416 MORPHOLOGY OF THE ABDOMEN IN THE ODONATA 



The Nervous System 

 (Plate XXV, figures 18 and 23) 



The nerve chain is normal. The ganglia are quite close to the 

 front end of the segment as a rule (between the posterior sternal 

 processes), and the connectives are slender and very close together. 

 No marked variation was noted in Megaloprepus. In all forms 

 the first abdominal ganglion is united with that of the metathorax; 

 the double nature of this body can usually be easily seen. 

 The Muscular System 

 (Plate XXV, figures 17, 20, 21, and 23) 



The only figure of the abdominal muscles of the Zygopterous 

 imago known to the writer is that of Plateau (1884), which shows 

 one whole segment and parts of two others adjacent, of the right 

 half of the abdomen. After an attempt to dissect these very 

 slender forms his difficulties can be appreciated. He says, "La 

 dissection des muscles abdominaux des Odonates est assez difficile 

 et ne m'a bien reussi que pour I'Agrion sanguineum." His 

 description is very brief. 



The muscles of all groups are fewer, smaller and simpler in the 

 imagoes than in the larvae. Except in the end segments the 

 tergals are the only ones which are not minute. 

 Segmental Muscles of the Sternum: (Plate XXV, figures 17 

 and 23)— 



A single pair of Longitudinal Sternal Muscles is located at the 

 hind end of each segment except the first, ninth and tenth. In 

 Calopteryx these decrease in length caudad, never being over one 

 third of the length of the segment and usually much less. They 

 are flat and thin, and are widest at their origin, an area near 

 the point where the sternum narrows to form the sternellum. 

 They insert upon the front of the succeeding sternite a short 

 distance a;: art and between the anterior processes. They are 

 widest at the insertion in some forms. These muscles are pro- 

 portionately larger and stronger in the Zygoptera than in the 

 Anisoptera. The females have the muscles of the second segment 

 normal but strong; the males have them modified to aid in the 

 movement of the penis. 



Segmental Muscles of the Tergum: (Plate XXV, figures 17, 20^ 

 21, and 23)— 



As the tergum forms the largest part of the skeleton of each 



