THE VENTRAL INTERSEGMENTAL THORACIC 

 MUSCLES OF COCKROACHES 



By L. E. CHADWICK i 



Department of Entomology 



University of Illinois 



The cockroaches, which have persisted in much their present out- 

 ward form since the Carboniferous, are admittedly rather primitive 

 in many structural respects. Although one dares not take for granted 

 that their musculature also retains a primitive configuration, there 

 is a good a priori chance that this is the case ; and comparisons with 

 other primitive types, such as Gryllohlatta (Walker, 1938, 1943), 

 the larvae of Dytisciis (Speyer, 1922) or Cybister, and larval or 

 adult Megaloptera, to my mind leave no doubt that the muscular 

 pattern of blattids displays many primitive characteristics. 



In this paper attention will be called to certain of these features, 

 as they are seen in the ventral intersegmental musculature of the 

 thorax. This system of muscles, though far from homogeneous 

 morphologically, provides a convenient segment of the total thoracic 

 musculature for analysis. It may also be regarded as itself a relatively 

 primitive component of pterygote anatomy, for study of the more 

 recently evolved, highly specialized flying insects shows the ventral 

 intersegmental muscles of the thorax gripped in an evolutionary trend 

 that has already led to the drastic reduction of these muscles and that 

 may ultimately result in their total disappearance. In contrast, the 

 cockroaches and other less advanced forms still exhibit a wealth of 

 muscles in this category, and thus afford some conception of this 

 portion of the ancestral basis from which the more adept flying insects 

 of today have developed. 



Descriptions of the thoracic musculature have already been pub- 

 lished for three blattid species: Blatta orientalis L,, by Miall and 

 Denny (1886) ; Periplaneta australasiae (L.), by Maki (1938) ; and 

 P. americana (L.), by Carbonell (1947). Miall and Denny purposely 

 gave only a general account ; and comparisons of the reports by Maki 

 and Carbonell discloses more numerous and in some instances more 



1 Formerly Chief, Entomology Erancli, Chemical Warfare Laboratories, Army 

 Chemical Center, Md. 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 131, NO. 11 



