NO. 2 THORACIC MUSCLES OF THE COCKROACH — CARBONELL 5 



tidae (III, p. 171) and the flexibility of the sclerites and membrane 

 contribute to make the movement freer. 



The nearly horizontal position of the coxa and leg make it also dif- 

 ficult to name the movements of the femur and the rest of the leg. 

 What would be a depression of the leg if it were in a vertical plane, 

 becomes here an almost backward movement of the leg which pushes 

 the body forward instead of lifting it. The opposite movement brings 

 the leg forward to its first position. 



On studying the muscles responsible for the described movements, 

 their number and complexity is amazing and it is not always possible 

 to tell the function of a particular muscle. However, according to its 

 origin and insertion it is possible in many cases to determine in a 

 general way the function that a set of muscles performs. The gen- 

 eral plan of the coxal and trochanteral muscles is almost identical in 

 the mesothorax and metathorax, and though a little different in the 

 prothorax, is similar enough to recognize the same scheme as in the 

 other two thoracic segments. On account of that, the following classi- 

 fication has been made : 



Coxal muscles. — The muscles of the coxa include promotors, re- 

 motors, adductors, and rotators. 



The promotors of the coxa are : (i) Muscles that have their origin 

 on the anterior edge of the episternum and insert on the anterior por- 

 tion of the coxal rim, between the articulation with the pleural and 

 trochantinal coxal processes. Present in mesothorax and metathorax 

 (126 in fig. 30; 16/ in fig. 46) ; absent in prothorax. (2) Muscles 

 that take their origin on the anterior edge of the episternum and under 

 side of pleural arm and pleural ridge, and insert on a small sclerite 

 isolated in the membrane, near the rim of the coxa and the pleuro- 

 coxal articulation. Present in the three segments (75, not shown in 

 figures, 12/ in fig. 31, 168 in fig. 48). (3) A muscle that originates 

 on the tergum and inserts on the said small sclerite. Present only in 

 the prothorax (74 in fig. 14). 



The remotors of the coxa are all the muscles inserted on the pos- 

 terior part of the coxal rim and meron, which have their origins on 

 the tergum. In the mesothorax and metathorax the powerful muscles 

 that extend between the meron and the subalar plate work probably as 

 remotors of the leg in addition to their function as alar muscles. In 

 this case both the meron and the subalar plate can function as insertion 

 and origin. A muscle that originates on the sternal arm and inserts on 

 the posterior part of the coxal rim (present in the three segments, 8^ 

 in fig. II, 1^2 in fig. 25, 1/2 in fig. 43) might probably act as a re- 

 motor too. 



