- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Very small–small, 21–30 mm (0.82–1.17 in.).
- Wing upper side: Males shiny blue, black-edged. Black margin of hind wing can fragment into separate patches. Females dark brown, basal part often slightly bluish, hind wing margin with red blotch.
- Wing underside: Grey with white-edged black blotches. Especially hind wing margins with red blotches.
- Habitat: Meadows, grazing land and wasteland.
- Flying time: First generation mid-May–July and second late July–August, occasionally also in September.
- Overwintering form: Caterpillar.
- Larval foodplant: Bird’s-foot trefoil (Lotus corniculatus) and red clover (Trifolium pratense).
- Endangerment: Vulnerable.
The short-tailed blue is very rare in Finland. It was not listed as a Finnish butterfly until a great migration occurred in 1999. It is the only wandering blue species that visits Finland, and is also the only member of the Blue subfamily to have a clear spur on its hind wing, making it easy to tell apart from around 20 close relatives. The spurs are not very long, hence the “short-tailed” part of its name. Some members of the Hairstreak subfamily also have short spurs.
Males fly around looking for females, who lay their eggs individually in the host plant’s flowers. 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.