MACHINERY OF INSECT FLIGHT 129 



with one occurring when the wings are loaded with wax. The movements 

 decrease after the cessation of excitation much more rapidly in the fly 

 without wings. The slow decrease found when the wings are loaded shows 

 the effect of inertia. The muscle loses its tension relatively slowly after 

 excitation is stopped, but will continue to vibrate only with an inertial 

 load (see below). 



In addition to improving our understanding of the mechanism moving 

 the wings, the foregoing examples give much information on how the 

 driving muscles must operate : ( 1 ) they shorten very little because of the 

 large mechanical amplification factor; (2) vibrations of high frequency 

 with practically no amplitude are possible; (3) the snap action is not 

 necessary for the operation of the asynchronous mechanism; and (4) a 

 shortened muscle may be easily relengthened for some time after it has 

 shortened. 



The Physiology of Fibrillar Muscle 



Having discussed in some detail the wing articulation and the thoracic 

 component of the flight machinery in one group of insects with the 

 asynchronous system, we may now turn to a consideration of the driving 

 muscles. These consist of two sets acting in opposition upon the thoracic 

 component, as described above. The evidence in the literature suggests 

 that these fibrillar muscles have unique physiological properties. They are 

 capable of very high frequency operation, up to 2,200 cycles per second 

 (Sotavalta, 1953). In flies with thorax open, electrical stimulation pro- 

 duces no visible shortening. That the action potentials from the thorax do 

 not correlate with wing movements has already been noted, as has the in- 

 crease in frequency when the wings are removed. Do these special proper- 

 ties of fibrillar muscle represent a modification of the excitatory or of the 

 contractile mechanism ? 



If the excitatory process is basically modified in fibrillar muscle, the 

 electrical properties of the membrane might show differences when com- 

 pared with other muscles. To test this possibility, microelectrodes were 

 inserted into the fibers of the dorsal longitudinal muscle of flies and both 

 resting and action potentials recorded by the method of Nastuk and 

 Hodgkin (1950). The resting potential was usually about 60 mV and the 

 action potential 80-100 mV, although values as high as 120 mV were ob- 

 served (Boettiger and McCann, 1953). The form of the action potential 

 is shown in Fig. 7D. Several species of v/asp gave very similar action 

 potentials. From the fibrillar muscle of Coleoptera, however, large action 

 potentials were not easy to obtain and did not show clear evidence of over- 

 shoot. In the best preparations, resting and acting potentials of 60 mV 

 were found. The microfibrillar muscle of a few species of moths pro- 



