Pure-tone songs in cicadas with special reference to the genusMagicicada

Abstract

1. The calling song ofMagicicada cassini consists of complex pulses of sound, each pulse being subdivided into about 9 sub-pulses, and is broadly tuned (Q3 dB = 5) around a peak frequency near 6 kHz. The calling song ofM. septendecim consists of a modulated pure tone, not divided into pulses or sub-pulses, and is sharply tuned (Q3 dB = 25) at a peak frequency of 1.3 kHz (Figs. 2, 3). 2. In bothM. cassini andM. septendecim the sound-producing tymbal consists of a membrane bearing 12 stiffening ribs anteriorly and an irregularly shaped tymbal plate posteriorly. The tymbal ofM. cassini has a much higher resting stiffness than that ofM. septendecim. 3. The tymbal muscles of both species produce single twitches in response to electrical stimulation and contract more rapidly as muscle temperature increases (Fig. 6). The tymbal muscle ofM. cassini is relatively more powerful than that ofM. septendecim. 4. InM. cassini, each cycle of tymbal movement produces one complex pulse, the succession of 9 sub-pulses being correlated with the successive buckling of 9 ribs. InM. septendecim, each tymbal movement produces one simple pulse, an unbroken train of sound waves, not obviously correlated with rib buckling (Fig. 5). 5. If, inM. septendecim, the rib buckling frequency determines the fundamental sound frequency, then the latter should increase with rising muscle temperature but this effect is not observed. It is more likely that sound is generated by the tymbal buckling in only 3 or 4 distinct stages, each generating a few oscillations at the fundamental sound frequency. This interpretation is also applicable to the pure-tone song ofChlorocysta viridis. 6. It is suggested that pure-tone songs in cicadas are made possible by a reduction in the stiffness of the tymbal. This permits the precise time of buckling of each rib to be influenced by the phase of oscillation in the abdominal resonator, thereby creating a coherent and continuous train of sound waves from one tymbal cycle to another.