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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



simplest of mechanical principles, for, with the exception of the 

 movements of the coxa on the body, the action at each of the flexible 

 joints is merely that of a hinge working in a single plane. 



In insects, and in most other arthropods, the functional base of 

 the leg is the coxa, and the appendage as a whole moves on the articu- 

 lation of the coxa with the body. In the more generalized orders, 

 where the coxa is suspended freely from the pleuron, the leg may 

 swing forward and backward on a transverse (or vertical) coxal axis 



— b 



Fig. 37. — The cardinal axes of motion, and the corresponding muscles of a 

 coxa freely articulated to the pleuron. 



A, diagram of the mechanism of coxal motion on the pleural articulation (t), 

 inner view. The cardinal movements are: (i) abduction and adduction on longi- 

 tudinal axis (b b) by means of abductor and adductor muscles {M, N) ; (2) 

 promotion and remotion on transverse axis (c c) by promotor and remotor 

 muscles (/, /) ; and (3) rotation on vertical axis {d d) by anterior and posterior 

 rotator muscles {K, L). 



B, diagram of coxal musculature, inner view of base of right coxa : E, F, 

 basalar and subalar muscles of wing attached on coxa (fig. 28) ; /, promotor of 

 coxa, tergum to trochantin ; /, remotor, tergum to coxa ; K, anterior rotator, 

 sternum to coxa ; L, posterior rotator, sternum to coxa ; M, abductor, episternum 

 to coxa; A'^, adductor, sternum to coxa. 



(fig. 37 A, cc), outward and inward on a longitudinal axis {hh) , or 

 it may turn in the plane of its base on an axis through the middle of 

 the coxa {dd). The possible elemental movements of the coxa and 

 of the leg as a whole are, therefore, promotion and remotion, abduc- 

 tion and adduction, and rotation; but, if the coxa has a free articu- 

 lation, the actual movements of the leg base are unlimited, since, by 

 simultaneous contraction of two or more sets of the coxal muscles, 

 there may result compound movements in any direction. In con- 

 formity with its motile possil)ilities, the coxa has a much more elabo- 

 rate musculature than that of any other .segment of the leg: where 



