156 COMPARATIVE ANATOMY 



meet at an acute angle at the sides of the body and are united 

 by a soft colourless cuticle. The tergum projects a little beyond 

 the sternum, and in the first eight abdominal segments a small 

 aperture guarded by a chitinous sclerite is situated under the 

 anterior corners of each tergum. These are the eight pairs of ab- 

 dominal stigmata leading into the respiratory tubes or tracheae. 



In the male cockroach the abdomen is narrower and the 

 terga more convex than in the female. The first tergum is 

 comparatively small, and the first sternum is very small, con- 

 sisting of an oval chitinous plate. Both the terga and sterna 

 of the next six segments are broad chitinous plates, but the 

 eighth and ninth terga are reduced to narrow bands of chitin 

 which in P. americana are overlapped and concealed by the 

 posterior margin of the seventh tergum, but in P. orientalis are 

 generally visible from the surface without disturbance of the 

 adjacent parts. The tenth tergum is a flat plate with a 

 truncated posterior margin projecting beyond the extremity 

 of the body. To its sides are articulated a pair of spindle- 

 shaped many-jointed cerci, and beneath it are two triangular 

 chitinous sclerites, the podical plates, between which the anus 

 opens in the middle line. Some authors regard the podical 

 plates as the representatives of the tergum of an eleventh 

 abdominal segment. 



Ventrally the abdomen narrows rather suddenly at the 

 seventh sternum. The eighth sternum, though narrow, is rather 

 long from front to back, but is largely overlapped by the seventh, 

 so that its exposed portion is short. The ninth sternum is 

 deeply convex ventrally and concave dorsally, is narrow, and 

 so much overlapped by the eighth, that more than half of its 

 length is concealed from view. Its posterior margin is divided 

 into a median and two lateral lobes by two shallow notches 

 in which a pair of slender unjointed styles are articulated. 



The concavity of the ninth sternum encloses a large pouch 

 formed by the infolding of the cuticle. Dorsally this pouch is 

 roofed in by the podical plates, and its anterior extremity is 

 continued into the male genital aperture. Surrounding the 

 aperture is a very complicated genital apparatus consisting of 

 asymmetrically disposed chitinous plates and hooks, the exact 

 relations of which need not be detailed here. Possibly the 

 elements of the tenth sternum are represented by some of 

 these plates. 



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