NERVOUS CONTROL OF INSECT MUSCLES 81 



The maximum power of the locust jumping muscle is as great as 20 kg. 

 per gm., nearly ten times better than for mammalian muscle expressed in 

 the same terms. The only satisfactory way of comparing power develop- 

 ment for animals as different as insects and vertebrates is to do it in terms 

 of the mean cross-sectional area of individual muscle fibers. A rough esti- 

 mate for this value compared with frog sartorius shows that each locust 

 fiber develops about the same maximum tension as frog fibers. The differ- 

 ence in length of the fibers accounts for the apparent colossal strength of 

 the insect muscles. 



The summation of successive contractions during an incomplete tetanus 

 is often very marked in insect muscle. The tetanus-twitch ratio in Decticus 

 muscles at 20° C was placed at 10/1 by Solf (1931), and it has a similar 

 value in all locust leg muscles for "fast" fiber stimulation. The ratio is in- 

 creased by lowering the temperature below about 15° C. The durations of 

 the twitches of leg muscles of Periplaneta, Dytiscus, Tettigonia, Locusta, 

 and ScJiistocerca all fall within the limits of 0.1 and 0.2 sec, i.e., of the same 

 order of magnitude as frog muscle. In Locusta the latent period between 

 the action potential peak and the onset of contraction in the relatively long- 

 fibered metathoracic flexor tibiae muscle is 2-3 msec. Peak twitch tension 

 is reached in 0.04 sec. 



The fusion frequency is usually of the same order of magnitude as that 

 of frog muscle, i.e., about 20 per second, but some very high values have 

 been recorded. Kraemer (1929) gives a value of over 50 per second for 

 Dytiscus and Pringle ( 1939) over 70 per second for Periplaneta. 



Stimulation of the second or "slow" motor axon was achieved by Pringle 

 (1939) in the cockroach metathoracic extensor tibiae muscle. After drying 

 out the metathoracic nerve trunk which he designated 3b and then remoist- 

 ening it, he found that the response of the muscle to maximal stimulation 

 of the nerve trunk was markedly changed. Initially there had been a twitch 

 following each stimulus and now there was no mechanical response at fre- 

 quencies below 30 per second, but a smooth extension of the tibia occur- 

 ring at higher frequencies. The response occurred at a precise threshold, 

 so was almost certainly due to a single axon. The rate of extension and 

 final tension increased with rate of stimulation up to frequencies of about 

 300 per second. The results showed clearly that the requirements of tonic 

 and slow actions could be met by a single axon, but they did not reveal the 

 mechanism of action of this axon. 



"Slow" axons supplying the pro-, meso-, and metathoracic extensor 

 tibiae muscles of the locusts Locusta migratoria and Scliistocerca gregaria 

 leave their respective ganglia by separate nerve trunks. Branches contain- 

 ing the axons meet in the coxae before final branches, carrying only the 

 extensor tibiae nerve fibers, leave to supply these muscles, as illustrated in 



