26 ILLINOIS BIOLOGICAL MONOGRAPHS [344 



is distinct but the limits of the somites are not so readily determined. The 

 cephalic limit of a segment is usually indicated by the distinct depression 

 on the dorsal and lateral aspects and by the short ventro-lateral depressions 

 which terminates at the cephalic end of the subspiracular lobe. Thus 

 the cephalic limit of a segment is not a straight line, but curves caudo- 

 ventrad and then slightly cephalo-ventrad of the subspiracular lobe. 

 The typical annulation and arrangement of setae, tubercles, and glandubae 

 on the typical abdominal segment are indicated elsewhere (Figs. 73-79). 

 The ninth abdominal segment is readily distinguishable because of its 

 location and shape, and by the absence of spiracles and larvapods. It is 

 typically smaller than the preceding segments, tapers more or less caudad, 

 and usually has one less annulet on the dorsum than have the preceding 

 segments. Its caudal limit is usually distinctly indicated by a deep 

 depression. The tenth, or the apparent ultimate, segment is modified and 

 differs from the other abdominal segments because of the presence of the 

 anus, anal larvapods, and other structures peculiar to this segment (Figs. 

 89-103). 



Tenth Urotergum. — The tergum of the tenth abdominal segment is 

 usually convex and often setiferous. It sometimes bears numerous spinous 

 processes, as in the Blennocampinae and Dimorphopteryx, or paired suranal 

 protuberances as in certain genera of Nematinae, or a median suranal 

 process {srp), as in the Cephidae and its allies. In these highly specialized 

 families the tenth abdominal segment is produced cephalad and fits into 

 the deep semicircular emargination of the ninth segment. The tergum 

 possesses a distinct, deep, median longitudinal depression extending 

 from the cephalic end of the tergum to the proximal end of the median 

 suranal process. In the Xyelidae the tergum is produced distinctly hunch- 

 like on the meson of the cephalic third caudad of the deep, broad transverse 

 depression. The tergum is produced caudad in certain species of Pachy- 

 nematus and forms a distinct caudal projection. In this genus the gland- 

 ubae are very conspicuous. The size, convexity, number, and arrangement 

 of the spinous protuberances, caudal processes, and setae are useful 

 characters in recognizing different subfamilies and genera. 



Suranal Lobe. — The membranous lobe (srl) of the Tenthredinidae, 

 which forms the dorsal wall of the anal slit may represent the rudiment of 

 the dorsal half of the ultimate segment, the so-called "telson." It bears 

 numerous setae of varying size and number and is usually fused with the 

 tergum of the tenth segment. In the larvae of the Xiphydriidae and its 

 allies, which possess a median suranal process, the suranal lobe is distinct, 

 more or less chitinized in part, usually separated from the tenth tergum by 

 a ridge or by an oblique suture which extends from the chitinized depres- 

 sion dorsad of the suranal process to the lateral end of the anal slit (au). 

 The area cephalad of this oblique suture is the pleuron of the tenth seg- 

 ment. 



