Evolution of ultrasonic signalling in wax moths: discrimination of ultrasonic mating calls from bat echolocation signals and the exploitation of an antipredator receiver bias by sexual advertisement

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Abstract
Pair formation in the lesser wax moth, Achroia grisella (Lepidoptera Pyralidae), is accomplished via male-produced pulses (100 µsec) of ultrasound (100 kHz) attractive to females. A. grisella are sensitive to a wide range of ultrasonic frequencies, enabling them to hear the echolocation signals of both aerialhawking and substrate-gleaning bats. Both flying and running moths exhibit defensive behaviors, dropping to the ground and cessation of locomotion, respectively, in response to simulated bat echolocations. We determined that female A. grisella discriminate the mating signals of advertising males from bat echolocations on the basis of the faster pulse rates (> 60 sec−1) of mating signals. Terrestrial defensive behavior is elicited by a slow rate of lengthy (≥ 1 msec) pulses, features of most searching phase bat echolocations. Unlike other insects that perceive and evade insectivorous bats, the perception of and negative responses to predators probably preceded acoustic signalling in A. grisella and other acoustic Lepidoptera. Thus, the acoustic signalling system may have originated via exploitation of an ancestral receiver bias, but development of a positive receiver response, and a possible shift in a signal character (pulse rate), must have occurred subsequently. We suggest various processes that may have allowed this evolutionary development and call attention to the potential role that the perception of predators and associated negative responses may have played in the evolution of signalling systems via exploitation of receiver bias.