Abstract
To function as a mechanism in premating isolation, the divergent and species-specific calling songs of acoustic insects must be reliably processed by the afferent auditory pathway of receivers. Here, we analysed the responses of interneurons in a katydid species that uses long-lasting acoustic trills and compared these with previously reported data for homologous interneurons of a sympatric species that uses short chirps as acoustic signals. Some interneurons of the trilling species respond exclusively to the heterospecific chirp due to selective, low-frequency tuning and “novelty detection”. These properties have been considered as evolutionary adaptations in the sensory system of the chirper, which allow it to detect signals effectively during the simultaneous calling of the sympatric sibling species. We propose that these two mechanisms, shared by the interneurons of both species, did not evolve in the chirper to guarantee its ability to detect the chirp under masking conditions. Instead we suggest that chirpers evolved an additional, 2-kHz component in their song and exploited pre-existing neuronal properties for detecting their song under masking noise. The failure of some interneurons to respond to the conspecific song in trillers does not prevent intraspecific communication, as other interneurons respond to the trill.
Highlights
Given the advantages of long-range signalling by airborne sound, it is not surprising that some insect taxa, such as grasshoppers, crickets, katydids, cicadas and even some groups of moths, have evolved airborne sound signals for acoustic communication
Insects were raised in a colony at the University of Graz
The insects were reared in a 12-h light/dark cycle at a temperature of 27 °C and 70% relative humidity
Summary
Given the advantages of long-range signalling by airborne sound, it is not surprising that some insect taxa, such as grasshoppers, crickets, katydids, cicadas and even some groups of moths, have evolved airborne sound signals for acoustic communication (von Helversen and von Helversen 1994; Gerhardt and Huber 2002; Greenfield 2002) These signals are usually produced by males, and their function has been thoroughly documented in contexts of male competition, spacing, courtship and mate choice (Gerhardt and Huber 2002; Bradbury and Vehrenkamp 2011; Pollack et al 2016). The females can subsequently use these modifications as a basis for discrimination and avoid being attracted towards heterospecific mates (Elsner 1983) This view is supported by the results of genetic analysis of quantitative trait loci in Drosophila and an acoustic moth, which suggest that an allele change at a single locus could result in the production of a markedly different signal (Gleason et al 2002; Limousin et al 2012)
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