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 extremity 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 



