Acoustic niche partitioning in two cryptic sibling species of Chrysoperla green lacewings that must duet before mating

Cryptic species may seem identical to us, but not to each other. The Chrysoperla carnea group of green lacewings has species-specific substrate-borne duetting songs that maintain reproductive isolation among its cryptic species. We propose that sympatric species of the group will partition acoustic niche space so as to minimize mating mistakes. Sibling species pairs could therefore occupy abutting regions of acoustic space without hybridizing in nature. One species pair, Chrysoperla plorabunda and C. adamsi, are not only closest relatives, but are also the most acoustically similar among sympatric congeners. Using F1 hybrids as a surrogate taxon occupying acoustic space between the parents, we tested the hypothesis that this space cannot support an additional species. Focusing on a critical song feature, volley period, we confirm firstly that the hybrid song occupies unfilled signal space between the parents. Secondly, we show that song discrimination is strong between parents, but weaker between parents and hybrids. Thirdly, each parent has the ability to respond to the other’s signal in a narrow region of overlap. Finally, preference functions for hybrids broadly overlap those of parents. These acoustic tests confirm that a species of this intermediate song phenotype could not persist.