- Family: Brushfooted Butterflies – Nymphalidae
- Subfamily: Emperors – Apaturinae
- Wing span: Large–very large, 60–75 mm (2.4–2.9 in.).
- Wing upper side: Brownish black, male with bluish violet sheen depending on angle of view. Forewing with white blotches, hind wings with white lateral strip. Tip of hind wing reddish. Orange-bordered eyespot around tip.
- Wing underside: Rust-red, forewing with black and white blotches. Hind wing with white strip. Back of forewing with large eyespot with blue centre, black between and white and rust-red outer margin. Back of hind wing with smaller eyespot.
- Habitat: Broad-leaved forests where willow grows in wetlands.
- Flying time: Mid-July–early August.
- Overwintering form: Caterpillar.
- Larval foodplant: Willows (Salix), e.g. goat willow (S. caprea) and eared willow (Salix aurita).
The Brushfooted Butterfly (Nymphalidae) family is the largest butterfly family in the world with over 6,000 species. In Finland it is represented by around 60 species, which are nowadays usually divided into five subfamilies, with the purple emperor and lesser purple emperor constituting the Emperors subfamily (Apaturinae). The other subfamilies are the Admirals and relatives, Browns, Heliconians and True brushfoots.
The purple emperor has appeared in Finland now and again over the last 100 years and has become more common again since the middle of the 1990s. In 2010 it was already more populous than the poplar admiral, which it resembles. It can also be easily confused with the white admiral and especially the lesser purple emperor. The white admiral and poplar admiral lack the eyespots that are characteristic of the purple emperor, however. Another identification marker between emperors and poplars is the colour of proboscis: emperors have yellow, admirals dark brown.
Males defend their territory, which is often on hilltops or in the crowns of trees, and wait for the females to arrive. Females lay their eggs individually on the leaves of the host plant.