24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



evolved groups. However, direct or serial homologs of all the ele- 

 ments occur in one or more other orders of winged insects. The most 

 extensive complements of these muscles are found in those forms, such 

 as larval dytiscids, Grylloblattodea, and Megaloptera, that are regarded 

 as primitive on the basis of other criteria. It is concluded, therefore, 

 that the blattids also are primitive in respect to the ventral interseg- 

 mental muscles ; and that possession of a rich ventral intersegmental 

 musculature was characteristic of the early Pterygota. 



ACKNOWLEDGMENT 



The writer is grateful to Dr. R. E. Snodgrass for helpful discussion 

 of several questions considered in this paper. 



GLOSSARY OF ABBREVIATIONS 



cv cervical sclerite 



ex coxa 



eps epistemum 



/m furca, furcal arm, segmental sternal apophysis 



Us intersegmental laterosternite 



ph phragma, or the primary dorsal intersegmental fold 



from which the phragma is derived 

 post, rot posterior rotator, a functional designation used to 



distinguish certain leg muscles 



s segmental sternum 



sept septum 



sps spinasternite or spina 



t segmental tergum 



tent tentorium 



ventr. diaphr ventral diaphragm 



X cruciate, used of a muscle whose origin and insertion 



are on opposite sides of the midline 



For the way in which these abbreviations are compounded into designations 

 of muscles, see section on Method and Material in text. 



REFERENCES 

 Barlet, J. 



1954. Morphologic du thorax de Lepisma saccharina L. (Apterygote Thy- 

 sanoure). II. — Musculature (a*"" partie). Bull. Ann. Soc. Ent. 

 Belgique, vol. 90, pp. 299-321. 

 Carbonell, C. S. 



1947. The thoracic muscles of the cockroach Pcriplancta americatta (L.). 

 Smithsonian Misc. Coll., vol. 107, No. 2, 23 pp., 8 pis. 

 Crampton, G. C. 



1926. A comparison of the neck and prothoracic sclerites throughout tlie 

 orders of insects from the standpoint of phylogeny. Trans. Amer. 

 Ent. Soc., vol. 52, pp. 199-248. 



