EVASION OF BATS BY MOTHS — ROEDER AND TREAT 457 



skeletal support, where the pair is joined by a third nerve fiber arising 

 from a large cell (B cell) in the membranes covering the support. 

 The three fibers continue their course to the central nervous system of 

 the moth as the tympanic nerve. 



The traffic of nerve impulses passing over the three fibers from A 

 cells and B cell to the nervous system of the moth can be followed 

 if a fine metal electrode is placed under the tympanic nerve. Another 

 electrode is placed in inactive tissue nearby. As each impulse passes 

 the site of the active electrode it can be detected as a small action 

 potential lasting about 1 msec. Since the tympanic nerve contams 

 only three nerve fibers, it is not difficult to distinguish and to read 

 out the respective reports to the nervous system from the pair of A 

 cells and the B cell. A similar experiment in a mammal is practically 

 meaningless since the auditory nerve contains about 50,000 nerve 

 fibers. 



This method of detection shows that the A cells transmit organized 

 patterns of impulses over their fibers only when the ear is exposed to 

 sound (Roeder and Treat, 1957) . The B cell transmits a regular and 

 continuous succession of impulses that can usually be distinguished 

 from the A impulses by their greater height. The B impulses are 

 completely unaffected by acoustic stimulation, and change in fre- 

 quency only when the skeletal framework and membranes lining the 

 ear are subjected to steady mechanical distortion (Treat and Roeder, 

 1959). The B cell behaves in a manner similar to receptors found 

 in other parts of the body that convey infonnation about mechanical 

 stress on joints, muscles, and skeleton. The role of such a receptor in 

 the ear of a moth is unknown. 



In the absence of sound, the A cells discharge irregularly spaced and 

 relatively infrequent impulses (pi. 2, fig. 2, A). A continuous pure 

 tone of low intensity elicits a more regular succession of more frequent 

 impulses in one of the A fibers (pi. 2, fig. 2, B). The other fiber is 

 not yet affected. Any slight increase in the intensity of the tone 

 causes a corresponding increase in the impulse frequency of the active 

 fiber. Wlien the intensity of the tone is increased to about tenfold 

 that producing a detectable response in the more sensitive A fiber, the 

 second A fiber begins to respond in like manner. Its action potentials 

 are superimposed on those of the first (pi. 2, fig. 2, C and D) by the 

 method of recording, but actually reach the central nervous system 

 over their own pathway. This experiment reveals two of the ways 

 in which the moth ear codes somid intensity. It is like an instrument 

 having a graded fine adjustment (the intensity-frequency relation) 

 and a coarse adjustment of two steps (the pair of A cells). Other 

 ways of coding intensity will appear later. 



