NO. I INSECT THORAX — SNODGRASS I5 



of that of the extinct ancestors of the Pterygota, than is to be found 

 in the thorax of modern winged insects, or in that of the earUest 

 winged forms known from the paleontological records. 



THE TERGUM 



The dorsal plates of the thoracic segments, though highly specialized 

 in the mesothorax and metathorax of the Pterygota, have been 

 affected less in the adaptive reconstruction of the thorax than have 

 either the sternal or the pleural parts. The reason for this is clearly to 

 be found in the fact that the evolution of the terga is correlated with 

 the development of the wings, organs of comparatively recent origin, 

 while the evolution of the sternal and the pleural parts began with the 

 differentiation of the thorax as the specialized locomotor center of 

 the body. In general, the dorsal plates of the thoracic segments have 

 preserved the structure characteristic of the terga of arthropods 

 having a secondary segmentation, in which the dorsal muscles extend 

 between antecostal ridges (fig. 5) derived from the primitive inter- 

 segmental folds (fig. 2). In the prothorax, however, the tergum 

 always lacks a true antecosta and precosta, and the principal dorsal 

 muscles are attached anteriorly on the back of the head, as are also 

 the ventral muscles. By this anatomical arrangement the head be- 

 comes movable on the body by the action of the prothoracic muscles. 



In the Apterygota the thoracic terga are comparatively simple 

 plates. In Japyx (fig. 6 A) the mesotergum and metatergum have 

 particularly large precostse (Pca Pcz), each set off by a distinct ante- 

 costal suture (oc), and marked by a median pair of prominent setae, 

 as is also the precosta of the first abdominal segment (IPc). Verhoeff 

 regarded the precostal sclerites of Japyx as representing terga of 

 rudimentary intercalary thoracic segments; Enderlein (1907) claimed 

 that they are but " apotomes " of the following terga ; it is now clear 

 that they are nothing more than unusually large precostae, since the 

 dorsal muscles are attached to ridges at their bases. Behind each 

 principal tergal plate is a membranous area (fig. 10, Mb) continuous 

 with the pleural area on each side of the segment. In the Protura the 

 tergum of the mesothorax and of the metathorax (fig. 8, T) does not 

 cover the entire back of the segment, there being a large membranous 

 or weakly chitinous area behind it. This area Derlese (iQio) regards 

 as " intersegmental," but Prell (1913) distinguishes in it two regions, 

 the first of which he calls the " nothotergite," while the second he 

 suggests is the homologue of the postnotal plate of certain winged 

 insects. The postnotum, however, as will be shown later, when 



