Gnominthus baitabagus

Description of the calling song
The calling song of G. baitabagus (Fig. 7) was recorded in the laboratory (24.5◦C, F1-male2, MNHN-EO-ENSIF612). Its echemes consist of short trills comprising 50 ± 8 syllables (n = 10 echemes). Each echeme has a rectangular amplitude profile and lasts for 0.9±0.1s, with a period of 7.5±0.4s. Syllables last 9.2±0.8ms, with a period of 17.1 ± 1.9 ms. The spectrum (Fig. 7g) shows a clear dominant peak at 21.7 ± 0.43 kHz corresponding to the third peak of an harmonic series (peak #1 ca. 7.2 kHz; peak #2 ca. 14.4 kHz).

Biomechanics of the calling song
The stridulatory file of G. baitabagus has a simple structure com- prising ca. 115 simple teeth among which ca. 80 are likely to be functional according to their distribution pattern (Fig. 5). In the functional part (ca. 1.215mm), the mean inter-tooth distance is 14.46  m (tooth density = 69 teeth/mm). Given the rather homoge- neous amplitude profile of the syllable, we assume that the closing movement of the forewings is more or less regular. Based on the measured syllable duration, the dominant frequency and inter- tooth distances, we can predict which frequency peak is generated by the tooth impacts.

Under the hypothesis H0 that stridulation generates the dominant peak at high frequency, the instantaneous velocity of the system would be very high (ca. 313 mm/s) and would involve ca. 190 functional teeth. This is far above the total number of teeth of the file, which invalidates this hypothesis.

The alternative hypothesis H1 is that stridulation generates the lower peak of the spectrum (ca. 7.2 kHz) and that the dominant, high-frequency peak, corresponds to the third harmonic of the low fundamental frequency, amplified by the resonance of the wing. Under this hypothesis, 80 functional teeth are sufficient to gener- ate this low-frequency peak, and the corresponding instantaneous velocity of the system is ca. 104 mm/s (721,000 teeth/s * 14.46  m).

The global estimated speed from syllable duration is 130.76 mm/s, which matches H1 and thus supports the hypothesis that the fundamental frequency is the low-frequency peak of the spectrum, i.e., that the tooth impact generates the low frequency and not the dominant peak at higher frequency. As a consequence, one tooth impact would generate three primary waves due to the vibratory properties of the resonator. [1]

References