March is a great time to observe the incredible insects inside ponds and pools. Even temporary puddles are host to some really cool characters. I found predaceous diving beetles (PDB’s) in a couple different places around Mount Pisgah recently, and they sport some flashy features like tail-breathing and larvae with 14 eyes!
Dytiscidae family beetles swim and hunt underwater, and they’re also good flyers! Above, you can see one that I observed getting out of the water and drying off for a minute. Just after the picture was taken, it flew away. Apparently, the run section of their swim-fly-run triathlon is the weak point. Their awkwardly long set of “oar” legs might have something to do with it.
By the way, you can tell PDB’s from other similar water beetles because they move those swimming legs in unison, whereas water scavenger beetles stroke one leg at a time. Predaceous diving beetles can also stash the four non-paddling legs into handy grooves while they’re submerged.
If you look closely at the underwater picture, you can see a bubble at the rear of the beetle. That’s its scuba tank. It has organs called spiracles at the tip of its abdomen, so that when it traps a bubble under its elytra, the beetle has reserves of air at the ready. Yes, they breathe through their bums.
Larvae of these beetles are called “water tigers” because they’re such good predators. They use sharp pincers to snare underwater prey, including snails or frogs bigger than themselves. They also have insanely redundant visual systems. PDB larvae have six simple-lens eyes on each side of their head, plus two “eye patches.” No one yet knows their exact functions or how all those eyes work.
Stay curious!
Karen Richards
Sources:
Amazing video of PDB’s underwater: https://thekidshouldseethis.com/post/natures-scuba-divers-how-beetles-breathe-underwater. Accessed 3/5/21.
Researcher Michael Bok on the PDB larvae’s visual system: https://arthropoda.wordpress.com/2010/03/26/beware-the-water-tigers/. Accessed 3/5/21.
