736 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 pleure nor ventral plates are 
usually developed in these segments, and even when sternal 
plates are developed, no pleura 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 emarginate 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 
DescrIPTION OF PLATE L. 
The invaginated segments of the abdomen of a male Blow-fly with the right side 
partially removed to show the male genital armature. v¢., vit. and witt., the 
sixth, seventh and eighth tergal plates. af, apodeme of penis ; e, left epipleuron ; 
g, anterior gonapophysis ; ¢’, posterior gonapophysis ; “f, hypophallus ; 7A, para- 
phallus ; s¢, genital sternum ; %, valvulee externoe. 
