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).