Adaptation or random change? The evolutionary response of songs to substrate properties in lacewings (Neuroptera: Chrysopidae: Chrysoperla)

Green lacewings of the carnea group of Chrysoperla constitute a large complex of cryptic species that differ principally in their substrate-borne, vibrational mating signals. Sexual selection has very likely contributed to their recent and rapid divergence. Some theoretical models require that evolutionary environmental adaptation assist sexual selection during speciation, whereas others do not. Here, we looked for evidence of adaptation having shaped the song phenotypes of lacewing species that have different habitat preferences. First, we looked for a pattern of convergent song evolution, predicted under a hypothesis of substrate adaptation during speciation. Focusing on species confined to conifers versus those on herbaceous vegetation, we found several examples where song phenotype showed a correlation with substrate that was independent of phylogeny. We also present results of experimental tests of environmental adaptation using C. downesi and C. plorabunda, which are conifer and meadow species, respectively. Experiments addressing purely biomechanical aspects of substrate transmission showed that the songs of C. downesi and C. plorabunda propagated equally well in either grass stems or conifer branches. Behavioural experiments showed, furthermore, that individuals did not discriminate between conspecific songs played through the ‘correct’ versus the ‘incorrect’ substrate. We conclude that the pattern suggestive of environmental adaptation of lacewing songs to their substrates was not supported by experiments that explicitly tested for adaptation. The carnea group of cryptic species may be one of the few examples where sexual selection has acted on arbitrary differences in premating signals, resulting in speciation.