56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



tera the scutum is strengthened by a median ridge on its inner 

 surface, forming a median suture externally (fig. 24 G). 



The mechanism of the wing-bearing tergum and its functional 

 evolution in connection with the wings have been well protrayed in 

 recent papers by Weber (1924, 1925). The tergum must be so con- 

 structed that it will bend upward in response to the contraction of its 

 dorsal longitudinal muscles^ in order to give the down-stroke to the 

 wings. The evident purpose of the parapsidal ridges, when present, 

 and of the V-ridge, as Weber points out, is to conduct the flexion of 

 the tergum in an even curve toward the middle from the two ends of 

 the segment, for without these gradient braces the pull of the muscles 

 would simply deflect the anterior and posterior parts of the tergal 

 plate, in most cases. In the higher Hymenoptera, however, the 

 tergum becomes strongly chitinized and rigid. Here flexibility is 

 supplied by the development of a secondary suture, which cuts across 



Epm 



Fig. 25. — Thorax and base of abdomen of larva of Scarites (Carabidae). 



a, a, pleural fold ; ac, antecostal suture of mesotergum ; Cx, coxa ; Epm, 

 epimeron ; Eps, episternum ; Sp, spiracle ; t, tergopleurites ; Tn, trochantin. 



the posterior part of the scutum (fig. 24 ], s) and divides the tergum 

 into two pieces movable upon each other. 



THE PLEURON OF A WINGED SEGMENT 



The nMe of the j^leuron in connection with flight is a more passive 

 one than is that of the tergum, the chief function of the pleura in 

 a wing-bearing segment being to support the bases of the wings and 

 the tergal plates. The pleuron of a winged segment, therefore, shows 

 fewer variations in relation to the wings than does the tergum, and 

 the pleura of the mesothorax and metathorax do not differ in basic 

 structure from the pleuron of the prothorax, though the actual differ- 

 ence may often be considerable on account of the degenerative ten- 

 dency of the prothorax, and especially of the prothoracic pleuron. 



The wing support of the pleuron of a winged segment consists of 

 a short thick arm, the pleural zving process (fig. 20, WP), extending 

 upward from the dorsal edge of the pleuron above the pleural suture 



