Since its discovery in Britain it has been thought to be a form or a subspecies of the brown argus (Aricia agestis) and as well as a species in its own right.

It usually has a small white dot in the centre of the upperside forewing and the black spots on the underside are missing leaving larger white spots on the light brown background.

The northern English populations belong to a form called salmacis (called the Durham argus in English[1]) and are very similar to the brown argus including the presence of black spots on the under-wing (see photo) hence all the confusion.

As far is known common rock-rose (Helianthemum nummularium) seems to be the sole food plant in Britain.

Pupation takes place at ground level in late May and butterflies are on the wing from mid-June to mid-July.