THE LANGUAGE OF ECHOES 



milliseconds. As illustrated in Fig. 11, a typical orienta- 

 tion sound contains only about 50 sound waves, no 

 two exactly alike. The wave length of the initial waves 

 is only half the wave length of those making up the end 

 of the emitted sound. These sounds are chirps, at least 

 that is what we call audible sounds made by certain in- 

 sects when they sweep through as wide a range of fre- 

 quencies within a fraction of a second. This type is 

 sometimes called a frequency-modulated pulse of sound, 

 and this group of bats may be thought of as "FM bats" 

 in contrast to the horseshoe bats with their much longer, 

 sharply beamed tones of nearly constant frequency, and 

 the faint but complex cHcks of the tropical "whispering 

 bats." 



Echoes of Insect Prey 



The FM or chirping bats have been studied much 

 more thoroughly than the other two groups; therefore, 

 more is known about them. They seem to be the most 

 highly specialized for a life of flight, very expert at ma- 

 neuvering under the most difl&cult conditions. The daily 

 (or nightly) business of catching insect food compels 

 them to be highly skilled in the detection of such small 

 moving objects and in the aerial acrobatics necessary to 

 intercept them. Since bats do almost all their hunting on 

 dark nights, often approaching insects from above or in 

 wooded areas where they would have to be seen against 

 a dark background, visual detection must be impossible. 

 And SpaUanzani, as we have said in Chapter 1, showed 

 before 1800 that blind bats catch as many insects as 

 normal animals. It has usually been thought that they 

 located insects by listening for the sounds of their wing- 

 beats, and this probably does occur in some circum- 

 stances when the flying insects make appreciable hum- 



87 



