These bright yellow butterflies are often the first butterflies to be seen in spring.

Physical description
Male brimstone butterflies have sulphur-yellow forewings and hindwings with an orange central spot. The female's fore- and hindwings are a delicate yellow or pale green with an orange central spot. Both butterflies have greenish veined underwings.

Caterpillars are bluish-green with a pale line low down on each side. The pupa looks like a curled leaf.

Distribution
They are widespread throughout Europe and North Africa.

Habitat
Brimstones are found in woodland and gardens.

Diet
The caterpillars feed on buckthorn and alder buckthorn. The adults can be seen in meadows, sipping nectar from teasel, knapweed and buddleia.

Behaviour
New adults emerge from their chrysalids in July and live until the following July. Adults hibernate over winter in woodlands and emerge on warm spring days. Last year's brimstones are often the first butterflies to be seen in spring.

Reproduction
Pairing in early spring is followed by a long courtship prior to mating. After mating the female lays pale green eggs on the undersides of buckthorn (Rhamnus catharticus) and alder buckthorn (Frangula alnus) leaves. If the leaves haven't yet opened, eggs will be laid on twigs or leaf buds. The eggs soon turn darker and finally become a dull grey before the tiny caterpillars hatch some 10 days later and feed hungrily on the fleshy leaves.

Conservation status
Brimstone butterflies are common in most of the UK. They have been spreading in recent years, particularly in northern England. They are protected in Northern Ireland but they are not listed as endangered on the IUCN Red List 2000.

Notes
It is thought by some that the term butterfly originally referred to this insect because of its butter-like yellow colouring. There is little agreement on this matter, other theories stating that the name refers to the butter-coloured faeces of butterflies or to the mediaeval myth that witches transformed into butterflies to steal butter.