36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



median ventral rim has either become membranous, or has united 

 with the edge of the sternum. In the Acarina, the ventral part of the 

 subcoxa has formed the large plate in the ventral wall of the body 

 to which the leg is attached, while the lateral part has been reduced 

 to a narrow arch over the coxal base. 



The assumed change in the coxal axis from a horizontal to an 

 oblique position, when the coxa became the functional base of the 

 limb, suggests a reason for the detachment of a supra-coxal piece, 

 the trochantin (fig. i8 B, Tn), from the body of the subcoxa, for 

 it is clear that the displacement of the axis would be facilitated if the 

 part of the subcoxa bearing the articular condyles became a free 

 sclerite. Borner's view that the ventral articulation of the coxa was 

 originally with the sternum, and that the trochantin is derived from 

 the sternum is contradicted by the fact that in Protura (fig. 8) and 

 in the prothorax of Plecoptera (fig. 13) the trochantin is a- free 

 sclerite bearing both the anterior (ventral) and the dorsal articula- 

 tions of the coxa. In scattered cases, in both the Apterygota and the 

 Pterygota, the coxa is articulated ventrally to the sternum, but in 

 most such instances this is clearly a secondary condition. That the 

 primitive axis of the coxa was horizontal is attested, furthermore, by 

 evidence, to be presented later, that the primitive musculature of the 

 coxa consisted of abductor and adductor muscles. In the head, the 

 mandible retains this form of articulation, and a simple abductor- 

 adductor musculature. 



It is not difficult to imagine the probable evolution of the basal part 

 of the subcoxa into the eupleural sclerites. In the Apterygota and in 

 the Chilo]>oda, this jjart of the subcoxa has broken up into small 

 plates forming various patterns in dififerent families. In Eoscntonwn, 

 probably three at least of the ventral series of pleurites (fig. 8, /, g, h) 

 belong to the leg base, and may be supposed to be remnants of the 

 eupleural part of the subcoxa (fig. 18 C). These sclerites, as Prell 

 (191 3) has pointed out, correspond in position with the divisions of 

 the pterygote pleuron (fig. 18 J), one being supracoxal in position 

 (Apl), and the other two (Acx, Pcx) precoxal and postcoxal. In 

 the Chilopoda (figs. 9. 32 B), the pleural pattern is variable, and 

 perhaps has little relation to that in any insect, but apparently there 

 are to be distinguished in it both eupleural and trochantinal sclerites. 

 In the Thysanura there is little uniformity in the pleural structure: 

 Lcpisiua (figs. II, 18 D) has a single eupleural plate; in Japyx 

 (figs. 10, 18 E) the only true jileural chitinization appears to be the 

 trochantin; in Machilis (fig. 12) one plate is present over the coxa, 

 but its relation to the pleurites of other Apterygota is not clear. 



