i6 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



The median coxal muscles are represented in insects by both dorsal 

 (M) and ventral (N) fibers. In the wing-bearing segments of ptery- 

 gote insects the first comprise three distinct groups of abductor fibers 

 (M, M', M") arising dorsally on the pleuron, and inserted ventrally 

 on the coxa both anterior and posterior to the dorsal articulation of 

 the latter (c). The second and third muscles of this group (M', M") 



Sex 



Ptar 



Fig. 12. — Thoracic pleural sclerites of Apterygota. 



A, lateral view of left side of mesothorax of Acerentomon doderoi (from 

 Berlese, 1910). B, thorax, base of head, and base of abdomen of Isotoma sp. 

 (from Ewing, 1928). C, left mesothoracic leg turned forward, and lateral re- 

 gion of mesothorax of Accrentulus barberi (from Ewing, 1928). 



c, dorsal articulation of coxa ; Cx, coxa ; d, ventral articulation of coxa ; Fm, 

 femur; H, head; IT, I IT, first and second abdominal terga; Ptar, pretarsus; 

 S, sternum ; Sex, subcoxa ; Ti, T2, Ts, thoracic terga ; Tar, tarsus ; Tb, tibia ; 

 Tr, trochanter. 



become wing muscles in the adult by the partial or complete detach- 

 ment of the epipleural areas on which they arise to form the basalar 

 and subalar plates of the wing base. The ventral median muscle of the 

 coxa (N) is present in insects that lack a ventral coxal articulation; 

 it arises on the sternal apophysis and functions as a coxal adductor. 



In the Apterygota the subcoxa becomes rudimentary. In most of 

 the Protura its sclerotization is reduced to two slender plates arched 

 concentrically over the base of the coxa (fig. 12 A, C, Sex), as shown 

 by Berlese (1910) and by Ewing (1928), though Prell (1913) has 



