Wingspan about 75 to 85 mm. The male butterfly is one of the most beautiful of all of the butterflies found in the British Isles. From certain angles it appears to have black wings intersected with white bands. However, when the wings are at a certain angle to the sun, the most beautiful purple sheen is displayed, a result of light being refracted from the structures of the wing scales. The female, on the other hand, is a deep brown and white and does not possess the purple sheen found in the male.

Deciduous woodland where the larval food plant - willow is present, particularly Goat Willow and Grey Willow.

Mid June to early August.

Its range is limited to central southern England, between South Wiltshire and South Hampshire in the west, Surrey and West Sussex in the east, and Oxfordshire and Hertfordshire in the north, with scattered colonies elsewhere. It is not found in the north of England, Scotland, Ireland, the Isle of Man or the Channel Islands and has not been seen in Wales since the 1930s.

Rare in Leicestershire and Rutland.