ECHOES OF BATS AND MEN 



mixture of sounds than merely their original chirp plus 

 a single echo returning from a single insect and having 

 the same wave form at a lower energy level. What really 

 impinges upon their ears is a whole series of echoes from 

 everything within several feet— the ground, other insects, 

 and every bush, twig, tree trunk, leaf, or blade of grass. 

 Many of these things contribute only small amounts of 

 echo energy, but the echo from an insect is itself a faint 

 one, and if it is audible so must the others be. How then 

 do bats sort out one class of faint echoes from all the 

 others? How do they hear the difference between echoes 

 that mean food to be caught and those that mean ob- 

 stacles to be dodged? 



If we knew how bats discriminate so expertly be- 

 tween faint insect echoes and the competing echoes ar- 

 riving within a small fraction of a second, we could make 

 more rapid progress toward solving the orientation 

 problems of blind people, to say nothing of developing 

 instruments that could emulate the bats more perfectly. 

 Unfortunately this is not yet possible, but it is interesting 

 to consider how well bats can make such discrimina- 

 tions. This cannot be easy, even for a bat, and faint 

 echoes from wire obstacles are less skillfully utilized 

 when stronger echoes arrive in the same small fraction 

 of a second. For example, we once performed an ex- 

 periment in which two rows of wires were stretched 

 across a flight room, one row at the middle of the room 

 and the other row 45 centimeters from the end wall, as 

 diagramed in Fig. 12. In both rows the wires extended 

 from floor to ceiling and were spaced 30 centimeters 

 apart. When the diameter of the wires was 0.46 milli- 

 meter, they were difficult echo targets, but the percent- 

 age of misses in a large number of flights through the 

 central row was 91 per cent. This represents a considera- 

 ble degree of success, and almost all the contacts were 



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