It’s been a while since I’ve done an identification post, so it’s time for a new one!  Today I’m going to focus on two aquatic beetle groups that a lot of people have a hard time telling apart from one another: the predaceous diving beetles (family Dytiscidae, the dytiscids) and the water scavenger beetles (family Hydrophilidae, the hydrophilids).  Once you know exactly which parts you should be looking for it’s easy, so let’s jump right in!

For these two groups of beetles, you really need to look at some body structures to properly ID them.  This means it’s a whole lot easier to ID them if you take them out of the water for a close look.  However, the body shape may give you a clue.  Let’s look at the dytiscid first:

The dytiscids are extremely streamlined and smooth.  Notice how the top of the beetle is rather domed?  And how the bottom is rounded?  These are characteristics of most of the dytiscids that allow them to swim very efficiently.  If you cut one in half across the middle of the abdomen, the shape of the cross-section would be nearly oval.  Now compare this to the hydrophilid:

The hydrophilids are also streamlined and smooth, but they are a different shape.  Notice how the underside of the beetle is mostly flat, compared to the broadly rounded belly of the dytiscid.  Also, you can’t see it in this view, but the beetle is shaped like an inverted V  or U along the abdomen so that the eltyra slope down away from the center line of the bug like the keel of a boat.  If you cut one of these beetles in half, it would be roughly triangular of semi-circular in cross-section.  So, the dytiscids tend to be very curvy while the hydrophilids are more angular and have some flat edges.  This doesn’t hold true for every member of either group, but it is a general trend.  With practice, you’ll start to notice general body shapes that will let you identify them without taking them out of the water.

Now let’s flip the beetles over for a moment.  First, I’ll draw your attention this structure:

Many hydrophilids have a long, sharp spine that runs down the center of the thorax and over the base of the abdomen.  It can be thick and heavy like this one or long and slender.  Not all hydrophilids have these spines, but if you see the spine, you can be sure that it is a hydrophilid and not a dytiscid.  Dead giveaway!  But let’s pretend for a moment that this beetle is one of the hydrophilids that don’t have a spine.  Then we need to look at a different part.

As you probably know, the abdomen and the thorax of insects are made up of several subsections.  The thorax is made up of three sections, with one pair of legs attached to each.  The abdomen is historically made up of 11 sections, but many insect groups have combined sections and now have less than 11.  The first section of the abdomen directly behind the thorax (also called A1) is an important section to look at when identifying beetles because it helps you determine which of the four suborders the beetle in question belongs to.  Dytiscids belong to the suborder Adephaga while the hydrophilids belong to the Polyphaga, so their A1 sections look different on the underside of their bodies.  Let’s look at this section on the hydrophilid first:

I know it’s hard to see in the photo, but the legs of this beetle sit on top of A1 and do not split it into two parts.  You can follow the line of the A1 section closest to the back end of the beetle across the entire beetle without interruption.  The dytiscids are different:

Their hind legs break the A1 section apart so that part of the section lies on either side of the legs.  In these beetles, you cannot follow the line of A1 closest to the back end of the bug all the way across the beetle without interruption because the legs get in the way.  If you look closely at the photo (which is admittedly not as clear as it could be), you can see the first section of the legs (those two little bumps the rest of the legs are attached to) extending beyond the A1 section and the two parts of A1 on either side.

It’s going to be impossible to figure out whether a beetle swimming in the water has a broken or unbroken A1.  Even if you scoop the beetle out of the water though, this structure can be difficult to see, especially on some of the smaller beetles.  My students have a very hard time with this characteristic and a lot of them never quite figure it out.  This is where the antennae come in handy!

The antennae of dytiscids and hydrophilids are very different, so it’s easy to tell the two apart.  Finding the antennae on these beetles is an entirely different matter though!  Insect mouthparts have a lot of little dangly bits called palps and it just so happens that a lot of aquatic beetles have their palps sticking out right where you’d expect to see antennae.  Let’s take a look, starting with the dytiscid.  This is not an antenna:

Nor is this:

The hydrophilids aren’t any easier.  This is definitely not an antenna:

Nearly all of my students eventually try to run through the identification key using these parts and invariably end up in the wrong place, especially with the hydrophilids.  I don’t blame them!  The palps in both beetle groups are in the right place to be antennae and look a lot like what you’d expect an antenna to look like.  In my experience, aquatic beetles are sneaky buggers and like to hide their antennae, especially if you preserve them for a collection or for identification later.  You frequently find them folded down under their heads alongside the inner margin of their eyes.   This is definitely true in the dytiscid pictured above.  I pulled the antenna out so you can actually see it in this photo:

See why I say they’re sneaky?!  These antennae look a lot like the palps, only longer.  The antennae of dytiscids are filamentous, which means that each segment of the antenna is about the same length and width as the segment before and after.  Now let’s compare that to the hydrophilid antenna:

Hydrophilid antennae don’t look anything like the palps!  They have clubbed antennae, which means that the segments near the tip of the antenna are much wider than the ones near the base.  This means that the antennae of the hydrophilids look absolutely nothing like the antennae of the dytiscids!  If you’re looking at the right part, the actual antennae and not the palps, it is very easy to tell these two beetles apart.  Clubbed antennae = hydrophilid.  Filamentous antennae = dytiscid.  Simple!

You often need to look at a combination of these characters to be sure you’ve correctly identified one of these beetles, but it’s not too bad when you know what to look for.  I’ll end the post with a handy dandy chart summing up what I covered above.  Happy identifying!

Characteristic	Dytiscidae (predaceous diving beetles)	Hydrophilidae (water scavenger beetles)
Shape of cross-section of abdomen	Approximately oval	Approximately triangular or semi-circular
Spine along center of thorax	Never has spine	Spine often present
A1 segment	Split into 2 parts by legs	Continuous, not split by legs
Antennae	Filamentous	Clubbed

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