The caterpillar is brownish grey, or purplish grey, dotted with black and dappled with dark brown; of the three lines along the back, the central one is black and swells out on the middle of each ring, the others are pale; a clear stripe of the ground colour below the black spiracles, and a slender line above them. The general colour is sometimes pale pinky brown or ochreous. It feeds on alder throughout the summer and autumn, and may be found in its domicile of spun-together dry leaves even in November, and sometimes later. Occasionally, a few caterpillars will feed up quickly, and attain the moth state in July or August, but the bulk do not become chrysalids until later in the year, and the moths emerge therefrom in May and early June. (Plate [87], Fig. 2.)
The species seems to occur, more or less freely, wherever there are alders throughout the greater part of the British Isles.
Abroad, the range extends to Eastern Siberia and Amurland.
Ruddy Highflyer (Hydriomena ruberata).
This species is most readily distinguished from the last by the short oblique black streak on the tips of the rather narrower fore wings; there are also black streaks between the veins and below the tips of the wings, as in the last species, but they are generally shorter and often hardly traceable.
The ground colour ranges from pale grey (sometimes with a green tinge), through brownish grey to reddish brown; usually central and outer marginal bands of a darker shade are present, but these characters may be very indistinct or entirely lost in the general coloration. (Plate [86], Figs. 11 and 12.)
The caterpillar is pale brown, dappled with grey; three dark greyish lines along the back; spiracles and the usual dots black, the latter with fine hairs; head, reddish brown, plates on first and last rings of the body light brown. It feeds, at night, during the summer and autumn, on sallow and willow, spinning together the leaves at the top of a twig to form a retreat during the day.
The moth is found in hedges, woods, and on heaths, in May and June; it may be occasionally beaten out of sallow bushes, but flies in the early evening, and is then more readily obtained. The species is widely distributed, but not generally common, in England and Wales, and in Scotland to Perthshire and probably further north, as it is found in Orkney, where specimens are numerous but rather small in size, and the caterpillars, according to McArthur, feed on heather as well as on sallow. Decidedly uncommon in Ireland, but it has been met with, in most instances singly, in Armagh, Tyrone, Westmeath, Kerry, Galway, and Sligo.