NERVOUS CONTROL OF INSECT MUSCLES 95 



It consists of implanting small fixed electrodes through the cuticle, with 

 fine, insulated copper wires attached. The animal is allowed the freedom 

 of a 2 ft. sq. "Perspex" double- walled cage within which it trails the leads 

 along with it. This makes possible the study of the muscle impulses (elec- 

 tromyogram ) , which are amplified and recorded on moving paper by an 

 electroencephalograph machine, and observation of the animal and the 

 electrical activity in a particular muscle or muscles at the same time. Two 

 pairs of leads are usually possible without seriously disturbing the animal 

 and recordings can be made from antagonistic muscles. 



Records from the metathoracic extensor tibiae muscle are particularly 

 easy to interpret, since they consist of a single series of action potentials 

 due to Si, except in the rare event of jumping. They show that impulses 

 effecting tonic contraction occur at the low rates of 10-20 per second but in 

 very irregular trains. Bursts of firing at rates up to 50 per second are 

 associated with maneuvering, and even in vigorous marching they do not 

 exceed peaks of 100 per second. 



Records from the antagonist flexor tibiae muscle are complex, reflecting 

 the compound nature of this muscle. Activity consists probably of "slow"- 

 fiber activity in one or more units reinforced by bursts of "fast"-fiber ac- 

 tivity, again with one or more units in action. 



The mesothoracic extensor tibiae muscle shows tonic activity at about 

 20 per second due to the "slow" fiber and occasionally increases up to about 

 100 per second during movement. The bursts of high-frequency activity 

 during movement are usually reinforced by brief bursts of 4-8 impulses 

 due to the "fast" fiber. The antagonist flexor muscle, like that of the meta- 

 thorax, reflects its multiple-unit nature by giving records of a similar, 

 complex nature. 



Antagonist muscles are frequently used against each other, as Elftman 

 (1941) has calculated that they must be used in human walking, in order 

 effectively to decelerate a rapid movement, and in other ways. 



Conclusion 



Most of the work described had been done on locust limb muscles, but 

 evidence has been presented which indicates that the mechanisms utilized 

 in locusts are possibly present universally in insects in the skeletal muscles. 

 This is particularly likely to be true of the "fast" system, and there seems 

 no reason why the locust "slow" systems could not be utilized by other 

 insects ; in fact it does not seem necessary to require much modification of 

 the locust machinery to work say a Drosophila leg. 



The "slow" system utilizes the same muscle fibers as the "fast" by bring- 

 ing a proportion of them into contraction, each in a progressive manner. 

 There is no justification for accepting Wilson's scheme of separate "slow" 



