28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



there is a small dorsal sclerite, or lamina supra-analis {sa), and two 

 lateroventral sclerites, or laminae infra-anales (la). These sclerites 

 are lost in adult Odonata, but a small supra-anal lobe, possibly a rem- 

 nant of the lamina supra-analis, projects from beneath the epiproct. 

 A similar lobe occurs in Nesomachilis (fig. 7 D, sa) , as well as in some 

 other Thysanura, and in larvae of Ephemerida. The supra-anal lobe 

 of these insects might be regarded, therefore, as a dorsal remnant of 

 the telson ( fig. 6, Tel) . In most insects, however, no trace of a twelfth 

 segment is to be found, and the periproct must be supposed to have 

 been reduced to the meml)ranous area at the end of the eleventh 

 segment in which the anus is situated. 



III. THE ABDOMINAL MUSCULATURE 



We do not have as yet a sufficient knowledge of the comparative 

 myology of Arthropoda to furnish a basis for any theory as to the 

 nature of the primitive body musculature in this group of animals, 

 in which mobility of the body is a characteristic feature. Widely 

 dififerent patterns of muscle arrangement are encountered in the several 

 arthropod classes, and even within a single class, while, among the 

 insects, extraordinary differences occur often between larval and 

 adult stages of the same species. 



In the Insecta the abdominal musculature consists typically of dorsal 

 and ventral longitudinal fibers, dorsal and ventral transverse fibers, 

 and lateral dorsoventral fibers ; but in none of these muscle groups 

 do all the fibers often retain their characteristic positions. 



The development of the body muscles has been described by Cholod- 

 kowsky (1891), Heymons (1895), and Nelson (1915). The dorsal 

 and lateral muscles are formed from the lateral somatic plates of the 

 mesoderm ; the ventral muscles arise from the median ventral parts 

 of the mesoderm where the somatic and splanchnic layers are united. 

 The muscle rudiments, or anlagen, according to Heymons, in insects 

 having open coelomic sacs (Blattidae, Gryllus, Acrididae), are formed 

 from sac-like evaginations of the mesodermal walls of the seg- 

 mental cavities, which are at first tubular, l)ut sooner or later l)ecome 

 solid strands of cells. In the higher insects, however, in which the 

 coelomic sacs are small or but little developed, the muscles either are 

 formed by the proliferation of cells from the mesoderm segments, or 

 they arise directly from mesenchyme tissue at points corresponding 

 with the position of the coelomic sacs of lower insects. 



Since the muscles are derived from the w^alls of the embryonic 

 coelomic sacs, or from the metameric divisions of the mesoderm, we 



