SEXUAL SIGNALING IN BLADDER GRASSHOPPERS - TACTICAL DESIGN FOR MAXIMIZING CALLING RANGE

Pair formation in the bladder grasshopper (Bullacris membracioides) is by duetting and male phonotaxis. Low-frequency stridulatory signals a re emitted by an abdominal resonator in the male and are answered by f emales using a species-specific time delay, Acoustic transmission in t he natural environment was studied using playback of sexual signals ov er distances of 450m under two atmospheric conditions (day and night), Upward-refracting sound conditions and a sound shadow zone beyond app roximately 50m prevailed during the day. Acoustic enhancement was demo nstrated at night when downward-refracting temperature inversions crea ted a tunnel effect with sound caught between the ground and zones of different temperatures. Transmission conditions are almost ideal at ni ght when the species actually calls; calling distances of 150m for the male signal in the afternoon increased to 1.5-1.9km at night, arguabl y the largest calling distance yet reported for insects. In contrast, female calls transmit over a maximum of 50m, signifying a marked discr epancy in the active space of sex-specific signals. Transmission dista nce may, however, be profoundly affected by levels of masking noise. A daptations to increase the signal range may variously be found in the signal itself, in behaviour patterns or in the sensory system. Here we demonstrate aspects of the first two types of adaptation in the sexua l signalling system of a grasshopper in which maximizing the calling r ange appears to be the major selection pressure, with lesser effects i mposed by inter-and intraspecific pressures and by the transmission ch annel.