NO. I INSECT THORAX SNODGRASS 45 



into freely movable wings, while those of the prothorax degenerated 

 and were lost. In harmony with this view is the difference in struc- 

 ture between the tergum of the prothorax and that of a wing-bearing 

 segment, and the differences in the segmental musculature. As the 

 skeletal parts of the mesothorax and metathorax responded to the 

 demand for mobility in the tergal lobes, the segmental muscles capable 

 of moving the latter were developed accordingly. 



The general structure of a wing-bearing segment is shown dia- 

 grammatically in figure 20. The dorsum of the segment may be 

 occupied only by the true tergal plate (T), or notion, as the tergum 

 of a thoracic segment is often called ; but the segment in which the 

 wings are best developed usually has also a second smaller plate, the 

 posfnofuiii (PN), lying immediately behind its true tergum. (3n the 

 sides of the segment are the usual jileural plates of pterygote insects, 

 the episternum {Eps) and epimeron {Epin), separated by the pleural 

 suture (Pis). At the upper end of the latter the dorsal edge of the 

 ]:)leuron is produced into a special pleural iving process (IVP), which 

 forms a fulcrum against the wing base. The tergum is often sup- 

 ported on the pleura by arms extending from its anterior lateral 

 angles to the corresponding dorsal angles of the episterna, each arm 

 constituting a prcalar bridge (Aiv). The postnotum, when present, 

 is generally connected likewise by lateral extensions with the epimera, 

 each extension forming a posfolar bridge (Piv). Between the wing 

 base and the upper edge of the pleuron there is an ample membrane 

 in which are situated several small epipleural plates (Ba, Sa). The 

 ventral parts have no distinctive features in the wing-bearing seg- 

 ments and have been sufficiently treated in the description of the 

 fundamental structure of the thoracic sterna (page 17). The details 

 of the wing-bearing tergum, and certain features of the pleuron, 

 however, need a more extensive special examination. 



STRUCTURE 'of A WING-BEARING TERGUM 



The tergum of a wing-bearing segment preserves in its basic struc- 

 ture the elemental composition of the tergal plate in any segment 

 where secondary segmentation has been established (fig. 2 B, C). 

 It includes the intersegmental antecostal ridge preceding (fig. 21 B, 

 Ac), and a precosta (A, B, Pc) demarked externally by the antecostal 

 suture (A, ac). Its posterior edge is reflected ventrally in a marginal 

 fold, or posterior reduplication (B, Rd). Between the tergum and 

 the following precosta there is a transverse membranous area 

 (fig. 23 B, Mb), functionally intersegmental, but which is morplio- 



