The greatest number among them have ten pro-legs; others have only eight; others only six—these may be called semi-loopers; others only four, one pair being situated on the last ring, and the other on the ninth, as in the case of looper caterpillars. And, lastly, there are others which have only two pro-legs.
The various forms, numbers, and positions of these organs, produce great differences in the mode of locomotion of caterpillars. Those provided with ten or eight membranous legs have in walking only a very slight undulating motion. Their bodies are parallel to the plane which supports them. They can walk very quickly; but their steps are short and quickly repeated. Others, on the contrary, in proportion as the number of their false legs diminish, and the spaces between the legs increase, walk in a more irregular and quaint manner.
Fig. 97.—Looper Caterpillar.
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.
| Fig. 98.—Caterpillar curved into an arch. | Fig. 99.—Caterpillar at full length. |
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.
