Sound radiation by the bladder cicada Cystosoma saundersii

Male Cystosoma saundersii have a distended thin-walled abdomen which is driven by the paired tymbals during sound production. The insect extends the abdomen from a rest length of 32-34 mm to a length of 39-42 mm while singing. This is accomplished through specialised apodemes at the anterior ends of abdominal segments 4-7, which cause each of these intersegmental membranes to unfold by approximately 2 mm. 

The calling song frequency is approximately 850 Hz. The song pulses have a bimodal envelope and a duration of approximately 25 ms; they are produced by the asynchronous but overlapping action of the paired tymbals. The quality factor Q of the decay of the song pulses is approximately 17. 

The abdomen was driven experimentally by an internal sound source attached to a hole in the front of the abdomen. This allowed the sound-radiating regions to be mapped. The loudest sound-radiating areas are on both sides of tergites 3-5, approximately 10 mm from the ventral surface. A subsidiary sound-radiating region is found mid-ventrally on sternites 4-6. Sound is radiated in the same phase from all these regions. As the abdomen was extended experimentally from its resting length to its maximum length, the amplitude of the radiated sound doubled and the Q of the resonance increased from 4 to 9. This resonance and effect are similar at both tergite 4 and sternite 5. 

Increasing the effective volume of the abdominal air sac reduced its resonant frequency. The resonant frequency was proportional to 1/(check)(total volume), suggesting that the air sac volume was the major compliant element in the resonant system. Increasing the mass of tergite 4 and sternites 4-6 also reduced the resonant frequency of the abdomen. By extrapolation, it was shown that the effective mass of tergites 3-5 was between 13 and 30 mg and that the resonant frequency was proportional to 1/(check)(total mass), suggesting that the masses of the tergal sound-radiating areas were major elements in the resonant system.

The tymbal ribs buckle in sequence from posterior (rib 1) to anterior, producing a series of sound pulses. The frequency of the pulse decreases with the buckling of successive ribs: rib 1 produces approximately 1050 Hz, rib 2 approximately 870 Hz and rib 3 approximately 830 Hz. The sound pulse produced as the tymbal buckles outwards is between 1.6 and 1.9 kHz. Simultaneous recordings from close to the tymbal and from tergite 4 suggest that the song pulse is initiated by the pulses produced by ribs 2 and 3 of the leading tymbal and sustained by the pulses from ribs 2 and 3 of the second tymbal.

An earlier model suggested that the reactive elements of the abdominal resonance were the compliance of the abdominal air sac volume and the mass of the abdomen undergoing lengthwise telescoping. The present work confirms these suggestions for the role of the air sac but ascribes the mass element to the in-out vibrations of the lateral regions of tergites 3-5 and the central part of sternites 4-6.