Functional morphology of tegmina-based stridulation in the relict species Cyphoderris monstrosa (Orthoptera: Ensifera: Prophalangopsidae)

Abstract

Male grigs, bush-crickets and field crickets produce mating calls by tegminal stridulation: the scraping together of modified forewings functioning as sound generators. Bush- (Tettigoniidae) and field-crickets (Gryllinae) diverged some 240 million years ago, with each lineage developing unique characteristics in wing morphology and the associated mechanics of stridulation. The grigs (Prophalangopsidae), a relict lineage more closely related to bush-crickets than to field-crickets, are believed to retain plesiomorphic features of wing morphology. The wing cells widely involved in sound production, such as the harp and mirror, are comparatively small, poorly delimited and/or partially filled with cross-veins. Such morphology is similarly observed in the earliest stridulating ensiferans, for which stridulatory mechanics remains poorly understood. The grigs, therefore, are of major importance to investigate the early evolutionary stages of tegminal stridulation, a critical innovation in the evolution of the Orthoptera. The aim of this study is to appreciate the degree of specialisation on grig forewings, through identification of sound radiating area areas and their properties. For well-grounded comparisons, homologies in wing venation (and associated areas) of grigs and bush-crickets are re-evaluated. Then, using direct evidence, this study confirms the mirror cell, in association with two other areas (termed ‘neck’ and ‘pre-mirror’), as the acoustic resonator in the grig Cyphoderris monstrosa. Despite the use of largely symmetrical resonators, as found in field-crickets, analogous features of stridulatory mechanics are observed between C. monstrosa and bush-crickets. Both morphology and function in grigs represents transitional stages between unspecialised forewings and derived conditions observed in modern species.