Acoustic emissions from insects.
Here is a recording of a conference concerning Cicadidae of Taiwan, by biologist Chen Chen-Hsiang. Taipei Artist Village, 21 Feb. 2008.
The ears under the water level : tympani are blocked and sounds transits through bone conduction. There, the audition is much less sensitive than in the air.
So I was trying to catch sounds from water bugs, I began to use home-made hydrophone. It's much better than spending too much time the head in ponds...