HEARING — GENERAL 7I 



sible; and it is convenient to begin the chapters on each 

 sense by deahng with the lower organisms first and then 

 working up to the higher forms of animal life. It follows that 

 it is important to point out that just as primitive light- 

 receiving organs exist in primitive creatures, so do those 

 same levels of animal life have simplified ways of detecting 

 vibrations which can scarcely be called "hearing" in its true 

 meaning. Indeed it is not always easy to tell whether the 

 response to vibrations in some lower forms is more nearly 

 related to a sense of hearing or to the sense of touch or feeling. 

 In some cases it may be both senses that produce reactions 

 of one kind and another. 



Many of the microscopic and near microscopic animals 

 are most susceptible to vibrations, and it is well known that 

 some of the colonial protozoa such as Vorticella, and some 

 of the more highly developed rotifers — particularly those 

 known as "tube-dwellers" — are so sensitive to vibrations that 

 the lightest touch on a jar or other vessel containing them 

 will immediately produce a kind of defensive action which 

 causes the former to retract their stalks, and the latter to 

 withdraw into their tubes. It would, in my opinion, be a 

 mistake to regard this as an elementary form of hearing and 

 such behaviour would seem to be nearer to the sense of 

 touch. 



Hearing is most important to many insects, though their 

 auditory organs are often placed in some weird situations — 

 on their bodies and on their legs. Numerous insects can make 

 mechanical noises by rubbing wing-cases and legs in such a 

 manner as to produce sounds which are necessary to keep 

 colonies together and to assist in mating. This method of 

 making sounds is known as stridulation and is most common 

 in grasshoppers and crickets. 



The humming noise made by bees, wasps, and beetles is 

 again mechanical, being brought about by the rapid move- 

 ment of the wings. It is of no significance to the insects them- 

 selves. Similarly, the so-called "squeak" of the Death's Head 

 Moth is not vocal, but is produced by air in the body being 

 forced out through the proboscis (feeding tube). 



In the amphibians sounds play an important role in 

 certain groups. Frogs and toads, which have already been 



