,— .^Tn ¥.^.- 



Fig. i8. — Diagrams outlining the possible origin and evolution of the thoracic 

 pleurites from a subcoxal segment of the leg. 



A, the theoretical subcoxa (Sex) as a primitive basal leg segment, with 

 anterior and posterior coxal articulations (a, b) on a horizontal axis. S, sternum. 



B, theoretical separation from subcoxa of a supra-coxal, trochantinal sclerite 

 (Tn) bearing the coxal articulations (a, b). 



C, subcoxa flattened into pleural wall of segment ; its basal chitinization 

 broken up into a series of eupleural sclerites (Acx, Apl, Pcx) forming an arch 

 over coxa concentric with the trochantin {Tn) ; posterior articulation of coxa 

 {b) dorsal in position. (Eoscntomon, fig. 8.) 



D, pleuron consisting of a eupleural sclerite (Epl) and a trochantinal 

 sclerite (Tn), concentric over coxa. (Lepisttia, fig. ii.) 



E, subcoxal part of pleuron with a single sclerite, the trochantin (Tn) ; coxa 

 articulated to sternum. (Japyx, fig. lo.) 



F, theoretical primitive pter^gote pleuron, consisting of a eupleural subcoxal 

 arch (Acx, Apl, Pcx) based on sternum, and of a trochantin (Tn) carrying 

 anterior and dorsal articulations of coxa (a, b). 



G, prothoracic pleuron of Plecoptera (fig. 13), consisting of anapleurite (Apl) 

 of eupleural arch, and of trochantin (Tn). 



H, usual structure of pterygote pleuron: trochantin (Tn) fused dorsally with 

 eupleural arch, and united areas divided by pleural suture (PIS) into episternum 

 and epimeron (Eps, Epni). 



I, ventral part of trochantin (Tn) a free sclerite by separation from part 

 reunited with eupleuron; postcoxal part of eupleural arch lacking. (Blattidae.) 



J, precoxal and postcoxal parts of eupleural arch forming independent plates 

 (Acx, Pcx). 



K, eupleural arch complete, united below with sternum. 



L, episternal, precoxal, and basisternal regions continuous ; postcoxal part of 

 eupleural arch lacking; sternum with secondary articulation with coxa (c). 



(35^ 



