Stridulation and tegminal resonance in the tree cricket Oecanthus nigricornis (Orthoptera: Gryllidae: Oecanthinae)

Summary

Study of the stridulatory mechanism of the tree cricket Oecanthus nigricornis F. Walker has yielded the following principal results:

• 1. The normal carrier frequency of the song is equal to the tooth-impact frequency throughout the range 3,000 Hz (at 15 °C) to 4,600 Hz (at 32 °C).

• 2. Under conditions of thermal equilibrium, pulse frequency and tooth-impact frequency are strongly correlated: however, the two frequencies are not controlled by the same underlying mechanism, as shown by marked differences in their reaction to step changes in temperature.

• 3. Tegminal free resonances have been identified in the range 1,000–6,900 Hz; between 3,800 and 4,600 Hz the resonances form a virtually continuous series, corresponding to modes of vibration in which the three major distal cells of the tegmen plus a variable portion of the harp oscillate as a single membrane, whose amplitude maximum is near the center of the tegmen distal to the harp (Fig. 8 A, B); these free resonances are excited by corresponding tooth-impact frequencies and are responsible for the normal carrier of the song at temperatures above 22 °C. Below 3,800 Hz free resonances have also been found and their modes of vibration identified: typically in this range a stationary node bisects each tegmen transversely, with maximum amplitude of oscillation at the distal margin of the tegmen, and a secondary maximum in the basal region near the file vein (Fig. 9 B, E). A continuous series of prominent tegminal resonances exists in the range from 5,100 to 5,850 Hz (i.e. outside the tooth-impact frequency band): these higher modes correspond to a characteristic vibration pattern in which the principal tegminal surface is trisected by two stationary nodes (Fig. 8 D, E, F). The free resonance of the harp membrane is at 6,000–6,200 Hz, well outside the range of carrier frequencies of the song; other major tegminal cells are tuned to similar frequencies. The resonance of individual tegminal cells may be excited as a secondary, low-amplitude component of the song, which is particularly evident during the early stages of each sound pulse when amplitude of the carrier is low.

• 4. Exceptional individuals stridulate with carrier frequencies in the range 5,200–5,750 Hz, i.e. in the range of the higher tegminal resonances: in this case the carrier does not equal tooth-impact frequency. This phenomenon has been reproduced experimentally by constraining the tegmina to vibrate in the modes identified for these resonances. In both normal and experimentally modified specimens, when the carrier frequency equals a higher tegminal resonance, the carrier is excited as an ultra-subharmonic resonance of the tooth-impact rate. This phenomenon indicates the presence of nonlinearities in the tegminal resonator.

• 5. A comparison of these results on Oecanthus with published data on Gryllus reveals significant differences, which are correlated with differences in structure between the two genera. Stridulation in Oecanthus is discussed in the light of published theories concerning the mechanism of tegminal resonance, and the concept of a resonator with continously variable tuning is supported.