XXII. 2 



ECHOLOCATION 



589 



but the inferior eorpora quadragemina and cerebellum are large. The 

 eyes are often moderately large and presumably used in twilight; the 

 retina contains mainly rods. The touch receptors are well developed, 

 especially on the wings. 



The echolocation is performed by discrete pulses of high intensity 

 and up to 120 kc frequency. These are produced by the very large 

 larynx, whose cartilages are ossified to make a rigid framework. The 



Fig. 368. Big-eared bat (Plecotus). 

 (After American Mammals, by W. J. Hamilton, McGraw-Hill Book Company.) 



strong cricothyroid muscles put great tension on the light vocal cords. 

 In the horseshoe bats there are special resonating chambers and the 

 face is elaborately modified to beam the sound forwards (Fig. 367). 



The ears of bats are greatly specialized, often with very large pinnae. 

 The cochlea is large and the basilar membrane narrow and tightly 

 stretched. The tensor tympani and stapedius muscles are large. 



The bat is almost entirely dependent on echolocation for avoiding 

 obstacles and catching insects. If the larynx is damaged or the ears 

 blocked it blunders against even large obstacles. The normal animal 

 can avoid wires less than 0.5 mm thick in complete darkness and if 

 blinded. The presence of loud noise at high frequency disturbs the 

 bat, but lower frequencies do not. This is evidence that the bat hunts 

 by echolocation and not (usually) by listening to the sounds made by 

 the insects. 



The mechanism adopted is not fully understood and certainly is not 

 always the same. In Vespertilionidae the sound is emitted by the 

 mouth in pulses of 1-4 m. sec. duration. The note falls through about 



