Water Scavenger Beetle

2012 May 12

While this was photographed on July 26, 2011, I actually found it some time earlier. It had died on the sidewalk on campus sometime earlier that summer, and had been sitting around in my office until I happened to bring my camera to work and take pictures of it. I didn’t want to risk taking it home, because it had dried out and trying to carry it was really likely to break off legs.

The beetle was a full five-eighths of an inch long (1.6 cm), which is pretty large for a beetle.

Looking at its underside, we can see that the four rear legs are kind of paddle-shaped. There are also a couple of odd-looking structures: a pair of nearly circular bulges that I think are muscles for the two front legs (which have tips that look like they might be adapted for grasping), and a long, straight ridge that looks kind of like a boat keel.

Here’s a picture of the “keel” from the side (I’ve also labeled what I think are maxillae, the supplementary mouthparts that most insects have. They seem to be unusually long for beetle maxillae. Then again, maybe they are the antennae? In which case, they are unusually tucked-in and protected for antennae.).

I first thought that this was one of the predaceous diving beetles, family Dytiscidae. However, that keel structure doesn’t seem to occur in many of the diving beetle species, and the few where it does look to be the wrong color. And, after posting this, Mike left a comment below saying that it was actually a Water Scavenger Beetle, family Hydrophilidae. Some of these are pretty large, and the ones in the genus Hydrochara look particularly close

In any case, I suspect that the keel is, in fact, what it looks like: a structure for helping the beetle swim straight, just like a keel of a boat.

Judging from the shape of the legs and the presence of the keel, I think this beetle is adapted for more or less sustained swimming. The glossy black color would make it stand out if it floated on the surface, so it probably stays submerged and cruises underwater looking for food, rowing with the four hind legs. They are called water “scavenger” beetles, but they evidently are happy to be predatory if they get a chance (but will eat pretty much anything).

3 Responses
  1. May 12, 2012

    Very neat find! It is actually a water scavenger beetle (Hydrophilidae), which are also aquatic beetles that can reach large sizes. The keel on the ventral side is one of the identifying characteristics.

  2. May 13, 2012

    I’m finding very small versions of this in our pond… what are they?

    pics near top of page (with snail), about pencil eraser size…

    there are smaller ones, about half the size of a grain of rice…

  3. May 15, 2012

    Thanks, Mike! I’ve rechecked and corrected the post based on your comment.

    Teanna: I don’t know what your beetle is. I assume you mean the one in the middle of the top row. It may be another species of water scavenger beetle, or it may be a whirligig beetle.

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