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!

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