The Large Blue is aptly named, being indeed the largest and also rarest of the ‘blue’ butterflies in Britain but sadly, absent from Dorset. It is easily distinguished from other blues by the row of black spots on its upper forewing, the underside consisting of a pale brown colour dotted with a further array of black markings.
The story of the Large Blue and it’s history in the UK is one of trials and tribulations, becoming extinct in 1979 but has been successfully re-introduced using European butterflies and their caterpillars, as part of a dedicated, long-term and highly successful conservation project.
The caterpillar of this species has an equally intriguing story, at first feeding on the flowers of it’s sole foodplant, Wild Thyme, on which the female laid her eggs over a week before. After feeding up, the half grown larva switches behaviour and descends to the ground where it is, in a sense, ‘adopted’ by red ants and taken underground into the heart of their nests.
The caterpillar, which now mimics an ant grub, has fooled the colony into believing this invader is in fact one of their own, much like a cuckoo in a birds nest. Here it will be tended by ants, providing them with a sweet liquid from a special gland on the caterpillar’s back whilst feeding on the ants own young. It is estimated that the Large Blue caterpillar will consume a staggering 1,200 ant grubs during it’s time in the nest, becoming a hundred times heavier than when it started off!
When fully grown, the caterpillar pupates inside the nest chamber, the chrysalis producing the same sugary liquid to ensure the ants care for it during the weeks before the adult butterfly emerges. The fresh insect, escaping to the surface with wings still crumpled before the ants realise their mistake!
If you are interested in seeing this rare butterfly, it is on the wing now, and can be seen at Collard Hill in neighbouring Somerset.