40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



While most of the lateral muscles are compressors of the abdomen 

 (fig. 14 B, cp), since they serve to approximate the sternum to the 

 tergum in each segment, some of them, at least in insects that make 

 active respiratory movements, serve as dilators (dlr). The lateral 

 dilators become mechanically antagonistic to the compressors by rea- 

 son of the fact that their points of origin are on the lower edges 

 of the terga ventral to their insertions on the overlapped edges of 

 the sterna. As in the case of the sternal protractor muscles of the 

 abdomen, the effectiveness of the dilators is commonly increased 

 by the dorsal extension of their points of insertion on apodemes of the 

 sterna. 



There is no evidence to suggest that any of the lateral abdominal 

 muscles of adult pterygote insects are derived from the primitive body 

 muscles of the lost appendages. In larval forms that retain appendage 

 rudiments on the abdomen, the lateral muscles lie mesad of the limb 

 bases (figs. 34 A, 36 C, /), attached above on the tergum and below 

 on the sternum. The persisting muscles of the abdominal appendages 

 pertain to the distal movable parts of the organs, and these muscles 

 take their origins within the limb bases (figs. 34 A, 36 D) . Exceptions 

 to this occur in the case of the muscles of retractile vesicles of holo- 

 metabolous larvae (fig. 36 C, D, rvs), which take their origin on the 

 dorsum, but these muscles are not retained in the adult. The branchial 

 muscles of ephemerid larvae (fig. 15 A, bmcls) are said to persist in 

 the adult stage, but they do not appear to correspond with any of 

 the lateral muscles in other pterygote insects. 



The transverse muscles. — The transverse muscles of the abdomen 

 are best known as the muscles of the dorsal and ventral diaphragms 

 (fig. 13 C, td, tv). It seems probable that primitively these muscles 

 were intersegmental in position, their fibers being attached on the 

 intersegmental folds, one set being dorsal, the other ventral. 



The fibers of the dorsal transverse muscles arise typically in groups 

 on the anterior edges of the lateral parts of the abdominal terga, and 

 spread mesally to their insertions along the ventral wall of the heart. 

 Only in a few insects are they evenly distributed along the entire 

 length of the tergum, or collected into anterior and posterior groups. 

 The usual anterior origin of the fibers, therefore, suggests that the 

 dorsal transverse muscles are primarily intersegmental. In the cater- 

 pillars (fig. 21 A, td) they practically have this position, except that the 

 diverging inner ends of the fibers spread into the anterior and posterior 

 parts of the adjoining segments. Usually the muscles of the dorsal 

 diaphragm extend from the second to the eighth or ninth abdominal 

 segment, but in the Blattidae they are said to occur not only in the 



