- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Small, 23–32 mm (0.9–1.25 in.).
- Wing upper side: Shiny light blue. Males with narrow black band on leading edge of forewing. Females with broader black edge especially on forewing. Hind wing with black margin fragmenting into separate spots.
- Wing underside: Black spots on very light-coloured greyish blue base and short stripe in middle of wing.
- Habitat: Forests, bogs, parks and gardens.
- Flying time: Mid-May–Mid-June. The rarest of the Finnish blues. Second generation (July–September) possible.
- Overwintering form: Cocoon.
- Larval foodplant: Alder buckthorn (Rhamnus frangula), common buckthorn (R. catharica) and bilberry (Vaccinium myrtillus).
Of the 17 members of the Gossamer-winged butterflies that can be found in Finland, the holly blue is the earliest, flying already at the end of April. It is quite common in southern and central Finland and can be found in parts of Lapland too. The holly blue is the only blue that has no black blotches on the shiny light blue upper side of its wings and the undersides are almost white with black spots. It has no red patterning.
Males fly around looking for females, who lay their eggs individually in the inflorescence of the host plant. The larvae, which hatch in a few days, have a gland that secretes a sweet, nourishing liquid that ants love to lick, so ants sometimes hang around the larvae and protect them.