- Name also: Idas Blue
- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Very small, 20–28 mm (0.78–1.09 in.).
- Wing upper side: Males with shiny blue, black-edged. Hind wing margin usually broken up into separate blotches. Females basal part blue, edges (especially forewings) broadly dark brown. Especially hind wing margin with red wavy strip or row of half-moon-shaped blotches.
- Wing underside: Males with pale grey-white. Females gingery. Both genders with white-edged black blotches. Wing margins with row of red blotches that can also form a continuous strip. Outer side of red margin blotches with shiny blue blotches.
- Habitat: Dry pine heaths, rocky outcrops and sandy land, but also bogs.
- Flying time: Late June–July.
- Overwintering form: Egg.
- Larval foodplant: Including heather (Calluna vulgaris), bog bilberry (Vaccinium uliginosum), crowberry (Empetrum nigrum), different Pea family plants (Fabaceae), Breckland thyme (Thymus serpyllum) and later ant larvae and cocoons.
The northern blue can be found commonly all across Finland except Lapland. It can be confused with the silver-studded blue and the cranberry blue, both of which have shiny blue blotches on the underside of their hind wings, just like the northern blue. The females of the species can be told apart by the different patterns that they have on the upper side of their wings. The upper side of the northern blue male’s wings are bluer than those of the cranberry blue, and their dark margin is narrower and has a sharper inside border than the silver-studded blue. The red blotches on the edge of the underside of the northern blue’s wings are spread over larger area than they are on the cranberry blue.
Females lay their eggs individually on the host plant. The following summer the hatching caterpillars initially stay on the host plant, but at the end side of caterpillar time they move towards ant hills. The caterpillars have a gland which secretes a sweet, nourishing liquid that ants like. Thus the ants protect the caterpillars, even though they eat ant larvae and cocoons.