70 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



middle coxa (fig. 34 B, C, D, M^ra) to the thoracic wall; the third 

 (D, B) is the oblique dorsal muscle attached ventrally on the lateral 

 extremity of the postnotum. 



The tip of each vibrating wing describes a figure-8 curve, if the 

 insect is held stationary, showing that the wing in motion undergoes 

 alternating partial rotations on its long axis. Some writers have 

 claimed that this movement results entirely from the pressure of the 

 air on the wing surfaces as the wing vibrates, the flexible posterior 

 margin of the wing being mechanically turned upward during the 

 down-stroke, and downward during the up-stroke. The rotary motion 

 of the wings, however, is necessary to give the forward movement 

 to the insect in the air, and, as may be seen well in the wing of a 

 fly, the nature of the wing articulation causes a deflection of the costal 

 margin to accompany the down-stroke produced by the muscles of 

 the tergum. In some insects, also, pressure at the anterior root 

 of the wing base deflects the costal margin, and for this reason the 

 basalar sclerite and its muscles have been termed the " pronator 

 apparatus" of the wing (Amans). A strong posterior deflection of 

 the wing accompanies pressure on the second axillary in Diptera, 

 and it cannot be doubted, therefore, that in swiftly flying insects the 

 muscle of the subalar sclerite, which pulls finally upon the second 

 axillary, plays an important part in the posterior rotation of the wing 

 during the up-stroke. The subalar muscle, therefore, would act as 

 extensor, depressor, and rotator of the wing. 



The mechanism for the control or modification of forward flight 

 is not definitely known. Those who attempt to explain the tropistic 

 reactions of insects usually assume a differential nervous regulation 

 of the muscles on the two sides of the body; but this explanation 

 could hardly apply to the indirect muscles of flight. It is probable, 

 then, that the insect determines its course through the air by the con- 

 trol of its direct wing muscles, or by changing the slant of the body 

 or the position of the legs, as suggested by Jousset de Bellesame 

 (1879). Some insects are able to arrest their forward flight and 

 suddenly reverse or go sidewise, without perceptibly changing the 

 position of the body, and some are well known for their hovering 

 powers. It is difficult to conceive how a wing structure and wing 

 mechanism so clearly adapted to forward flight can also propel the 

 insect backward or sidewise. The act of hovering on vibrating wings 

 is explained by Straus-Diirckheim (1828) as accomplished through 

 a continued contraction of the subalar muscles, thus checking the 



