NO. I 



INSECT THORAX SNODGRASS 



87 



crowded the base of the coxal abductor (M) to the forward part of 

 the eupleural region of the subcoxa, which later becomes the epister- 

 num, and upon this plate arise the abductors of the coxa in all 

 modern insects, except where the pleurum becomes rudimentary. 

 Placed at a mechanical disadvantage by the new position of the coxal 

 articulation, the abductor has regained efficiency in part from the 

 inflection of the pleural articular surface toward the center of the 

 coxal base (fig. 37 A). 



The above outline of what may have taken place in the theoretical 

 transformation of the leg base is purely hypothetical, but it accounts 



Scy: 



B 



Fig. 38. — Theoretical derivation of modern coxal muscles from primitive 

 coxal and subcoxal muscles. 



A, Diagram of theoretical primitive musculature of subcoxa and coxa : 

 a, b, horizontal axis of coxa on subcoxa ; /, /, tergal promotor and remotor 

 muscles of subcoxa ; K, L, sternal promotor and remotor muscles of subcoxa ; 

 M, N, abductor and adductor muscles of coxa, arising within subcoxa. 



B, muscles transposed, posterior coxal articulation (b) shifted to dorsal posi- 

 tion : /, becomes promotor of coxa ; /, becomes remotor of coxa ; K, L, become 

 rotators of coxa ; M , N. remain abductor and adductor of coxa. 



for the present comj)licate(l musculature of the coxa in the more 

 generalized pterygote insects, and it explains those features that give 

 the coxal musculattire an appearance of having been secondarily 

 adapted to the purposes of the coxa. 



A typical generalized coxal musculature of the leg of a wing- 

 bearing segment is shown at figure 37 B ; it is approximately that 

 of the middle leg of a grasshopper (Acridid?e), except that some of 

 the single muscles in the figure are represented by several groups of 

 fiber bundles in the grasshopper. The principal tergal promotor (/) 

 is nearly always a single muscle inserted below on the trochantin 

 (T»), or on the anterior coxal rim when the trochantin is absent. 



