My Favorite Aquatic Insect

Water scorpion

Water scorpion, Family Nepidae, Ranatra quadridentata

I was planning to do another post in my water quality series, but I am not feeling it tonight.  Instead, I am going to discuss my favorite insect, Ranatra quadridentata.  You don’t mind, right?  We’ll get back to the water quality stuff next week.

I got excited about the possibility of pursuing entomology as a career because of dragonflies.  I adore dragonflies and love everything about them.  They were my favorite insects for a long time.  But then I went out collecting with a friend a semester or two after I moved to Arizona.  We were looking for giant water bugs for my research and mucking about in a stream.  Our strainers flashed through the little pools that had formed in the streambed during the dry season and came up full of treasures.   I hadn’t taught my first aquatic entomology course yet, so I didn’t yet know most of what I was looking at.  At one point, I was shifting the leaves, sticks, and other non-insect junk around in my strainer and pulled a stick out.  I was just about to toss it back into the pool when it moved unexpectedly!  I may have been a wee bit startled.  I jumped badly and the contents of my strainer ended up in my lap, the “stick” back in the water.  I scooped it back out and took a closer look.  My first Ranatra quadridentata!  From this improbable beginning, my love affair with the water scorpions began.

Respiratory siphon

Respiratory siphon

Water scorpions belong to the true bug family Nepidae, which is very closely related to the giant water bugs in the family Belostomatidae.  Although the species in the genus Ranatra are long and narrow (they are commonly called water sticks), those in other genera, such as the genus Nepa, look a lot like the larger giant water bugs in the genus Lethocerus.  It’s easy to see how closely related they are just by looking at them.  They are both predators, both large aquatic insects, and they both have respiratory tubes that extend off the back end of the bugs.  The easiest way to tell the two families apart is by looking at the respiratory siphon: giant water bugs can retract theirs into their bodies while the water scorpions cannot.

Some of the water scorpions are big, impressive bugs.  But I don’t love those.  In fact, I haven’t ever seen a live one.  No, I love the water sticks, the little skinny guys.  They’re all over Arizona and I’ve seen thousands, maybe millions.  They are one of those special aquatic insects that have a pollution tolerance value of 11, right off the top of the chart, so they can live in some of the most disgusting water you can imagine.  They can be incredibly abundant in those kinds of waters because they are able to outcompete nearly everything else that might try to live in there.  At one pond we take our aquatic entomology students to, you might get a hundred water sticks every time you swish your strainer through the water.  Dropping a stick or a rock into the pond results in hundreds or thousands of water scorpions boiling up to the surface.  It’s impressive!  (Wish I had a photo of it…)  The bugs also play dead when you pick them up.  They really look like sticks.

Being able to live in horrifically gross water and playing dead when you pick them up isn’t enough to make someone love them though.  I love the water sticks for two reasons: 1) they are phenomenally cute, and 2) of all the predatory aquatic insects, I think they are the most ill-adapted to life in water.  It doesn’t seem like these bugs should be able to survive at all!  Let me give you some examples.  Let’s start with their raptorial forelegs:

raptorial forelegs

Raptorial forelegs

The water scorpions are predators and have a diet similar to the giant water bugs (other insects, fish, tadpoles, frogs, etc).  Now take a look at the forelegs on the water stick at left.  Just look at how weenie those legs are!  Most predatory insects have big bulky forelegs (think giant water bugs or mantids) that allow them to strike quickly and powerfully.  Water sticks don’t have those broad, muscular forelegs, so they are slow and comparatively weak.  I’ve never seen them eat anything bigger than a small mealworm and though there are plenty of small things they can eat in the wild, they are pretty pathetic as far as predators go.  At least they have a decent beak for piercing and sucking their food once they manage to get a hold of something:

Piercing-sucking mouthpart

Piercing-sucking mouthpart

Next, they really depend on their breathing tube to survive.  They are very narrow and have very little space under their wings, so they need to stay in contact with the surface most of the time to be able to breathe.  While this allows them to live in some of those very dirty habitats (i.e they don’t need to extract oxygen from the water), it means they can’t stay completely underwater long.  And they call themselves aquatic insects!  Sad…

Then there’s those skinny little legs:

Skinny legs

Skinny legs

Just look at those!  Most aquatic true bugs have broad, flat legs that allow them to swim through the water efficiently.  Not the water sticks!  They’ve got long, terribly skinny legs instead.  That’s not ideal if you are an aquatic insect.  Because they stay near the surface, these bugs are usually latched onto the bank or a rock or some other solid object and don’t need to swim much.  But, they are aquatic and things want to eat them, so they do occasionally need to dive.  It’s hilarious to watch these guys try to swim:

Just look at all the flailing!  And how slowly they move!  It looks more like they flounder about and hope they end up where they want to go rather than actually swimming.  Worst.  Swimmers.  Ever!

So, water sticks are predators with wimpy forelegs, aquatic insects that can’t breathe underwater, and they live in water but can barely swim.  In spite of these shortcomings, water sticks are incredibly abundant, so they’re obviously managing somehow.  You have to love any animal that clings to life the way a water stick does!  And I do.  I really do!  Hopefully you love them just a little bit now too.  :)


Unless otherwise stated, all text, images, and video are copyright © 2011