This is the unusual Valezina form of the female. Typically only between 5% and 15% of the females in a colony are of this form, and at one time they were confined to the southern coastal counties of Dorset, Hampshire and West Sussex. In recent years however, the Valezina form appears to have been spreading further northwards, though whether this is purely the result of natural expansion, or has been influenced by the deliberate release of captive-bred butterflies, is unclear.