Searching for ultrasonically singing katydids in the Colombian rainforest, Fernando Montealegre-Z and Glenn Morris of the University of Toronto found some insects who can really hit the heights. Having transported the insects back to his hotel room in Cali, Colombia, Morris switched on his ultrasound detector and was astonished to discover that the insects were singing at an ultrasonic frequency of 130 kHz, higher than other ultrasonic singers he had recorded before at 83-106 kHz. Intrigued by the insect's 130 kHz ultrasonic song, Montealegre-Z and Morris teamed up with Andrew Mason to examine the song in more detail and find out how it was produced (p. 4923).
Almost all male katydids sing to serenade the ladies by rubbing their forewings together, and many sing ultrasonically above the 20 kHz upper limit of human hearing. One wing has a `file', a modified vein with teeth on it, while the other wing acts as a `scraper'. As the insect closes its wings, the scraper moves over the teeth at a certain frequency, causing the wings to oscillate and produce sound at the same frequency. As the wings close faster, the scraper travels faster over the teeth, producing higher frequency sounds, but this is limited by the speed of the wing muscle contraction. Would this method of sound production work for the high ultrasonic insects?
To investigate, the team recorded the katydid song using sound recording equipment capable of picking up bats' ultrasonic calls, and analysed the sound waves. They found that the insects sang a song containing trains of pulses, separated by silent intervals, at 123-129 kHz. This was in contrast to katydids singing ultrasonically at frequencies below 40 kHz, which produce calls made of continuous pulses.
To find out how the high ultrasonic singers' wings were moving to produce the sound, Montealegre-Z explains that the team used high-speed video recordings and stuck small pieces of reflective tape on the wings of some animals, using light-sensitive diodes to pick up the reflections off the moving wings while the insects were singing. This way, the team could correlate the speedy wing movements with the waves of sound. They found that the wing movements of high ultrasonic singers were too slow to account for the high frequency sound, so the team looked at the structure of the scraper more closely to see if anything in its structure could help explain the discrepancy.
Scrutinising the scraper under an electron microscope, they found that the scraper in high ultrasonic katydids was attached to a much larger piece of bendy cuticle than the scrapers in katydids that sing below 40 kHz. They suspect that this feature is responsible for a different sound producing mechanism: rather than continuously moving the scraper over a large portion of the file, the scraper gets stuck behind one of the teeth and bends, storing elastic energy as the wings slowly close. When the scraper can't bend any more, it springs forward very quickly over a group of teeth, generating a single ultrasonic pulse. This might help save the energy needed to contract the wing muscles at high speed, but does mean that the katydids can't produce a long-lasting pulse, as they need to pause the scraper between sound pulses to store up elastic energy.
One mystery still remains, though. Ultrasonic sounds don't travel well in dense, wet jungle, so the team are planning to investigate if the katydids are somehow exploiting their jungle environment to get the message across, or if their songs are falling on deaf ears.
- © The Company of Biologists Limited 2006