100 INVERTEBRATE PHYSIOLOGY 



Attempts in Cambridge to isolate an indirect flight muscle in order to 

 elucidate the mechanism of the rhythmic activity were unsuccessful, and 

 attention was therefore directed to the sound-producing muscle (the 

 tymbal muscle) of cicadas, which appeared to be histologically similar to 

 the indirect flight muscles. The results of this work (which was done in 

 Ceylon on Platypleura capitata) have now been published (Pringle 

 1954a,b). The hypothesis there put forward is somewhat different to that 

 suggested by the flight-muscle work in 1949 ; it is that these fibrillar muscles 

 do not differ from ordinary striated muscle in the way in which they de- 

 velop tension and can shorten after the arrival of motor nerve impulses, 

 but that, when they are connected to a nonlinear elastic system in the 

 skeleton which produces sudden shortening at a critical tension, a process 

 takes place in the contractile machinery ("de-activation by release") which 

 changes its properties for a short time to those of the unexcited muscle. 

 In this "de-activated" condition the muscle can be passively extended by 

 small forces such as the residual elasticity of the tymbal skeleton, and the 

 redevelopment of tension after the deactivation interval (aided perhaps 

 by a sudden stretch at the end of the interval) continues the activity in a 

 myogenic rhythm. It was suggested that Boettiger's click mechanism in 

 the dipteran thorax makes this explanation adequate also for the myogenic 

 rhythm of the indirect flight muscles. 



It may perhaps be useful to discuss these ideas in more detail in order to 

 clarify some of the terminology used. Fig. 1 shows diagrammatically the 

 course of events which are supposed to take place when a long-fibered 

 muscle is excited through its motor nerve. The impulse travels along the 

 nerve fiber (1) to the neuromuscular junction (2) where it initiates the 

 junctional phenomena ; we are not here concerned with the nature of neuro- 

 muscular transmission. The junctional processes lead to a local depolariza- 

 tion of the surface membrane of the muscle fiber (3), which in turn may 

 produce a propagated action potential. In arthropods this appears to be 

 rare, but instead a multiplicity of nerve endings on each muscle fiber pro- 

 duce local depolarizations over a wide enough area of surface to have much 

 the same effect ; in any case the depolarization appears to be the event which 

 initiates further changes inside the muscle fiber. These surface changes 

 lead to events of largely unknown nature (4) which finally induce activity 

 in the contractile machinery (5) and may move the skeletal elements (6) 

 and the external environment. 



In this complicated and only partially understood sequence of phe- 

 nomena it is easy to introduce confusion by inexact terminology. I would 

 like to suggest that we should agree on a definition of words. Excitation, by 

 analogy with the similar phenomenon occurring in nerve fibers, should be 

 limited to the processes taking place in the muscle surface membrane ; 



