22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



coxal ligament ^ils-fug is either lacking or very frail in several genera. 

 Cryptocercus alone of the species investigated possesses muscles as 

 yet found in no other blattid ; these are the semiligamentous transverse 

 bands isps-iils and 2sps-2ils, parts of which seem to have survived 

 as septa in some other cockroaches. With these minor exceptions, the 

 differences among the several species are merely variations in rela- 

 tive size and proportions of the various muscles, or occasionally in 

 their position. Such differences, some of them quite conspicuous, ob- 

 viously indicate shifts in functional emphasis, although in most in- 

 stances the details of their interpretation from this viewpoint are 

 obscure. 



In contrast with most modern insects, the cockroaches enjoy a 

 relatively rich ventral intersegmental musculature. Some authorities 

 would be inclined, perhaps, to regard this as a consequence of 

 secondary reduction of sclerotization in the ventral regions of the 

 blattid thorax ; but acceptance of this view would make it very hard 

 to account for the presence of homologous muscles in a number of 

 other groups in which the thorax is extensively sclerotized. There 

 is no muscle recorded in this paper for which either a direct or a serial 

 homolog has not been found in at least one other order of pterygote 

 insects, and most of them are known from several. Coupled with the 

 fact that those other orders that display a similar degree of com- 

 plexity in the ventral intersegmental musculature are the ones con- 

 sidered highly primitive in various other respects, the evidence seems 

 more consistently interpreted in the conclusion that the cockroaches 

 also are primitive in this feature, and that the primitive state of the 

 ventral intersegmental muscles was a complex one. As already indi- 

 cated in the introduction, the various structural patterns preserved 

 for our scrutiny among the more recently evolved orders of insects, 

 which constitute a progressive series of specializations toward greater 

 efficiency in flight, show that improvement in the flight mechanism 

 has been accompanied regularly by reduction in the ventral inter- 

 segmental thoracic musculature. These facts also favor the view that 

 the possession of numerous discrete muscles in this category is a 

 primitive characteristic. The results of a more comprehensive inquiry 

 into these problems will be reported elsewhere. 



SUMMARY 



I. A comparative study was made of the ventral intersegmental 

 musculature of the thorax in 19 species of cockroaches. The observa- 

 tions produced a few corrections, mainly additions, to earlier de- 



