Introduction

Intraspecific acoustic communication in grasshoppers or katydids appears to be a very simple and straight forward behaviour: one sex – usually the male – produces an acoustic signal, and the female, once perceiving and recognizing the signal as species specific, shows some kind of response, either an acoustic reply or a phonotactic movement to the male. However, the system is far from being that simple and involves more than just a sender and receiver. First, communication usually takes place in a physically complex environment, where sound signals are subject to attenuation and degradation, depending on the carrier frequencies, which are often in the high-sonic or ultrasonic range because of the small size of the sound radiating structures (Wiley & Richards, 1978; Michelsen, 1992). In addition, the physical conditions of the transmission channel for the sound may vary strongly during day or night and with weather conditions; consequently, the ability to detect and localize a signal undergoes strong variations. Second, insects often aggregate and communicate in areas rich in resources or at periods of the day or night favouring mate attraction. As a result of many signallers calling in close proximity, masking interference will take place at the site of receivers, depending on the spacing, as well as the kind and extent of signal timing. Since such favourable areas and times for signalling are similar for different species, heterospecific choruses may be formed with impressive sound pressure levels of biological background noise, which further complicates the detection of a signal (reviewed for katydids by Schatral (1990)).