They produce sound by raking a file on the base of their left forewing with a scraper on right forewing. The file is a modified vein with file-like teeth. As the insect closes its wings the scraper is raked over the file. The frequency of their sound increases as the scraper more rapidly rakes the file. When necessary, they can increase the volume by slightly raising their forewings, forming a small cavity in which the sound resonates and is amplified. Our hearing is not sensitive enough to hear their entire melody because their ultrasonic range extends above the 20 kHz upper limit of our hearing! They apparently achieve these high frequencies by straining the scraper against the file in such a fashion that when it breaks loose and glides down the file it is under greater pressure and moving significantly faster. http://msucares.com/newsletters/pests/gloworm/2008/glow0508.pdf
