NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 73 



STRUCTURE OF THE LEGS 



The general form of a grasshoppper's leg is shown in the illustra- 

 tion of the middle leg of Dissostcira (fig. 40). The appendage con- 

 sists of a coxa {Cx), a trocJmnter (Tr), a femur (Fm), a tibia (Tb), 

 a three-segmented tarsus (Tar), and a pretarsus comprising a pair of 

 lateral claws (Un) and a median arolimn (Ar). In the articular mem- 

 brane before the base of each coxa there is a small trochantinal sclerite 

 (Tn), best developed in the prothorax. 



Each leg is set into a membranous area, or coxal corium (fig. 26), 

 occupying an oval' interruption in the sclerotic wall of the body between 

 the pleuron and the sternum, known as the coxal cavity, the rim of 

 which is reinforced by a submarginal inflection. The coxa is hinged 

 to the body wall by only a single articulation, which is with the pleuron. 

 The rudimentary trochantin (fig. 40, Tn) does not restrict the move- 

 ment of the coxa. The anterior and middle coxae are free to move in 

 any direction, but the hind coxae, which are directed posteriorly, have 

 a more limited range of motion. The number of muscles inserted upon 

 the hind coxae, however, suggests that what little movement these 

 coxae possess is of much importance in the function of the hind legs. 



It should be noted that the articulating surfaces of the pleuro-coxal 

 hinge are formed by inflections of the body and coxal walls, and there- 

 fore lie on the inner surfaces of the latter (fig. 41 D). In this respect 

 the basal joint of the leg differs from the basal articulations of the 

 head appendages with the head wall, for the latter are external sur- 

 faces of contact lying outside the articular membranes. The peculiar 

 character of the pleuro-coxal (subcoxo-coxal) articulations attests, 

 therefore, that these articulations are not homologous with the basal 

 articulations of the gnathal appendages on the edge of the epicranium. 



The coxo-trochanteral joint and the articulations between the seg- 

 ments of the telopodite, except the trochantero-femoral joint, which is 

 but little movable, are all of the dicondylic hinge type with anterior 

 and posterior articulating points on a horizontal axis transverse to the 

 length of the leg segments. Movement at these joints is approximately 

 in the same vertical plane. The trochanters are closely attached to the 

 femora, but the hinge lines lie in a vertical plane, and the presence of a 

 posterior femoral muscle in the trochanter of the first and the second 

 leg (fig. 36 A, 72) shows that the primitive motions at the trochantero- 

 femoral joint were movements of production and reduction. The seg- 

 ments of the tarsus are movable on each other, but since they have no 

 musculature, they can be moved only as they are influenced by the 

 tendon of the ungual retractor which passes through them (fig. 44, .r). 



