Comparative forewing ultrastructure of high-frequency singing crickets (Eneopterinae)

Male crickets belonging to the tribe Lebinthini (Eneopterinae) produce high-frequency, sometimes ultrasonic, calling songs characterized by a dominant harmonic frequency -a phenomenon known as harmonic hopping. While cricket sound production is well understood, the mechanism allowing the dominant frequency to move to the harmonics of the spectrum is little studied. The wing region known as the harp corresponds to the primary resonator of the cricket tegmina. Here we hypothesize that the surface ultrastructure of this region could play a role in the physical properties of the wings and in their vibratory behavior. In this study, we used scanning electron microscopy to explore the diversity of wing membrane ultrastructure in the harp region of 33 species of Eneopterinae, including species producing both low-frequency and high-frequency calling songs. We highlighted a great diversity of ultrastructure within the subfamily. We defined and measured 5 morphological traits concerning the hexagonal cells visible on the dorsal face of the wing, and the microtrichia, filiform sensory structures present on the ventral face. Significant differences are found in hexagonal cell size and density between males and females, and between the males of the species of the Lebinthini tribe and the non-Lebinthini species. These results suggest a link between the presence of these hexagonal structures and the song frequency value. Surfacic structures could play a role in the stiffening of the wing, a property directly related to vibration frequency of the wing. The observation of these structure and more precisely their size and distribution between high-frequency songs producing species and Low-frequency species constitute a new avenue for the understanding of harmonic-hoppings in Lebinthini crickets.