CHAPTER IV 



Sound Reception 



The to-and-fro motion of a particle repeatedly displaced from equili- 

 brium is termed vibration. In a continuous medium a vibrating particle 

 causes periodic displacement of neighbouring particles by elastic 

 forces. The resulting disturbance takes the form of periodic com- 

 pressional v^aves. Sound reception is the process of detecting these 

 waves whether they occur in gases, liquids, or solids. Whether one 

 defines all vibration detection as 'hearing', restricts the term 'hearing* 

 to the detection of air-borne sounds by specialized receptors and 

 relegates other sound reception to a 'vibration sense' (von Budden- 

 brock, 1952), or limits 'hearing' to that condition where an animal 

 behaves as if it has located a sound source (sound being defined as any 

 mechanical disturbance whatever which is potentially referable to an 

 external locaHzed source) (Pumphrey, 1950) is not important to an 

 understanding of how the receptors work. The crucial question, 

 physiologically speaking, is to what extent insects can detect periodic 

 motion or vibration in their environment and which parameters of the 

 wave are detected. In this respect it is helpful to inquire first what kind 

 of sounds or vibrations insects are exposed to normally. 



Many sounds are produced by insects themselves. Many are inci- 

 dental to ordinary movements, but nearly all orders of insects have 

 members that produce specialized sounds. Fringsand Frings (1958), 

 in their review of the subject, have listed the following specialized 

 methods of sound production: (1) tapping the substrate; (2) explosive 

 expulsion of air or other material through a small orifice ; (3) snapping 

 a prosternal spine from a cavity in the mesosternum ; (4) vibrating the 

 body without flight to produce a buzzing, whining, or piping; 

 (5) snapping the wings in flight; (6) snapping tymbals or tymbal-like 

 organs; (7) stridulation (rtibbing specialized surfaces of the elytra 

 together or stroking the elytra with the hind femora) ; (8) expulsion 

 of air over a specialized vibrating membrane. The sounds produced 

 by insects are principally connected with sexual activity, territoriality, 

 defence against predators, and maintenance of cohesion in flying 

 swarms (Haskell, 1957). In addition to responding to sounds of these 



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