Mechanisms of sound-production and muscle contraction kinetics in cicadas

Summary

• 1. The mechanisms of sound-production are described in 7 species of Australian cicadas:Abricta curvicosta, Arunta perulata, Chlorocysta viridis, Psaltoda argentata, P. claripennis, P. harrisii andTamasa tristigma. In all these species, sound is produced by a pair of tymbals, each of which is buckled by a large muscle (Figs. 1–7). The tymbal muscles are all of the synchronous (= neurogenic) type.

• 2. There are great differences between species in the range of sound frequencies generated by their tymbal mechanisms and in the extent to which their songs are divided into pulses and subpulses. The most extreme case is the calling song ofChlorocysta viridis, in which there are no pulses and the sound produced is a modulated pure-tone (Fig. 3).

• 3. In most species the left and right tymbal muscles contract alternately and so the muscle contraction frequencies during singing are half the observed pulse repetition frequencies. InT. tristigma, the two tymbal muscles contract only a few percent out of phase in calling but in full antiphase in protest song (Fig. 7). InA. perulata, there is some evidence that the two tymbal muscles contract in synchrony during calling even though they clearly alternate in protest song.

• 4. Muscle contraction frequencies during calling songs vary from 56 Hz inC. viridis to 224 Hz inPsaltoda claripennis. Contraction frequencies during protest songs are somewhat lower than in calling.

• 5. The tymbal muscle behaves as a single motor unit in all species, giving all-or-nothing twitches with a single, sharp threshold. The durations of isometric twitches are strongly correlated with the inferred cycle period (= reciprocal of contraction frequency) in a total of eleven species with synchronous tymbal muscles (Fig. 8).