86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



articulation being at the ventral anterior angle, and the axis extend- 

 ing obliquely dorsally and posteriorly to the rear articulation. The 

 third segment, a small trochanter-like piece (iTr), is hinged to the 

 second on a longitudinal axis with typical coxo-trochanteral articu- 

 lations. The structure of the proximal part of the acarine leg, there- 

 fore, strongly suggests that the ventrally expanded basis is the sub- 

 coxa, and that the following two segments are the coxa and the first 

 trochanter. Though the axis of the subcoxo-coxal joint is somewhat 

 oblique, its movements are essentially those of abduction and adduc- 

 tion, since the coxal muscles consist of dorsal and ventral antago- 

 nists, both attached here on the ventral plate of the subcoxa, probably 

 on account of the reduction of the lateral wall of the latter. The 

 obliquity of the subcoxo-coxal hinges in the Acarina is clearly an 

 adaptation to allow the legs to move in the plane of the flattened body. 



Assuming, then, a subcoxal segment functioning as the leg base 

 at some remote time in the ancestry of insects, we must postulate 

 either that the present coxal musculature is a new development, or 

 that it has been evolved through a transfer of the subcoxal muscles 

 to the coxa. There is no evidence in support of the first supposition ; 

 there is nothing to contradict the possibility of the second. Abundant 

 evidence is at hand to show that the bases of insect muscles may 

 undergo considerable migrations. If the dorsal promotor and remotor 

 muscles of the subcoxa (fig. 38 A, I, J) were transferred to the coxa, 

 they would become promotors and remotors of the coxa (B) when 

 the posterior coxal articulation (A, b) assumed its dorsal lateral 

 position {B, b) ; but a transfer of the ventral promotors and remotors 

 of the subcoxa (A, A', L) to the coxa (B) would convert these 

 muscles into anterior and posterior rotators of the coxa. The dorsal 

 promotor (/), however, remains attached to the trochantin (B, Tn), 

 a piece of the subcoxa, as long as this sclerite persists ; only when the 

 trochantin is greatly reduced or is lacking does it become attached 

 to the coxa. The flexibility of the free part of tli£ trochantin, or 

 its detachment from the rest of the pleuron allows the trochantinal 

 muscle to function as a coxal promotor. 'Jlie trochantinal hinge with 

 the coxa (a) thus becomes a lifting point rather than an articulation, 

 as has been noted by Crampton. 



The primitive abductors and adductors of the coxa (fig. 38 A, 

 B, M, N) retain their origmal function, but the second becomes 

 attached to the sternum with the suppression of the ventral wall of 

 the subcoxa and is eventually supported on the sternal apophysis 

 (SA) to give it more efficient action. The forward migration of 

 the posterior coxal articulation to a mid-lateral ]K)sition ( l'>, b) has 



