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 extinct in Britain but has been successfully reintroduced with new conservation methods. Currently large blue 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 among the behavioural ecology field as it exhibits a unique parasitic relationship with a single species of red ant, Myrmica sabuleti.[2] The species also feeds on thyme and wild marjoram oregano as food plants early in its life cycle.