THE EXTERNAL GENERATIVE ORGANS. 72? 



elusion 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 Muscidse 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. 



