How an extinct butterfly fluttered back to life

The life cycle of the large blue butterfly is one of the more extraordinary in the animal kingdom. Its survival is dependent upon a combination of pure chance, the grazing habits of cattle and its ability to trick its way into the nests of an entirely different species, whose young it devours greedily. Unsurprisingly, the butterfly became extinct in Britain in 1979.

Now, the large blue, endangered throughout Europe, is thriving again in parts of southern Britain, due to an intensive but secretive programme of re-introduction at sites in southern Britain in a process made more complicated by its total dependence upon other plants and creatures.

According to a report by Butterfly Conservation released yesterday, around 10,000 large blues were estimated to have flown in Britain last summer, an increase of more than 50 per cent over the previous two years and almost certainly the largest number since the Fifties. "It is a fantastic success,'' said a spokeswoman for Butterfly Conservation.

The location of all but one of the five sites have been kept secret to deter collectors. Only the site at Collard Hill in Somerset has been made public. The butterfly has now spread naturally to another six places.

All of those large blues that flew during the summer will now be dead, since the butterfly season lasts for just three weeks at the height of the summer and individual large blues live for just four or five days. During the season, the large blue mates and the female lays her eggs - but only on wild thyme flowers. Once the caterpillars hatch, they begin eating the thyme, before dropping to the ground where, if they are lucky, they are discovered and adopted by a Myrmica sabuleti, a species of red ant. The red ant is "attracted" to the caterpillar and begins stroking it, whereupon the caterpillar exudes a sugary substance from special glands on its back, which the ant eats. This "courtship" results in the ant carrying the caterpillar back to its nest. At this point, the caterpillar changes from being a vegetarian to a carnivore and begins devouring the ant grubs, growing in size until it dwarfs its hosts.

Right about now, in late September, the large blue caterpillar is about a third of its way through its 10-month squat in the ants' nest. As the weather cools, it becomes dormant, but revives and begins gorging again in Spring. Sometime in late May or early June, the caterpillar turns into a chrysalis, from which the butterfly emerges three or four weeks later. The ants, despite the fact that the butterfly has been gorging on their young, are believed to play some part in warding off predators until it is ready to fly away on its brief existence during the hottest part of July.

The large blue is utterly dependent upon the existence of the wild thyme together with the M. sabuleti ants - none of the other four species of red ants in Britain provide suitable food and conditions for it to survive.

The ants only thrive in areas where the grass is just the right height and are dependent on regular grazing by cattle. The decline of the large blue was therefore linked to changes in agricultural practices in the 20th century, which resulted in far less grazing land - as well as more use of pesticides.

Nerys Coward, spokeswoman for Butterfly Conservation said that re-introduction of the large blue, using breeding stock from Sweden, was complicated by the need to find sites that fitted all the requirements - thyme, red ants and grazing cattle. She added: "We have also discovered that managing a site for one endangered species is good for others - pearl-bordered fritillary butterflies, tiger beetles and pale heath violets have all benefited."

The re-introduction programme has involved 11 different organisations, co-ordinated by the Centre for Ecology and Hydrology, based in Dorset.

The improbable life of the large blue

* July: The large blue butterflies mate. Females lay eggs on flowers of wild thyme.

* Late August: Eggs hatch and caterpillars begin eating thyme. The caterpillar waits to encounter a Myrmica sabuleti red ant. When it does it secretes a substance which the ant likes; the ant carries the caterpillar to its nest.

* Late August-late May the following year: The caterpillar remains in the nest eating ant grubs and growing. Some species fed by the ants themselves. During the winter, the caterpillar hibernates.

* May-June: The caterpillar turns into a chrysalis, still inside the nest.

* July: The adult butterfly emerges from the nest and lives for four to five days.