Short Communication a Paradoxical Problem in Insect Communication: Can Bush Crickets Discriminate Frequency?

Many bush crickets (Tettigoniidae) produce calls that consist of trains of transient high-frequency pulses (Fig. 1). Each pulse is generated by a file tooth on one wing being struck by a plectrum on the other, causing the wing resonator to produce harmonic transients each composed of only two or three cycles of oscillation. In animals, high-frequency sound can only be detected by means of some form of auditory filter, since the refractory period of the nervous system restricts direct measurement using periodicity to frequencies below 1 kHz. The necessity of using a filter produces a paradoxical situation with these brief transients. If the bandwidth of a tuned filter is such that frequency can be accurately discriminated, the response time may exceed the duration of the signal and then there would be no output. To respond to such a signal, the filter bandwidth must be increased but then frequency information is lost. The stridulatory apparatus of bush crickets is an elaborately constructed device with design features peculiar to each species. When activated, the resonating area produces a characteristic modal vibration the so-called carrier frequency of the call. This is inbuilt and cannot be altered by the insect. Other features of the call tooth strike rate, amplitude modulation, syllable length and syllable sequence could be altered, since they are theoretically controlled by the insect's nervous system. If the tooth strike rate is such that the wing resonator is synchronously excited then a long harmonic call can result, as in Ruspolia differens (Serville) (previously Homorocoryphus) (Bailey, 1970) and Mecopoda elongata (Linnaeus) (J. C. Hartley and R. O. Stephen, personal observation). Such calls present no problem in frequency determination. If, however, the tooth strike rate is low, each impulse may produce only a few oscillations which decay well before the next impulse, as shown by the examples in Fig. 1 and given for many other species (Table 5.1, Sales and Pye, 1974). The use of brief harmonic transients is therefore by no means a rare occurrence in the Tettigoniidae. If these brief transients are to have a communication value, is their frequency important or is it just the pulse