THE INTERNAL ANATOMY OF INSECTS 



145 



small crystals that reflect the light ; and in insects it is a mass of fine 

 tracheae surrounding the retinula of each ommatidium. 



XIII. THE ORGANS OF HEARING 



Fig. 159. — Side view of a locust with the wings 

 removed; t, tympanum. 



a. THE GENERAL FEATURES 



The fact that in many insects there are highly specialized organs 

 for the production of sounds indicates that insects possess also organs 

 of hearing; but in only a few cases are these organs of such form 



that they have been gen- 

 erally recognized as ears. 

 The tympana. — In 

 most of the jimiping 

 Orthoptera there are 

 thinned portions of the 

 cuticula, which are of a 

 structure fitted to be put 

 in vibration by waves of 

 sound. For this reason these have been commonly regarded as organs 

 of hearing, and have been termed tympana. In the Acridiidae, there 

 is a tympanum on each side of the first abdominal segment (Fig. 

 159); and in the Locustidee and in the Gryllidas, there is a pair of 

 tympana near the proximal 

 end of each tibia of the first 

 pair of legs (Fig. 160). 



The chordotonal organs. — 

 An ear to be effective must 

 consist of something more than 

 a membrane that will be put 

 in vibration by means of 

 sound; the vibrations of such 

 a tympantmi must be trans- 

 ferred in some way to a nerv- 

 vous structure that will be 

 influenced by them if the 

 sound is to be perceived. Such 

 structures, closely associated 



with the tympana of Orthoptera, were discovered more than a half 

 century ago by Von Siebold (1844) and have been studied since by 

 many investigators . The morphological unit of these essential auditory 



Fig. 160. — Fore leg of a katydid; /, tympa- 

 num. 



