- Latin synonym: Aricia eumedon, Plebejus eumedon
- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Small, 24–32 mm (0.94–1.25 in.).
- Wing upper side: Brown. Narrow dark patch in middle of forewing. Margins of females’ hind wing with reddish patches.
- Wing underside: Brownish, basal part shiny turquoise. White-edged black blotches and margin with red blotches that can also form a continuous strip. Hind wing with white stripe that runs from black central blotches to wing margin.
- Habitat: Roadsides and overgrown fields where cranesbill grows.
- Flying time: June–early July.
- Overwintering form: Caterpillar.
- Larval foodplant: Wood cranesbill (Geranium sylvaticum) and bloody cranesbill (G. sanguineum).
- Endangerment: Near threatened.
The geranium argus is common in central and southern Finland and can be seen in most parts of northern Finland. The upper sides of both genders’ wings are brown, even though the Blues subfamily that it belongs to usually follow the pattern that the males have blue wings on top and the females have brown. The geranium argus can be differentiated from its close relatives by the white stripe on the underside of its hind wing which reaches from the black central blotch to the edge.
Like the larvae, the adult geranium argus is also strongly attached to the foodplant and they are seldom seen very far from cranesbill stands. The best flying time is when cranesbills are flowering. Females lay their eggs individually on cranesbill flowers and leaves. The developing larvae have a gland that secretes a sweet, nourishing liquid that ants love to lick, so ants are often around them and other closely related larvae.