“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.) [Illustration: 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. ] While we look upon the dermal tube of worms as a single but flexible lever, the body of the arthropods, as Graber states, is a linear system of stiff levers. We have here a series of stiff, solid rings, or hooks, united by the intersegmental membrane into a whole. When the muscles, extending from one ring to the next behind contract, and so on through the entire series, the rings approximate each other. The ectoskeletal segments bend to one side by the contraction of the muscles on one side, the point of the outer segmental fold opposite the fixed point becoming converted into the turning-point (_C_). The usual result of the arrangement of the locomotive system is the simple curving of the body (_C_), and then the alternate bending of the body to right and left, which produces the serpentine movements characteristic of the earthworms, the centipede, and many insect larvæ. The most striking example of the wonderful variety of movements which can be made by an insect are those of the Syrphus larva. When feeding amid a herd of aphides, it is seen to now raise the front part of the body erect and stiff, then to bend it down, or rapidly turn it to either side, or move it in a complete circle. (Graber, pp. 23–26.)