NO. II THORACIC MUSCLES OF COCKROACHES — CHADWICK I3 



nate muscles {"eps^'-fuiX, "epSi'-cxiX) that insert on the contra- 

 lateral profurca and procoxa. These conditions in P. hrunnea are 

 illustrated in figure 14. Here it will be noted that the origin of the 

 cruciate furcal muscle {8) is more central than that of the cruciate 

 coxal muscle (p). In P. australasiae (fig. 15: 8) the more dorsal 

 bands of the furcal muscle originate so near the midline that they 

 appear to arise from the spina; and they are so recorded in Maki's 

 (1938) description. 



The contrasting arrangement of the corresponding muscles in P. 

 americana is apparent in figure 16. In this species, there is no visible 

 remnant of the ligament isps-iils, and the cruciate muscles {8, p) 

 originate far laterally, on the anterior margin of eps^, as they were de- 

 scribed by Carbonell (1947). Hence the structure of P. australasiae 

 and P. americana is superficially quite distinct. Access to intervening 

 forms, such as P. hrunnea, etc., shows, however, that in P. australasiae 

 and P. americana we are merely confronted with rather extreme vari- 

 ations in the arrangement of morphologically identical elements. 



Cryptocercus is unique among the cockroaches studied in that the 

 most dorsal fibers of the transverse muscle of the first intersegment, 

 isps-epS2, continue across the body without attachment on the spina 

 (fig- 13: 5)- This development is almost certainly secondary, for 

 the more ventral fibers of this muscle have the usual spinal con- 

 nection. 



The several variants we have seen in the cruciate muscles are such 

 that all of them could have been derived, by gradual transition, from 

 any one chosen as a starting point. There is also at present no bar 

 to the alternative assumption that any or all of them might suddenly 

 have arisen de novo from each other or from an unknown basic 

 pattern as a result of gene mutation or recombination. Therefore, a 

 decision as to which of the existing configurations portrays the most 

 primitive condition is not warranted on the basis of the evidence so 

 far presented. Although Cryptocercus shows some very primitive 

 features in the first thoracic intersegment, it is even here less primi- 

 tive in other respects than certain other cockroaches, and should not 

 be regarded as the prototype for the arrangement of the cruciate 

 muscles unless independent confirmation can be produced. Other data 

 that bear on this question are cited in this section, b, below. 



At the present time, it also does not seem possible to decide whether 

 muscles such as isps-iils and isps-eps^ are fundamentally distinct, or 

 whether the episternal branch is no more than a hypertrophied off- 

 shoot from an originally single transverse band. Both elements are 

 present simultaneously in a few other insects, not all of which are 



