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 Tabanidae, 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, valvse externse, covered 

 by stiff setae; they are separated from each other by a deep 

 emargination in the tergum, roofed over by thin membranous 



