lOO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



the pleuron), but the second being transferred to the sternum, still 

 act in their original capacities. The dorsal promotor is the last sub- 

 coxal muscle to be given over to the coxa, being retained by the 

 trochantin until this sclerite becomes rudimentary or disappears. 



7. Before the wings appeared, or while they were developing into 

 organs of flight, the tendency of the subcoxa, now the chitinous 

 pleuron of the segment, apparently was to break up into various pieces 

 that would allow flexibility to the lateral segmental wall. In the 

 Apterygota, the eupleural and trochantinal arches remain separate, 

 and both have become variously reduced. In the Protura the eupleural 

 arch has broken up into. a number of sclerites corresponding in a 

 general way with the eupleural plates of the Pterygota, perhaps indi- 

 cating that the proturan ancestors were nearly related to the ancestors 

 of the Pterygota ; but in the other Apterygota the eupleuron has 

 become degenerate, and suggests no evolution toward the pleural 

 pattern of the Pterygota. In the early Pterygota, the eupleural arch 

 became divided into a dorsal plate, an anterior plate before the coxa, 

 and a posterior plate behind the coxa. The trochantin, still carrying 

 the coxal articulations, remained an independent sclerite for a while, 

 but its dorsal part later united with the dorsal eupleurite. The com- 

 ])ound plate thus formed then became strengthened by an internal 

 ridge extending upward from the dorsal articulation of the coxa, 

 and the corresponding external suture divided the plate into epister- 

 num and epimeron. The anterior part of the trochantin has become 

 detached in many insects to form a free sclerite still bearing the 

 anterior coxal articulation. In the higher pterygote groups the evolu- 

 tion of the pleuron has been toward a reunion of the sclerites to form 

 a solid segmental wall, especially in the wing-bearing segments. 

 Where the free remnant of the trochantin is lost, the coxa may ac- 

 quire a ventral articulation with the sternum, as in some of the 

 Apterygota. 



8. When the thorax became set apart as a locomotor region, the 

 overlapping of its segmental plates and the telescoping of its segments 

 became a handicap to its functional development. To counteract its 

 mechanical weakness, the muscle-bearing parts of the sterna were 

 transferred from the anterior margins of the sternal plates to the 

 posterior margins of those preceding, thus bringing about a reversal 

 in the segmental relations of the ventral muscle attachments in the 

 thorax from those in the abdomen. When f ureal arms were later 

 developed, the muscle attachments were farther transferred in part 

 or entirely to these processes, and the poststernal parts were then lost 

 or absorbed into the posterior margins of the ])rinciiial sternal plates. 



