This striking moth rests with wings open and half-raised and forewings slightly curled. Can be distinguished from the similar Lunar Thorn by a dark central spot towards the outer edge of the hindwing upperside. The central silver crescent on the forewing is also larger on the Purple Thorn. Second generation moths are typically smaller and paler.
The caterpillars can be found from late May to early July and again in from August to September in southern England.Overwinters as a pupa, just below ground.
The adult moths are rarely seen apart from when attracted to light.