- Family: Gossamer-winged butterflies – Lycaenidae
- Subfamily: Hairstreaks – Theclinae
- Wing span: Small, 23–33 mm (0.9–1.29 in.).
- Wing upper side: Brown, red blotch in back corner of hind wing.
- Wing underside: Brown. Thin white stripe across wings forming a W-shape. Hind wing margin with black-edged red strip.
- Habitat: Woods that include elm trees.
- Flying time: Mid-July–mid-August.
- Overwintering form: Egg.
- Larval foodplant: Elms (Ulmus glabra).
The Hairstreaks (Theclinae) are one of three subfamilies of the Gossamer-winged butterflies (Lycaenidae) family that can be found in Finland. The two others are the Blues (Polyommatinae) and the Coppers (Lycaeninae).
The white-letter hairstreak can be found rarely in the south-western corner of Finland. The species always rests with its wings pressed together. It differs from the black hairstreak, which it looks quite like, by the W-shaped stripe on its hind wing (which has given the species its scientific name: w-album = white W) and the continuous black stripe that runs inside the red strip along its edge. Like all the hairstreaks (except the green hairstreak), the white-letter hairstreak also has a small spur on its hind wing.
White-letter hairstreak males defend their territory in trees while waiting for females, who lay their eggs individually or in pairs in elm buds and branches. The following spring the developing caterpillars first eat the flower buds and later the leaves. The species is rare, but in the 1990s it began to spread eastwards from Turku and its neighbourhood.