A Physiological Analysis of Cicada Song

ABSTRACT The neuromuscular mechanism of sound-production in cicadas has been elucidated by a detailed anatomical and physiological study of Platypleura capitata (Oliv.) and by the analysis of magnetic tape recordings of the song of eight other species in Ceylon. In all cases the song consists of a succession of pulses, the repetition frequency being between 120 and 600/sec. Each pulse is composed of a damped train of sound waves whose frequency is determined by the natural period of vibration of the tymbals. A pulse of sound is emitted when the tymbal suddenly buckles or is restored to its resting position by its natural elasticity; in the song of some species both movements are effective. The tymbal muscles, which are responsible for the buckling, have a myogenic rhythm of activity, initiated, but only slightly controlled in frequency, by impulses in the single nerve fibre supplying each muscle. The two tymbals normally act together. The curvature of the tymbals can be increased by the contraction of accessory muscles, the chief of which are the tensor muscles. This increases the volume of sound emitted at each click and lowers the pulse repetition frequency; the abdomen is raised from the opercula by contraction of the tensor muscles. The tracheal air sacs form a cavity which is approximately resonant to the frequency of tymbal vibration and can be varied in size by expansion of the abdomen. Cicada songs, to the human ear, appear to be of great variety. The differences arise largely from the properties of the mammalian cochlea as a frequency analyser; the degree of coherence of phase between pulses, which is probably without significance to the insect, is of great importance in determining the quality of the sound to a human observer. The songs of three species which resemble respectively a bell, a musical phrase and a strident chatter are analysed from high-speed oscillograms, and the difference in quality of sound is explained by reference to the wave-forms. Some species emit a regular succession of pulses. Others have a slow pattern to their song, produced by the co-ordinated nervous excitation of three functional groups of muscles: (a) wthe tymbal muscles, producing the sound; (b) the tensor muscles, controlling the amplitude and pulse repetition frequency; (c) the muscles controlling the resonance of the air sacs. Of the nine species recorded in Ceylon, those belonging to the genus Platypleura produce their pattern by using (b) and (c), the tymbal muscle being in continuous rhythmic activity; those of the genus Terpnosia use mainly (a) to interrupt the continuity of emission of sound pulses, with some accompanying change in amplitude and pulse frequency. The remaining species use all three muscle groups, but different patterns of co-ordination produce great differences in song. In one species (Platypleura octoguttata) a distinct courtship song was recorded from a male in close proximity to a female; this ends with attempted copulation. Preliminary electrophysiological experiments show that the chordotonal sensilla associated with the tympana are extremely sensitive to high-pitched sounds. When the song of another cicada is played back through a loudspeaker the impulse pattern in the auditory nerve corresponds to the pulse modulation envelope, with some after-discharge, as in other insect ‘ears’ (Pumphrey, 1940). The function of the song is to assemble the local population of a cicada species (males and females) into a small group. It remains to be determined whether it is the main intersexual stimulus in mating behaviour.