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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



and it is only in the Protura that the last of these segments is well 

 developed (fig. 5 B, XII). In some of the lower insects apparent 

 traces of the terminal segment are to be found, but in most of the 

 true Insecta there are only two postgenital segments present. A still 

 further reduction, however, has taken place in many insects, with the 

 result that but one segment is to be recognized beyond the ninth. 

 Where two postgenital segments are present there is little doubt that 

 they are the tenth and the eleventh ; and where the number of post- 

 genital segments is reduced to one, it is usually to be assumed that the 



XIT 



Cer 



Papt 



^-An 



Cer 



Eppt(XIT) 



/ ^ An- 



Cer 



Cer 



Eppt 



Fig. 7. — Terminal abdominal structures of Thysaiiura. 



A, end of abdomen of male Ncsoinacliilis maoricus. B, caudal filament and 

 cercus of same removed, showing their origin from small eleventh segment nor- 

 mally retracted into the tenth. C, dorsal view of same. D, ventral view of 

 eleventh segment, with bases of caudal filament and cerci. E, dorsal view of 

 eleventh segment and terminal appendages of Thcnnohia. F, ventral view of 

 same. 



An, anus; Cer, cercus; cf, caudal filament; Eppt. epiproct ; LB. limb basis; 

 Papt, paraproct; .fa, possible rudiment of lamina supra-analis ; .S7», primary 

 sternum ; Sty, stylus. 



single end-segment is the tenth somite, and that the eleventh has dis- 

 appeared, though it is probable that the tenth and the eleventh somites 

 are in some cases combined in the definitive terminal segment. 



The tenth segment. — The tenth somite of the abdomen is clearly 

 retained as the numerical tenth segment in insects in which there are 

 two distinct ])ostgenital segments. It is a complete annulus in the 

 Thysanura (fig. 7 A, X) , quite distinct from the eleventh segment 

 (B, A7), which is mostly concealed within it. In the Odonata the 

 tenth segment is a continuously sclerotized ring (fig. 12 B, C, X) , 



