NO. II THORACIC MUSCLES OF COCKROACHES CHADWICK I9 



attachment sites for almost all the ventral musculature, including, of 

 course, the usual longitudinal intersegmental muscles. We have al- 

 ready indicated our belief that the entire endosternal complex, which 

 is ligamentous in consistency, is of muscular derivation ; and we sug- 

 gest here that the sternal ligaments are merely another example of 

 transformed muscles. We may then regard the endosternum schemati- 

 cally as a point of junction of various intersegmental muscles, among 

 which are the usual longitudinal bands and a muscle to the preceding 

 segmental sternum. 



The configuration thus summarized is, however, exactly what is 

 seen in the musculature associated with the sternal arms of pterygote 

 insects. True, the number of elements that impinge upon this focus 

 is less than in the Apterygota ; but those elements that do occur in 

 the Pterygota all have their counterparts in muscles that are attached 

 on the apterygote endosternum in proximity to the attachments of 

 the sternal ligaments, or in similarly directed portions of the endo- 

 sternum itself. Only the fact that the sternal arm of Pterygota is 

 usually a heavily sclerotized ingrowth of the ventral body wall gives 

 the superficial impression of a fundamental difference between the 

 two subclasses. 



In cockroaches, even this distinction breaks down ; for in the 

 prothorax of blattids the paired furcal pits do not give rise at once 

 to sclerotized apodemes, as they do in the mesothorax and metathorax, 

 or in the prothorax of most Pterygota. Instead, there extends inward 

 from the pit a flexible, fibrous ligament that connects with the apex 

 of a sclerotic bar whose other end articulates firmly with the pleural 

 apodeme. Upon this bar, at or near its junction with the sternal liga- 

 ment, are inserted the usual muscles of the furcal complex. 



On the basis of these facts and the considerations outlined above, 

 we suggest that the sternal arms of pterygote insects represent muscles 

 that formerly ran from the segmental sternal region posteriorly to a 

 common junction of various other intersegmental muscles, including 

 the forerunners of the present longitudinal furcal muscles. In the 

 course of evolution, the sternal muscles were replaced first by non- 

 contractile ligaments, a condition still manifest in the Apterygota and 

 in the prothorax of blattids, and finally by sclerotized apodemes, the 

 form in which they now appear in the pterothorax of cockroaches and 

 in all three thoracic segments of most winged insects. These changes 

 in the sterno junctional muscle have not altered the morphological rela- 

 tionships of the other muscles attached at the junction, which may 

 still be regarded as an intersegmental locus in the morphological sense. 



