NO. I 



INSECT THORAX — SNODGRASS 



73 



articular surface on its outer basal rim (fig. 33 A, b) to the coxal 

 process of the pleuroii (fig. 4). In most of the lower orders, the 

 coxa has also an anterior articulation with the ventral end of the 

 trochantin (fig. 31, a), but when the trochantin is absent, the coxa 

 is suspended from the pleural process alone, except where it becomes 

 articulated ventrally to a process of the furcisternum. Sometimes 

 the coxa is not movable, as in the thoracic legs of caterpillars, and in 

 the meta-thoracic leg of adult beetles. The leg, however, is always 

 movable at the coxo-trochanteral joint, and if the coxa is distin- 

 guished as the leg basis, the part of the limb beyond it is the 

 telopodite. 



The joints of the leg consist of membranous rings of the leg wall 

 between the chitinized areas that constitute the leg segments. Some- 



Tar 



Fig. 32. — Leg of a caterpillar, and of a centipede. 



A, left prothoracic leg of Estigmenc acraca, anterior view; B, leg of Lithob'ms 

 sp. Note similarity of structure in the coxse (C.r), and in the dactylopodite-like 

 terminal claws {Dae) ; trochanter (7V) rudimentary in the caterpillar, repre- 

 sented by two segments in the centipede (iTr, sTr). 



times there is no close association between adjacent segments, but 

 usually at one or two points the segments are hinged by chitinous 

 processes, or condyles, or by other articulating surfaces on their 

 opposing margins. Hinged joints are either monocondylic or dicon- 

 dylic, according as they have one or two artictilar points. A single 

 hinge is typically dorsal ; in dicondylic joints, one hinge is anterior 

 and the other posterior, except at the trochantero-femoral joint where 

 the hinges, if present, are dorsal and ventral. 



The structure of the hinges between the leg segments varies much 

 at dififerent joints and in dififerent insects. Sometimes the two oppos- 

 ing surfaces simply touch by their points. In other cases the hinge is 

 of the ball-and-socket type, a condyle of one surface fitting into a 

 socket of the other, and in dicondylic joints of this kind the two hinges 

 are frequently reversed in structure, but the condyle of the anterior 



