Asymmetry of Tymbal Action and Structure in a Cicada: a Possible Role in the Production of Complex Songs
Authors: Fonseca P. J. , Bennet-Clark H. C. 
Published: April 2021

The type 1 echeme of the song of the small European cicada Tympanistalna gastrica consists of a pair of loud IN–OUT pulses followed by a train of soft IN–OUT pulses. In all nine insects investigated, the right and left tymbals buckled inwards and outwards alternately, but the echeme started with the buckling of the right tymbal. Both the inward and the outward buckling movements produced single discrete sound pulses.

The loud IN pulses were produced with the tymbal tensor muscle relaxed. They were approximately 10 dB louder than the loud OUT pulses and than the soft IN and OUT pulses. The period between the right loud IN and OUT pulses (3.75±0.31 ms) (mean ± S.D.) was significantly shorter than between the left loud IN and OUT pulses (4.09±0.28 ms). The period between the loud IN and OUT pulses was significantly shorter than the period between the soft IN and OUT pulses, which was similar on both sides (mean for the right tymbal 5.54±0.20 ms, mean for the left tymbal 5.30±0.51 ms).

Measured at the tymbal, the power spectrum of the right loud IN pulses showed major components between 4 and 8 kHz as well as around 11.7 kHz. That of the left loud IN pulse had approximately 10 dB less power at 4 kHz and similar power at 7–8 kHz, with a further louder peak at around 10.8 kHz. The loud OUT pulses and all subsequent IN and OUT soft pulses showed very little power at 4 and 8 kHz, but all showed a spectral peak at approximately 13 kHz. The soft OUT pulses had similar pulse envelopes to the preceding IN pulses, which they closely mirrored.

Measured at the fourth abdominal sternite, only the right loud IN pulse produced peak power at 4 kHz. The transfer function between the tymbal sound and that at sternite 4 was maximal at 4 kHz for the right loud IN pulse and showed a peak at this frequency for both loud and soft IN and OUT pulses. The 4 kHz components of all pulses, and particularly that of the right loud IN pulse, which has the loudest 4 kHz component, excited sympathetic sound radiation from the abdominal sternite region.

Measured at the tympanal opercula, both loud IN pulses produced peaks at 7–8 kHz of similar power. The transfer functions between the tymbal sound and that at the tympanal opercula showed peaks of power at this frequency range for both loud and soft IN and OUT pulses, suggesting that this component excites sympathetic radiation via the tympana.

Components of the sound pulses produced by one tymbal are also transmitted via the contralateral tymbal. The pulses transmitted during both loud IN pulses had ragged envelopes, but the soft IN pulses and all OUT pulses were transmitted as clean coherent pulses with slow build-up and slow decay, suggesting that the ipsilateral tymbal excited a sympathetic resonance in the contralateral one.

The tymbals of T. gastrica have two unusual features. At the dorsal end of rib 2, there is a horizontal bar that extends anteriorly over rib 3 and posteriorly over rib 1 to the dorsal end of the tymbal plate. This bar appears to couple the three ribs so that they buckle in unison. The resilin sheet at the ventral ends of ribs 1, 2 and 3 was significantly wider, dorso-ventrally, in the right tymbal than in the left in eight insects that were measured (mean right-to-left ratio, 1.37).

The asymmetry between the right and left loud IN pulses correlates with the morphological asymmetry of the tymbals. The complexities of the song in T. gastrica appear to result from the preferential excitation of sound radiation from the abdomen surface or via the tympana by components of the distinct pulses produced by the asymmetrical tymbals and from the tymbals themselves.

Moribund or fatigued insects were successively unable to produce the right loud pulse and then the left loud pulse. The complex song may in this way act as an honest signal of male fitness.