SOUND RECEPTION 109 



sumed to be reacting primarily to stimuli outside the normal biological 

 range and doing so through the agency of organs not finely adapted for 

 this function. Coating the legs with paraffin did not abolish the res- 

 ponse so it is unlikely that hairs or spines mediate the response. As 

 already pointed out, other chordotonal organs in the leg are associated 

 with articulations. The articulations are also equipped with hair plates. 

 Either of these groups could be concerned. Whichever types they are, 

 it is probably the tarsal ones that are important in perception, since 

 response to vibration is lost following amputation of the tarsi (Antrum 

 and Schneider, 1948). 



In flies (CalUphora and Eristalis), which are among the least sensi- 

 tive insects, there is no subgenual organ of any sort. Responses are 

 probably mediated through a tibial chordotonal organ which con- 

 sists of three sensilla (Schneider, 1950). The upper frequency limit of 

 this organ is about 100/cs. At frequencies up to this, the discharge in 

 the nerve is synchronized with the stimulus. At higher frequencies it 

 is asynchronous. 



Antrum and Schneider (1948) decided that sensitivity is connected 

 with the anatomical structure of the subgenual organs. Where these 

 are fan-shaped, thresholds are low (e.g., Orthoptera); where they are 

 conical and compactly constructed (e.g., Hymenoptera), thresholds 

 are higher. On the other hand, the beetles Geotrupes and Melolontha, 

 which lack true subgenual organs, are very sensitive to frequencies be- 

 tween 600 and 1,000 c/s and 600 and 1,500 c/s respectively and still are 

 responsive to 4,000 and 8,000 c/s respectively (Schneider, 1950). 

 Recordings obtained following successive amputations of lengths of 

 leg suggest that the receptors are the ten chordotonal sensilla in the 

 distal end of the tibia. In any case, sensitivity does not depend upon 

 the number of sensilla. Camponotus sp? with a subgenual organ 

 containing about twenty sensilla is as sensitive as Apis with about 

 forty. 



The curious anatomical position of the subgenual organ with res- 

 pect to the blood sinus of the leg invites speculation about the func- 

 tional association of this organ with the blood. Work with a model has 

 shown that vibrations would establish vortices in the blood such that 

 a rather constant pressure would be applied to the outer surface of the 

 subgenual organ (Antrum, 1942). It has been postulated that the effect 

 is a rectifying one and that it serves to permit the nerve to transmit 

 frequencies higher than the 200-500 c/s that represent the top limit 

 permitted by the refractory period of nerves (Antrum, 1942; Antrum 

 and Schneider, 1948). While it is possible that vibrations act indirectly 



