Phonoreception 



495 



are auditory. In many insects the scolopidia are arranged in a row (the 

 crista acoustica) near the edge of the tympanum in such a way that move- 

 ment of the tympanum produces a relative movement between the sense 

 cell and the scolopale. In other insects the scolopidia are in direct contact 

 with the central portion of the tympanum (Fig. 170), so that the maximal 

 amplitude of vibration is available for stimulation. 



Cuticle 



Distal or cap cell 



Scolopale or sense rod 



Terminal filannent 



Middle or sheath cell 



Sense ce 



A B 



Fig. 171. Schematic diagram of scolopidia. A, Scolopidium from a chordotonal organ 

 (from Debauche/ after Snodgrass^'')- B, Scolopidium from Johnston's organ (from 

 Debauche,** after Eggers")- From Wigglesworth.^ 



Function of Tympanic Organs: Auditory Function. The range of 

 sensitivity of the tympanic organs has been studied by recording electrical 

 changes in the sensory nerves or ■ central nervous system in response to 

 stimulation by sounds of known frequency and intensity. Typical results 

 are shown in Fig. 172. In one of the Acridiidae a response was obtained 

 for stimuli of between 300 and 20,000 cycles, and the response did not occur 

 after destruction of the abdominal tympanic organ. In the lower part of 

 this range the sensitivity is very much less than that of man, but at 10,000 

 cycles it is almost equal to the human sensitivity. In two katydids a response 

 was present between 800 and 45,000 cycles; ^^ in Grylliis from 300 to 800 

 cycles;''^ in Locusta from 300 to above 10,000 cycles. '^^^ The threshold as 

 a function of frequency for the tympanum of Locusta is shown in Fig. 172, 

 in which it may be compared with that for the human ear and for the cercus 

 of Gryllus. The response of the tympanal nerve is asynchronous; i.e., the 

 various fibers respond apparently at random, and the character of the 

 response does not change with frequency. These observations afford no 

 evidence that the tympanic organ is capable of frequency discrimination. 



Regen -^^ demonstrated that a female cricket was attracted to a telephone 

 which was transmitting the chirp of a male cricket. This response was 

 obtained with adult unmated females but did not occur with females which 

 had been mated.'" Also two male katydids in the same vicinity tended to 

 sing in concert." The concerts consisted of introductory and conclusional 



