Page:Jardine Naturalist's Library Exotic Moths.djvu/265

207 GEOMETRIDÆ.

family admits of a more explicit definition than most of those previously considered. Although it is not in the perfect insect that the most marked peculiarities appear, we can seldom fail at once to recognise them, even in that state, by their general weakness of appearance, attenuated form of body, and comparatively ample expanse of wing. But the singular mode in which the caterpillar advances over a surface, as if measuring it by successive spans, is the most distinguishing peculiarity, and has suggested the name. In typical examples they have three pair of pectoral legs, a single pair of abdominal prolegs, and a pair of anal ones. With the two latter of these they fix themselves to the plane of position, then move the body forwards as far as possible and rest on their pectoral legs, when the hinder part is drawn forwards to the head and the body curved upwards into a kind of loop; the same process is again repeated, and a rapid progression thus effected. "Their muscular power is very great, and hence their positions during repose are striking. Fixing themselves by their anal feet alone, they extend their bodies in a straight line, holding them in that position for a great length