- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Very small, 20–25 mm (0.78–0,98 in.). Females slightly larger than males.
- Wing upper side: Shiny bluish violet with different sizes of blackish brown patches. Margins with row of black blotches. Hind wing with black wavy strand or row of half-moon-shaped blotches next to margin blotches. Oval black blotch in middle of forewing and more faintly on hind wing.
- Wing underside: White with clear black blotches. Orange strip (or row of blotches) close to edge of hind wing.
- Habitat: Open rocky terrain in forests.
- Flying time: Late May–mid-June.
- Overwintering form: Cocoon.
- Larval foodplant: Orpine (Hylotelephium telephium) and its relatives, and white stonecrop (Sedum album).
- Endangerment: Critically endangered, protected in all of Finland.
The chequered blue has declined sharply. It is very rare and can only be found in a few places in southern Finland.
The chequered blue can be confused with the baton blue which also has arcing blotches on the upper surface of its forewings and hind wings. The chequered blue can be recognised by its large margin blotches and strong black blotches on the underside of the wing.
Males defend their mating territory among rocky outcrops, trees and bushes. Females lay their eggs individually on the leaves of the host plant. After about a month the developing larvae have a gland like many of their close relatives that secretes a sweet, nourishing liquid that ants like to lick, so there are sometimes ants around, which protects them. Caterpillars make their cocoons under stones and overwinter there.