NO. 6 INSECT ABDOMEN — SNODGRASS II3 



Discussions on the morphology of the abdominal appendages of 

 insects, and speculations on their possible homologies with the limbs 

 of Crustacea have continued for half a century without leading to 

 definite conclusions. They began at least with Wood-Mason (1879), 

 who, in a paper on the origin of insects, interpreted the stylus-bear- 

 ing plates of the abdomen of Machilis as the protopodites of primi- 

 tively biramous appendages, of which the endopodites are represented 

 I by the gonapophyses, and the exopodites by the styli. The eversible 

 I vesicles, Wood-Mason suggested, may be homologues of nephridia, 

 ; those of the eighth and ninth segments being converted into the 

 I genital ducts. 



Diversities of opinion soon followed the publication of a more 

 widely read paper by Haase (1889) on the morphology of the seg- 

 mental appendages, containing not only a clear exposition of the 

 appendicular nature of the stylus-bearing plates in the abdomen of 

 Thysanura, but also a demonstration of the triple origin of the de- 

 finitive abdominal sterna of insects in general from the union of the 

 i rudimentary embryonic limbs with the median sternal area in each 

 segment. The styli, Haase claimed, are not the leg rudiments, but 

 secondary structures of the nature of hairs, which have been con- 

 : verted into locomotory organs from sensory organs. The eversible 

 j vesicles he believed function as blood gills, but are to be traced back 



in all cases to coxal glands. 

 j Wheeler (1893) and earlier students of the embryology of insects 

 j regarded the gonapophyses of the genital segments as the direct 

 representatives of the appendages of these segments. Considering 

 the late development of the gonapophyses, however, and their in- 

 variable median position on the body of the insect, Heymons (1896a) 

 contended that the genital processes are secondary integumental out- 

 growths having no relation to the appendages, and that the latter 

 are preserved in the styli and cerci. Heymons' heterodox opinion 

 brought a severe criticism from VerhoefT (1896), who defended the 

 limb nature of the gonapophyses as an established fact, and main- 

 I tained that the identity of the abdominal styli with the thoracic styli 

 I of Machilis could not be disputed, and that therefore both are merely 

 appendicular processes of the legs. In reply to VerhoefT, Heymons 

 (1896&) emphasized his former statements in evidence of his view 

 ! concerning the nature of the appendicular parts of the abdomen, 

 i pointing out that, during embryonic development of the Orthoptera, 

 I the abdominal appendages disappear, and the gonapophyses are later 

 ■ formed quite independent of the limbs. He argued that if the 

 I gonapophyses are the limbs, intermediate stages should be found some- 



