108 THE PHYSIOLOGY OF INSECT SENSES 



from 50 to about 1,000 c/s; in the sensitive group, up to 8,000 c/s. In 

 both groups the threshold drops with increasing stimulus frequency; 

 however, in the sensitive group it shows a minimum in the 1,000 c/s 

 band of the frequency spectrum (Fig. 69). The rise in threshold at 

 higher frequencies following the minimum may be an artefact 

 (Antrum and Schneider, 1948; Schneider, 1950). The smallest meas- 

 ured amplitude that causes any response (middle leg of Decticus at a 

 stimulus frequency of 2,000 c/s) is 0'36A, a distance equal to about 

 one-third the diameter of the hydrogen atom. From this it follows 

 that the sense organs involved are developed to the physical limit, but 

 the minimum energy required is greater than the energy content of the 

 unitary elemental process (i.e., collision of one molecule) of which the 

 stimulus consists (Antrum, 1943). 



Threshold can be recorded either as amplitude or in terms of 

 acceleration. The acceleration b which a substrate vibrating sinusoid- 

 ally imparts to an object lying on it is given by the expression —a-cxi^ 

 sin out (maximum aoj ^) in which a is the amplitude and w (= lirv) is the 

 angular frequency. A comparison of the thresholds as accelerations 

 of the two groups of insects, sensitive and insensitive, shows that a 

 different range of acceleration is necessary for each group. For the 

 sensitive group it ranges from 2- 10~^* to \0~^g{g = 981 cm. sec. 2); for 

 the insensitive group, from S-IO"^ to g). On the other hand, for a 

 given insect threshold acceleration is more nearly constant over the 

 range of stimulating frequencies than is threshold amplitude. For 

 this and other reasons Antrum and Schneider (1948) and Schneider 

 (1950) felt that acceleration was the crucial physical parameter to 

 which the sense organs respond and that regardless of the stimulus 

 frequency the organs must undergo equally large accelerations. 



There is only indirect evidence as to which of the several organs in 

 the leg are actually responding to the vibrations. Of the species 

 studied by Antrum and Schneider (1948), the least sensitive of those in 

 the insensitive group lack true subgenual organs, while the sensitive 

 species possess them. When an operation designed to destroy sub- 

 genual organs was performed on those species possessing them the 

 threshold of response to vibration rose and the effective frequency 

 band became narrow so that these sensitive insects now resembled 

 insensitive ones (Antrum, 1941). 



From these experiments it was concluded that the sensitive insects 

 possessed an organ especially adapted to detect vibrations in the 

 substrate and that this organ was the subgenual collection of chordo- 

 tonal sensilla. The insensitive insects, on the other hand, were pre- 



