- Latin synonym: Maculinea arion, Glaucopsyche arion
- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Small–middle-sized, 29–42 mm (1.13–1.64 in.). Largest of the Finnish blues. Females usually larger than males.
- Wing upper side: Shiny blue, forewing with black blotches and black stripe. Shiny blue hue decreases and wings become darker with age.
- Wing underside: Light-coloured, brownish grey, with white-edged black blotches. Basal part of hind wing shiny turquoise.
- Habitat: Dry, sandy landscapes with scant vegetation.
- Flying time: Late June–late July.
- Overwintering form: Caterpillar.
- Larval foodplant: Young caterpillar eats Breckland thyme (Thymus serpyllum), later it lives as a parasite in the nests of Myrmica sabuleti ants.
- Endangerment: Critically endangered, protected in all of Finland.
The large blue can only be found in Taipalsaari and Liperi, and it has declined recently in Finland. The large blue can be differentiated from its close relatives by the black blotches on the upper side of its wings, a feature that is only shared by the baton blue and the chequered blue. All of these species are rare. Large blue females lay their eggs individually on the buds of Breckland thyme. When they have partly grown and developed their skin a few times the caterpillars fall from the host plant to the ground. The larvae have a gland that excretes a sweet, nourishing liquid that ants (in this case harvester ants) like to lick. Ants carry the young caterpillars to their nest, where they become predators and eat the ants’ eggs, larvae and those in the pre-cocoon stage. In spring the caterpillars enter a cocoon and emerge as butterflies and dig their way out of the nest in late June.