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First published online November 14, 2008
Journal of Experimental Biology 211, vi (2008)
Copyright © 2008 The Company of Biologists Limited
doi: 10.1242/jeb.011718
Outside JEB |
STRIDULATORY SWEET SOMETHINGS
Mount Holyoke College
srachoot{at}mtholyoke.edu
|
A warm summer night in many places is often accompanied by a symphony of insect songs and calls. The chirps of crickets and the buzz of katydids are usually made by a process known as stridulation where the insect scrapes a `pick' across a series of ridges on its body. Male insects use sounds to announce their presence, their species and their abiding interest in mating. But, with a boisterous song comes the risk of attracting unwanted attention from rival males, predators or parasites. Now, researchers have found a secret world of quiet stridulatory communication occurring privately between mating moths. The sound is imperceptible to the human ear, hardly traveling beyond its recipient, and is picked up by the insect's highly specialized tympanic ear. Ryo Nakano at the University of Tokyo and his colleagues in Japan and Denmark have discovered that the male Asian corn borer moth, Ostrinia furnacalis, makes low-intensity ultrasound at frequencies beyond human hearing by rubbing specialized scales on the wing and thorax together. By doing this the males suppress the escape behavior of otherwise skittish females.
According to Nakano and his colleagues, butterflies and moths usually employ several structures to make sounds – such as stridulation scraping of leg spurs across thickened wing veins, and percussive wing clapping – but none of the previously known means of sound production are found in O. furnacalis. Nakano and his colleagues first recorded vibrations generated by the male moth and realized that the insect was emitting sounds in the ultrasound range of frequencies. Having recorded the sounds, the team searched for their source and discovered sex-specific scales on the male's forewing that rub against similar scales on the thorax. These had not been noticed before, because the sex-specific scales are hidden when the moth is at rest. Scanning electron micrographs that the team gathered show that the morphology of the scales is different from that of ordinary scales on the moth's body, with ridges that are significantly thicker and more narrowly spaced. Removing the sex-specific scales reduced the level of ultrasound considerably, and the insects' mating success decreased as well, indicating the importance of these ultrasonic whispers.
Using high-speed videography and recordings of courting pairs, the team showed that the ultrasound pulses generated by the males correspond with the vibrations of the forewings. The wings also move faster than they do in flight, suggesting that this mode of communication, though subtle, is energetically costly. And finally the team showed that the right and left wings never touch each other, but contact the region of the thorax coated in the specialized scales.
Having discovered this novel stridulatory behavior, the authors checked courting behavior in other moth families and found that they also detect ultrasonic noise above the background level.
This work adds `ultrasonic production' to the varied functions of moth scales and will likely lead others to find similar cryptic systems among singing insects. The tympanic ear, once tuned to help the insect avoid predators, is now used in more intimate conversations.
References
Nakano, R., Skals, N., Takanashi, T., Surlykke, A., Koike, T.,
Yoshida, K., Maruyama, H., Tatsuki, S. and Ishikawa, Y.
(2008). Moths produce extremely quiet ultrasonic courtship songs
by rubbing specialized scales. PNAS
105,11812
-11817.
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