When a homo sapien male makes a loud noise rubbing his penis against his abdoment, it usually leads to a court appearance. When the male micronecta scholtzi does the same, it’s hailed as a new record in nature.
The river bug, also known as a lesser water boatman, has been recorded at a hair under 100 decibels when issuing the genital-based mating call. Given the insect is around 2.3 millimeters — approximately one-tenth of an inch — long, that gives it the highest sound-to-size ratio in the natural world. (The image above is an example of a larger member of the water boatman family.)
The action may be difficult for human to emulate as the water boatman has a couple of advantages. As well as having a defined ridge on its penis, it also has a series of ridges on its abdomen: consider it the ultimate in washboard abs. But given the area where the rubbing takes place is only around 50 micrometres (one twentieth of a millimeter, barely half the thickness of a human hair), and there’s no obvious signs of natural amplification, it’s still something of a mystery how the noise is so loud.
To put it in perspective, not only is the original sound roughly as loud as a passing motorbike, but even when it has lost most of its power passing from the water to the air, a human walking alongside a river can still hear the sound made by a water boatman at the bottom of the river.
One theory is that the water boatman is either easily able to evade predators that would be attracted by the sound, or that no such predators exist. That would remove any incentive to quieten the mating call in the way of many other animals.
Another factor in the volume could be the fact that the water boatman has a small pocket of air along its front: it may be using the contrast between this air and the surrounding water to boost the noise.
(Image credit: Flickr user TiggerT, used under Creative Commons licence)