Arachnoscelis arachnoids, a rare species of katydid from Central Northeast of Colombia, uses elastic energy and wing movement to reach volumes greater than 110 decibels, which is louder than a diesel truck or a subway train.
Benedict Chivers from the University of Lincoln, UK, with colleagues found a region in Colombia where Arachnoscelis arachnoids (previously known from only one specimen) are very abundant, offering a unique opportunity to study this rare species. They then used ultrasound-sensitive equipment and high-speed video to determine that Arachnoscelis arachnoids males sing at about 74 kHz.
To call distant females, male bushcrickets produce songs by ‘stridulation’ where one wing 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 arachnoids produce their calls during the opening phase of the wing.
“There is strong selection in katydid singing for pure-tone (high quality) calls to increase the effectiveness of the signal for communicating information,” said Benedict Chivers, who reported the findings in a paper published in the Journal of Bioacoustics.
“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.”
Chivers examined the stridulatory apparatus as a function of the high frequency calls used by this Arachnoscelis arachnoids 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.
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 Arachnoscelis arachnoides is vital to understand the evolution of ultrasonic communication in this group, from ecological interactions to the neurophysiological process of ultrasonic hearing.
Bibliographic information: Benedict Chivers et al. Ultrasonic reverse stridulation in the spider-like katydid Arachnoscelis (Orthoptera: Listrosceledinae). Bioacoustics: The International Journal of Animal Sound and its Recording, published online July 23, 2013; doi: 10.1080/09524622.2013.816639