30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



the marginal vein, so that in the kymographic figures (pi. 15, fig. 

 41 ; pi. 16, figs. 42, 43, 44) only a single tracing is visible. Arrived 

 above the wing glides somewhat forward, not nearly so much, how- 

 ever as in the lower position, and with the marginal vein in front 

 begins anew a downward stroke. 



The modus operandi of this movement may be interpreted as 

 follows: The downward movement is executed with great force 

 while the surface of the wing lies nearly horizontal, only the anal 

 marginal portion being bent slightly upward. The direction of this 

 movement is nearly vertically downward, or downward and some- 

 what forward. There can be no doubt that the relief of the wing 

 surface, the longitudinal folds and the hair increase the working 

 pressure during the downstroke. The fly thus obtains during the 

 downstroke a strong impulse upward, which overcomes its weight. 



The great breadth of the lower loop of the 8 (pi. 15, fig. 41 ; pi. 

 16, figs. 42, 43, 44, 45; pi. 18, figs. 50, 51) shows that the wing 

 moves upward much slower than downward. During this rather 

 slow upstroke the wings become — if, as is usual, the insect flies for- 

 ward — gliding surfaces, the fly glides forward, the slightest impulse 

 to forward movement being sufficient to make the force of gravity 

 efifective for translation. At the same time the fly, of course, also 

 slightly descends. 



During the backward movement above mentioned the wing op- 

 poses only very little resistance to the air, but still enough to pro- 

 duce the necessary forward impulse. During this phase, also, there 

 is descent. The upper horizontal movement backward is short in 

 comparison with the lower forward one, and is in the main to be 

 considered as the turning and adjustment necessary 10 make the 

 wing ready for the next downstroke. 



The rapidity of the forward progression chiefly depends on the 

 angle of the long axis of the 8 with the horizon, and the insect flies 

 fastest when the downstroke approaches a vertical direction. It 

 can also be said that the progression is the slower the narrower the 

 lower loop of the 8 becomes ; and if it once becomes as narrow as 

 the upper loop the beat of the wing will merely overcome the force 

 of gravity and the insect will hover at the same spot, as the Syrphidae 

 so often do. 



The Function of the Muscles and the Joints 

 The downward movement of the wing is caused by the contraction 

 of the two powerful indirect dorsal muscles. Jurine (1820, p. 95) 



