398 MORPHOLOGY OF THE ABDOMEN IN THE ODONATA 



Segmental Muscles of the Tergum: — 



The Primaiy Longitudinal Tergal Muscles (plate XXI, 

 figures 1 and 4, pit) are similar to the opposing sternals. They 

 are widest anteriorly and diverge a little toward their insertion, 

 leaving an inverted triangular space in which the chamber of the 

 heart lies, and into which the Tertiary Longitudinal Tergal 

 Muscles project from above and forward. They are replaced 

 in the Aeshninae by two pairs as in the Sternals. They occupy 

 the dorso-lateral aspects of the tergum, and are pre'^ent in all but 

 the tenth segment. They attach to the suture-folds as do the 

 Sternals. 



The Secondarj^ Longitudinal Tergal Muscles are absent. 



The Tertiarj' Longitudinal Tergal Muscles (plate XXI, figure 

 1, tit) lie in pairs beneath and between the last named, along the 

 mid-dorsal line flanking the heart. They are very short, not over 

 one-third the length of the segment, and about two-thirds as wide 

 as long. Their origin is upon the face of the pleurite.and their 

 insertion is upon the suture-fold dorsal and slightly internal to 

 that of the last named muscles. 



The Quaternary Longitudinal Tergal Muscles (plate XXI, 

 figure 1, qlt) are larger than the Tertiary being approximately 

 half the length of the segment and half as wide as long. They 

 originate on the surface of the tergite above the ventral side of the 

 Primary Longitudinal Tergals and run straight caudad to insert 

 upon the usual region of the suture-fold. They are in position 

 opposed to the sternal muscles of the same name. 



In the first, ninth, and tenth segments many of the regular 

 sternal and tergal muscles are replaced by others for the anchoring 

 of the abdomen to the thorax and the movement of the anal ap- 

 pendages. In all probability these were originally derived from 

 the Primary Sternals and Tergals (unless we take the ground that 

 the latter were derived from the intersegmental muscles common 

 in other groups of insects) but they are now mostly intersegmental 

 and separate names are desirable. 



The Ventral Sternal Thoracico-Abdominal Muscles (plate XXI, 

 figure 1, vsta). There are two pairs of these ventral muscles, an 

 outer and slightly dorsal one, and an inner one partly covered by 

 the outer. The inner has its origin just lateral to the median 

 line of the central part of metasternite ; the outer a little posterior 



