NO. 8 INSECT ABDOMEN SNODGRASS I07 



The seventh segment forms the last completely exposed annulus of 

 the abdomen. Its tergum (fig. 35 A, VI IT) resembles the terga of 

 the preceding segments, but the sternum (VI IS) is extended as a 

 subgenital plate beneath the base of the ovipositor, and ends in two 

 small lobes embracing the ventral valvulae. Above the seventh sternum 

 is a vestibular cavity, in the anterior wall of which is the gonopore. 

 The eighth tergum (VIIIT) has the same shape as the seventh, but 

 its lower ends are overlapped by the posterior lateral angles of the 

 seventh sternum. The venter of the eighth segment is represented 

 only by the membranous integument forming the anterior wall of the 

 vestibulum containing the genital aperture. The ninth tergum (IXT) 

 is narrowed dorsally, but is expanded on the sides of the ninth seg- 

 ment, where its lower margins overlap the second valvifers {2Vlf). 

 Between the valvifers the membranous venter of the ninth segment 

 forms a deep concavity in which is lodged the shaft of the ovipositor. 

 The abdomen terminates with a conical proctiger (Ptgr) bearing ven- 

 trolaterally a pair of slender processes (Soc). Beneath the proctiger 

 the third valvulae (3VI) project as a pair of short, broad, dark- 

 colored lobes, normally embracing the tip of the ovipositor {Ovp). 



The ovipositor of Pteronidea ribesii is rather weak, since the eggs 

 of the currant sawfly are deposited on the surface of the leaves of 

 the food plant, but it has the characteristic form of the sav^y ovi- 

 positor. The shaft of the organ is composed of the broad, laterally 

 compressed first and second valvulae (fig. 35 B, iVl, 2VI), which are 

 respectively ventral and dorsal to each other. The dorsal blades are 

 united for their entire length by a narrow median membrane (E). 

 The outer surface of the distal part of each valvula is crossed by a 

 series of strong, oblique ridges (C, D) ; the membranous proximal 

 part is traversed by a narrow flexible ramus {C, rivl, E, r2vl) which 

 curves upward to the basal attachment of each valvula with the cor- 

 responding valvifer. 



The first valvifer is a small triangular sclerite lying anterior to the 

 lower end of the ninth tergum (fig. 35 B, iVlf), where it is mostly 

 concealed beneath the lower part of the eighth tergum (A). It is 

 definitely articulated by its dorsal posterior angle (B, C, a) to the 

 ventral anterior angle of the ninth tergum, and by its ventral angle (&) 

 to the dorsal edge of the second valvifer. The ramus of the first val- 

 vula is continuous with the first valvifer at the anterior angle of 

 the latter (c). 



The second valvifer is a large elongate plate (fig. 35 B, E, 2Vlf) 

 lying beneath the eighth and ninth terga (A) . It is connected by mem- 

 brane with the ninth tergum, but has no articulation with the latter, 



