NO. I INSECT THORAX — SNODGRASS 25 



thorax immediately below the edge of the tergum, above and behind 

 which is the segmental spiracle (Spo). Arching over the base of the 

 coxa is a chitinized fold which should represent the trochantin (Tn), 

 though it is continuous basally with the sternum before the articulation 

 of the latter with the coxa (Cr). Anterior to the trochantin are two or 

 three chitinized pleural areas (A, B, /, m) continued from the apoto- 

 mal folds of the sternum {i, j, k) . The apotomal folds constitute a 

 specialization in the Japygidse, but the true pleural parts apparently 

 are reduced to the trochantinal fold over the coxa. 



In Lepisma the thoracic pleural regions have well-defined chitinous 

 plates. In the mesothorax and metathorax, two sclerites (fig. ii, r, 

 Tn) arch concentrically over the base of the coxa, while a third 



Fig. II. — Mesothoracic pleuron, edge of tergum, and base of leg of Lepisma. 



Cx, coxa; r, eupleural sclerite; s, sclerite on base of coxa; Tn, trochantin; 

 T, tergum. 



smaller one {s) is attached to the rim of the coxa itself. The first is 

 evidently a sclerite of the eupleural series. It is a narrow chitinous 

 band, with a short dorsal arm projecting upward and posteriorly, to 

 the end of which is attached a nuiscle. The second sclerite {Tn) has 

 the position of the trochantin in Euscntouwn (fig. 8), though it fits 

 closely over the upper end of the coxa, and has no special articular 

 points with the latter. It is larger than the dorsal sclerite, triangular 

 over the coxa, and expanded again where it overlaps the anterior 

 angle of the coxal base. The third sclerite {s) is a small triangular 

 piece closely attached to the base of the coxa, and is probably a 

 detached piece of the latter. It suggests VerhoetY's " coxopleure " 

 in Lithobius (fig. 9, q), but is apparently not the sclerite so designated 

 by Verhoeff in Lepisma. In the i)rothorax, the same pleural sclerites 

 are present, though they are here less distinct and not easily separated. 



