NO. 14 INSECT ABDOMEN SNODGRASS 49 



of Chilopoda and Hexapoda. There can be no question as to the 

 identity of the first body segment in the Symphyla, since, though 

 reduced, it bears a pair of legs. These first legs are small and lack 

 one of the distal segments present in the other legs. Behind the bases 

 of the third pair of legs is situated in each sex the median genital 

 opening, which evidently belongs to the third body segment, though 

 it may invade the venter of the following segment. If, therefore, we 

 count the absent second antennal somite as segment /, the genital 

 opening in Symphyla is on segment VII, which is the third body 

 segment. 



In the Pauropoda and Diplopoda a direct determination of the 

 gnathal and postcephalic segmentation becomes impossible by reason 

 of the facts that in the adult there is only one appendage in the maxil- 

 lary region of the head, and the first apparent body segment is reduced 

 and lacks appendages. As above pointed out, the evidence deduced 

 from developmental studies is conflicting and inconclusive. By com- 

 parison with Symphyla, however, the genital segment of the pauro- 

 pods and diplopods should be segment VII, since there is no evidence 

 of variability in the position of the genital outlets in any of the 

 progoneate myriapods. The male exit ducts of the Pauropoda open 

 on a pair of small penes situated at the bases of the second pair of 

 fully developed legs ; the first legs, if present, are rudimentary and 

 arise from an apparent neck region, which latter, therefore, must be 

 regarded as the reduced first body segment. The single female outlet 

 duct opens by a median aperture between the bases of the legs on the 

 same segment as in the male. (See Silvestri, 1902; Attems, 1926.) 



The Diplopoda have a distinct " neck segment " intervening between 

 the head and the first leg-bearing segment of the body, which typically 

 contains a large dorsal plate, the collum, and a small ventral plate, 

 but apparently has no appendages. In the adult there is only one 

 postmandibular appendage on the head, the gnathochilarmm, which 

 appears to have a composite structure. The first three body segments 

 following the neck segment have each only one pair of legs, and the 

 genital apertures are always on the segment of the second pair. In 

 most of the diplopods (the Proterandria) the eighth or the eighth 

 and ninth pairs of legs (those of the third "double segment") are 

 modified in structure to serve as intromittent organs, and are termed 

 the gonopods. 



The dispute on the question of diplopod segmentation concerns the 

 number of postoral somites in the head, the nature and composition 

 of the gnathochilarium, and the relation of the plates of the neck seg- 

 ment to the cephalic capsule and the gnathochilarium on the one hand, 

 and to the body on the other. 



