The Brimstone is a fairly large butterfly and one of the easiest to identify. The male has clear yellow wings which, in flight, can be mistaken only for the Clouded Yellow. The latter is smaller, darker, and a deeper orange-yellow. The female Brimstone has much paler upper wings compared to the male, with a green tint that, at a distance can easily be misidentified for a Large White. Both sexes are unmistakable at rest. They always sit with their wings closed, so only the undersides are seen. These look extraordinarily like a pale yellow leaf, with pointed corners, prominent veins, and even a spot of 'mould' in the centre. The body and legs are the same pale yellow, but the eyes are large, black and shiny. Note, too, the beautiful clubbed antennae which sprout from between the eyes like a pair of pink stalks.

Dimensions:
Adult wingspan: 60-74mm.

National:
This is a relatively common butterfly found in England and Wales although population densities are usually low. It occurs in woodland, hedgerow and scrub wherever its foodplant Buckthorn Rhamnus cathartica and Alder Buckthorn Frangula alnus occur.

At Durlston:
At Durlston it is regularly recorded along both butterfly transects. Numbers early in the year tend to be less than those recorded later but this may be due to the fact that the survey does not start until mid April, long after many overwintering adults have emerged from hibernation.

The butterfly overwinters as an adult and emerges early in the year, usually March. The eggs are laid singly on the leaves of the foodplant. Once the larva has matured it pupates low down in the vegetation. This generation then emerges in the late summer and early autumn with adults consequently overwintering, usually in woodland.