NO. II LEGS OF PRIMITIVE ARTHROPODS EVVING 29 



In Pauropus the tergites (pi. 9, fig. 27) are simple, oval-shaped 

 plates, seven in number, which fit the lateral curvature of the body 

 and extend down less than half way on the sides of the same. There 

 is no tergum over the first body segment. The first tergal plate 

 covers dorsally the second body segment. The next four tergal plates 

 are allotted to body segments 3-10, all of which bear legs. The last 

 two tergal plates cover roughly the remainder of the body. In Pau- 

 ropus the tergites do not cover entirely the dorsal surface. There 

 is present between adjacent tergites a considerable area that is mem- 

 branous (pi. 9, fig. 27). 



In Symphyla the terga are very similar to those of Pauropus, being 

 simple plates that do not completely cover the dorsal surface. These 

 plates mav be variously shaped. In the genus SyiuphylcUa the tergum 

 of the first body segment is reduced and divided both transversely 

 and longitudinally, making four smaller plates (pi. 9, fig. 28). Terga 

 vary in size in Symphyla according to the size of the segment each 

 covers. 



In the Protura the terga no longer have the simple structure found 

 in Pauropoda and Symphyla. Typically each of the thoracic, as well 

 as the abdominal, terga possesses a strong transverse apodeme near 

 the anterior margin. This apodeme (pi. 10, fig. 29) may be simple 

 or may be singly or doubly branched laterally. The writer (Ewing, 

 1921) has utilized this variation as a generic character. Just posterior 

 to this apodeme is a transverse groove. Prell (1913) has represented 

 both this transverse suture and the apodeme, but he also shows two 

 poorly chitinized regions posterior to the terga proper. The writer 

 has not located such regions in our American proturans. Snodgrass 

 (1927) considers the apodeme as representing the antecostal suture 

 and the small area in front of it as being the precosta, and both 

 structures as being typical of an insect tergite. 



The tergal region of the prothorax is much modified, suggesting a 

 condition found in the tergal region of the first body segment of 

 Symphyla. The tergum proper (pi. 10, fig. 29) is reduced and poorly 

 chitinized in front, while laterally there is a prominent fold over the 

 coxa. This fold corresponds to the intersegmental lobe of the neck 

 as regarded by Berlese (1910). 



In Eosentonwn a median apodeme is present on the thoracic 

 tergites. This median ridge is found in many pterygote insects. 



The thoracic terga of Japy.i' are taken as a sam])le representing the 

 Thysanura for two reasons. They have been much di.scussed pre- 

 viously, and they show considerable variation. In Japyx isahellac 

 (pi. 10, fig. 30) the thoracic terga are oval plates that -almost join 



