Nature · Insects & other invertebrates

Purple emperor

One brood, late June to mid-August · wingspan 70–92 mm

Apatura iris — Nymphalidae

The most sought-after of British butterflies, a large woodland insect whose male flashes an intense iridescent purple-blue as he turns in the light and shows plain dark brown from any other angle. He spends most of his life in the canopy of a tall "master" oak, drinking aphid honeydew, and comes down to the ground only for salts — from puddles, dung, carrion and sweat. Females, larger and wholly brown, lay single eggs on sallow. Victorian collectors called him His Imperial Majesty.

How to spot them

▸Male flashes brilliant purple-blue only at certain angles; from others he is plain dark brown — structural, not pigment, colour
▸Female larger, with no purple at all; both sexes have a white band across each wing and an orange-ringed eyespot on the hindwing
▸Almost never at flowers: it feeds on honeydew, sap, dung, carrion and damp ground
▸Males patrol and spar high around one tall 'master tree', usually an oak
▸Larva slug-like, green with two horns and yellow chevrons, on sallow; overwinters on a twig, turning bark-brown

How to know it

Its male flashes intense purple-blue as he banks, and looks plain brown the instant the angle changes

Purple emperor — the colour is structural interference from the wing scales, not a pigment.

Ignores flowers entirely and is baited by collectors with dung, carrion or a rotting fish

Purple emperor, which takes salts and honeydew rather than nectar.

Males spar in the air around a single tall 'master tree', usually an oak, high above the wood

Purple emperor — the master-tree habit makes it hard to see and easy to miss.

Its slug-like green caterpillar has two horns, sits on a sallow leaf, and overwinters as a bark-brown mimic on a twig

Purple emperor; the foodplant is goat and grey willow, not oak.

Victorian collectors called it His Imperial Majesty and hunted it with high nets on long poles

Purple emperor — the 'emperor' of British butterfly collecting.

Named for the goddess of the rainbow, for the shifting colour of the male's wings

Purple emperor — Apatura iris.

Quick-fire facts

•Apatura iris; the specific epithet iris is for the rainbow goddess, after the male's iridescence
•Larval foodplants are sallows — chiefly goat willow (Salix caprea) and grey willow (Salix cinerea)
•The adult feeds on aphid honeydew in the canopy, tree sap, and minerals from dung, carrion and damp tracks
•Butterfly Conservation and the Woodland Trust both note it is severely under-recorded because it lives in the treetops
•Strongholds include Bentley Wood and the woods of Wiltshire, Hampshire, Sussex and Northamptonshire; it is spreading and has recolonised sites in the Midlands
•Only the male is iridescent; the purple is produced by microstructure in the scales and disappears with the viewing angle
•It overwinters as a small larva spun to a sallow twig, changing colour to match the bark
•The lesser purple emperor, Apatura ilia, is a continental species not resident in Britain

Drill it — 7 cards

1 / 7new card

Sources

Butterfly Conservation — Purple Emperor
Woodland Trust — Purple emperor