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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 80 



an antecosta, but the ventral muscles are never attached to this ridge. 

 The furcisternum lies between the coxal bases ; it is usually narrow, 

 sometimes reduced to a mere base for the furca. When the coxal 

 cavity is closed behind by a postcoxal bridge from the pleuron ( fig. 3 

 A, Pcx), the bridge unites below with the furcisternum and is usually 

 continuous with it. In some of the Apterygota and in some species of 

 the higher pterygote orders the coxa of the mesothorax and of the 

 metathorax is articulated ventrally to the sternum by a condyle on 

 the lateral margin of the furcisternum. 



The sternal arms, whether they are independent apophyses of the 

 sternum, or are united upon a common base, extend upward and out- 



FiG. 4. — Diagrammatic cross-section of wingless thoracic segment of a 

 pterygote insect. 



Cx, coxa; C.vP, pleural coxal process; PI, PI, pleura; PI A, pleural apophysis; 

 PIR, pleural ridge; S, sternum; SA, sternal apophyses; T, tergum. 



ward toward descending apodemes of the pleuron (figs. 3 B, 4, PIA), 

 and the two sets of processes are almost always closely associated, in 

 some cases fused, but more generally united by short muscle fibers 

 (fig. 28, G). The sternal and pleural arms thus form bridges across 

 the coxal cavities, and the similarity of their position and their com- 

 plimental function suggest a correlation in origin. Neither pair is 

 present in the Apterygota. 



The pleuron, though subject to an endless number of minor varia- 

 tions, shows a general plan of structure in the Pterygota which may 

 be simplified to the diagrammatic scheme shown in figure 3 A. In the 

 Apterygota the pleuron is little developed, and does not indicate an 

 evolution toward that of the winged insects, but it does suggest, as 

 will be shown later (page 22), the nature of the structure from which 



