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Fig. 17.—Diagram of the integument and arrangement of the segmental muscles: A, relaxed; m, muscle; g, membranous articulation; r, chitinous ring. B, the same contracted on both sides. C, on one side.—After Graber.

“Fig. 17 makes this still clearer. The muscles (m) extend between two segments immediately succeeding each other. Supposing the anterior one (A) to be stationary, what do we then see when the muscle contracts? Does it also become shorter? The intersegmental fold is drawn forwards, and hence the entire hinder segment moves forward and is shoved into the front one, and so on with the others, as at B. Afterwards, if the strain of the muscle is relieved by the diminishing action of the tensely stretched, intersegmental membrane, it again returns to a state of rest.” (Graber.)

Fig. 18.—Diagrams to demonstrate the mechanism of the motion of the segmented body in the Arthropoda: One larger segment (cf) and 4 smaller. The exoskeleton is indicated by black lines, the interarticular membranes by dotted lines. The hinges between consecutive segments are marked at, tergal (dorsal) skeleton; s, sternal (ventral) skeleton; d, dorsal longitudinal muscles = extensors (and flexors in an upward direction); v, ventral longitudinal muscles = flexors. In B, the row of segments is stretched; in A, by the contraction of the muscles (d) bent upward; in C, downward; tg, tergal; sg, sternal interarticular membranes.—After Lang.