Giant Black Water Beetle

A co-worker recently brought in this enormous beetle, understandably curious as to its identity. I knew right away it was one of the "predaceous diving beetles", as I have been seeing them of late in some of the vernal pools while seeking salamanders.

She had found the beetle in a parking lot - no water nearby - which isn't too surprising. These beetles, water-loving as they may be, fly well and are often attracted to well lit places at night.

After a bit of shuffling through some literature, I was able to pin down the ID as Hydrophilus triangularis, the so-called giant black water beetle (at least I think it is; let me know if I am mistaken). I suspect that the specific epithet triangularis stems from the almost perfectly triangular plate on the beetle's carapace, shown above.

It's hard NOT to notice one of these things - they are about an inch and a half long! Beetles in the family Hydrophilidae are called water scavenger beetles, and can easily be confused with a similar family, the Dysticidae, the predacious water beetles.

Even though this species and others of its ilk are termed scavenger beetles, many are predatory.

If you've ever noticed these large beetles in the water, you'll quickly see what good swimmers they are. A beetle in the drink will occasionally surface and trap a small air bubble under its body, then descend into the depths hunting for food. The water scavenger beetles differ from the predacious diving beetles in their method of surfacing. They pop to the surface head first, while predacious diving beetles surface tail first. Another easy way to tell the two groups apart in the water, at least if it is a larger species, is their method of leg locomotion. The scavengers row their legs alternately, while the predacious beetles row them in tandem.

One feature that is apparent with the beetle in hand is the incredibly waxy smoothness of the carapace, or upper shell. It's like a freshly waxed surfboard, and such slickness undoubtedly greatly reduces drag while swimming.

These feathery brushlike projections on the oar legs are conspicuous with the animal in hand, and are presumably yet another adaption for helping the animal propel itself through the H2o.

This long spine, running lengthwise down the center of the ventral surface of the beetle, is an intimidating feature. It suggests a large hypodermic syringe.

A closer look at the spine. It'd be cool if the beetle used that lance to spear its prey, but it doesn't. I don't know the purpose of this feature, and could only speculate that it serves to provide structural reinforcement.

Watch for these interesting beetles if you are around the margins of ponds, vernal pools or nearly any shallow vegetated wetland.