NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 61 



the hind leg there are two muscles in the abductor group (fig. 38 C, 

 12^, 126), one being a small anterior muscle (12^), apparently acces- 

 sory to the promotor (A, 118), the other a large posterior muscle 

 (C, 126) which unquestionably functions as an abductor. 



The pleural muscles associated with the functional abductor mus- 

 cles of the coxa in the wing-bearing segments are attached on the outer 

 rim of tlie coxa (fig. 43 B, M', M") and, in adult insects, arise typi- 

 cally on the epipleural basalar and subalar sclerites (figs. 37 A, p8, pp ; 

 38 B, 128, I2p), and function as wing muscles. In some adult insects, 

 as in Panorpa (fig. 14 B), the first of these muscles (-M') arises on a 

 dorsal lobe of the episternum (Ba), which is clearly the homologue of 

 the basalar plate or plates in other insects, such as are present in the 

 adult of Dissosteira (fig. 26, Ba). The posterior epipleural muscle 

 (M") is always attached to the subalar sclerite in adult insects (figs. 

 14 B, 2,7 A, 38 B, Sa). Both muscles, however, in the nymph of 

 Dissosteira (fig. 2y C, M', M") and in other nymphal Orthoptera, 

 arise directly from the upper edge of the pleuron, one on the epis- 

 ternum, the other on the epimeron, and, if they act together, they 

 must be abductors of the coxa. The epipleural muscles, therefore, 

 appear to be groups of coxal abductor fibers that have become specially 

 developed as secondary wing muscles in the adult. The first is a 

 pronator-extensor of the wing; the second is the depressor-extensor 

 of the wing (fig. 49, M', M"). 



The foregoing analysis of the basal leg musculature of Dissosteira 

 shows that the coxa is provided with six sets of muscles, including 

 an anterior and a posterior group of fibers arising on the tergum 

 (fig. 43 B, I, J), an anterior and a posterior group arising on the 

 sternum (K, L), a lateral group arising on the pleuron (M, and also 

 M' and M" in the wing-bearing segments), and a mesal group arising 

 on the sternum (A''). The anterior and posterior dorsal and ventral 

 muscles may be supposed to represent the theoretical primary tergal 

 and sternal promotors and remotors of a primitive limb basis (fig. 6, 

 /, /, K, L), which have become transferred to the coxal region (fig. 

 11) after the subdivision of the basis into subcoxa and coxa. The 

 lateral and mesal muscles, therefore, are subcoxo-coxal muscles, the 

 fibers of the first {M) retaining their origins on the subcoxal pleuron, 

 those of the second {N) having been transferred to the sternum, 

 perhaps by the incorporation of the ventral rim of the subcoxa into 

 the definitive sternal plate. 



Muscles that move the telopodite. — The muscles that operate the 

 telopodite, or that part of the leg beyond the coxo-trochanteral hinge, 

 6 



