- Family: Gossamer-winged butterflies – Lycaenidae
- Subfamily: Hairstreaks – Theclinae
- Wing span: Small, 25–37 mm (0.98–1.44 in.).
- Wing upper side: Base colour brown, shiny bluish violet from a certain angle. Females bluish violet only at base of wings but it is easier to see – although it still depends on the angle.
- Wing underside: Grey, with white stripe traversing forewings and hind wings. Back corner of hind wing with two orange blotches.
- Habitat: Woodland where oak trees grow.
- Flying time: Mid-July–mid-August.
- Overwintering form: Egg.
- Larval foodplant: Oak (Quercus robur).
The purple hairstreak can be found on the Åland Isles and the southern coast of Finland. It is the only hairstreak whose wings have a shiny blue upper side and a grey underside. It differs from the Blues in that a white lateral stripe traverses the underside of its wings and it has no white-edged black blotches.
The purple hairstreak’s favourite territory is the crown of oak trees. Females lay their eggs individually in buds on oak tree branches. In spring the hatching caterpillars make their way into the buds, and they make their cocoons in the trunk of the tree, protected by lichen. The species has spread during the 1990s towards the east and north-east from Turku and its close neighbourhood.