492 Comparative Animal Physiology 



of the source of the reflecting material a bat might be able to locate an object 

 with considerable accuracy. This method of echo location is directly com- 

 parable to that of a radar ranger and localizer which broadcasts pulses of very 

 high frequency radio waves and from the echo permits very accurate deter- 

 mination of both the distance and direction of a reflecting surface. The system 

 of the bat diff"ers from radar primarily in that the waves emitted are high fre- 

 quency sound rather than high frequency radio waves. 



Hartridge's theory of avoidance of objects by bats was proved in a series 

 of papers, -"• ^^- ^^ and has since been confirmed by other investigators. 

 It is definitely known that during flight bats of the genera Myotis, Eptesicus, 

 and Pipistrelliis emit brief cries of a sound frequency between 30 and 70 kc. 

 and perhaps also of higher frequencies, with the highest intensity usually 

 at 50 kc. Each cry has a duration of 0.001 to 0.002 second, and the cries 

 are emitted about 10 times a second when the bat is hanging on a wall, about 

 30 times per second when it is flying in unobstructed space, about 50 times 

 per second when it is approaching an object, and usually only 30 times per 

 second just before it flies past an object. The frequency of each cry begins 

 at 70 kc. and drops to 30 kc.''* The emission of cries is facilitated by 

 modification of the pharyngeal ventricles into pockets which act as drumlike 

 acoustic resonators for ultrasonic frequencies. Bats also emit audible sounds 

 of 7 kc. and audible "clicks." These sounds are apparently used for commu- 

 nication with each other. The ultrasonic frequencies used are not of unusual 

 occurrence in our daily life (e.g., key jingling, up to 98 kcr air moving 

 through a small opening, up to 80 kc), but bats are the only animals known 

 to make such use of them. 



Bats which have one or both ears covered or which have their mouths 

 covered so that their ability to receive or emit sounds is impaired are unable 

 to avoid obstacles as well as normal bats do. In fact, such bats show Httle 

 inclination to fly at all, whereas normal bats can fly between 1 mm. wires 

 stretched 30 cm. apart, with relatively few collisions. In studies of the 

 microphonic eff^ect in the cochlea Galambos has found that responses are 

 obtained to an upper limit of 98 kc, the limit of the apparatus but below 

 that of the bat. 



There has been a considerable amount of discussion recently on how a bat 

 is able to locate objects so close that the reflected sound would be heard at 

 almost the same time as the emitted sound; i.e., in the ear the cry and the 

 echo would overlap. For instance, a bat can detect a glass plate at a distance 

 of 2 feet. One suggestion is that bats might use the "trailing edge" rather 

 than the "leading edge" of the series of waves which constitute a cry.'" 

 Others are that bats (1) may use the Doppler eff'ect, (2) may decrease the 

 duration of the signal as they approach an object, or (3) may have the ears 

 screened from the source, i.e., from the larynx.-' The two latter possibilities 

 seem most likely. 



PHONORECEPTION BY ARTHROPODS 



Receptors in Insects 

 Aside from the vertebrates, the insects comprise the only group of animals 

 in which phonorcccption has been intensively studied. I lowever, although 

 insects probably comprise the only group in which it is highly developed, 



