28 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



to the furcisternum. This chitinoiis arch constitutes the cupleuron 

 (Crampton, 1914), and it is clear that it corresponds closely with the 

 eupleural series of sclerites in Eosentomon (fig. 8), though not neces- 

 sarily in identity of individual plates. The " typical " trochantin of 

 the pterygote pleuron (fig. 3, Tn) is excluded from the dorsal, or 

 pleural, articulation of the coxa {CxP), but there are many insects in 

 which it extends posteriorly to the articulation and takes part in the 

 formation of the articular condyle (figs. 7, 14, 15 B, 7';/), and, as we 

 have seen, in the plecopteran prothorax (fig. 13), it intervenes entirely 

 between both episternum and epimeron. and the coxa, and carries the 

 dorsal condyle of the coxal articulation as well as the anterior one. 



^TIR 



^Apl 



Fig. 13. — Prothorax of Plecoptcra. (Proc. U. S. Nat. Mus., 1909-) 



A, left side of prothorax and base of leg of nymph of Ptcronarcys; pleuron 

 composed of a dorsal anapleurite {Apl) of eupleural arch, and of a trochantin 

 (eutrochantin of Crampton) carrying both anterior and dorsal articulations of 

 coxa (C, a, b). 



B, same parts of nymph of PcrJa. 



C, inner view of right prothoracic pleural plates of nymph of Perl a; a, anterior 

 articulation of coxa ; Apl, anapleurite ; b, dorsal articulation of coxa ; PIR, 

 pleural ridge ; Tn, trochantin. 



We may conclude, therefore, that the basic structure of the pterygote 

 pleuron, as shown at A of figure 15, is identical in plan with that 

 of the apterygote pleuron, as exhibited in Eoscittoinon (fig. 8), and 

 that the pleuron in each group consists of a eupleural series of scle- 

 rites, and of a trochantinal sclerite arching concentrically over the 

 base of the coxa. 



Though we may see, then, a fundamental identity of structure in 

 the pleuron throughout the entire hexapod group, the evolution of 

 the pleural sclerites has been quite different in the Apterygota and 

 Pterygota. In the former, the sclerites show a tendency to reduction 

 in different ways in different families, and the reduction, usually 

 ending in obliteration of some of the sclerites, has produced the 

 various and apparently unrelated pleural patterns characteristic of 



