456 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



companied by the simultaneous activity of millions of sense cells, nerve 

 cells, muscle libers, and glands. Even if it were possible to register the 

 traffic of nervous and chemical information generated and received by 

 each and all of these neural elements during the behavior, it is doubtful 

 whether the record would provide a meaningful description of the 

 action. 



Even though these problems cannot be solved directly at the present 

 time, they become less formidable if the behavior selected for study is 

 simple and stereotyped, and only a small number of nerve cells are 

 concerned in its execution. These conditions are partly fulfilled by the 

 sensory mechanisms whereby certain nocturnal moths detect the ap- 

 proach of insectivorous bats. 



ECHOLOCATION AND COUNTERMEASURE 



Bats detect obstacles in complete darkness by emitting a sequence of 

 high-pitched cries or chirps and locating the source of the echoes. As 

 Griffin (1958) and others have shown, this form of sonar is unbeliev- 

 ably precise. By means of it, insectivorous bats locate and track flying 

 moths, mosquitoes, and small flies (Griffin et al., 1960) . North Ameri- 

 can bats, such as Myotis lucifugus and E'ptesicus fuscus^ emit chirps 

 about 10 times a second when they are cruising in the open. Each chirp 

 lasts from 10 to 15 milliseconds (msec.) with an initial frequency of 80 

 kilocycles (kc.) dropping about one octave in pitch toward its end 

 (see pi. 3, fig. 2). 



The frequencies in these chirps are ultrasonic, that is, inaudible to 

 human ears, which cannot detect tones much above 15 to 18 kc. The 

 higher frequencies used by bats make possible more discrete echoes 

 from smaller objects. The chirps can be rendered audible by detecting 

 them with a special microphone and rectifying the ultrasonic com- 

 ponent. They then can be heard through headphones as a series of 

 clicks. These clicks fuse into what Griffin has called a "buzz" when 

 the bat is chasing an insect or avoiding an obstacle. 



Several families of moths (in particular the owlet moths or Noctui- 

 dae) have evolved countermeasures enabling them to detect the chirps 

 of bats. A pair of ultrasonic ears is found near the "waist" of the 

 moth between thorax and abdomen (pi. 1, fig. 1). An extremely thin 

 eardrum or tympanic membrane is directed obliquely backward and 

 outward into the recess (dark area) found at this point (pi. 1, fig. 2). 



Internal to the eardrum is an air-filled cavity that is spanned by a 

 thin strand of tissue running from the center of the eardrum to a 

 skeletal support (pi. 2, fig. 1). This tissue contains the sound- 

 detecting apparatus, consisting of two acoustic sense cells (A cells). 

 A single nerve fiber arises from each A cell and passes close to the 



