THE LANGUAGE OF ECHOES 



frequency sound. Often they dart out from such a posi- 

 tion to seize an insect that flies within range. 



Another group of bats, confined to the tropics, feed 

 mostly upon fruit, but some also eat insects, which 

 they may pick off the vegetation. These bats emit 

 much fainter sounds than the horseshoe bats— ex- 

 tremely brief clicks, lasting from a fraction of a milU- 

 second to 2 or 3 milliseconds. The sound waves making 

 up these very short pulses are compUcated, with a va- 

 riety of frequencies from as low as 10 to as high as 150 

 kc, again depending on the species. The vampire bats, 

 which feed on the blood of Uving animals and men, be- 

 long to this group. Without causing the victim to awake 

 from his sleep, they feed by making small cuts with their 

 very sharp teeth and drinking the blood that flows for a 

 time before clotting. All these bats seem to refrain from 

 the active pursuit of flying insects, and the intensity level 

 of their sounds is so low that only the best of micro- 

 phones and sound-analyzing equipment will register 

 them. They may be called whispering bats to distinguish 

 them from the other two groups. 



The third major category includes the common in- 

 sectivorous bats that are well known in North America 

 and Europe. With a very few exceptions, these bats all 

 hunt flying insects in the open, tracking their elusive 

 moving prey on the wing, maneuvering through com- 

 plicated spUt-second turns and other acrobatics to follow 

 and intercept the erratic flight of moths and flying 

 beetles, May flies and mosquitoes. The sounds used by 

 this group, only a few miUiseconds in duration and al- 

 most as intense as those of the horseshoe bats, have a 

 characteristic frequency pattern. Each orientation soimd 

 starts at a very high frequency and drops rapidly during 

 its brief life, to end about an octave below the frequency 

 at which it started. The common little brown bats of the 



85 



