NO. 8 INSECT ABDOMEN SNODGRASS 3I 



fibers, apparently intersegmental muscles, are attached near the base 

 of the gonapophysis and evidently act as protractors and retractors 

 of the latter. An exact study of the gonopod musculature could not 

 be made on the material at hand, but there is no doubt that each 

 gonopod, as indicated somewhat diagrammatically at F of figure 6. 

 is movable as a whole by the tergal muscles inserted on its base (LB). 

 The gonapophysis, on the other hand, is movable individually on the 

 basis by the group of short fibers (gmd) arising in the latter, and 

 inserted within its proximal end. The stylus (.S"^v), being set in a 

 membranous socket on the distal margin of the basis, is freely movable 

 by its long muscles (smcl) arising on the proximal margin of the 

 basal plate. In Thermobia the gonapophysis of the ninth segment is 

 attached directly to a small sclerite lying in a notch of the base of the 

 large stylus-bearing plate, but the small sclerite is evidently a secon- 

 dary subdivison of the larger one. 



The postgenital segments of the Thysanura are more generalized 

 in some members of the group than they are in any of the Pterygota. 

 The tenth segment is a complete annulus (fig. 6 A, X), but it lacks 

 appendages in postembryonic stages, as it does in all the more gener- 

 alized Pterygota. The small eleventh segment (A, XI) is normally 

 concealed within the tenth segment, but it bears the cerci (Cer) and 

 the long median caudal filament (cf). In Ncsomachilis the body of 

 the eleventh segment, when removed from the tenth (D), is seen 

 to consist of a dorsal arch (IXT) produced into the caudal filament 

 (cf), of two lateral lobes (LB), evidently the bases of the cereal 

 appendages (Cer) which they support, and of a narrow, membranous 

 venter bearing the two paraproctial lobes. The cerci appear to have 

 no muscles arising in their own bases, but the tenth segment is filled 

 with a mass of intersegmental fibers (D, xmcls) attached posteriorly 

 on the anterior margin of the eleventh segment. Some of these fibers, 

 inserted at the bases of the cerci, serve to move the cerci. The twelfth 

 segment, or periproct, is practically obliterated, but in some species 

 of Thysanura there is a small, fleshy dorsal lobe beneath the base of 

 the caudal filament, possibly a remnant of the lamina supra-analis 

 of the telson. 



In order better to compare the structure of the pterygota ovipositor 

 with that of the ovipositor of Thysanura, we may briefly summarize 

 the principal features of the simple thysanuran ovipositor as follows : 



I. The thysanuran ovipositor consists of two pairs of processes (the 

 first and second gonapophyses), and of two pairs of basal plates be- 

 longing to the eighth and ninth abdominal segments, respectively. 



