Male spacing and the influence of female courtship behaviour in the bladder cicada, Cystosoma saundersii Westwood

1. Experimental choruses of males of Cystosoma saundersii with manipulated densities -12 times that usually found in bushes of the same size were set up and compared with choruses with natural densities of males. Greater numbers of both males and females were attracted to the louder, denser choruses.

2. There were higher rates of dispersal away from and movements within the experimental choruses than those found at natural densities. This indicated some form of repulsion occurring between males in close proximity.

3. Playback experiments to calling males in the field showed that males would stop singing and move away in response to high intensities of a conspecific calling song. The intensities eliciting this reaction cor- responded to a conspecific male singing at separations of less than or equal to 0.5 m. Aggression between males or aggressive songs have not been observed.

4. Females fly in to the chorus and alight generally within 0.5 m of the male of their choice. They then produce a pheromone which they disperse by wing-flicking. The rate of wing-flicking was used as a quan- titative measure of the willingness of a female to mate.

5. Twice as many females were attracted to higher densities of males than to natural densities. However, in the experimental situation, the probability of an attracted female actually mating was half that occur- ring in natural densities. This appeared to be due to an inhibition of the female wing-flicking behaviour.

6. Playback experiments showed that females will respond by wing-flicking to a male only when his song is at least 9 dB louder than that of his nearest neighbour (when the sound pressure levels of the sources are measured separately at the position of the female). Using this figure as a threshold, volumes in which females would respond were calculated for males with a series of nearest-neighbour distances. At the average separation found between males sharing bushes in the field, the volume in which a female would respond covered most of the volume in which a female is likely to land. Thus, the discriminating capabilities of the female were shown to govern the minimum separation occurring between males.