Sexing Giant Water Bugs

Female giant water bug

Female giant water bug

I am asked how to tell male and female giant water bugs apart more often than I would have ever imagined.  It would seem that there are a limited number of people in the world who can sex water bugs, so I have a rather steady stream of people asking me how to do it.  A few weeks ago I started looking around online to see if there was any information about sexing water bugs, but there’s really nothing available.  Well, there’s one article that is absolutely and completely wrong (I’m not even going to link to it, but don’t listen to anything the person who wrote the eHow article on the subject says!), so that’s no help at all.  It’s high time that this information be made available online!

Sexing belostomatids (aka, giant water bugs) isn’t always trivial.  The males and the females have the same coloration, so you can’t use color differences.  The females are often a little bigger than the males, but there’s a lot of variation in size in both sexes and it can be difficult to tell whether an individual is “big” or “small” unless you have many other individuals to compare with it.  You can sometimes use behaviors to distinguish sexes, especially during the mating season.  For example, if a bug is carrying eggs on its back or climbing up a stick to tend to eggs, it’s a male.  But… you also can’t be sure you have a female just because one doesn’t have eggs, even during the peak of the mating season.  Instead, you need to rely on a structure that varies reliably between the sexes.

Giant water bugs DO bite, but you’re going to have to hold them in place if you want to tell which sex you’ve got.  That usually involves picking them up, so if you do choose to sex water bugs yourself, be careful!  When I pick up water bugs, I use my middle finger and my thumb and press the bug down onto the bottom of whatever habitat/container they’re in.  I then grasp water bugs by the sides of the thorax, as it’s a very rigid part of the body where you can get a firm grip, and use my index finger to support the bug (I apologize that I only have a grainy black and white photo of this…):

Holding giant water bugs

Holding giant water bugs

You need to hold on tight, especially with the larger species, because they are surprisingly strong and will try to wiggle free.  Once you get a good hold on one, you’ll need to flip it over.  The part you need to look for is on the bottom of the abdomen:

Genital plate

Genital plate

This structure is called the genital plate, and it conceals the reproductive organs underneath.  Don’t worry!  You don’t need to go digging around to find internal parts and can use the shape of the genital plate itself.  The shape of the plate varies from genus to genus, but there are some general rules.  The genital plates of male water bugs are smoothly rounded at the tip (the part closest to the back of the bug) and complete (i.e., have no splits or gaps).  For example, the genital plate of males in the genus Abedus look like this:

Abedus male genital plate

Abedus male genital plate

The arrow points to the continuous and smoothly rounded tip of the genital plate.  Here’s a drawing in case the shape is difficult to see in the photo:

Abedus male genital plate

Abedus male genital plate

See?  Rounded at the tip, no splits or gaps.  The females are different.  In some water bug genera, there are splits, notches, or gaps at the tip of the genital plate, so that the line around the tip of the plate is broken.  Many of them have flattened areas at the tip of the genital plate so that they are not completely round.  Most have two distinct little tufts of hairs, either at the edges of a notch at the tip of the genital plate or alongside the midline of the genital plate near the tip.  For example, here’s a female Abedus:

Abedus female genital plate

Abedus female genital plate

In this image, the arrow pointing up from the bottom indicates the flattened, slightly notched part of the genital plate while the arrow coming from the side points to the tuft of hair on the right side of the midline.  The drawing:

Abedus female genital plate

Abedus female genital plate

The little tufts of hair can be quite small and the shape of the genital plate only subtly different from the male, but there are definitely two little tufts of hair on either side of the midline, a flattened area at the tip, and a small split at the tip of the genital plate in Abedus females.

These structures can vary in appearance from species to species, and especially between genera.  For example, this is the genital plate of a male Lethocerus:

Lethocerus male genital plate

Lethocerus male genital plate

Even though it’s much longer and narrower than Abedus, the arrow indicates the same sort of rounded, unbroken tip of the genital plate.  There is a fringe of hair along the tip of the plate in this species, but note that there are no distinct tufts of hairs anywhere.  There’s a groove that runs the length of the middle of the plate, but it doesn’t leave a gap at the tip.  In contrast, here’s the female:

Lethocerus female genital plate

Lethocerus female genital plate

The arrow here points to the tuft of hair to the right of the midline.  In this species, the tufts are at the very tip of the genital plate rather than on the upper surface and recessed from the edge as in Abedus.  There is also a small notch between the two tufts, though it’s a little difficult to see in the photo.

And that’s it!  Smooth, round genital plates in males and flatter, sometimes broken genital plates with two tufts of hairs in females.  In the US, the genital plates of the genus Belostoma are very similar to those shown here for Abedus and other American Lethocerus are similar to the Lethcerus pictured here, so the images here will help with sexing US water bugs.  Other genera in other countries follow similar patterns, but may have some slight differences.

Good luck!

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