NO. I INSECT THORAX — SNODGRASS 99 



localized center of gravity from now on determined the general form 

 and proportions of the body parts. The specialization of the thorax 

 before wings appeared consisted of structural changes better adapting 

 its segments to the functions of supporting the legs and of giving 

 more efficient attachment and action to the leg mu.scles. 



4. The limbs of primitive arthropods were probably rather simple 

 appendages growing outward from the lateral or pleural areas of the 

 segments between the dorsum and the venter. They probably turned 

 forward and backward on their bases, each being moved by promotor 

 and remotor muscles, fibers of each set arising on the dorsum and on 

 the venter, or on the tergum and the sternum when segmental plates 

 were developed. The second segment of the limb most likely moved 

 in the opposite direction from the basal one, i. e., it turned dorsally 

 and ventrally by a longitudinal axis on the first, and was provided 

 with abductor and adductor muscles. The third segment moved in 

 the same plane as the second, on a longitudinal axis with the latter. 

 The proximal three segments of the primitive limb were the subcoxa. 

 the coxa, and the first trochanter. 



5. As the evolving limb came to need more solid support, the sub- 

 coxa became flattened out in the pleural wall of its segment, and lost 

 its power of motion. A dorsal piece of the distal rim of the subcoxa, 

 bearing the anterior and the posterior articulations with the coxa, 

 separated from the basal part, and the ventral region of the latter 

 degenerated, or united with the edge of the sternum. The subcoxa 

 thus became reduced to a basal eupleural arch and a distal trochantinal 

 arch, the two lying concentrically over the base of the coxa, and 

 having now the status of chitinous elements in the pleural wall of 

 the body segment. 



6. The coxa, thus forced to re])lace the subcoxa as the functional 

 base of the limb, had to adapt itself to its new responsibilities. By 

 a shifting of its posterior articulation with the trochantin to a dorsal 

 position, it preserved for the limb the power of torward and back- 

 ward movement. With the change, however, the coxa, acquired in 

 addition the possibility of a i)artial rotary motion, but, while it re- 

 tained its transverse movements, its abductor muscles lost efficiency 

 through the altered position of the articulation. The assumption by 

 the coxa of the former duties of the subcoxa involved a transfer of 

 the subcoxal muscles to the coxa. The dorsal promotor and remotor 

 muscles functioned still as such when attached to the coxa, but the 

 ventral muscles became rotary muscles, since the coxa was now free 

 to turn on its dorsal articulation. The abductor and adductor mus- 

 cles of the coxa, the first retaining its origin on the subcoxa (now 



