NO. 14 INSECT ABDOMEN SNOUGRASS 35 



genital chamber (B). This organ is described by Borner (1902, 

 1904) as a complex of folds produced from the walls of the genital 

 chamber ("uterus externus"), which enclose a ventral space into 

 which opens the saccus internus (" uterus internus"). The organ is 

 termed a " penis ", though only its distal part is entirely free from 

 the genital chamber wall, and its use has not been observed. Borner 

 regards the lateral lobes of the organ as derivatives of the segmental 

 appendages of the genital segment, but of this there appears to be no 

 actual evidence, and, judging from Xiphosura, the genital appendages 

 should be merged in the sternal plate of the eighth segment. 



Among the Phalangida (Opilionida) the most generalized condi- 

 tion of the external genitalia is preserved in the Cyphophthalmi, 

 where, as shown by Hansen and Sorensen (1904), there is a small 

 sternal plate exposed before the genital opening, and a broad plate 

 behind the latter. If we assume that the true first abdominal sternum 

 is obliterated in the phalangids, as in most other Arachnida, then the 

 pregenital plate must be sternum VIII, and the postgenital plate 

 sternum IX. In the Phalangidae the anterior part of the venter of 

 the opisthosoma is produced forward into the venter of the prosoma 

 to such an extent that the genital opening comes to lie between the 

 bases of the second pair of legs (fig. 13 A, Gtr), and the sternum of 

 the postgenital segment forms a long quadrate plate (IXS) between 

 the bases of the third and fourth pairs of legs. Lying before the 

 gonotreme is a transverse fold which may be the sternum of the 

 genital somite, but it appears to belong rather to the segment of the 

 last pair of legs. 



The gonotreme of the phalangid Liobunum (fig. 13 A) is a wide 

 transverse cleft (Gtr) above the anterior margin of the postgenital 

 sternum (IXS). It leads into a large sacklike genital chamber with 

 membranous walls (fig. 12 A, GC), which in each sex extends far 

 posteriorly in the ventral part of the body. The genital chamber con- 

 tains a long, tapering, strongly sclerotized genital organ, which is an 

 ovipositor (Ovp) in the female and a penis in the male. The organ 

 is a tubular evagination of the posterior wall of the genital chamber, 

 and is transversed by the outlet duct of the gonads (Dct) . On its base 

 are inserted two pairs of muscles, a long anterior pair (pmcl) arising 

 on the posterior angles of the postgenital sternum (IXS), and a 

 shorter posterior pair (rmcl) arising on the lateral areas of the ter- 

 gum of the penultimate body segment (XVT). The genital organ 

 has the same essential structure in each sex, though there are differ- 

 ences in its size and shape (fig. 13 B, D) to be noted presently. The 

 lateral walls of the genital chamber are strengthened by two long. 



