120 INVERTEBRATE PHYSIOLOGY 



this group shows an intermediate condition with respect to the formation 

 of the muscle, the flight muscles developing from both preformed fibers 

 and free myoblasts. 



The asynchronous system presents a basic problem in muscle physiology, 

 for excitation and contraction appear to be dissociated. The key to the solu- 

 tion of this problem lies in the peculiar mechanical system in which the 

 muscle operates. We must therefore begin our analysis with the mechanics 

 of the flight. 



The Mechanics of Flight 



When flies are placed in a jar containing CCI4 vapors, flight movements 

 are induced at a certain level of anaesthesia. A number of other volatile 

 compounds will produce a similar result, but only in CCU does the flight 

 tone sharply increase in frequency. At a certain level the movements be- 

 come erratic and sputtering, suddenly coming to a stop with the wings 

 either up, as at the end of a normal up stroke, or down, as at the end of a 

 down stroke. All intermediate positions are unstable. Considerable resist- 

 ance is encountered if one attempts to move the wings from the stable po- 

 sitions. However, when a critical point is reached, the wings snap without 

 further aid to the end of the stroke (Boettiger and Furshpan, 1952). Be- 

 cause of its similarity to the action of a common noise maker this was called 

 the click mechanism (Boettiger and Furshpan, 1950). 



Movements of the wings under CCI4 can best be achieved by pressure 

 upon the scutellum. The remarkable feature of these wing movements is 

 that the wings, on being snapped up and down, appear to follow the normal 

 flight path with proper changes in angle of attack and in direction during 

 the stroke. Evidence for a similar snap action during flight was obtained, 

 and so it was suggested that CCI4 by its effect on the flight muscles, direct, 

 indirect, or both, sets the articulation as in normal flight. Therefore, if the 

 positions of the structural components are studied in CCl4-treated flies 

 with the wings in the up and in the down positions, the mechanics of flight 

 may be worked out (Boettiger and Furshpan, 1952). 



The mechanical system moving the wings in flight is composed of the 

 articulation and the thoracic component. As these are bilateral structures, 

 their movements will be described on only one side. The thoracic com- 

 ponent shown in Fig. 1 consists of the scutellar lever and the tergopleural 

 elements that move it, the anterior notal process with the parascutum to 

 which it is attached, and the mesopleural process. 



Scutellar Lever : The tergum or upper part of the thorax is hinged 

 at j-k and rests at its posterior end on the joint at i, Fig. lA. On the down 

 stroke contraction of the longitudinal indirect muscle (5, Fig. 2) pulls the 

 articulation i of the scutellum a forward and slides the tergum posteriorly, 



