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TEXT-BOOK OF ENTOMOLOGY 



impediment to flight, but compensated for by the action of the large 

 wings. 



The arthropodan limb is a compound leverage system. It is, says 

 Graber, a lateral outgrowth of the trunk, which repeats in miniature 

 that of the main trunk, its single series of joints or segments form- 

 ing a jointed dermo-muscular tube. Yet the lateral appendages of 

 an insect differ from the main trunk in two ways : (1) they taper to 

 the end which bears the two claws, and (2) their segments are in the 

 living animal arranged not in a straight line, but at different angles 

 to each other. The basal joint turning on the trunk acts as the first 

 of a whole series of levers. The second joint, however, is connected 

 with the musculature of the first or basal joint, and thus each suc- 

 ceeding joint is moved on the one preceding. Each lever, from 

 the first to the last, is both an active and a passive instrument. 

 (Graber.) 



While, however, as Graber states, the limbs possess their own sets 

 of muscles and can move by the turning of the basal joint, the labor 

 is very much facilitated, as is readily seen, by the trunk, though the 

 latter has to a great extent delegated its locomotive function to the 

 appendages, which again divide its labor among the separate joints. 



Graber then calls attention to the analogy of the mechanics of 

 locomotion of insects to those of vertebrates. An insect's and a 

 vertebrate's legs are constructed on the same general mechanical 

 principles, the limbs of each forming a series of levers. 



Fig. 22, A, represents diagrammatically the knee joint of a verte- 

 brate, and B that of an insect ; a, the femur or thigh, and 6, the tibia 



or shank. In the verte- 

 brate the internally situ- 

 ated bones are brought 

 into close union and bend 

 by means of a hinge-joint ; 

 so also in the chitinous- 

 skinned insect. 



The stiff dermal tube 

 of the insect acts as a 

 lever by means of the 

 thin intersegmentul mem- 

 brane (c) pushed in or 

 telescoped in to the thigh joint, a special joint-capsule being super- 

 fluous. The muscles are in general the same in both types ; they 

 form a circle. In both the shank is extended by the contraction of 

 the upper muscles (d) and is bent by the contraction of the lower 



FIG. 22. Diagram of the knee-joint of a vertebrate i .1 1 

 and an insect's limb (B) : a, upper; ft, lower, shank, united 

 at A by a capsular joint, at B by a folding joint ; <> ' . extensor 

 or lifting muscle ; a 1 , flexor or lowering muscle of the ln\\er 

 joint. The dotted line indicates in A the contour of the leg. 

 After Graber. 



