RELATION BETWEEN PREY AND PREDATOR — ROEDER 289 



in man-made alarm systems, and that analysis of evasion systems 

 might be likely to lead to a more complete picture of the neural basis 

 of behavior of obvious survival value. The physiology of three eva- 

 sion systems in insects has been partially explored. Although our 

 information about them is still most inadequate, they will serve as 

 a basis for the present discussion. 



The abdominal nerve cord of the cockroach, Periplaneta americana, 

 contains several so-called giant fibers which occupy about 12 percent 

 of its cross-sectional area (Roeder, 1948). The largest of these fibers 

 is about 30 microns in diameter, exceeding in this respect the largest 

 (alpha) nerve fibers in the mammalian nervous system. In addition 

 to the giants the nerve cord shows a relation of fiber diameter to fre- 

 quency which is not dissimilar to that found in vertebrates (pi. i), 

 and it must be concluded that the total number of fibers in the cock- 

 roach nerve cord is much less. The giant fibers, which take up so 

 much space in proportion to their number, are the internuncial units 

 in an alarm reaction (Pumphrey and Rawdon-Smith, 1937; Roeder, 

 1948) which will be discussed below. A second example of a small 

 number of units participating in an alarm reaction and at the same 

 time performing more complex tasks is to be found in the jumping 

 muscle of the locust. Hoyle (1955a) has shown that the largest 

 muscle in the body of Locusta migratoria, the extensor tibiae of the 

 metathoracic leg, is supplied by three motor nerve fibers, only two of 

 which have a clear motor function. A third example, this time of a 

 sensory nerve, is provided by the auditory organ which enables cer- 

 tain families of moths to detect the approach of predatory bats (pi. 2, 

 fig. i). It contains only two acoustic receptor cells (Eggers, 1938) 

 and one accessory cell of undetermined function (Roeder and Treat, 

 1957). Analogous mechanisms in vertebrates involve thousands of 

 separate nerve fibers. 



In order to appreciate the relative merits of nerve systems composed 

 of a few large units versus those composed of many small units we 

 must consider the factors afifecting impulse transmission in a single 

 nerve fiber. A single axon, however large, can convey only a limited 

 amount of information from one point to another because of the on-ofif 

 or all-or-none nature of the nerve impulse. Therefore, several small 

 fibers are able to convey a much greater range of information than a 

 single large fiber simply because with several units the coding possi- 

 bilities are greater. To offset this disadvantage the single large fiber 

 conveys its information more rapidly, the velocity of impulse conduc- 

 tion being roughly proportional to fiber diameter. Since the three 

 systems mentioned above are all involved in the detection of, and 



