24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, 1 23 



says of the adult anisopterous thoracic musculature that it is "highly 

 specialized, powerful, efficient, and is greatly simplified, when com- 

 pared with the larval musculature or the more primitive musculature 

 of an orthopteroid insect." 



The Odonata as a group clearly stand apart from all other winged 

 insects ; the adults were perfect dragonflies in Carboniferous times, 

 but they must have been in the course of their evolution for a vast 

 length of time before their remains are known in the paleontological 

 record. Early in their flying history the dragonflies probably adopted 

 the simple and logical way of moving the wings by the direct pull of 

 muscles on opposite sides of their bases. The method of moving the 

 wings by rhythmically altering the curvature in the wing-bearing ter- 

 gum depends on several adjustments and modifications in the thoracic 

 skeleton to make it practicable. Yet this second method is that evolved 

 by most of the other insects, and probably in the first place was the 

 simplest method of flapping primitive paranotal lobes. 



The legs of an adult dragonfly have two apparent segments in the 

 trochanteral region, but the trochanter of the larva is a single seg- 

 ment with only a suggestion of a proximal constriction. 



V. THE ABDOMEN 



The abdomen of a larval dragonfly varies in shape and relative size 

 in the various groups, but in every respect it is always much different 

 from the adult abdomen. In the larva of Anax Junius (fig. 9 A) the 

 abdomen is about twice the length of the head and thorax together. 

 It is expanded toward the middle and tapers posteriorly to the tips 

 of three horny processes enclosing the anus. There are 10 annular 

 segments ; the anal processes perhaps represent an eleventh segment. 

 The tergal plates are strongly arched, the sterna slightly convex in 

 the passive position (F). 



The chief mechanical functions of the abdomen of an anisopterous 

 larva are respiration and locomotion, both functions being served by 

 the huge gill-bearing sac of the intestine that occupies most of the 

 posterior compartment of the abdominal cavity (fig. '/,rspC). In a 

 live larva the branchial chamber may be seen by transmitted light 

 through the abdominal wall making regular expansions and contrac- 

 tions. Water is drawn into the chamber and expelled by way of the 

 rectum and anus. If the water is slowly released, the larva is merely 

 breathing ; when it is suddenly and forcibly expelled, the larva darts 

 rapidly forward by jet propulsion from the anus. 



The musculature of the branchial chamber consists of bands of 

 longitudinal fibers in its walls running between the gill rows, and of 



