The males of this species possess one of the most striking shades of blue of any of the European species, fringed with neat black margins and white borders. The females conversely are a rather dull brown with variable amounts of blue suffusion near the wing bases and variable orange marginal spotting, The underside is a pale greyish-buff colour with plentiful black spots. In many ways it is similar to the Common Blue (Polyommatus icarus), although the current species is only found at high altitudes. The underside of the Eros Blue is usually paler, with less intense orange markings and a ‘spottier’ appearance. Whilst localised, it can be rather abundant where found.

Distribution: Limited to the higher mountains of southern and central Europe, including the French and Spanish Pyrenees, Massif Central, Alps of SE France, N Italy, S Switzerland and W Austria. Also found in the Apennines (C Italy) and the higher mountains of the Balkans, including SW Croatia, S Bosnia, S Serbia, Albania, North Macedonia, Bulgaria and N Greece. An isolated population exists in the Tatras in Slovakia.

Flight Period: Flies in a single generation, generally mid-June until mid-July in the Balkans, early July-early September elsewhere.

Habitat: Generally found in subalpine meadows, alpine grassland, limestone slopes and sparsely wooded rocky slopes, usually between 1800-2400m. May occur closer to 900m in the Balkans, usually on acidic soils.

Foodplants: Oxytropes (Oxytropis campestris and O. halleri) and Genista depressa.

Nectar Sources: I observed this species visiting flowers of houseleeks (Sempervivum) and sainfoins (Onobrychis). Males will frequently visit damp ground.