Identifying American Giant Water Bug Genera

In my last post, I went over what to look for to distinguish a giant water bug (remember: they belong to the family Belostomatidae) from other true bugs.  Today, I’ll go over what to look for to tell the different American genera of water bugs apart by showing you examples ofArizona’s giant water bugs.

This is Abedus herberti:

Abedus herberti

Abedus herberti

This bug is near and dear to my heart since he was the subject of my Master’s thesis.  I think Abedus is definitely the cutest of the giant water bugs (if any of them can be considered cute)!  This is a medium sized water bug, a little over an inch long, and most of the American Abedus are about the same size.  So, how do you tell this bug apart from the other water bugs?  It’s easy!  Take a look at the back end of the bug.  First, you’ll notice that it doesn’t have the long respiratory siphons you find in some water bugs.  If you recall from my last post, these are called air straps, the shorter of the two types of respiratory appendages in the belostomatids.  You should also notice that this bug is broadly rounded, particularly in the back.  The other two American genera of giant water bugs have pointed tips at the end of their abdomens.  So, the combination of the rounded back end and short air straps lets you know that this water bug belongs to the genus Abedus.

You’ve seen him before, but let’s take a look at Lethocerus medius one more time:

Lethocerus medius

Lethocerus medius

You should be able to tell right away that this bug is very different from the cute little Abedus above.  First, you’ll notice the long respiratory siphon at the back end.  This bug has his siphon fully extended (they do look quite a bit shorter when they’re retracted), so the different between the respiratory siphon of this bug and the air straps of Abedus should be immediately obvious.  The shape of Lethocerus is also distinctive.  This bug is robust and strong, so he has huge raptorial forelegs.  He’s also pointed at the back end.  Lethocerus species are BIG bugs.  The biggest true bug in the world is a species of Lethocerus in fact.  L. medius is actually fairly small for a Lethocerus, only about 2 inches long, but it’s still a formidible looking bug.  Some of the biggest members of this genus can be close to 5 inches long!  So, if you find a water bug that is large, pointed at the back end, and has a respiratory siphon instead of air straps, you know you’re looking at a Lethocerus.

The last genus we have in the U.S. is Belostoma.  This is Belostoma flumineum:

Belostoma flumineum

Belostoma flumineum

The American Belostoma tend to be smaller than Abedus and Lethocerus.  This particular species is about 3/4 of an inch long.  It’s a little hard to tell from the photo, but this bug has air straps and not the long respiratory siphon.  Belostoma is more rounded overall than Lethocerus as well, but it is also pointed in the back.  You can tell Belostoma apart from Lethocerus easily by looking at the size of the bug and the presence of air straps.  Belostoma is easy to distinguish from Abedus simply by looking at the shape of the back end: pointed in Belostoma and rounded in Abedus.

While the U.S. species of giant water bugs tend to fall into nice, neat little size categories (Belostoma is smaller than Abedus which is smaller than Lethocerus), the water bugs in other locations show a lot more variation.  For example, in South America you will still find Lethocerus, Abedus, and Belostoma, but the size ranges will be quite different than they are in the U.S.  The South American Belostoma, for example, include some of the smallest and largest species of the genus.  They range in size from a little over 1/4 inch to almost the size of Lethocerus!  In South America, you wouldn’t be able to use the same size characteristics to distinguish the different genera of water bugs.  However, the type of respiratory appendages a bug has and the shape of the tip of the abdomen will still tell you which genus you have, regardless of the size.

Next time, I’ll go over the behavior that has made the belostomatids famous among entomologists, parental care.  Water bugs have some seriously cool behaviors and parental care is one of the best.  Prepare to be shocked and amazed!


Text and images copyright © 2009


8 thoughts on “Identifying American Giant Water Bug Genera

  1. Hi
    I am a biologist in Tucson, AZ. My specialty are AZ Coleoptera, not true bugs, but I am photographing a lot of those, too. I had a big Lethocerus land at my blacklighting sheet in Silvercity, NM, and my friend Arlene found the same (?) sp in her pool in Cochise Stronghold, AZ. It looks very similar to your L. medius, but it is larger than 2 in – 56mm without the siphon.
    I don’t know how to build a link here, so I’m just including the address

    What do you think?

    Margarethe Brummermann

    • I looked it up and unless something has changed in the last 40 years since the last serious survey of Lethocerus in the Americas was published, your specimen is Lethocerus medius based on the distribution information you provided. You can also take a look at the forelegs to be sure. If the femurs on the front legs have a pair of grooves on the inner side for reception of the tibia, it’s medius for sure!

  2. Pingback: How to Identify Belostomatids « Myrmecos Blog

    • Yes, I know! I look for water bug images to use in talks every once in a while and it’s amazing how many are mislabeled. That’s why I thought I should do a post on them. They’re really not hard to tell apart if you know what to look for.

  3. Hello!

    I’m an undergraduate student at Florida State University, and I’ve been doing some individual research on Belostoma Lutarium. Your information has always been helpful, thank you for keeping such a detailed and interesting log.

    I wanted to ask you about a behavior I saw in the lab today from one of the bugs. As I picked up the plant one of them was perching on and took it out of the tank, it did the most bizarre thing! It parted its ‘air straps’ and squirted this clear fluid onto the wall ~2 feet away!

    I have never seen one do this before! There is a book out there that mentions that Lethocerous squirts an ‘obnoxious fluid’ from its anus. I was wondering if you knew anything more about this behavior, where the fluid comes from or is stores, and maybe what it’s made of? Have you ever seen this happen with any of your bugs?

    • This happens all the time! Lethocerus is very big on squirting people when they handle them. It is probably a defensive mechanism against predation because they never do it unless threatened. What is it, where it stored, that I’m not so sure about. Some researchers think it’s digestive contents, but it could also be related to the trail marking chemicals that Lethocerus uses. Either way, the fluids would come out the back end of the bug and I think they have to get the respiratory siphon out of the way to squirt, hence the parted air straps you observed. Fun!

      I actually got squirted this weekend when I was doing an outreach activity at the Biosphere and the kids totally missed it! I was so sad to miss the opportunity to talk about it. Kids LOVE it when things excrete feces or anything that looks like urine on you. :)

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