Electromyograms (EMGs) of the timbal muscles were recorded during the calling songs and 'protest songs' (also referred to as alarm signals) in five species of cicadas: Cicada barbara lusitanica, Tettigetta josei, Tettigetta argentata, Tibicina quadrisignata and Tympanistalna gastrica.
The timbal muscle contraction rates of all species ranged from 50 to 250 Hz. The basic timbal cycle generating sound during the inward and outward buckling of the timbal was maintained in calling song and protest song for all five species. Comparison of the temporal parameters and the corresponding timbal muscle activity in both sound signals revealed that the timbal muscle period as well as the intertimbal delay showed the same mean values. Major differences between calling song and protest song are apparent in the pattern determining schemes and phrases as well as in the sound amplitude modulation.
The amplitude of the sound pulse during the inward movement of the timbal was positively correlated with the time lag from the timbal muscle activity to the IN sound pulse in the calling songs of Tettigetta josei, Tettigetta argentata and Tympanistalna gastrica, species with a one-to-one correspondence between a single contraction of the timbal muscle and the production of a sound pulse as the timbal buckles inwards. These relationships did not hold for the other two species, where a single contraction of the timbal muscle causes the timbal to buckle inwards in steps to produce a train of sound pulses. This indicates the presence of different mechanisms responsible for sound amplitude modulation. There was no evidence that the period of the timbal muscle contractions correlated with changes in sound pulse amplitude during singing.
Cicadas, sound production, calling song, EMG recording, amplitude modulation.