NO. 6 INSECT ABDOxMEN — SNODGRASS 21 



beyond which are the parts of the eleventh segment, and apparent 

 remnants of the twelfth (A). Likewise, in larvae of Plecoptera the 

 tenth segment is cylindrical in form (fig. 8 A, A'), and the parts of the 

 eleventh segment (Eppt, Papt) are quite distinct from it. The struc- 

 ture in an adult plecopteron is essentially the same as that of the 

 larva, but the tenth segment is smaller. In the Ej^ihemerida, both 

 larval and adult forms, the tenth somite is a well-developed segment, 

 which, because of the reduction of the eleventh somite, forms the 

 terminal segment of the body and appears to carry the long cerci. 

 Its tergal region is i^roduced posteriorly in a median lobe, and thus 

 resembles the supra-anal ])late of other insects, but the small true 

 epiproct of the eleventh segment lies beneath the lobe of the tenth 

 tergum and carries the median caudal filament. The venter of the 

 ephemerid tenth segment a])pears to contain the anal o])ening, but 

 it is evident that the anus is drawn forward and that the paraprocts are 

 united with the liases of the cerci. 



Among ortho])ter()id insects the tenth segment is \-arial)le ; its ven- 

 tral region is usually membranous, and in some families its tergal plate 

 is fused with the eleventh tergum, or epiproct. In the T'hasmidae, 

 however, the tenth segment is large and normally developed ( fig. 

 8 E, G, A'). In Diaplicroniera it has distinct tergal and sternal plates, 

 the tergum overlap]iing the edges of the sternum in the female (F), 

 though the two plates are ankylosed in the male to form a strong 

 support for the clasjier-like cerci (E). The paraprocts are united 

 ventrally with the tenth sternum ( F, Papt) and appear to be lobes of 

 the latter. The ventral region of the tenth segment is membranous 

 in most other Orthoptera (fig. 8 D), though the dorsum usuall\- 

 contains a distinct plate {XT). In Acrididae the tenth tergum is a 

 narrow transverse sclerite fused laterally with the ninth tergum, 

 but it is separated from the epiproct by a complete suture. In Blat- 

 tidae (fig. 40 A), Tettigoniidae, and Gryllidae (fig. 8 1)), however, 

 the tergum of the tenth segment (A'), is generally fused with the 

 epiproct {Eppt). and since the paraprocts become continuous with 

 the membranous ventral wall of the segment, the tenth somite in 

 these families loses the status of an inde]:)endent bodv segment. 



It must be noted here that the principal muscles of the cerci arise 

 on the tergum of the tenth segment. The size of the tenth segment, 

 therefore, generally varies according to the size of the cereal muscles, 

 the segment being large in insects having long, mobile cerci (fig. 8 A, 

 XS), and strongly develo])ed in insects in which the cerci have 

 a grasping function, as in Japyx ( fig. 40 O.and in Diaphcrouiera (fig. 

 8 E, X). When the cereal musculatm-e is weak the tenth segment 



