ARTHUR DEWITT WHEDON 397 



(jrandis is the only exhaustive treatment of the subject, the writer 

 of the present paper has worked out the Anisoptera first, using 

 Wallengren's nomenclature, and what follows on the Zygoptera is 

 the result of comparison with this higher suborder. 



Segmental Muscles of the Sternum: — 



The Primary Longitudinal Sternal Muscles (plate XXI, figures 

 1 and 4, pis.) lie on either side of the nerve chain in segments two 

 to eight, one pair to each. Each muscle is about two-fifths the 

 width of the sternum, the margin of which is a little lateral to that 

 of the nmscle. They arise on the extreme cephalic end of each 

 segment and run parallel to the nerve cord to a similar position on 

 the next segment. The mode of attachment is clearh^ shown in 

 plate XXIII, figure 12. They are very thick, rectangular bands, 

 one and one-half to two times as long as wide, which serve to bend 

 the abdomen ventrally when the members of the pairs work to- 

 gether, and to bend the body to one side when one member con- 

 tracts as the other relaxes. In the Aeshninae they are replaced 

 by two pairs, one of which, the Primary (Plate XXIII, figure 8, 

 pis. and Ipsp) lies nearer the sternum and runs as in the Zj'goptera, 

 the other, the Secondary, dorsal to it runs obliquely outward to 

 the edge of the sternum posteriorly. In certain segments there is 

 also a third pair. 



The Secondary Longitudinal Sternal Muscles hre absent in the 

 Zygoptera. ' 



The Tertiary Longitudinal Sternal Muscles (plate XXI, figure 

 1 , tls) of which there is one pair in each of the first eight segments, 

 are very small and of varying proportions in the different seg- 

 ments; in the first they are very minute and nearly square; in the 

 eighth they are half the length of the segment. There is a gradual 

 change from front to back. The insertion is always on the pos- 

 terior suture very near the median line, and the origin upon the 

 surface of the sternum. The greater part of the muscle, especially 

 in the anterior segments is covered by the Primary Longitudinal 

 Sternal Muscles. 



The Quaternary Longitudinal Sternal Muscles (plate XXI, 

 figure 1 , qls) are still more minute and lie, one pair to a segment, 

 near the pleuro-sternal suture. Their origin and insertion is like 

 that of the last named muscles. They are always completely 

 covered by the Primary Longitudinal Sternal Muscles. 



TRANS. AM. ENT. SOC, XLIV. 



