22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



to facilitate their use as organs for entrapping flying prey on the wing, 

 and the wings come to have a posterior position on the back of the 

 thorax, where the weight of the head and thorax is balanced against 

 that of the long, slender abdomen. The union of the two segments 

 converts the pterothorax into a strong framework for the support of 

 the large muscles of flight. The distortion of the thorax is compen- 

 sated by the extension of the mesothoracic episterna on the back in 

 front of the wings, and that of the metathoracic epimera upon the 

 venter behind the legs. The obliquity of the wing-bearing segments 

 gives to the wing muscles also a strong posterior slant, in contrast to 

 the more usual forward slant of these muscles in other insects. It is 

 well known that the wing mechanism of the Odonata differs from that 

 of other insects in that the muscles that produce the up-and-down 

 movements of the wings are inserted in such manner that they pull 

 directly on the wing bases on opposite sides of the supporting pleural 

 fulcra. A good account of the odonate wing mechanism, briefly stated, 

 is given by Sargent (1951). It is, therefore, possible, but undemon- 

 strated, that the posterior slant of the wing segments has some favor- 

 able relation to the special mechanism of the wings ; it at least gives 

 greater length to the wing muscles. As a flying machine, the dragon- 

 fly is unsurpassed. 



Inasmuch as the essential characters of the adult thorax are already 

 present in the thorax of the larva, and are evidently not of any special 

 use to the larva, it would seem that the larval thorax represents an 

 early developmental stage of the imaginal thorax, and, therefore, by 

 contrast with the head and the abdomen, undergoes merely a progres- 

 sive change rather than a reconstructive change at the transformation 

 to the adult. 



The larva possesses the usual two pairs of thoracic spiracles, but 

 only those of the first pair are functional for respiration. These spira- 

 cles (fig. 8 A, iSp) are relatively large and lie in the upper parts of 

 long peritremal plates {B,Ptr) between the pronotum and the meso- 

 thoracic episterna. The external orifice of each spiracle is a vertical 

 slit opening into an oval atrium {C,Atr), from which is given off 

 directly a large tracheal trunk (removed in the figure), and, from its 

 lower end, a smaller branching trachea. A short occlusor muscle 

 (ocmcl) inserted on the ventral margin of the atrium has its origin 

 on the peritreme below the spiracles. Dragonfly larvae sometimes 

 crawl partly out of the water, or entirely at their transformation, and 

 at such times they must breathe through these open anterior spiracles. 

 The other spiracles of the thorax and the abdomen serve on the newly 



