MACHINERY OF INSECT FLIGHT 131 



rapidly, the spike height decreases, and the membrane does not completely 

 recharge between responses. The regular discharge patterns arise in the 

 thoracic ganglion. A record of the action potentials of a single muscle 

 fiber and of thoracic movements is included in Fig. 7E, to show beyond 

 all doubt that there is no phasic relation between the membrane events and 

 the mechanical movements produced by the muscles. 



Response to Electrical Stimulation . 



It is possible to drive the flight mechanism of a fly through stimulating 

 electrodes pushed into the vicinity of the thoracic ganglion (Boettiger, 

 1951). In the best cases a series of wing beats follows each stimulus, 

 making the sound of a short buzz. With increasing frequency of stimula- 

 tion, these fuse into continuous movements, though the buzz is louder and 

 higher in pitch immediately following each stimulus. At 15-20 stimuli per 

 second in Sarcophaga, the movements are of normal amplitude and fre- 

 quency w^ithout modulation. As the frequency of stimulation is increased 

 still further, the flight tone sputters and suddenly the wings stop in either 

 the up or down position. The fly now shows the same responses as one 

 treated with CCl4. 



A record of the thoracic potentials with external leads is shown in Fig. 

 7 before (A) and during (B) driven flight. The normal irregular firing of 

 the muscle fibers gradually changes to a synchronous one as the muscle 

 fibers are recruited to the driving stimulus frequency. During the recruit- 

 ment there was no noticeable change in flight movements. After a short 

 period of driven flight the fly stopped and folded its wings back but the 

 action potentials remained (C). 



These experiments with driven flight prove that, even with simultaneous 

 excitation of the muscle fibers of both sets of antagonistic muscles, the 

 shortening and lengthening cycle of the contractile elements are not altered. 

 Also we may conclude that the high-frequency behavior of these muscles 

 is not the result of an alteration in the number of fibers responding, since 

 the excitatory processes of all fibers were synchronized with the stimuli. 



Under the conditions of driven flight the direct muscles must also be 

 excited. In certain cases it seems that the indirect driving muscles are acti- 

 vated while the direct muscles are not. In Fig. 6A the small vibrations in- 

 dicate activity of the fibrillar muscle, the rapid rise in amplitude at the 

 start resulting from contraction of the direct muscles. Movements of the 

 scutellum and muscle potentials were recorded in an unusual start under 

 external stimulation (Fig. 8). The stimulus is shown to control the firing 

 of the indirect muscle fibers, for only one large action potential follows the 

 stimulus artifact. At the beginning of the record, each stimulus produced 

 only a small slow movement of the scutellum due to its action on some direct 



