RELATION BETWEEN TREY AND PREDATOR — ROEDER 293 



in a trajectory 30 cm. high and 70 cm. long, wherefore it develops 

 the astonishing tension of 20,000 g. per g. of muscle. At other times 

 it develops quite gradual and gentle contractions while participating 

 in walking and running. In vertebrates a similar range of tensions 

 in a comparable muscle, such as the gastrocnemius, depends upon the 

 presence of several hundred motor nerve fibers each supplying a small 

 group of muscle fibers. Graded tensions are produced when varying 

 numbers of these motor units are excited in the central nervous sys- 

 tem. However, the locust extensor tibiae is supplied with three motor 

 fibers, only two of which appear to be directly excitatory to the muscle 

 fibers. The fast or F fiber innervates all the muscle fibers and pro- 

 duces a near-maximal twitch when an impulse passes along it. A short 

 burst of impulses produces a slightly higher tension, but in any case 

 the locust can only jump when the F fiber is excited, and the F system 

 is clearly concerned with the all-out effort of escape. The slow or Si 

 nerve fiber innervates only 30 percent of the muscle fibers, and single 

 impulses in it fail to produce any significant degree of muscle shorten- 

 ing. However, the Si fiber operates through temporal summation — 

 the reciprocal of the intensity to frequency coding encountered in 

 sense organs. Thus, 10 impulses per second in the Si fiber cause a 

 smooth, gradual tension to develop in the muscle. Further increases 

 in impulse frequency cause corresponding smooth increases in muscle 

 tension, which becomes maximal at about one-quarter of the F-gen- 

 erated tension when 150 impulses per second are passing down the Si 

 fiber. Combinations of activity in F and Si seem able in this way to 

 provide the wide range of tensions and speeds encountered when 

 this muscle participates in predator evasion and normal walking. 



From this brief discussion it seems clear that the great survival 

 value of speed to the prey and the inherently slow conduction in insect 

 nerves have played a part in reducing the number of nerve units con- 

 cerned in a startle reaction to an extreme minimum. Although much 

 discriminatory capacity is thereby sacrificed, a considerable amount 

 of information can still be coded by the two-unit systems of the moth 

 and locust. 



Conduction along nerve fibers consumes only part of the time re- 

 quired for an animal to respond to a stimulus. The rest is taken up 

 by synaptic and neuromuscular delays, temporal summation at syn- 

 apses, and possibly by hitherto unrecognized neural phenomena. It 

 may well be asked if that fraction of the response time occupied by 

 impulse conduction is sufficiently large so that differences in conduc- 

 tion velocity could alter the startle response time to a significant 



