of ecology at Oxford University, worked out exactly what it needed to survive; and Simcox, a conservation consultant for the Centre for Ecology and Hydrology, drove his VW Campervan to an island in Sweden, collected some eggs from the large blues that fly there, and released caterpillars in Devon and Somerset.

The large blue, which is globally endangered, now flies at sites in Somerset in greater numbers than anywhere else in the world. After pioneering that first ever successful reintroduction of a butterfly driven to extinction in Britain, Thomas and Simcox, with assistance from everyone from the National Trust and Gloucestershire Wildlife Trust to Holland & Barrett, are this summer attempting an ambitious second phase. They want to help the large blue move north, to the Cotswolds, where ithasn't flown for 50 years. So far, so straightforward, but establishing a new colony of these unique butterflies is an almost unimaginably delicate and labour-intensive operation.

Simcox's base camp is a stone cottage with rough whitewashed walls close to one of two secret sites where the butterflies are being reintroduced. Every day, he rises at 6am to check on the caterpillars. This time he did not have to drive to Sweden but obtained eggs from a thriving colony on Green Down nature reserve, Somerset, where a record 135,000 eggs were laid this year. Virtually invisible to the naked eye, and smaller than a pinhead, these eggs are attached to thyme. Over several weeks in June and July, Simcox and Thomas, helped by a sharp-eyed conservationist, Sarah Meredith ("not many