728 THE GENERATIVE ORGANS. 
The evidence that the number of segments in the basal 
portion of the abdomen does vary is very convincing. In the 
common Cockroach—Periplaneta orientalis—there are only 
seven visible abdominal sterna in front of the sexual orifice of 
the female, but in Periplaneta americana I find two very rudi- 
mentary sterna in front of these, so that there are nine sterna 
in front of the genital orifice, and the same number is common 
to the males in both species. It is obvious, therefore, that in 
these Insects the number of basal segments varies in the 
abdomen, and the two sterna nearest to the thorax are always 
feebly developed in Cockroaches whenever they are present, so 
that one or both may be easily overlooked. 
With the exception of Periplaneta, in which the genital 
aperture is normally situated behind the ninth somite, in all the 
other large Orthoptera which I have examined it is behind the 
eighth. 
I propose to term the sternum, which is situated imme- 
diately in front of the genital aperture, the progenital sternum. 
It is usually the eighth counted from the base of the abdomen, 
rarely the ninth as in Periplaneta americana. Occasionally it 
appears to be the sixth, as in the males of Calliphora ; in this 
case, however, eight terga exist in front of the sixth sternum, 
and there is evidence to show that the sterna which are absent 
are the two immediately in front of the progenital sternum. 
The number of segments behind the progenital sternum 
in mature insects varies from one to three. When only one 
is present it surrounds the anus; I therefore term it the Anal 
Somite. The two possible somites which intervene between the 
progenital and anal I term the Meso- and Meta-genital Somites ; 
one or both are usually represented by a pair of appendages ; 
and the progenital and anal somites frequently also exhibit 
more or less developed appendages. These appendages are 
termed Gonapophyses, or Cerci, according to the form which 
they assume. 
In the male Hymenoptera there are manifestly two pairs of 
gonapophyses, which surround the penis; they are developed 
from the pro- and meso-genital somites. In the female these 
