Water Strider

Stop the presses: the Bug of the Week is actually a bug!

More specifically, it’s a water strider, in the family Gerridae.  If I had to guess, it’s in the genus Aquarius, but I don’t know water striders all that well.  (Since the absolute best that I can hope for is ID to genus, I can’t help thinking that I should have written “Generically, it might be Aquarius…” but that doesn’t really convey what I want it to.  The English language is a crazy, crazy beast.)

The most distinctive feature of the Gerridae is their ability to move across the surface of a body of water.  They’re very lightweight, and they’ve evolved the ability to spread their weight across such a wide area of water that they are completely supported by its surface tension.  Like all bugs (Hemiptera), their mouthparts are adapted for piercing and drinking, so if you think of a water strider as a self-propelled drinking straw that walks on water, you’ve got the gist of it.  

As usual, though, there are enough interesting things going on to warrant a closer look.


There are a couple of things going on in this photograph.  The obvious one is that Gerridae are territorial predators, and this is an example of one that is pushing its limits, and possibly its luck.  

Their typical prey are insects that have fallen into the water.  Most insects float but cannot swim (much less walk on the surface), and their struggles with surface tension create ripples on the water.  The striders can feel the ripples and follow them back to their source, where they can use their superior mobility to approach the other insect safely, grab it, and treat it like an insect Slurpee.

In this case, the strider has two problems: the ant hasn’t fallen completely into the water, and it’s dangerous.  The floating grass gives the ant enough purchase to turn and face the strider, and from the strider’s point of view, an ant this size is unsafe from either end.

I watched the pair for a few minutes, and this is the strider’s closest approach.  It very nearly got stung for its trouble, and soon moved off to find easier prey.

The other interesting thing about this strider is that it has wings.  Water striders are very unusual in that the adults will develop wings, or not, in response to environmental conditions: the more unstable the environment, the longer the wings.  This one has wings that run the entire length of its body, and sure enough, it’s in a seasonal pond – really not much more than a puddle – fed by seepage from a seasonal stream, a few miles from the Columbia River.


The second photograph shows the same strider, moving off to continue its hunt.  I’ve included it because it shows a couple of cool optical effects of the way that the striders interact with the surface tension of the water.

First, you can actually see the strider’s legs deform the surface of the water, from the way that the curved surface around each foot refracts the light that’s reflected off the bottom of the pond.  (I’ve enhanced the contrast a bit to highlight the effect.)

Second, the water is just the right depth so that the sunlight refracted by the same curved surface casts a large shadow beneath each foot.  The size of the shadow tells you how much the surface has been stretched, which tells you in turn that the strider bears most of its weight on its front and back legs.  The central pair, splayed well off to each side, is mostly used for propulsion and control.  (The shadows below the central pair get larger when the strider is, well, striding, but my camera doesn’t record video so I don’t have a good way to show it.)


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