The Large blue (Phengaris arion) is a species of butterfly in the family Lycaenidae. The species was first defined in 1758 and first recorded in Britain in 1795. In 1979 the species became mostly extinct in Britain but has been successfully reintroduced with new conservation methods. The species is classified as "near threatened" on the IUCN Red List of Threatened Species. Today P. arion can be found in Europe, the Caucasus, Armenia, western Siberia, Altai, north-western Kazakhstan and Sichuan.

The large blue can be distinguished by its unique speckled black dots on its wings with a blue background.

The large blue butterfly is well known in behavioural ecology as it is a brood parasite of a single species of red ant, Myrmica sabuleti. The discovery was made by Captain Edward Bagwell Purefoy along with F. W. Frohawk and others.

...