A large, bright sulphur yellow butterfly, with a distinctive hook on the leading edge of its front wings, and a point on its rear wings. It has an orange coloured spot, or small ring, in the centre of the four parts of its wings. The male is brighter than the female, which can have a greenish tinge.
The Brimstone actually appears very early from hibernation and is single brooded. If weather conditions are suitable it is often seen in mid-March, and certainly in April.
The new generation appears in late July and August and is on the wing until September when it hibernates.
The adult Brimstone nectars on a variety of plants but is particularly partial to purple flowers such as Betony and Clover. Nectaring is particularly important for the Brimstone, which has a very long life, (often almost eleven months), and so it has to maintain a very high energy level by intake of nectar before it goes into hibernation.