Predaceous Diving Beetle

We found several of these beauties while netting in the ponds for newts. Barb immediately identified them as predaceous diving beetles (PDB).  With the name predaceous, I decided they weren't to be handled.  Good decision... their "bite" injects digestive juices, like many other beetles and bugs such as the assassin bug, digesting their prey within its body before  sucking the nutrition out of them.  Having your finger partially digested is said to be a rather unpleasant experience so we slid them into containers using a no touch technique.

Cybister fimbriolatus

With a quick trip to the computer, we were able to identify these as Cybister fimbriolatus in Bugguide.  They are members of the family Dytiscidae which are all commonly called predaceous diving beetles.  They quickly scrambled over the net and are equally quick under water.  

The hind legs, which are flattened and have coarse hair-like appendages, act like paddles as it swims using a rowing technique.  They tend to hangout under water, clinging to vegetation, waiting for dinner to swim by.  They will occasionally chase their prey but usually just wait to be served.

Rowing legs- Patrick Coin

And what do they eat?  Anything they want to.  PDBs are voracious predators, consuming worms, insect larvae, tadpoles, snails and even fish bigger that the beetle itself.  There are videos of PDB consuming a hamster embryo that were too gross for this family page.

The adult has an interesting method of getting its oxygen.  Lacking gills, it swims to the surface, raises its elytra (hard wing covers) and breathes through spiracles on its abdomen.  It hangs on the surface, butt up while keeping its head below the surface to look for prey and predators.  It traps air under its elytra and then dives quickly back down where it breathes in the trapped air, staying underwater for prolonged periods. Click here for pictures of a dramatic dive.

PDB larva

C. fimbriolatus larvae are equally predaceous and aggressive, earning their name "water tigers."  Like most beetle larvae, they look totally un-beetlelike with their long slender bodies and six legs scrambling along the bottom of their pond.  Click here for some PDB larvae scrambling around to the background music of Tuvan throat singers.

Several sources say that PDBs have few predators themselves as they "taste bad."  This always makes me ask "how do we know that?"  Are there biologists who specialize in taste testing?  Do they sip, lick or nibble?  Do they rinse their mouths between each taste?  And how do we know that it tastes bad to a potential predator?  After all, a fish will gobble up a worm or a slimy leach with relish.

Life for C. fimbriolatus isn't totally a bowl of tadpoles.  An article in Pubmed describes a nematode parasitoid which infests it, just like the horsehair worms we recently described.  The worm, Drilomermis leioderma apparently only infests our beetle.  The good news is that 40% of the affected beetles survive for several days, even if several worms have emerged from it.

You may find them occasionally around your porch light.  They are capable fliers, out looking for another pond or a mate.  Are PBDs good or bad?  The answer to that question is always "it depends."  They eat fish and aren't eaten by many other creatures.  However they (a) do eat mosquitoes larvae and (b) in many cultures they are in turn eaten by humans.  I think I will pick (a).

On Bull Creek, no diving beetles were harmed or consumed.
Raising PDB as pets
Life cycle of Dytiscus (PDB) 
PDB information from MDC.

August 31, 2013
I can now attest to the reputation of the water tiger.  I reached into a net today to grab a little squirming critter and felt an intense pain in the tip of my finger.  It swelled up over a minute and continued to throb for an hour.  I think I will recognize this creature next time.