Abstract
Stridulation patterns and stridulatory organ were rarely investigated in the parasitoid family Mutillidae, and data still lack for any sex or species in many subfamilies. We described the morphology of stridulatory organ and the distress call of Myrmilla capitata (Myrmillinae), a small mutillid wasp in which both sexes are apterous. As in other mutillids, the stridulation is produced by rubbing a scraper on the gaster tergite II against a file on gaster tergite III and consists of sequences of pulse trains organized in two subunits (disyllabic chirps) characterized by pulses with opposite phase. Overall, sexes differed in few morphological and acoustical traits. In particular, females had wider and longer files (due to the larger tergite III), smaller ridge thickness, greater inter-ridge distance and produced sound with lower maximum frequencies. This is in accordance to the fact that individuals with wider files, and files with thicker and more separated ridges, tend to emit sounds reaching lower frequencies. Both sexes, on average, seemed to "use" about 50% of the ridges while stridulating, and the file is moved against the scraper with an average speed of about 6 mm/s. Syllable duration and pulse rate (number of pulses/s) were also affected by some morphological traits of the file. Thus, intra-specific variation in the morphology of the stridulatory organ affects stridulation features in this small velvet ant with relatively weak sexual dimorphism.