There are two families of macro-moths in the British Isles which are much larger then the others: the Geometridae, with over 300 species, and the Noctuidae, with over 400 species. The Geometridae comprises at least 20,000 described species in total, and is distributed worldwide. The British members are divided very unevenly into six subfamilies: The Archiearinae – orange underwings (two species); the Alsophilinae – March Moth (one species); The Geometrinae – emeralds (ten species); the Sterrhinae – mochas and waves (about 40 species); the Larentiinae – carpets, pugs and allies (nearly 170 species); and the Ennominae – thorns, beauties, umbers and allies (about 95 species).
The Geometridae are a varied group, but most have broad, rather triangular forewings and rather light, slender bodies, enabling low-energy flight rather than power and speed. Probably as a consequence, only three species regularly arrive in the British Isles as immigrants in any numbers, and it is likely that these are carried on jet streams for much of their journey.
They are readily distinguished from butterflies by their antennae, which are feathered in the males and slender in the females, but never clubbed.
Many species fly at dusk, before light-traps are fully effective, and are best searched for with a net, tapping vegetation to disturb them. Others come readily to light, but sometimes only late at night, and some have dawn or daytime flights. The females of a number of species have reduced or vestigial wings, and walk to disperse eggs rather then flying. This uses less energy than flight and enables them to remain active on winter nights at temperatures at or below freezing. These females are seen at light-traps only on the rare occasions they crawl in, and instead are best searched for on woody vegetation. May geometrids have functional tongues and can drink moisture, but relatively few can be found reliably by searching flowers or by trying to attract them to sugary baits, probably because low-energy flight requires less frequent re-fuelling.
Another feature of the family which distinguishes it is that the caterpillars have only two pairs of hind legs, or prolegs. These consist of a pair of claspers at the back end and another pair a short distance in front of them. A few species, notably the two orange underwings and the Light Emerald, have traces of a much smaller third pair of prolegs, indicating that they have evolved from forms with more than two. The central region of the body has no legs and is looped up when the caterpillar moves, drawing the hind end up to the three pairs of legs at the head end. Some can cover distances quickly, which is likely to be an advantage when moving about in trees, and a large proportion eat the foliage of woody plants. The geometrids are often referred to as ‘looper’ or ‘inch-worms’ after this mode of progression, while ‘geometrid’ means ‘ground-measurer’ in Greek. If dislodged, the caterpillars are likely to fll on a silken thread; they then clamber back up this or up the plant stem, or may be wind-blown on these thread. Some pupate in cocoons attached to the plant and need never leave the tree canopy, but the majority pupate in the earth below.
(Taken from Waring and Townsend’s Field Guide to the Moths of Great Britain and Ireland)