THE THORACIC EXO-SKELETON. 193 



margin receives the malleolar pegs laterally, and is prolonged 

 internally as an apodeme, which lies within the inner coxal 

 sclerite, and gives attachment to the extensor muscles. 



The Coxo - Trochanteric Articulation is very complex, and is 

 capable of flexion and extension, also of rotation on the long 

 axis of the femur, and of rotation at right angles to the long 

 axis of the femur on the long axis of the coxa. 



The anterior malleolar peg is horizontal, and the posterior 

 vertical. The elevation of the anterior edge of the trochanter 

 throws the tibia forwards, and draws the posterior malleolar 

 peg from its socket ; this permits of the depression of the 

 distal end of the femur on the anterior malleolar peg. The 

 elevation of the posterior edge of the trochanter rotates the 

 femur backwards on its long axis, and carries the insect 

 forwards over the tarsus ; this movement withdraws the anterior 

 malleolar peg from its socket, and replaces the posterior ; the 

 femur is then swung forward on the vertical posterior malleolar 

 peg. In extreme flexion of the femur on the coxa both pegs are 

 in their sockets, and the coxo-trochanteric articulation is locked. 

 Thus in running, when the foot and tibia are thrown back by 

 rotation of the femur, the latter swings forward in the horizontal 

 plane on the posterior malleolar peg; but when the foot is 

 brought to the ground in front of the femoral plane, the distal 

 end of the femur is depressed — that is, the femur is extended 

 on the anterior malleolar peg ; a rotation of the femur then 

 urges the insect forwards, and unlocks the anterior and locks 

 the posterior coxo-trochanteric diarthrosis. 



The movements of the shank of the femur on the trochanter 

 are apparently very slight, and depend entirely on the elasticity 

 of the integument uniting these parts. 



The Tibia is sub-cylindrical, slightly curved on its long axis, 

 and thickest at its proximal extremity. It forms a hinge with 

 the femur, the distal extremit}^ of which is hollowed out below 

 for its reception in extreme flexion. 



The proximal extremity of the tibia ends in two curved 

 ridges (Fig. 7), which articulate with two grooves in the 

 interior of the lateral portion of the distal extremity of the 



