HEARING 179 



doubtedly do hear sounds; they emit sounds, and can be 

 observed to respond to them. 



The noisiest insects are the crickets and grasshoppers, and 

 the unrelated cicadas ; the well-known songs of these insects 

 appear generally to be produced by the males, and serve 

 to attract the females to them. It is in these insects, too, that 

 auditory organs are most highly developed. In the grass- 

 hoppers the "ears" are contained in the middle sections of 

 the front legs. They consist of rows and clusters of sensory 

 cells connected with nerve fibres lodged in cavities which 

 communicate with the outside through small slits. The 

 sensory cells are graduated from small to large along the 

 length of the row, and present an appearance reminiscent 

 of the arrangement of fibres in the cochlea of a vertebrate. 

 It is indeed possible that they are tuned to respond to sounds 

 of different wave-lengths. However this may be, the insects 

 do not appear to have very acute discrimination for it is 

 possible to attract the females of many species with regularly 

 repeated chirps of wave-lengths widely different from those 

 produced by the males. In the cicadas the song is so astonish- 

 ishingly loud that it is difficult to believe that the sense of 

 hearing is particularly acute, especially as the auditory organ 

 is closely associated with the vocal organ situated on the 

 underside of the body. No one who has not heard it can 

 imagine the deafening noise that can be produced by some 

 of the tropical cicadas ; it is usually produced by the males 

 when they are sitting on a tree-trunk, but at least one 

 Brazilian species can sing also when it is flying about. The 

 female has an auditory organ like the male's, but she has 

 no sound-producing structures and consequently is dumb — 

 "Happy the cicadas' lives, for they all have voiceless wives." 

 One must suppose that the male's ear is useless to it while it 

 is singing for it must be impossible for it to hear anything 

 above its own voice. 



Many other insects that are not known to make any sounds 

 apart from the buzzing of their wings have structures that 

 appear to be auditory organs in various parts of their bodies ; 

 many moths, for example, have such structures in their 

 thorax or abdomen. It is possible that these insects make 

 sounds of so high a frequency that they are inaudible to us, 



