The thorax ([Fig. 6]), the second primary division of the body of insects, plays almost as important a part as the head. It consists of three segments or rings, which are in general joined together—the prothorax, the mesothorax, and the metathorax, each of which bears a pair of legs. The wings are attached to the two posterior segments.
All insects have six true legs. There is no exception whatever to this rule, though some may not be developed.
From the segments to which they are attached, the legs are called anterior, posterior, and intermediate. The legs are composed of four parts: the trochanter, a short joint which unites the thigh to the body; the thigh or femur; the tibia, answering to the shank in animals; and the tarsus, or foot, composed of a variable number of pieces placed end to end, and called the phalanges.
We take as examples the hind leg of a Heterocerus ([Fig. 7]), and the front leg of a Zophosis ([Fig. 8]) (genera of beetles).
We shall not dwell on the different parts, as they perform functions which will occupy us later, when speaking of the various species of the great class of insects.
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| Fig. 7. Hind leg of a Heterocerus. | Fig. 8. Front leg of a Zophosis. | |
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| Fig. 9.—Posterior leg of a jumping insect. | ||
The functions which the legs of insects have to perform consist in walking, swimming, or jumping.
In walking, says M. Lacordaire, insects move their legs in different ways. Some move their six legs successively, or only two or three at a time without distinction, but never both legs of the same pair together, consequently one step is not the same as another. The walk of insects is sometimes very irregular, especially when the legs are long; and they often hop rather than walk. Others have one kind of step, and walk very regularly. They commence by moving the posterior and anterior legs on the same side and the intermediate ones on the opposite side. The first step made, these legs are put down, and the others raised in their turn to make a second.
Running does not change the order of the movements, it only makes them quicker—very rapid in some species, and surpassing in proportion that of all other animals; but in others the pace is slow. Some insects rather crawl than walk.