In its typical form the male of this species (Plate 50, Figs. 1, 2) is ashy grey, with a darker central band on the fore wings; and the female is dusky greyish-brown, also with a darker band. The colour of the male varies in shade from almost whitish (var. _pallida_, Tutt), to blackish grey; in the paler forms the central band of the fore wings is often of a purplish tint, and in the darkest forms the band is almost black. The female var. _pallida_, is pale buff. The eggs, which are brownish, inclining to reddish on the micropylar area, are covered with dark grey hairs from the body of the female and laid side by side in a chain-like arrangement on a twig of hawthorn or sloe (those figured on Plate 51 were deposited in a box, and not securely attached). From eight to twelve is said to be the usual number in a batch, and each female will deposit an average of 160 eggs. The caterpillars do not hatch out all at the same time, but by ones and twos, at intervals spreading over a period of two, or perhaps three, weeks. Several forms of the caterpillar have been described, but the ground colour is generally more or less black above and greyish on the sides; the ornamentation comprises interrupted white or whitish stripes, streaked or clouded with reddish, and reddish warts; the hairs are reddish brown. The example figured on Plate 51 was from eggs laid by a female moth in Selkirk, South Scotland. From the age of three weeks until it became full grown it was black marked with yellow on the back and orange on the sides; hairs pale greyish mixed with black ones, especially on the back towards the black, glossy, and somewhat hairy head. It hatched on April 26, was reared on plum, pupated early in June, and the moth, a darkish grey female, emerged on July 31. Another caterpillar that hatched on May 1, and two others from still later hatchings, were then in chrysalis. [Illustration] Pl. 50. 1. PALE OAK EGGAR, _male_; 2 _female_. 3. DECEMBER MOTH, _female_; 4 _male_. 5. SMALL EGGAR, _male_; 6 _female_. [Illustration] Pl. 51. PALE OAK EGGAR. _Eggs enlarged, and caterpillar._ {113} The caterpillar may be found from April to June on hawthorn and sloe, and it is said also on birch, oak, sallow, apple, bramble, etc. Those that I have found resting by day on shoots of hawthorn, apparently enjoying the sunshine, have almost invariably been "ichneumoned"; but others that came up after sunset to feed on the shoots were generally healthy. Usually the caterpillar feeds up and pupates the same year, but on the moors in Aberdeenshire and some other parts of Scotland it is said to hibernate and to complete its life cycle the following summer and autumn. Furthermore, the moths from these winter larvæ are much darker than normal, and have been doubtfully referred to var. _ariæ_, Hübn., a form found in the Alps, Scandinavia, and Finland.