NO. II THORACIC MUSCLES OF COCKROACHES — CHADWICK 7 



discussed under the spinabdominal muscles. (See e, this section, 

 below). 



c. Spinacoxal muscles. — The first and second spinae both carry 

 posterior rotators (or remotors) of the preceding coxa (isps-cxi, 

 2sps-cx2) in all blattids examined. A corresponding muscle of the 

 third spina is absent as such, but may be represented, as already sug- 

 gested by Maki (1938), in the muscle jug-cxs post, rot., which fre- 

 quently shows signs of a dual composition. In some specimens, a few 

 of the fibers of fus-cxs post. rot. appear to be continuous with those 

 of the ligament jn^-jus. Only in larval Dytiscus (Speyer, 1922), in 

 Gryllohlatta (Walker, 1938, 1943), in Zootermopsis (fig. 6: 2p), and 

 in the larvae of Cybisfer and Corydalus has a distinct muscle, Ssps- 

 cxs, been found ; and in these species the muscle jus-cxs post, rot., 

 which is also present, seems to be a single band. 



Spinal promotors of the mesocoxa and metacoxa (isps-cxs, 2sps- 

 CX3) also occur frequently in cockroaches, as they do in other primi- 

 tive forms. However, the mesocoxal promotor is absent in Blaberus, 

 Diploptera, Leucophaea, Macro pane sthia, Nauphoeta, and Pycnosce- 

 lus; and the last four of these genera also lack the metathoracic 

 homolog. Both spinal promotors are likewise missing in Cryptocercus, 

 which shares to some extent the tendency of these genera toward 

 hypertrophy of the transverse muscles of the first spina although it 

 differs markedly from them in certain other respects. In some other 

 cockroaches, e.g., in Blattella, the spinal promotors, though present, 

 are weak. Thus, the trend toward obliteration of these muscles, which 

 has gone far among higher orders of insects, is evident even among 

 the Blattariae. 



d. Spinaspinal muscles. — The muscles isps-2sps and 2sps-" ^sps" 

 were found in all the species studied here, although they are at times 

 weakly developed and easily overlooked; this is particularly true of 

 2sps-"ssps." The first of these muscles is present also in most Or- 

 thoptera {sensu stricto) and Mantodea, but not in other orders with 

 the possible exception of Isoptera, where it was recorded by Fuller 

 (1924, fig. 9, muscle n) for Termes latericius Hav. Since this muscle 

 does not occur in other termites studied by Maki (1938) and the 

 writer, it may be that Fuller misjudged the posterior attachment of 

 his muscle n, which perhaps represents isps-fus, a muscle missing 

 from Fuller's account but present in all Isoptera studied by others. 



The muscle 2sps-"ssps" has so far been recorded only from blat- 

 tids, where its general occurrence may be taken as one more indica- 

 tion of primitive structure. What is probably a vestige of this muscle 



