NO. 6 



INSECT ABDOMEN— SNODGRASS 



45 



into internal and external layers of fibers, and in most cases a well 

 marked separation between median and lateral fibers in each of these 

 groups. In a general statement on the abdominal musculature of 

 orthopteroid insects, Ford (1923) says: "In the common ancestors 

 of the orthopteroid insects the tergal musculature probably consisted 

 of two broad layers, an internal longitudinal and an external oblique, 

 with the inner layer approximately equaling the length of the terguni, 

 and the outer layer much shorter. Of the present-day orders the 

 Blattaria approach closely this hypothetical type." Of the ventral 



PN7 



14-0 



/ / / 

 Pc 14-5 14-3 



Fig. 16. — Musculature of the right half of the first five segments of the ab- 

 domen of Dissostcira Carolina, together with dorsal muscles (//-') of meta- 

 thorax. (Compare with fig. 15 B.) 



140-igj, median internal dorsals; ijO-igS, lateral internal dorsals; 143-202, 

 median ventrals ; 144, first lateral internal ventral ; 775, first lateral external 

 ventral. 



musculature Ford says : " The hypothetical type of sternal muscu- 

 lature is similar to the tergal, having the two-layered arrangement 

 of longitudinal ental and oblique ectal muscles. The Blattaria again, 

 resetnble the hypothetical type." 



Since the external muscles of the dorsum and venter are not alwavs 

 oblique, being often parallel with the internal muscles, and since, 

 furthermore, the dorsal and ventral muscles are not always differ- 

 entiated into external and internal layers (fig. 15 A), it would seem to 

 the writer more probable that all the dorsal and ventral fibers were 



