The incredibly loud world of bug sex

Micronecta scholtzi

The hills of the European countryside are alive in the chorus of amorous, screaming, male aquatic bugs. The little insect above, Micronecta scholtzi (Corixidae), measures in at a whopping 2.3mm and yet produces a clicking/buzzing sound easily audible to the human ear above the water surface. To put that in perspective: trying to hear someone talk underwater while standing poolside is nearly impossible, yet this minute insect generates a click loud enough to be mistaken for a terrestrial arthropod. While that doesn’t sound too impressive when we are surrounded by other loud insects like the cicada, M. scholtzi turns out to be a stunningly loud animal when you take into consideration the body size and medium the sound is propagating through to reach our ear. Put into numbers the intensity of the clicks underwater can reach up to 100 dB (Sound Pressure Level, SPL). Shrink us into the insect world and this sound production is equal to a jackhammer at the same distance! So what on earth has allowed this little bug to make this noise and get away with it in a world full of predators?

The authors naturally point out how surprising these results are. The first thing that becomes apparent is that the water boatmen must have no auditory predators since they are basically swimming around making the most noise physically possible for any small animal anywhere. Really this isn’t too surprising since most underwater predators are strictly visual hunters (dragonfly larvae, water bugs and beetles etc…). It is very likely that sexual selection has guided the development of these stridulatory calls into such astounding levels. The second most surprising thing is clear once you graph just how loud these insects are relative to their body size. At the top of the graph is the bottlenose dolphin (T. truncatus) with its famous sonar. But the greatest outlier is actually our little insect in the bottom left with the very highest ratio between sound and body size (31.5 with a mean of 6.9). No other known animal comes close. It is likely though that further examination of other aquatic insects may yield similar if not more surprising results!

To be more accurate about the “screaming”, the bugs (bugs in this instance is correct; the Corixidae belong to the order Hemiptera – the true bugs) are likely to be stridulating – rubbing together two parts to generate sound instead of exhaling air, drumming, stb.… In the article the authors speculate that the “sound is produced by rubbing a pars stridens on the right paramere (genitalia appendage) against a ridge on the left lobe of the eighth abdominal segment [15]”. Without pulling up their citation, it appears that stridulation by males in the genus is well documented for mate attraction. And as you would expect, news outlets and science journalists read “genitalia appendage” and translate that to penis: and you end up with stories like this. The function of the parameres can be loosely translated to similar to mandibles in that they are opposing structures (usually armed with hairs) for grasping. A exact use of them may differ by species or even orders, but they are very distinct form the penis (=aedeagus) since they simply help facilitate mating and don’t deliver any sperm. So in reality you have genital “claspers” with a “pars stridens”. And the best illustration of a pars stridens is over on the old blog Archetype. This structure is highlighted below in yellow (and happens to exist on the abdomen of the ant). De röviden – it’s a regular grooved surface akin to a washboard. In the end the sentence quoted above should be translated to “two structures at the tip of the abdomen that rub together like two fingers snapping”.

Detail of the pars stridens (in yellow) on the forth abdominal tergite in a Pachycondyla villosa worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Sueur J, Mackie D, & Windmill JF (2011). So small, so loud: extremely high sound pressure level from a pygmy aquatic insect (corixidae, micronectinae). PloS one, 6 (6) PMID: 21698252