MYOGENIC RHYTHMS 103 



(1) How is functional continuity preserved in the change over from 

 a 1 : 1 to a myogenic system in flight and sound production ? 



(2) Is deactivation by release a potential property of all muscle, re- 

 alizable whenever a suitable arrangement is developed in its skeletal attach- 

 ments, or is there a fundamental difference between the contractile mech- 

 anisms of fibrillar and nonfibrillar muscles ? 



Let us examine the first question, dealing first with sound-production. 

 Functionally, it is not difficult to suggest reasons why a higher pulse fre- 

 quency is advantageous in the cicada system. Insect ears respond to sound 

 pulses with impulses at the pulse frequency ; and, if excitation of the female 

 through this nerve has a sexually exciting function, more intense excite- 

 ment should be produced by a higher pulse frequency in the male song. 

 Hagiwara (1953) and Hagiwara and Watanabe (1956) have described 

 an alternative method of at least doubling the pulse frequency in the song 

 of Graptopsalfna nigrojuscata, where the tymbals on opposite sides click 

 exactly in antiphase, the motor nerve discharge being driven by a gang- 

 lionic pacemaker. Physiologically, the requirement for effective deactiva- 

 tion by release appears to be a sufficiently rapid movement of the tymbal 

 at the in click ; since there is no antagonistic muscle whose contractions 

 must be coordinated with those of the tymbal muscle, extra clicks above 

 the 1 :1 ratio with nerve impulses introduce no difficulties in the mechanism 

 as the system evolves toward greater speed of tymbal movement. 



With the flight motor the position is different. The orthopteran case may 

 be taken as typical of the primitive flight systems in insects, and fortunately 

 we have a considerable knowledge of the dynamics and physiology of flight 

 in locusts from the work of Ewer and Ripley (1953) and of Weis-Fogh 

 and Martin Jensen (1956), and in Periplaneta from Roeder (1951) 

 and Sotavalta (1954). The physiological picture of locust flight muscles 

 has been much confused by the experiments of Voskresenkaya (1947), 

 discussed by Chadwick (1953). This worker gave electrical stimuli 

 to a locust thorax in which the motor nerves were still attached to the 

 intact ganglion, and reported various types of rhythmic activity from 

 the flight muscles, not always correlated with the stimuli. Ewer and Ripley 

 (1953) were able to show that some of Voskresenkaya's results were due 

 to after-discharge from the ganglion, but they confirmed that there is not 

 an exact correlation between stimulus and contraction frequencies even in 

 the isolated nerve muscle preparation. Electrical recording from the tergo- 

 sternal muscle showed, however, that the most important phenomenon is a 

 long relative refractory period in the motor nerve so that, with near-thres- 

 old intensities of stimulation through platinum wire electrodes, impulses 

 may be generated at only every second or third stimulus as the stimulus 



