The Moths of the British Isles/Chapter 7

This species has the fore wings of the male brownish clouded and suffused with ochreous; there is a white patch at the base, and some white marks including three spots towards the apex, on the outer marginal area; two black cross lines, the first inwardly and the second outwardly, edged with white; the space between the lines is sometimes clouded with whitish, and there is an almost central black -shaped mark. Hind wings tawny with a black central line, some brownish marks beyond, and sometimes two white spots at the upper angle. The female is much larger in size, without ochreous suffusion on the fore wings, and the hind wings have the ground colour whitish. It varies in the tone of the brown colour, and, in the male, in the amount of ochreous suffusion (Plate 65).

The eggs are laid in rows, generally two deep, on a birch twig. At first they are greenish, but soon change to brownish olive or shining purplish brown.

When young the caterpillars cluster together on the twigs, as shown on Plate 64. They are at first black with glossy dots, and later, greenish, but still dotted with black. After the third skin change, they are without the black dots, and the colour is then pretty much that of the mature caterpillar, which is green, rather whitish on the back, and with a dark green central line; a series of seven creamy oblique stripes along the sides, and on the sides of the first three rings there is a whitish stripe broken at the divisions; these markings are often edged with dark green; on the eleventh ring there is a somewhat horn-like prominence, striped with creamy white, and below it a yellow stripe; the spiracles are white, ringed with black. Head small, paler green, with whitish marks. Feeds on birch, from late May to July. Alder, sallow, and lime have also been mentioned as food plants.

The rough, blackish, or sooty-brown chrysalis is enclosed in a coarse netted cocoon, dark brown in colour, and more or less covered with moss, leaves, or other material, among which it is spun up, generally on the ground, but sometimes just under the surface. Assisted by the points on the rings of the body, the chrysalis is able to work itself partly out of the cocoon, and this it does some days before the moth emerges.

The moths usually emerge in late March and in April, earlier or later in some seasons. They do not always come up the year after pupation, but often remain two or more winters in the chrysalis.

The males fly in the sunshine, and are very strong on the wing; the females are not active until dark. This sex has been found resting on the twigs of birch, also on heather, and occasionally on a tree trunk. The males "assemble" freely to a freshly emerged female. The species inhabits the more open parts of woods and forests, moors and hillsides where birches flourish. It is probably more plentiful in its Scottish localities, such as Rannoch and Forres, than elsewhere, but it occurs also in Aberdeenshire, Kincardineshire, and Argyllshire. In England it seems to be not uncommon in Wyre Forest, Worcestershire, and the Reading district in Berkshire. It used to be so plentiful in Tilgate Forest, Sussex, that over a hundred males were brought to the net in one day by a bred female put down to allure them. This happened some fifty years ago, and compares curiously with a record of one male attracted by a female in Tilgate Forest, April 13, 1869. Other localities in Sussex that have been mentioned are St. Leonard's Forest and near Petersfield; it has also been found in Herefordshire and in some parts of Suffolk. Distributed over Central and Northern Europe, the range extending to North Italy.