Differentiation between left and right wing stridulatory files in the field cricket Gryllus bimaculatus (Orthoptera: Gryllidae)

Abstract

Male crickets produce acoustic signals by wing stridulation, attracting females for mating. A plectrum on the left forewing's (or tegmen) anal margin rapidly strikes along a serrated vein (stridulatory file, SF) on the opposite tegmen as they close, producing vibrations, ending in a tonal sound. The tooth strike rate of the plectrum across file teeth is equal to the sound frequency produced by the cricket (i.e., ∼5k teeth/s for ∼5 kHz in field crickets) and is specific to the forewing's resonant frequency. Sound is subsequently amplified using specialised wing cells. Anatomically, the forewings appear to mirror each other: both tegmina bear a SF and plectrum; however, most cricket species stridulate using right-over-left wing overlap making the stridulatory mechanism asymmetrical by default, rendering the left tegmen's SF unused. Therefore, we hypothesised structural differences between functional and unfunctional SFs. Three-dimensional mapping was used to accurately measure SF structures in Gryllus bimaculatus wings. We found that the left SF shows significantly greater variation in inter-tooth distance than the right, but less variation within the first sixty teeth (the functional part) than the right file. The left SF's slow evolutionary change over millions of years is discussed considering modern molecular phylogenies and fossil records.