The moths emerge in May or June. The males are very active on the wing in the afternoon sunshine, and later on, and may often be seen in numbers dashing hither and thither in an apparently erratic flight over heaths and open spaces, in search of the females. The latter do not fly till night, and occasionally they are attracted to a bright light.
Except that it has not been noted in the Shetlands, the species occurs throughout the British Isles. Abroad its range extends over Europe, and it is found in Amurland.
The Drinker (Cosmotriche potatoria).
The male is reddish brown, more or less clouded on the forewings with ochreous; and the female is yellow, or whitish ochreous. Sometimes this colour distinction of the sexes is reversed, and the males are pale whilst the females are dark. In the fens of Cambridgeshire notably, pale or yellowish males are not altogether uncommon. Such specimens would seem to accord better with the Linnean type than the more usual form indicated above. Barrett mentions, among other aberrations, male specimens from South Wales with the whole of the fore and hind wings deep rich glossy purplish chocolate.
There is variation in the two whitish or silvery marks on the fore wings, the upper one is often very small, sometimes quite absent, and the lower one reduced to a crescent. The chocolate brown cross lines, of which there are usually two on the fore wings, are sometimes faint or entirely missing. Tutt has recently named nine forms, chiefly colour aberrations, and two others were previously named. (The moth is figured on Plate [61], and the early stages on Plate [60].)
The eggs, which are white with bluish grey markings, are laid in clusters on grass stems, etc.
The caterpillar is slaty grey inclining to blackish; the lines on the back are formed of yellowish dots and dashes; two rows of tufts of short black hairs on the back, with longer brown hairs between; low down on the sides are shaggy tufts of white and yellowish hairs and longer brown hairs; an erect pointed tuft of brown hair on second ring, and a similar one on ring eleven but the latter inclines backward. Head greyish, striped and lined with brown and yellowish brown, and clothed with brown hair. It feeds on coarse grasses, including the ribbon grass grown in gardens, in August to September or October.
In the latter month it goes into hibernation, being then but little over an inch in length. About April it resumes feeding and becomes full grown in June or thereabouts. The long yellowish or whitish brown cocoon in which it changes to a brown chrysalis is more or less pointed at the lower end, and generally attached to a culm of grass or a reed. A showery season seems to suit these caterpillars better than a hot, dry one. The partiality of the caterpillar for a drop of dew, mountain or otherwise, has frequently been noted. The old English name of The Drinker Caterpillar (1682) is therefore not only an appropriate one but shows that this larval habit was observed even at that early date. The specific name potatoria given to the moth by Linné is of similar significance.