NO. 6 INSECT ABDOMEN — SNODGRASS 



79 



cles arising in the latter. The gill bases are interpreted by Borner as 

 the subcoxae of the abdominal appendages, since they follow exactly 

 in line with the subcoxal. or " pleural," plates of the thorax. The 

 gills, therefore, Borner contends, are the equivalents of the legs, 

 and the proximal end of each represents the coxa. On the other 

 hand, we might assume that the gill basis includes the equivalents 

 of both the subcoxa and the coxa of a thoracic leg, in which case the 

 gill shaft or plate might be supposed to be the telopodite with its 

 proximal end representing the first trochanter. The presence of the gill 

 muscles arising in the gill basis suggests this homology, since the 

 muscles are comparable with the trochanteral muscles of a leg (fig. 

 26, 0, Q). l)Ut again, the gill resembles the stylus of a thysanuran 

 appendage, and there is doubt as to whether the abdominal styli 

 are true telopodites or secondary appendicular processes of the coxae. 

 Leaving aside, for the present, the question of homologies between 

 the parts of the abdominal appendages and those of the thoracic 

 appendages, a comparison of the abdominal appendages of the ephem- 

 erid larva with the abdominal appendages of Thysanura leaves 

 little doubt that the gill-supporting lobes of the former are the 

 equivalents of the stylus-bearing plates of the latter, and that the 

 gill stalks or plates are the homologues of the styli. Neither the 

 abdominal gills of ephemerid larvae nor the styli of Lcpisina arc 

 present on the early postembryonic stage of the insect. In the newly 

 hatched larva of EphcmcreUa vnlgata, according to Heymons (1896), 

 there are lateral protuberances of the abdominal segments, especially 

 prominent on segments II and VII, which are derived during em- 

 bryonic development from the embryonic limb rudiments. The gills 

 first appear as outgrowths from these abdominal lobes about four 

 days after hatching. The styli of Lcpisina sacchariiia. Heymons 

 (1897) says, appear likewise a considerable time after hatching, and 

 arise from the parts of the ventral plates of the eighth and nintli 

 abdominal segments that are derived from the " .\nlagen " of the 

 embryonic appendages. 



LATERAL ABDOMINAL AI'PENDAGES OF SIALID AND COr.EOI'TEROUS 



LARVAE 



The larva of Sialis (fig. 35 A) presents at least an excellent imita- 

 tion of an insect that has carried the ])rimitive polypod condition 

 into a postembryonic stage. The long, tapering, segmented, ap- 

 l)endicular organs, usually termed " gills," projecting from the sides 

 of the first seven abdominal segments have a striking resemblance to 



