MACHINERY OF INSECT FLIGHT 133 



physiology of the fibrillar muscle must be studied in an isolated muscle 

 prep^iration by the classical methods. 



The Meclianical Properties of Fibrillar Muscles 



The discovery of the snap action in the articulation and the demonstra- 

 tion of its importance in normal flight suggested to us the early experi- 

 ments of Gasser and Hill (1924) on the responses of the frog muscle to 

 quick changes in length. The theory was proposed by Boettiger and Fursh- 

 pan (1950) that the opposing longitudinal and vertical indirect flight 

 muscles were both in complete tetanus. By this it was not meant that the 

 tension is smoothly maintained, but that the muscle is kept in the active 

 state as described by Hill (1949). In such a tetanus or maintained active 

 state, tension is a function of length and of the velocity of shortening. The 

 opposing muscles cannot usually neutralize each other, since the position at 

 which they would have the same length is unstable ; because of the setting 

 of the articulation, one muscle would be lengthened and the other short- 

 ened when flight begins. In the shortening muscle, tension would be ex- 

 pected to fall rapidly after the critical period. On the return stroke the 

 antagonist would quickly lengthen the muscle. To obtain work from such 

 a system the reappearance of tension during the lengthening must be 

 delayed, so that at each length the muscle has greater tension while short- 

 ening than lengthening. 



The first step in testing this theory was to show that fibrillar muscle 

 could be put into typical isometric tetanus. Since we already had a body 

 of information on flies, the first experiments were performed on one of the 

 anterior vertical muscles of Sarcophaga. McEnroe (1952) reported that, 

 with one end of the muscle detached and coupled with a recording lever, a 

 steady tension of 250 mg. could be obtained during tethered flight. The 

 tension increased before the start, was continued during flight, and slowly 

 disappeared at the end of flight. Upon stimulation the muscle went into 

 tetanus. Much larger isometric tensions were later obtained in large Taba- 

 nids (McEnroe, 1954). 



The original theory was therefore substantiated to some extent. As far 

 as the excitatory process was concerned, the muscle was in a maintained 

 active state similar to tetanus during flight. The changes in tension neces- 

 sary for the production of the wing cycle must result from the changes in 

 length, in the speed of shortening, and in the speed of lengthening. Pringle 

 (1954) found similar responses to stimulation in the tymbal muscles of 

 those species of cicadas possessing fibrillar muscle to produce high-fre- 

 quency tones. He accounted for the relengthening of the muscle without 

 the usual rise in tension by assuming a deactivation by release. The de- 

 activated state was considered to last a short time, requiring immediate 



