MACHINERY OF INSECT FLIGHT 141 



chronous type the usual direct correlation exists between the nerve im- 

 pulses and the muscle response. In those insects possessing fibrillar flight 

 muscle the mechanism operates in a different manner. During the active 

 state initiated by each nerve impulse a variable number of contraction cycles 

 may occur. Studies of this mechanism were made on the large fly Sarco- 

 phaga bullata. Recordings of action potentials from the muscle with ex- 

 ternal and internal leads show that processes involved in building up the 

 active state are the same in fibrillar muscle as in many other striated 

 muscles. 



The special properties of the asynchronous mechanism result from the 

 nature of fibrillar muscle and the mechanical system with which it functions. 

 A snap action is demonstrated in the articulation that is shown to depend 

 upon the relations of the first two axillary sclerites with three components 

 of the basic thoracic structure. Records of movements of these components 

 during flight reveal their action under a variety of conditions and features 

 of the physiolog}' of fibrillar muscle as well. 



Using the longitudinal flight muscle of the bumble bee, Bombiis, it was 

 possible to study the mechanical properties. Isometric twitch is small. A 

 smooth tetanus is obtained at 40-60 stimuli per second. Tension-length 

 relations of maximal isometric contractions are typical but show that 

 shortening is limited to 10% of muscle length. In situ the shortening is 

 mechanically limited to 1%. 



Only when the active muscle is allowed to shorten with an inertial load, 

 or is rapidly stretched after a rapid shortening, are its special properties 

 revealed. In the first case the dynamic tension-length relation is a loop, 

 the area of which represents the work done in one cycle against viscous 

 damping forces such as the movement of the wings through the air. The 

 response to rapid stretch when the muscle has less than the tension charac- 

 teristic of its length (as after rapid shortening) , in contrast to that of other 

 muscles, shows that the muscle may be stretched to full length while ten- 

 sion rises to only a fraction of its initial value. Following the attainment of 

 full length the tension rises rapidly. 



The presence of special mechanical properties in the mechanism moving 

 the wings is correlated with the operation of fibrillar muscle. Together 

 these features constitute the principle machinery to move the wings. 



REFERENCES 



Boettiger, E. G., 1951. Stimulation of the flight muscles of the fly. Anat. Rec. Ill, 443. 

 Boettiger, E. G., and E. Furshpan, 1950. Observations on the flight motor of Diptera. 



Biol. Bull. 99, 346. 

 Boettiger, E. G., and E. Furshpan, 1950. Observations on the flight motor of Diptera. 



Fed. Proc. 10, 17. 



