- Latin synonym: Plebejus icarus
- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Small, 23–35 mm (0.9–1.37 in.). Males slightly larger than females.
- Wing upper side: Males shiny sky-blue, margins with narrow black band. Females darker blue, edges dark brown or wings completely dark brown. Forewing and hind wing margins with red blotches.
- Wing underside: Light grey or light brown with white-edged black blotches. Red blotches on edges of both forewing and hind wing. Hind wing with red blotches in middle rows of black blotches, and white wedges that reach inside inner row of black blotches. Area at base of wings shiny turquoise.
- Habitat: All kinds of open but not damp habitats.
- Flying time: Mid-June–mid-August. Second generation (end-July–September) possible during a warm summer in southern Finland.
- Overwintering form: Caterpillar.
- Larval foodplant: Different Pea family plants (Fabaceae) such as genera bird’s-foot trefoil (Lotus corniculatus) and Clover (Trifolium).
A white wedge-shaped area that surrounds the red blotches is characteristic of the common blue and is a feature that is only shared by the northern brown argus, although the upper side of its wing has brown and red patches and it can only be confused with common blue females. Common blue’s wedge is not as clearly defined and large as the northern brown argus’s. The common blue’s forewings and hind wings have black blothches that are almost equally large; the blotches on the northern brown argus’s forewings are clearly larger.
The common blue is common across almost the whole country. Males wait for females guarding their mating territory in trees and bushes. Females lay their eggs individually on the host plant on leaves or in flowers. The larvae have glands which secrete nourishing, sugary fluid that ants like to lick. In this way ants are attracted to the larvae, and their presence protects them.