NO. II THORACIC MUSCLES OF COCKROACHES — CHADWICK 23 



scriptions of 3 of these species. In general, there are very minor 

 differences among the species in respect to the presence or absence 

 of individual muscles in the category studied, although there are 

 numerous differences, some conspicuous, in the relative size and pro- 

 portions of the various muscles. 



2. The present ventral intersegmental thoracic muscles had their 

 primary attachments on the spinae, on the intersegmental latero- 

 sternites, or on the forerunners of the furcal apophyses. Cockroaches 

 still have two typical spinae and definite vestiges of a third. They 

 possess an extensive spinasternal musculature. Remnants of the 

 musculature of the intersegmental laterosternites are present, but some 

 of these muscles now have segmental attachments, and others are 

 represented by noncontractile ligaments. The longitudinal furcal 

 muscles are equivalent to those of other pterygote insects. 



3. Attention is called to the postcoxal ligaments that run between 

 the furcal apophysis and the immediately following intersegmental 

 laterosternite in each of the thoracic intersegments, and to the sig- 

 nificance of these former muscles in the development of the postcoxal 

 bridges of higher insects. 



4. Evidence and arguments are presented for the hypothesis that 

 the furcal apophyses represent former muscles that have been replaced 

 in phylogeny by sclerotized apodemes. It is suggested that one at- 

 tachment of these muscles was on the segmental sternum, the other 

 at a common intersegmental junction of several other muscles, among 

 them elements of the longitudinal ventral group. Loss of movement 

 at the sternal insertion led first to transformation of the stemo- 

 junctional muscle into a fibrous ligament and eventually to the scle- 

 rotization of the ligament. Analogous events have occurred fre- 

 quently in the evolution of the pterygote thorax. The blattid 

 prothorax exemplifies a stage in the evolution of the typical furcal 

 apophysis when the postulated sternal muscle was still in a liga- 

 mentous condition. The subsequent sclerotization of this ligament, 

 which has occurred in the other thoracic segments of cockroaches and 

 in all three segments of most Pterygota, in no way alters the morpho- 

 logical relationships of the other muscles inserted upon its central 

 end; morphologically, then, the present attachments of the longitudi- 

 nal ventral muscles on the furcal arms are still intersegmental, and 

 it is therefore unnecessary to invent mechanisms whereby they might 

 have been shifted from an intersegmental to a segmental site of 

 attachment, 



5. The ventral intersegmental thoracic musculature of cockroaches 

 is rich in number of elements, compared to that of more recently 



