Caterpillars of this family of moths, with very few exceptions, have only two pairs of claspers or prolegs; when there are more than four claspers, the extra ones are only rudimentary and therefore useless. In moving from place to place the caterpillar stretches out to its full length, first to one side and then to the other, as though measuring the distance. When a hold is secured with the true legs the body is arched and the claspers are brought up almost to the point held by the true legs; the latter are then thrust forward and the measuring business proceeds as before. Some kinds perform the looping manœuvre very deliberately, but others at a quick rate. In common parlance among British entomologists the caterpillars are called "geometers" or "loopers," but to our American confrères they are known as "measuring-worms" or "span-worms."
Most of the caterpillars feed openly on the foliage of trees, shrubs, or low-growing herbs, and the majority remain upon their respective plants during the day.
A large proportion of the moths may be obtained in the daytime, either by beating or otherwise disturbing the foliage or herbage among which they hide; several kinds rest on tree trunks, palings, rocks, walls, etc., where they are sometimes conspicuous, but more frequently not easy to distinguish from their surroundings. On the whole, members of this family are more available to the day collector than are those of the Noctuidæ. Although several species occasionally visit the sugar patch, such species are, as a rule, obtained more readily and in larger numbers by other methods. Brilliant light has a great attraction for many of the moths, some are more often captured at gas or electric lamps than in any other way, and among these are the migratory species.
Staudinger divides the family into the following subfamilies:—
Geometrinæ (= Geometridæ, Meyrick). Acidaliinæ (= Sterrhidæ, Meyrick). Larentiinæ (= Hydriomenidæ, Meyrick). Orthostixinæ (not represented in Britain). Boarmiinæ (= Selidosemidæ, Meyrick).
Except as regards the Larentiinæ, I have largely adhered to Staudinger's arrangement of genera in each of the above subfamilies.
The typical genus of Larentiinæ would be Larentia, Treit, to which something over two hundred species are referred by Staudinger, among which are upwards of sixty that occur in the British Isles. Following some of the later generic changes, I find that none of our species are left in Larentia, but a few fall into Hydriomena, Hübner, and therefore Hydriomeninæ has been adopted for this subfamily.
This greyish-brown moth has two darker, sometimes reddish, cross lines on the fore wings, and one such line on the hind wings. It is presumably only to be regarded as an accidental visitor to England. The first record was of a specimen captured in the Warren at Folkestone in July, 1866, and since that year others were obtained in the same locality, but apparently not more than about half a dozen altogether. None seems to have been recorded for over thirty-five years. The specimen, whose portrait is shown on Plate 38, Fig. 3, was obtained from Dresden.
Abroad, the range includes Central and Southern Europe, Asia Minor, Syria, and Armenia.
When freshly emerged from the chrysalis, the species represented by Figs. 6 to 8 on Plate 38 is of a beautiful blue-green colour, but in course of time a greyish shade creeps over the wings. The dark cross lines vary in intensity; in some specimens well defined and blackish, in others very faint, and hardly discernible; occasionally, the space between the lines on the fore wings is dark shaded; the whitish submarginal line is not always present. This species is the cythisaria of Schiffermiller, and the cytisaria of other authors.
The caterpillar (figured on Plate 41, Fig. 1, from a coloured drawing by Mr. A. Sich) is green, with three lines along the back, the central one dark green, the others whitish; a pinkish stripe low down along the sides, the points on the head and the first and last rings of the body are often pink also. It feeds on pettywhin (Genista anglica), also on broom (Cytisus scoparius) and gorse (Ulex); in captivity it seems to thrive on laburnum. Most frequently obtained in the springtime after hibernation.
The moth is to be found in June and July on moorlands and commons pretty well throughout the British Isles, but it seems not to have been noted north of Perthshire, in Scotland.
Odd specimens have been known to occur in late August or early September, but this is quite exceptional.