So what is it- A bird? A rodent? A dog? Here’s a hint- it lives in rotting wood, and has six legs.
Yes, it is an insect. Odontotaenius disjunctus, commonly known as a bess beetle (or crying beetle), and it has the most advanced sound-based communication system known in arthropods. You can find it under a moist rotting log anywhere from here in the Amazon up to New York. This beetle is large (~2 inches) and widespread and would be quite noticeable if it weren’t for its rather hidden natural history.
|A bess beetle family, under a log.|
Flip enough rotting logs, dig through enough decaying leaves, and you’ll eventually find a group of bess beetles get a look inside their fascinating life.. You’ll see a few adults, many grub-like larvae, and some round clumps of dirt that contain developing pupae. Poke around a bit, and the sounds begin. The beetle is known locally as a ‘crying beetle’ due to its ability to make a high-pitched whiny alarm call if you disturb it (like by picking it up). But the alarm noise from the adults isn’t the interesting one; what’s fascinating are the fourteen almost inaudible squeaks that the larvae make below.
All insects have six legs, including their larvae. But take a close look at these beetle larvae, you’ll see only two pairs of legs, for a total of four- so what happened to the other two? Over the course of evolution and thousands and thousands of years, the back pair of legs got smaller. Much smaller. They’re no longer used for moving around in dirt; rather they’re used to meaningfully strum a structure called a plectrum.
|Bess beetle larva. Note only 2 pairs of legs. |
Photo by Whitney Cranshaw, Colorado State University, Bugwood.org.
Imagine a tiny stump of a leg strumming a comb in various directions- that’s what is happening between the hind leg and the plectrum. As the stump hits the teeth of the comb at different angles or different timespans, the frequency of the sound changes ever so slightly, resulting in overall different noise, or call. Researchers listening in found that there were 14 distinct noises made in a variety of situations via this stump-and-plectrum combo.
What we don’t yet have is a translator. Does one larva squeak mean “feed me!” (which adults do by defecating mostly-broken down wood)? Another angle strummed, “Protect me! Someone lifted the log” or “Help me molt”?
Who knows. What we do know is this is a remarkably advanced sound-based communication system for an insect. Other insects stand out by their use of things like body movements or scented pheromones (ants and bees) to deliver complex bits of information, so it’s no surprise that an insect has evolved the ability to do that by making a noise, especially when living in a medium like soil in which movement is hardly visible and pheromones slow to disperse.
Some day, some scientists will figure out what these larvae are saying. And how fascinating that will be. Until then, flip a log, take a peak at this miniature advanced world, appreciate the wonders of nature, and don’t forget to (gently) put the log back in place.