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TEXT-BOOK OF ENTOMOLOGY 



mental 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, inter- 

 segmental m e m b r a n e , it 

 again returns to a state of 

 rest." (Graber.) 



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 hoops, united 

 by the intersegmental mem- 

 brane 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 con- 

 traction of the muscles on 

 one side, the point of the 

 outer segmental fold oppo- 

 site the fixed point becoming 

 converted into the turning- 

 point (C). 



The usual result of the 

 arrangement of the locomo- 

 tive 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 larvae. The most strik- 

 ing 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 



FIG. 18. Diagrams to demonstrate the mechan- 

 ism of the motion of the segmented body in the 

 Arthropoda : One larger segment (ct) and 4 smaller. 

 The exoskeleton is indicated by black lines, the inter- 

 articular 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) ; n. ventral longitudinal 

 muscles = flexors. In , the row of segments is 

 stretched ; in A, by the contraction of the muscles (d) 

 bent upward ; in f", downward ; fy/. tergal ; xg, sternal 

 interarticular membranes. After Lang. 



