Do large bushcrickets have more sensitive ears Natural variation in hearing thresholds within populations of the bushcricket Requena verticalis (Listroscelidinae : Tettigoniidae)

The auditory spiracle of tettigoniid Orthoptera influences hearing threshold and, for the most part, individuals with larger auditory spiracles have lower hearing thresholds; they are more sensitive. Hearing thresholds of both sexes of the bushcricket, Requena verticalis Walker (Orthoptera; Tettigoniidae; Listroscelidinae), were measured at the male call's carrier frequency and were found to correlate with spiracle dimension. In turn, spiracle dimension correlates with the size of the insect as measured by pronotum length. The best frequency of hearing is close to 16 kHz and this appears to be independent of size. Males show a higher variation in threshold than females and this was reflected in a trend toward lower variance in spiracle size in females. To test the effects of size on sensitivity, spiracle size was manipulated by partially blocking it. Blocking the spiracle decreases sensitivity to high rather than low frequencies. As in other tettigoniids, the spiracle and associated auditory system act as a high-pass filter. Within and between sex differences in hearing sensitivity were compared with differences in male call intensity. It is argued that sensitivity to sounds associated with mating should be as much under sexual selection as the sexual calls of males.