- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Small, 26–34 mm (1.01–1.33 in.).
- Wing upper side: Shiny sky-blue, wing edges narrowly black. Females’ wing colour can be slightly darker with broader black margin.
- Wing underside: Light grey (male) or brownish grey (female), with white-edged black blotches. Hind wing blotches smaller or completely lacking. Base of hind wing turquoise–greenish.
- Habitat: Dry and open landscapes with stones, gravel and sand.
- Flying time: Late May–early July.
- Overwintering form: Cocoon.
- Larval foodplant: Different kinds of Pea family plants (Fabaceae) such as genus Lotus, vetch (Vicia) and clover (Trifolium).
- Endangerment: Vulnerable.
The Gossamer-winged butterfly family is the second largest in the world with over 6,000 species (only the Brushfoot family is larger). The Blues is one of the Gossamer-winged butterfly family’s 3 subfamilies existing in Finland.
The quite rare green-underside blue can be found in southern and especially south-eastern Finland and it has good distinguishing marks that set it apart from other Blues: (1) the shiny blue upper side of its wing, (2) the black blotches on the underside of the forewing that grow larger towards the hind wing, and (3) the shiny greenish turquoise basal area of the hind wing.
Males fly around looking for females, but they can also defend their mating territory in bushes, where they wait for the females, who lay their eggs in the foodplant’s buds. The colour of the larvae can change according to the foodplant. The larvae have a gland that secretes a sweet, nourishing liquid that ants love to lick, so ants sometimes hang around and protect the larvae.