73 6 THE GENERATIVE ORGANS. 



The Cerci. One or two pairs of simple or jointed appen- 

 dages frequently exist, which arise from the ventral edges of 

 the metagenital and anal terga. These were regarded by 

 Lacaze-Duthiers as dorsal appendages, and he termed them 

 tergo-rhabdites. As neither pleurae nor ventral plates are 

 usually developed in these segments, and even when sternal 

 plates are developed, no pleurae have been discovered, 

 there is apparently no sufficient reason to regard them as 

 dorsal rather than ventral appendages. In Locusta and 

 Blatta, I think it is obvious that the cerci are the ventral 

 appendages of two terminal somites. 



The Podical plates of Blatta and some other Insects are per- 

 haps parts of the anal annulus, possibly sternal plates ; but 

 their homology is very doubtful. 



d. The External Generative Organs of the Male Blow-fly. 



In the male Blow-fly the abdomen exhibits only five rings 

 externally, and the fifth sternum is deeply ernarginate behind. 

 The notch in the fifth sternal plate is the external opening of 

 a large cavity, the genital sinus. Four obvious segments are 

 invaginated within this sinus, represented by distinct tergal 

 plates, but only two sternal plates can be distinguished ; the 

 anterior of these is in front of, and the posterior behind, the 

 genital armature. 



There are therefore nine visible somites represented by 

 more or less complete annuli in both the male and female of 

 this insect; and if, as appears probable, the male and female 

 genital apertures correspond, they are both situated behind 

 the eighth sternal plate, which is the progenital sternum. In 

 the female this is perfectly obvious, but in the male it is more 



I M SCRII'TION OK 1'l.ATE L. 



The invnginatcd segments of the abdomen of a male Blmv-lly with the right side 



partially removed to show the male genital armature. </'., rii. and r//i., the 



sixth, seventh and eighth tergal plates. <;/, apodeme of penis ; t, left epiplcuron ; 



Mierior gonapopliyxis ; g f t po-termr <M>nap(iphysis ; ///, hypophallus ; //, para- 



phallus ; ./, genital sternum ; v, valvul.e uxterna:. 



