THE THORACIC EXO-SKELETON. 191 



distally in a peg — the anterior malleolus ; this malleolus (;») 

 articulates with a depression in the trochanter. 



The posterior sclerite always exhibits a thin fenestra (/), 

 surrounded by a thick rim ; and the latter supports a second 

 peg — the posterior malleolus, which also articulates with the 

 trochanter. When the limb is fully flexed on the coxa, the 

 trochanter lies in the fenestra. 



The internal coxal plate is smaller than the others, and 

 completes the inner aspect of the coxal joint. 



The Anterior Coxa is united by a loose syndesmosis with the 



sclerites of the prosternal area, and with the margin of the 



anterior sterno- coxal foramen. Its anterior plate forms a 



diarthrosis with the projecting process of the epitrochlear 



, sclerite. 



^ The movements of the anterior sterno-coxal articulation are 

 very free : the anterior limbs are not only used as legs, but 

 almost as arms, serving to clean the face and proboscis, and in 

 climbing. These actions depend chiefly on the great mobility 

 . of the sterno-coxal articulation, as the prothorax — the manu- 

 truncus of Kirby — is immovable on the mesothorax. In the 

 predacious Diptera the anterior legs are used in seizing their 

 prey, as in Empis and Dolichopus. 



The Intermediate Coxa is lodged in an elliptical depression 

 between the meso- and metasternum (Plate VII., Fig. i, p, q), 

 so that its movements are greatly restricted. The anterior 

 sclerite articulates at its outer extremity by a peg (Fig. 2, a) 

 with a deep socket in the mesosternum, on which the coxa 

 rotates about its long axis ; this joint permits of a rowing 

 movement of the femur in the horizontal plane in running, and 

 forms what I call a roller-joint. This kind of articulation is 

 very highly developed in some of the geodephagous Coleop- 

 tera, as, for example, in Passalus. 



The wide-open proximal margin of the coxa is connected 

 with the edges of the sternal foramen by a loose syndesmosis, 

 which forms a kind of conjunctiva, in which the coxa moves. 

 The inner plate articulates with the capitellum at the posterior 

 extremity of the mesothoracic entosternum. 



