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.