatosis is no longer effective," says Dr Bell. "The virus mutates and still kills about 95 per cent of the rabbits in my study population."

Over the years, Dr Thomas solved the problem of why the Large Blue was becoming extinct. He realised, however, that in order to reintroduce the butterfly, both ant and butterfly would have to be bred together in captivity. The goal was to re-establish the Large Blue on six sites. Pilot tests on five of them have re-established small numbers of butterflies using caterpillars from Swedish populations. Last summer a sixth site was established, using Swedish caterpillars and butterflies. This proved such a success that Dr Thomas will, for the first time, release butterflies and caterpillars which have been bred in Britain to a new site.

The Large Blue is a relic of a time 10,000 years ago when the south of England was as hot as the Dordogne. The butterfly hung on by the skin of its teeth because, 1,000 years before the climate changed, people were starting to make clearings - creating warmer microclimates for the ant and the butterfly.

Now, the Large Blue has become a symbol for other endangered creatures which have survived not in spite of, but because of, human beings. Others on the danger list are animals and plants such as the rare pale heath violet, the wood lark and other butterflies like the Purple Fritillary, the Dark Green Fritillary and the Grayling.

Returning to the Aesop analogy, what can the moral of this story be? It is that, in today's world, there is simply no room for creatures that become too specialised and co-dependent. It is the generalists, the rats and the pigeons, that are having a field day, while our beautiful and bizarre quirks of nature are being lost. !