30 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



part. The part dorsal to the coxa unites partially or completely with 

 the eupleural region above it, in the second case becoming a part of 

 the definitive episternum, while the part before the coxa may separate 

 from the siipracoxal part to become a free plate (fig. 7 B, TW2), the 

 sclerite ordinarily known as the trochantin (fig. 3, Tn). In the higher 

 orders of the Pterygota, the trochantin becomes rudimentary or disap- 

 pears entirely, and it is subject to much variation even in those orders 

 where it is best developed. It is clear that the trochantin is a sclerite 

 that has played a much more important part in the pleural mechanism 

 of the earlier insects, and that it is now in process of degeneration. 



Pc- 



ac- 



PIS- 



Acx 



-Pcx 



Fig. 15. — Theoretical composition of the pleuron, suggesting its origin from a 

 subcoxal segment of the leg. 



A, diagrammatic lateral view of a wingless segment, showing pleuron consist- 

 ing of a eupleural arch (Acx, Eps, Epm, Pcx), and of a trochantinal arch (Tn), 

 the latter fused dorsally with anapleurite (fig. 13, Apl) to form the definitive 

 episternum and epimeron (Eps, Epm). 



B, mesothoracic pleuron and base of coxa of mature nymph of cicada (Tibicina 

 scptcndechu), showing parallelism in structure with A, and suggesting the 

 origin of the pleuron from a subcoxal leg segment. 



The foregoing review shows that there is a basic unity of structure 

 in the pleural parts in both the Apterygota and the Pterygota, since 

 in each group the pleurites fall into a eupleural and a trochantinal 

 arch concentric over the base of the coxa. For practical purposes it 

 is perhaps enough that we can trace the approximate evolution of the 

 pleurites in their various modifications, but a deeper understanding 

 of the thorax demands an explanation of the origin and nature of the 

 pleurites themselves. 



The pleural sclerites have been regarded as intrinsic elements of 

 the lateral walls of the segments, but their variability and their general 

 weak development in the Apterygota, would indicate that the plates 



