Mechanisms of high frequency song generation in brachypterous crickets and the role of ghost frequencies

Abstract

SUMMARYSound production in crickets relies on stridulation, the well-understood rubbing together of a pair of specialised wings. As the file of one wing slides over the scraper of the other, a series of rhythmic impacts causes harmonic oscillations, usually resulting in the radiation of pure tones delivered at low frequencies (2-8kHz). In the short-winged crickets of the Lebinthini tribe, acoustic communication relies on signals with remarkably high frequencies (>8kHz) and rich harmonic content. Using several species of the subfamily Eneopterinae, we characterised the morphological and mechanical specialisations supporting the production of high frequencies, and demonstrated that higher harmonics are exploited as dominant frequencies. These specialisations affect the structure of the stridulatory file, the motor control of stridulation and the resonance of the sound radiator. We placed these specialisations in a phylogenetic framework and show that they serve to exploit high-frequency vibrational modes pre-existing in the phylogenetic ancestor. In Eneopterinae, the lower frequency components are harmonically related to the dominant peak, suggesting they are relicts of ancestral carrier frequencies. Yet, such ghost frequencies still occur in the wingsʼ free resonances, highlighting the fundamental mechanical constraints of sound radiation. These results support the hypothesis that such highfrequency songs evolved stepwise, by a form of punctuated evolution that could be related to functional constraints, rather than by only the progressive increase of the ancestral fundamental frequency. Supplementary material available online at

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

“…7C). Although the pre-mirror was reduced or lost in this lineage (Béthoux, 2012), and the mirror is maintained, the main radiating structure is composed of an area encompassing h1 and h2 (Bennet-Clark, 2003;Montealegre-Z et al, 2011a;Robillard et al, 2013), which have a minor or no contribution in sound production in Cyphoderris spp. (Fig.…”

Gnominthus gen. nov., a new genus of crickets endemic to Papua New Guinea with novel acoustic and behavioral diversity (Insecta, Orthoptera, Gryllidae, Eneopterinae)

“…This picture is however questioned by several recent studies which suggested that this is perhaps a simplification of a larger spectral diversity. First, the biomechanical study of Agnotecous robustus showed that the dominant frequency of this species is in fact not a harmonic of the first peak, but a high fundamental frequency generated by an accelerated tooth impact rate (Robillard et al, 2013). Second, a few Lebinthus species presented songs that did not possess the same frequency spectrum as for the species previously described.…”

“…et al, 2009). However, if the wing velocity better matches a higher instantaneous velocity, it suggests that more than one waveform is produced per tooth strike, because of the harmonic vibration of the wings, as suggested for some other eneopterine crickets (Robillard et al 2007(Robillard et al , 2013. Also, when the main peak of song frequency corresponds to a second or third harmonic, there is generally a corresponding modulation of the sound wave, so that every third or fourth sound wave is slightly higher in amplitude, followed by some dampening.…”

“…This is the case for the eneopterine fauna of New Guinea Island, which is known only through a few briefly described species (e.g., Brunner von Wattenwyll, 1898, Chopard, 1931, Bhowmick, (1979)1981, Otte, 2007. The eneopterine species from New Guinea all belong to the tribe Lebinthini Robillard, 2004, which are the first crickets known to communicate with high frequencies (>10 kHz), and in some species with ultrasonic signals (26-28 kHz) (Robillard and Desutter-Grandcolas, 2004a;Robillard et al, 2007Robillard et al, , 2013. Recent results supported the hypothesis that high-frequency songs may have evolved in a stepwise manner rather than only by a progressive increase of the ancestral dominant frequency (Robillard et al, 2013).…”

“…The eneopterine species from New Guinea all belong to the tribe Lebinthini Robillard, 2004, which are the first crickets known to communicate with high frequencies (>10 kHz), and in some species with ultrasonic signals (26-28 kHz) (Robillard and Desutter-Grandcolas, 2004a;Robillard et al, 2007Robillard et al, , 2013. Recent results supported the hypothesis that high-frequency songs may have evolved in a stepwise manner rather than only by a progressive increase of the ancestral dominant frequency (Robillard et al, 2013). The basis for this resides in the preexisting harmonic structure of songs generated by the resonant properties of wing radiators.…”