72 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



V. THE LEGS y\ND THEIR MUSCLES 



In discussing the anatomy of an insect's leg, it will be convenient 

 to limit the application of the term " leg " to that part of the appen- 

 dage which ordinarily forms the free movable limb, ignoring for the 

 present the theoretically l)asal subcoxa, but including the coxa in all 

 cases, even when the latter is firmly fixed to the body wall. 



STRUCTURE OF AN INSECT S LEG 



The typical and usual parts of the leg of an insect (fig. 31 A) are 

 the coxa (Cx), the trochanter (Tr), the femur (F), the tibia (Tb), 



Ac 



Fig. 31. — Structure of an insect leg, as represented by that of a roach 

 ( Periplaneta ainericana ) . 



A, left mesothoracic leg and trochantin, anterior view ; B, base of prothoracic 

 coxa, with trochantin. a, articulation of coxa with trochantin ; Ar, arolium ; 

 b, articulation of coxa with pleuron ; he, basicostal suture ; Cla, claw ; Cx, coxa ; 

 d, anterior coxal suture ; e, anterior coxo-trochanteral articulation ; F, femur ; 

 Mer, meron ; q, ventral pads of tarsal segments, tarsal euplantulae ; Tar, tarsus ; 

 77), tibia; Tn, trochantin; Tr, trochanter. 



the tarsus (Tar), and the pretarsits (Ptar). The several divisions of 

 the leg are known as segments, or joints ; the term segment will here 

 be used for each piece of the leg. and the word joint limited to the 

 articulation between adjoining segments. The surfaces of the leg 

 are oriented for descriptive purposes when the limb is extended at 

 right angles to the body, the outer surface then being dorsal, the 

 inner ventral, and the other two anterior and posterior. 



The movable base of the leg is ordinarily the coxa. The base of 

 the coxa is inserted into a membranous area of the body wall between 

 the pleuron and the sternum, which permits of whatever motions the 

 closer articulations of the coxa with the surrounding chitinous parts 

 allow. In pterygote insects the coxa is nearly always attached by an 



