THE EXTERNAL GENERATIVE ORGANS. 737 
difficult to arrive at this conclusion, as the progenital sternum 
is immediately behind the fifth sternum. It is therefore ap- 
parently the sixth, and two pairs of gonapophyses intervene 
between it and the anal, somite. If, therefore, the male and 
female genital orifices correspond in being behind the eighth 
somite, there are traces of one, or possibly of two, additional 
somites in the male between the eighth or progenital, and the 
ninth or anal, somite, which are entirely absent in the female 
insect. The abdomen of the male has, if this is the case, 
eleven somites in its composition, whilst that of the female 
presents only nine. 
In the Tabanidz, Tipula, and the majority of the Diptera, 
the genital armature of the male is obviously behind the eighth 
abdominal somite, as it is in the female, and, as has been already 
stated, this is the usual position of the anterior gonapophyses 
and the genital orifice. A detailed study of the invaginated 
segments of the male Blow-fly leads to the conclusion that 
two sterna are undeveloped between the fifth and progenital 
sternum, and that two somites are represented by appendages 
only between the progenital and anal somites. 
The largest element in the invaginated portion of the 
abdomen is the tergal plate of the eighth obvious segment. I 
shall term it the progenital tergum (Pl. L., viii.). 
The Progenital Tergum is similar to the terga of the abdominal 
somites of the Crayfish in having its inferior edges prolonged 
as epipleura (see p. 156). The inner edge of each epipleuron 
has a thickened margin or ridge, the epipleural ridge. These 
ridges on either side are connected by a thin membranous 
integument which forms a roof over the penis. The anterior 
extremities of the epipleural ridges are prolonged forwards, 
are overarched by two tergal plates, the sixth and seventh, 
and articulate with a cordate sternal plate, the progenital 
sternum. 
The posterior angles of the progenital tergum are prolonged 
as a pair. of broad, hollow processes, valve externz, covered 
by stiff setee; they are separated from each other by a deep 
emargination in the tergum, roofed over by thin membranous 
