NO. 14 INSECT ABDOMEN— SNODGRASS 57 



ments behind the locomotor region, or thorax, is inconsequential, as 

 is also the matter of whether the abdominal segmentation is com- 

 pleted before or after hatching. 



The hexapods are always opisthogoneate in that the genital ducts 

 extend posteriorly from the gonads and open near the end of the body, 

 but there is considerable variation in the number of somites that pre- 

 cede the genital somite. In the Collembola the genital opening in each 

 sex is at the posterior end of the fifth abdominal segment, which is 

 segment XII from the mouth, and there is only one postgenital seg- 

 ment. The genital openings in Protura are likewise at the end of the 

 penultimate segment, but this segment is somite XVIII (eleventh 

 abdominal segment). In insects other than Collembola and Protura 

 the primitive paired gonopores were probably in the female on 

 somite XIV (seventh abdominal segment), and in the male on 

 somite XVII (tenth abdominal). With most of the insects, however, 

 the definitive genital outlet is the aperture of a secondary median 

 duct, and is subject to migration in the female from the seventh 

 abdominal segment to the eighth, ninth, or tenth, but in the male 

 appears to be always between the ninth and tenth segments. 



Considering the fact that somite formation is teloblastic in the 

 arthropods, and that the generative zone lies just before the end- 

 segment, or telson, it is evidently impossible that the genital segment 

 can be the same somite in Collembola, Protura, and the other insects. 

 With the establishment of the genital ducts in Collembola and Protura, 

 somite formation has ceased, but the genital segment in the former 

 is the fifth abdominal somite, and in the latter the eleventh. With the 

 other insects, four embryonic somites may be generated in the female 

 behind the segment of the primary genital ducts, and one in the male. 

 The abdomen of all the hexapods except Collembola has thus become 

 standardized with a maximum segmentation of eleven somites between 

 the thorax and the telson ; but the telson, except in Protura, is sup- 

 pressed in the adult and the eleventh somite forms the usual proctiger, 

 though it too is often reduced or united with the tenth. The reason 

 for regarding the twelfth abdominal segment of the Hexapoda as the 

 telson is that, when present in the embryo, it never has any of the 

 appurtenances of a somite, such as appendages or nerve ganglia. 



The lateral gonoducts of the male have separate openings to the 

 exterior among the Hexapoda in Protura, Ephemeroptera, some 

 Dermaptera, and perhaps secondarily in certain Diptera ; but in gen- 

 eral the lateral ducts discharge into a median ductus communis. 

 Embryological evidence (see Heymons, 1895, 1897; Wheeler, 1893) 

 gives reason for believing that the primary genital ducts of male 



