88 THE PHYSIOLOGY OF INSECT SENSES 



of contiguous particles away from the source of disturbance. It follows 

 that the waves resulting from this disturbance may be detected either 

 by a device sensitive to pressure change or by one sensitive to displace- 

 ment. As Pumphrey (1940) has stated in his discussion of the physics 

 of sound detection, the prerequisites of a pure pressure receptor are a 

 massive, opaque (to sound) chamber closed by a stiff diaphragm whose 

 displacement is vanishingly small. A recording system coupled to the 

 diaphragm would measure the excursions which are proportional to 

 the pressure amplitude. The orientation of the instrument with respect 

 to the source of sound is unimportant. The auditory mechanism of 

 the mammal is undoubtedly a receiver of this type, as are most 

 commercial microphones. 



A displacement receiver requires a diaphragm or moving vane 

 whose mass and hinges are so slight as not to offer resistance to 

 motion. For maximum efficiency a receiver of this type should be 

 oriented in such a way that the incidence of sound is normal to the 

 plane of the moving element. 



Judging from their structure, all insect sound-detecting organs are 

 displacement rather than pressure receptors. There is one ingenious 

 experiment reported which was designed to ascertain whether or not 

 the sound receptors were indeed displacement receptors (Antrum, 

 1936). Ants {Formica rufa and Myrmica spp.) can respond to loud 

 artificial sounds. Antrum directed sound vertically downwards upon 

 a reflecting surface, and so set up standing waves. In one experiment 

 ants were allowed to walk upon the reflecting surface ; in another, they 

 walked upon gauze suspended at a critical level above the surface. 

 Under these conditions of a standing wave the reflecting surface was 

 the region of maximum pressure change and minimum displacement, 

 whereas at the gauze, this being an antinode, the reverse was true. 

 Ants responded most vigorously when walking on the gauze ; that is, 

 they were most sensitive to displacement. Unfortunately, as Pumphrey 

 (1940) pointed out, the sounds employed were extremely intense 

 (10^1 times human threshold at 1,000 c/s), and there is no indica- 

 tion as to whether the ants were responding to air-borne sound 

 waves or to vibrations in the substrate. It is not known what organs 

 were involved. As Pumphrey further pointed out, it is not necessarily 

 true, as Antrum maintained, that velocity was the critical factor, since 

 displacement, velocity, and acceleration are all maximal at an 

 internode. 



In any event, there is at this time absolutely no conception in terms 

 of structure of the manner in which chordotonal sensilla operate. 



