Common features and species-specific differences in stridulatory organs and stridulation patterns of velvet ants (Hymenoptera: Mutillidae)

Due to the mostly qualitative, scarce and scattered available data, it is still debated if stridulation patterns and morphology of stridulatory organ are species-specific traits in velvet ants (Hymenoptera: Mutillidae). Here, we test such an hypothesis, analysing the stridulation patterns and the associated morphology in females of six species spanning five genera and three subfamilies. In all the species, sound is produced by rubbing together a morphologically well-defined file composed of parallel ridges (on metasomal tergite III) and a weakly defined scraper (on tergite II). In all species, the resulting sound is made of sequences of pulse-trains organized in two subunits (disyllabic chirps) characterized by pulses with opposite phase. We found important differences among species in the morphology of the file concerning its size, the inter-ridges distance, the ridge thickness and the density of ridges. We also found some differences in distress-call patterns, mostly concerning the duration of syllables and the number of pulses/ms. Tergite III size seems to have some effects on file morphology and limited effect on sound patterns, though a larger sample size is required to confirm these results. Classification and Regression Tree analyses revealed that five morphological and five acoustical variables had, overall, a role in assigning correctly most individuals to their species. Overall, taxonomic relatedness among species seemed to weakly account for inter-specific morphological similarity and not to account for acoustical similarity, suggesting that ecological factors may have been important in shaping species-specific stridulation in velvet ants.