350 INSECTA. 



formation, one side atrophying after the paired terminal hypodermal 

 growth has fused. In most cases, however, the unpaired terminal 

 section must be regarded as a secondary integumental invagination. 

 This point has not yet been investigated in all groups of the Insecta. 



The agreement thus found to exist in the position of the genital 

 apertures in Phyllodromia (Heymoxs) and in the Ephemeridae 

 (which, according to Palmen, the Perlidae also resemble) may 

 perhaps justify us in concluding that the opening of the genital 

 glands on the boundary between the seventh and eighth segments 

 corresponds to the primitive condition for all groups of the Insecta, 

 and that the more posterior position of the apertures found in many 

 forms has arisen secondarily by a backward displacement. If this 

 is the case, we must assume that the condition in the Thysanura, 

 in which the genital aperture is paired and opens between the 

 eighth and ninth abdominal segments, or on the last, is a secondary 

 modification (cf. Haase, No. 153). 



The external genital appendages arise in most Orthoptera (as 

 Dewitz has proved for the Locustidae) out of two pairs of cone-like 

 projections belonging to the eighth and ninth abdominal segments, 

 the posterior pair of which very soon become double. The six parts 

 of the ovipositor of the female arise in this way, while, in the male, 

 corresponding shorter projections are found. The ovipositor of the 

 female Ichneumonidae and Cynipidae come under the same category, 

 as well as the sting of Apis (Kraepelix, Dewitz, Xo. 103). Since 

 the first rudiments of these paired appendages closely resemble the 

 imaginal discs of the Dipterous larvae, they have repeatedly been 

 regarded as abdominal limbs (p. 299, and footnote p. 300). The 

 ovipositors of many Diptera and Coleoptera, on the contrary, as well 

 as the penis of the Coleoptera, are to be derived from the most 

 posterior abdominal segment, which is invaginated and telescopic. 



The male genital gland, in Phyllodromia, at first develops in a 

 way similar to that described above in connection with the female 

 organs. Only in later embryonic stages can sexual differentiation 

 be recognised. It is then found that, in the male, four accumula- 

 tions of genital cells become surrounded with an epithelium. These 

 accumulations, which represent the four testicular follicles of Phyllo- 

 dromia, are closely connected with the rudiment of the efferent 

 genital ducts (vasa deferentia), and, in later stages, shift with the 

 latter somewhat backward and away from the original genital rudi- 

 ment. A remnant of the genital rudiment still remains attached 

 to the terminal filament, and this, according to Haase, is the female 



