36 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



dorsals (di), and function as abdominal protractors, since tlieir con- 

 traction lengthens the abdomen by decreasing the overlap of the 

 segments. 



di' de Pc Ac Q d 



Pc Ac 



Fig. 14. — Diagrams illustrating more evolved types of musculature, and seg- 

 mental mechanisms. 



A, dorsal muscles differentiated into internal and external median dorsals 

 {dim, dcin), and internal and external lateral dorsals {dil, del) ; lateral muscles 

 (0 differentiated into tergo-sternal (t-s), tergo-pleural (^/>), and sterno- 

 pleural (s-p) groups; ventral muscles differentiated into internal and external 

 median ventrals (vim, vein), and into internal and external lateral ventrals 

 (vil, vel). 



B, illustrating the compressor and dilator mechanism of an abdominal seg- 

 ment, in which some of the lateral muscles function as compressors {cp), and 

 others, attached ventrally on edge of tergum, become dilators {dlr). 



C, usual arrangement of dorsal muscles as seen in longitudinal section, with 

 internal dorsals {di) attached intersegmentally on antecostae (Ac), and external 

 dorsals {dc) arising on posterior parts of terga, both sets acting as retractors 

 of the terga. 



D, modification by which external dorsals (de), attached in posterior folds 

 of terga, become antagonistic to internal dorsals {di) and act as protractors of 

 the terga. 



The division of the dorsal muscles into median and lateral groups 

 of fibers afifects both the inner and the outer layers (fig. 14 A, B, dim, 

 dil, and dem, del) , but it is not always apparent, and the lateral dorsals 

 are sometimes absent. The line of division of the inner dorsals into 



