its full strangeness, which took scientists 100 years fully to understand. In the days when butterflies were collected and pinned up, bringing colour to Victorian drawing rooms, the large blue was the most desirable prize of all. This was not merely for its beauty, or rarity, but because no one could breed it in captivity. Collectors put the caterpillars on thyme only to watch them fall off, shrivel up and die.

During the 20th century, lepidopterists finally discovered the large blue's secret: when the caterpillar hurls itself from the thyme it never eats leaves or flowers again. Instead, it secretes a sweet fluid that attracts ants. They are fooled into thinking the caterpillar is an ant grub and haul it into their underground nests. Here, the caterpillar sings tothe deluded ants and feeds voraciously on ant grubs for 10 months of the year, gaining 98% of its body weight this way before pupating. Every June, the butterfly hatches out underground, folds its wings and squeezes up a tunnel and out into the sunshine.

A large blue larva on thyme. Photograph: Butterfly Conservation

This parasitic miracle was understood by the 1950s but the large blue continued to decline. Butterfly collectors were blamed but the reason for its extinction was only discovered by Thomas in the 1970s, just too late to save the species. He worked out that the large blue could perfectly impersonate only one species of red ant, and this ant required particularly hot, well-grazed meadows to survive. The loss of traditional grazing caused its decline – and