Lepidoptera The Brimstone Butterfly – Gonopteryx Rhamni
This, the last member of the present family, is remarkable for the graceful outline of its wings. The costal margins of the fore pair are most beautifully arched, and both pairs are sharply angled on the hind margins. This latter characteristic is quite unique among British butterflies, though we shall presently meet with instances of angular projections on hind wings only.
The ground colour of the male (Plate II, fig. 4) is a rich sulphury yellow, with a greenish tinge. That of the female is a very pale greenish yellow. The only conspicuous markings are small saffron spots, one near the centre of each wing. The antenna are red, short, and beautifully curved; but, unlike the two preceding species, the legs are almost white.
The eggs of this butterfly are laid in April on the two species of buckthorn (Rhamnus catharticus and R. Frangula) by the females that have successfully weathered the winter. They are of a bright yellow colour, and are usually hatched in about a fortnight.
The body of the caterpillar is green, and it is thickly covered with little black wart-like projections. A pale stripe also runs along each side. During May and June it may be found on its food plants, and toward the end of the latter month it attaches itself by a silken carpet and belt to the under side of one of the leaves.
The chrysalis is of a very peculiar shape, the body being curved, and the wing cases standing out prominently beyond the general surface. Its colour is a bright apple green marked with yellow, and it is so transparent that certain of the structures can be seen through its skin.
The perfect insect emerges in about three weeks after the change to the chrysalis; and may be looked for from July to the end of the summer. This period may be regarded as the best time in which to hunt for Rhamni, but it is to be noted that this butterfly makes its appearance during all months of the year, even though it is single-brooded.
A large number seem to hybernate, and their winter sleep is so light that the welcome rays of the sun on a mild day, even during the bleak months of November to February, will often call them out from their hiding places. Then, as a rule, the hybernating butterflies do not live long after depositing their eggs for the future brood; but the Brimstone often lives on till its offspring have themselves attained the perfect state, so that it is possible to capture the insects of two different years both on the same day. In such a case it is generally easy to distinguish between the two, for the newly emerged specimens are beautifully bright and fresh in colour, while those of the previous year are more or less faded and worn, their wings being often semi-transparent through the loss of scales, and frequently disfigured by the stains of mildew.