NO. I INSECT THORAX — SNODGRASS 7I 



rotatory motion of the wings and preventing their forward drive. 

 The lifting power, then, needs only to counterbalance the weight of 

 the insect's body. 



The wing musculature of the Ephemerida shows clearly that the 

 mayflies belong to the non-odonate branch of the Pterygota. Their 

 thoracic musculature, according to the account of Diirken (1907), 

 includes typical longitudinal and oblique tergal muscles (fig. 28, 

 A, B), and tergo-sternals (C), the two sets constituting indirect de- 

 pressors and elevators of the wings. The mayflies, however, do not 

 flex the wings, and as a consequence the episternal and epimeral coxal 

 muscles retain their primitive function as movers of the coxae. The 

 principal difference in the thoracic musculature of the Ephemerida 

 and that of Orthoptera is in the greater number of muscles that arise 

 on the epimeron, or on the epimeral region of the pleuron. There 

 is no question of the truth of Diirken's statement that the musculature 

 of the mayflies clearly separates the Ephemerida from the Odonata, 

 but his claim that it separates them also from the Orthoptera does 

 not appear to be warranted by his own descriptions. The Ephemerida 

 are certainly, however, the most primitive of the non-odonate branch 

 of the Pterygota. 



The highly specialized wing musculature of Odonata has been de- 

 scribed by Poletaiew (i88r) and by Lendenfeld (1881). The usual 

 tergal and tergo-sternal muscles are completely lacking in the thorax 

 of the dragonflies, and each wing is provided with a set of direct 

 muscles which efifect all its movements. These muscles are of secon- 

 dary development in the nymph, according to Poletaiew. They com- 

 prise identical sets of muscles in each of the wing-bearing segments, 

 alike in all dragonflies. According to the elaborate descriptions of 

 Lendenfeld, there are eight muscles to each wing. One of each set 

 arises on the tergum, the others arise from ventral marginal ridges 

 of the pleura or from processes of these ridges ; they are all inserted 

 either on the bases of the wing veins or on plates directly associated 

 with the wing base. 



In the Isoptera the dorsal longitudinal muscles of the thorax are 

 degenerate, but the direct wing muscles, which are highly developed, 

 show that the termite thoracic musculature is of the orthopteroid 

 type. Fuller (1925) describes the muscles of the thorax of winged 

 termites, but he does not explain how flight in these insects is sus- 

 tained, even feebly, by a mechanism in which the principal motor 

 elements appear to be lacking. 



