MACHINERY OF INSECT FLIGHT 127 



shifted to the same axis. The complete cycle took 8 msec. The anterior notal 

 process does not show double movement each stroke, but is rigidly con- 

 nected to the terguni and consequently moves with it, there being no 

 hinged parascutum. Removal of the wings in wasps reduces the amplitude 

 of muscle movement to about one-half, while in flies there may be httle 

 or no change (Fig. 5B). This suggests that wing inertia plays a role in 

 maintaining wing amplitude in wasps, while in flies the snap action in the 

 articulation is more important. 



If the various articulating parts are not held in the proper relation, 

 erratic flight movements result. The sputtering flight under CCI.1 already 

 referred to is an example. Frequently a mounted fly will show similar be- 

 havior, apparently trying difi^erent settings in an effort to attain free flight. 

 In Fig. 4C movements of the scutellum and anterior notal process were 

 caught in a moment when the articulation was not properly adjusted, and 

 so the relation between these erratic movements and the mechanics of 

 normal flight can be determined. In this case the scutellum moved little at 

 the beginning of the down stroke. Not until the anterior notal process 

 moves inward, to allow the union k of this process and the 2nd axillary 

 sclerite to attain the critical point, does the scutellum move rapidly as the 

 result of the recoil of the strained elastic elements. The fast movement 

 begins at the instant the anterior notal process reverses direction. When 

 the inhibition to movement is somewhat greater, the wings may be brought 

 to a sudden stop, as shown by the scutellar movement record of Fig. 5E,F. 

 The stop may last the duration of several cycles. The inhibition to move- 

 ment develops gradually, being greater each cycle until more force is re- 

 quired to move the articulation past the critical point than is generated 

 by the indirect muscles. Movement then stops until balance is again 

 achieved. On the down stroke the stop appears at a different point than on 

 the up stroke. These unusual movements can, therefore, be readily under- 

 stood with our information on the mechanics. 



One of the simplest ways to reveal the snap action is to study scutellar 

 movements with the wings removed. After the removal of the wings, the 

 only resistance to movement is that in the articulation. Once this is over- 

 come at the beginning of each stroke, the movement accelerates to the end, 

 driven by the stored elastic energy- (Boettiger and Furshpan, 1951). 

 Normally the wing acts as a governor to smooth out the stroke. The accel- 

 erating movement, beginning at the critical period, is brought to a sudden 

 halt by mechanical stops. The articulation allows only limited movement 

 of the scutellar lever and consequently of the driving muscles. Mechanical 

 limits to the movement in such a vibrating system may be necessary to pre- 

 vent the tearing of the muscle by the build up of amplitude due to inertial 

 forces. That the setting of the articulation may be altered by injury to 



