94 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



ones of which are so arranged as to bring the peak of the curvature 

 in the tergum on a transverse Hne between the wing bases. 



The Odonata are commonly said to have a wing mechanism quite 

 different from that of other insects. On the basis of von Lendenf eld's 

 (1881) description of the odonate wing muscles, the dragonflies have 

 been supposed to be equipped with a special set of muscles inserted 

 directly on the wing bases. A study of the thoracic musculature in 

 either the Anisoptera or the Zygoptera, however, will show that there 

 are ony two small muscles that can be regarded as special wing 

 muscles ; one of these is accessory to the pronator of the wing, the 

 other to the depressor. The large pronator and depressor muscles, 

 though they arise ventrally on the lower edge of the pleuron and are 

 inserted directly on the two basal plates of the wing, are evidently the 

 homologues of the basalar and subalar muscles of other insects. Two 

 smaller muscles lying mesad of the pronator are clearly leg muscles 

 since they have their origins on the coxa and their insertions on the 

 extreme lateral edge of the tergum. Von Lendenfeld ascribed these 

 muscles to the wings ; he describes them as arising on the pleuron and 

 as inserted on the wings. Each wing has a homologue of the flexor 

 muscle in other insects, though it does not function as such because 

 of the lack of a flexor mechanism in the base of the odonate wing. 

 The tergo-sternal muscles are highly developed, their ventral attach- 

 ments are on the sternum and their dorsal attachments on the antero- 

 lateral lobes of the tergum. The dorsal longitudinal muscles are re- 

 duced to a pair of small, divergent fiber bundles attached anteriorly 

 on the median apodemal spine of the tergum, and posteriorly on the 

 anterior margin of the following tergum. The wing mechanism of 

 the dragonflies is thus merely an extreme modification of that common 

 to all insects. 



A wing, in order to be an efficient organ of progressive flight, must 

 be capable not only of an up-and-down movement, but also of anterior 

 and posterior movements accompanied by a partial rotation on its 

 long axis. The anterior margin of the wing must be brought forward 

 and deflected during the down-stroke, and lifted with a posterior 

 movement during the up-stroke. The rotary movement of the insect's 

 wing is caused partly by the structure of the wing itself and its re- 

 sponse to air pressure, and partly by the nature of the wing articula- 

 tion on the body, but it is greatly augmented by muscles that pull 

 downward on the base of the wing, one before the pleural fulcrum, 

 the other behind it. These muscles are inserted on the basalar and 

 subalar sclerites beneath the wing base (fig. 48, Ba, Sa). Two of them 



