EVASION OF BATS BY MOTHS — BOEDER AND TREAT 463 



selective advantage of evasive action was 40 percent, meaning that for 

 every 100 reacting moths that survived, there were only 60 surviving 

 nonreactors. 



This figure is very high when compared with similar estimates of 

 sui-vival value for other biological characteristics. It seems more than 

 adequate to account for the evolution of the moth's ear through natural 

 selection even if the detection of bats turns out to be its only function. 



CONCLUSION 



As with most investigations, this work raises more questions than it 

 has answered. The role of the B cell remains completely obscure. 

 There is no evidence to connect it with the auditory function even 

 though it is located in the ear, and its regular impulse discharge is a 

 characteristic feature of the tympanic nerve activity of many species 

 of moth (Treat and Boeder, 1959. See also pi. 5). The manner in 

 which the A cells transduce sound waves recurring 100,000 times a 

 second into the much slower succession of nerve impulses remains a 

 mystery, and the synaptic mechanisms whereby information from the 

 A fibers is translated into action by the nervous system of the moth, 

 await investigation. 



During the field experiments it was noticed that many other natural 

 soimds initiated impulses in the A fibers. These included the rustling 

 of leaves, the chirp of tree and field crickets, and, in one instance, ultra- 

 sonic components in the wingbeat sounds made by another moth. 

 Occasionally, the A fibers discharged regularly as if detecting a 

 rhythmic sound, though none was audible to the observers and its 

 source (if any) remains a mystery. There is no evidence that these 

 identified and unidentified sounds are important in the life of a moth, 

 yet it must be said that a moth can detect them, and a careful study 

 of moth behavior in their presence would be of value. 



Several families of moths lack ears and show no response to ultra- 

 sonic stimuli. Some of these, such as the sphinx or hawk moths and the 

 larger saturniid moths, are probably too much of a mouthful for the 

 average bat, and might find no survival advantage in a warning device. 

 Others are of the same size and general habits as the noctuids and 

 might be expected to suffer attacks by bats. Included in this group 

 are some common pests such as the tent caterpillar. It will be inter- 

 estmg to learn whether these forms owe their success in survival to 

 some structural or behavioral countermeasure that compensates for 

 the lack of a tympanic organ. 



In spite of these unanswered questions, we believe that some progress 

 has been made in putting together the sensory information received 

 by an animal, and relating this to what the animal does. That this has 

 been possible in moths is only because of the small number of channels 



625225— C2 31 



