If the reader will glance at Fig. 97, taken from Réaumur's "Mémoire sur les Chenilles en général,"[37] which represents a looper caterpillar, with four membranous legs, he will see that there is a considerable space between the posterior legs and the first pair of pro-legs, along which the body has no points of support. If one of these caterpillars, lying quiet and at full length, determines to walk, in order to take its first step (Fig. 98) it begins by humping its back, curving into an arch that part which has no legs, and finishes by assuming the position seen in Fig. 99. In the former position it has its two intermediate legs against the posterior legs, and, in consequence, it has brought forward the hinder part of its body, a distance equal to the interval of the five segments which separate them. There it hooks on by its _intermediate_ and _hind_ legs. Then it has only to raise and straighten the five rings which had formed the loop, and to advance its head to a distance equal to the length of five segments. The step is thus made, the caterpillar making the same movements in taking the second and following steps. [37] Tome i., p. 49, Plate I., Fig. 6. This sort of gait has gained for them the name of Geometers, because they seem to measure the road over which they travel. When they make a step, they apply the part of their body which they have just curved up to the ground, in exactly the same way as a land surveyor applies his chain to it. [Illustration: Fig. 100.--Caterpillar of the Canary-shouldered Thorn (_Eugonia alniaria_).] These looper caterpillars cannot shorten nor lengthen their segments at will, as other caterpillars, but only bend their bodies. There are many species whose bodies are cylindrical, stiff, and of the same colour as bark. Their attitudes deceive even the close observer. They embrace the stem of a leaf or twig with their hinder and intermediate legs, whilst the rest of their body, vertically elevated, remains stiff and immovable for hours together. Fig. 100 shows the caterpillar of the Canary-shouldered Thorn (_Eugonia alniaria_) in this strange position. Now, this is a feat of strength which the most skilful of our acrobats, ordinary and extraordinary, which all the Leotards of the present day, and those who are to succeed them, can never accomplish. With such a persistency, this caterpillar can sustain its body in the air for a considerable time, in all the positions imaginable, between the vertical and the horizontal, and downwards again in any incline from the horizontal to the vertical. "If one considers," says Réaumur, "how far we are from having in the muscles of our arms a force capable of supporting us in such attitudes as these, we must own that the power of the muscles in these insects is prodigious."