Rare bushcricket’s chirp as loud as a power saw
An undergraduate researcher is joint first author of a new paper detailing why a recently rediscovered species of bushcricket has an unusually loud and ultra-high frequency calling song.
Ben Chivers, who is studying animal behaviour at the University of Lincoln, UK, co-authored the paper which illustrates the process in which the katydid or bushcricket Arachnoscelis arachnoides produces sound.
Using ultrasound-sensitive equipment and high-speed video the team determined that the males sing at about 74 kHz, using elastic energy and wing movement to reach such high ultrasonic frequencies involving sound levels of about 110dB – comparable to that of a power saw.
To call distant females, male katydids produce songs by ‘stridulation’ where one wing (the scraper) rubs against a row of ‘teeth’ on the other wing. This is a multiplication process by which the slow motion of the wings is multiplied to the high frequency vibrations produced by scraper and file-teeth encounters.
For sound production the male opens and closes the wings but universally in most species the songs are only produced during the closing phase. Different from most katydids, male Arachnoscelis produce their calls during the opening phase of the wing.
Arachnoscelis was originally described in 1891 based on a single specimen. Since then they have not been seen and remained a mystery in the rainforests of Colombia.
Several similar katydids were erroneously allocated in Arachnoscelis, but Dr Fernando Montealegre-Z, lecturer at the University’s School of Life Sciences, discovered an abundant population of A. arachnoides in Colombia. This offered a unique opportunity to study these exotic creatures which are close relatives of an unusual group of high ultrasonic insects.
This information was recently released in another publication, where Ben and Dr Montealegre-Z proposed that Arachnoscelis should be a genus incorporating only the one species arachnoides.
Ben examined the stridulatory apparatus as a function of the high frequency calls used by this species which were recorded at around 74 kHz. At such high frequencies, high quality pure-tone calls are too short to be effectively heard by the females.
The paper ‘Ultrasonic stridulation in the spider-like katydid Arachnoscelis’ was published in the Journal of Bioacoustics on Wednesday, 24th July.
Ben said: “There is strong selection in katydid singing for pure-tone (high quality) calls to increase the effectiveness of the signal for communicating information. The file and the scraper are the first step in sound production and my undergraduate research on nearly 50 species of katydids revealed a correlation between the quality of the acoustic signal produced and the structure of the stridulatory file. Arachnoscelis arachnoides exhibits a file with a tooth distribution consistent with the broadband (low quality) calls associated with singing in the high ultra-sonic range.”
Arachnoscelis arachnoides artificially lengthens the call through the introduction of silent intervals. This process allows effective signal transmission to the females while maintaining high ultrasonic frequencies. A side effect is that the purity of the tone is lost and the call covers a wide spectrum of frequencies.
Therefore A. arachnoides is vital to understand the evolution of ultrasonic communication (hearing and singing) in this group, from ecological interactions to the neurophysiological process of ultrasonic hearing.
Dr Montealegre-Zapata, who has been mentoring Ben, said: “These katydids usually exhibit small body sizes and their muscular mass is so minute they might not be able to generate the necessary power to close the wings at elevated speeds. Therefore, they use scraper deformation (stored elastic energy) to achieve an increased high-frequency tooth strike rate. This means that during the opening phase the wings are paused each time when pushing the scraper behind a tooth to store elastic energy by deformation (this is what causes the silent intervals and call lengthening) – energy is transferred when the outside force of the wing deforms the scraper.
“We have evidence that ultrasonic signalling in A. arachnoides involves sound levels of about 110dB which are considered unusually loud for such a small insect. The mechanism of scraper distortion is therefore a good candidate to be responsible for the high sound intensity observed in the calls: at such ultrasonic frequencies, you need to be loud to be detected by females.”
The paper describes how this discontinuous scraper motion does not require the systematic organisation of teeth as observed in other species. As a novelty, the paper also documents the relatively rare method of reverse stridulation (producing sound on the opening of the wings) at the high ultrasonic frequencies used by this species.
Credit to Natasha Mhatre for the image used.