THE EXTERNAL GENERATIVE ORGANS. 927 
clusion that they are developed from the eighth and ninth 
somites of the abdomen, and first suggested that they are 
homologues of the parts of the sting in the female. 
In the Muscide the male organ is, however, apparently 
situated behind the fifth or sixth abdominal somite, whilst in 
the female the generative aperture is obviously between the 
eighth and ninth. The question therefore arises, Are these 
appearances deceptive, or is the position of the genital orifice 
differently situated in the two sexes? Similar discrepancies 
in the position of the generative apertures in the two sexes are 
of common occurrence. Thus, Huxley, after describing the 
sternal plates of the abdomen in the two sexes of Periplaneta 
orientalis, wrote: ‘While in the female the opening of the 
recto-genital chamber lies between the tenth tergum and 
seventh sternum, in the male it lies between the tenth tergum 
and ninth sternum ;’ and he concludes, speaking of the gona- 
pophyses of the male, by stating that ‘though they are of the 
same nature as the gonapophyses of the female, they are not 
their exact homologues.”* 
It follows, therefore, that either the number of abdominal 
somites is incorrectly estimated, or that there is no constancy 
in the position of the genital apertures and their armature in 
the two sexes. 
Nomenclature of the Abdominal Somites.—I shall now endeavour 
to show that such discrepancies are the result of the manner 
in which the identification of the abdominal somites is usually 
attempted. In my opinion the method adopted, that of 
counting these from before backward, is wrong in principle, and 
has given rise to much confusion. In order that such a system 
should give results of any value, it must be assumed that the 
variation in the number of somites which occurs in the abdomen 
in different Insects is entirely due to the non-development of 
the apical segments. If the number of basal segments varies, 
as I shall show it undoubtedly does, then the fifth abdominal 
somite of one Insect may be the morphological representative of 
the eighth, ninth, or tenth of another. 
* ¢The Anatomy of Invertebrated Animals,’ p. 406. 
