l8o THE SENSES OF ANIMALS 



and there is evidence that some moths can "hear" such 

 frequencies, as we shall see in the next chapter. On the other 

 hand some tropical butterflies [Ageronia] make a loud click- 

 ing noise, like that made by drawing a finger along the teeth 

 of a comb, as they fly — not a continuous sound but short 

 bursts at irregular intervals. No structure that can be identi- 

 fied as a hearing organ has been identified in any butterfly, 

 and yet it is impossible to believe that the production of 

 this sound is without any meaning in the biology of these 

 creatures. 



On the other hand insects possess many structures that 

 are obviously sense organs, but whose function is unknown. 

 These "sensilla" take the form of rods of cells, often of 

 considerable complexity, associated with nerve fibres. They 

 are found on almost any part of the surface of the body 

 and may be lodged in pits, or project as hairs, or merely lie 

 in contact with the cuticle. Some certainly serve the sense 

 of touch, others probably those of smell and taste, but some 

 may perhaps serve the sense of hearing. Whatever the truth 

 may be it seems probable that when insects perceive sound 

 vibrations the majority do so more by the sense of touch than 

 of hearing as we know it. Even the most highly elaborated 

 insect ears cannot, from their structure, be supposed to 

 analyse sounds of different wave-lengths as can the ear of a 

 vertebrate nor, from a consideration of the insect's nervous 

 system, could they be expected to do so. There is not room 

 in an insect for the vast number of nerve cells that make 

 up that part of the brain which interprets the messages 

 received from the ear in a vertebrate. 



The fishes possess, in addition to the ear, another means 

 of perceiving vibrations or changes of pressure in the water 

 surrounding them. We have no such sense and consequently 

 it is impossible for us to know the nature of the sensations 

 produced by it. Nevertheless experiment has revealed much 

 of the function of this sense in the life of fishes. In most 

 fishes it is easy to see a line running along the side of the 

 body from head to tail. A small tube filled with fluid runs 

 under the skin beneath the line and is connected with a 

 number of branched tubes that ramify over the head. The 

 tube — the lateral-line canal — opens through the skin by 



