Footprints on the water

It was a sunny Sunday afternoon in Cougar Creek, and the water striders were out enjoying the sunshine. In every calm pool, they scooted about, like teams of rowers in outrigger canoes.

They're hard to see, even when the light is good. They're small (about half an inch long), they're fast (up to 1.5 metres per second, or 3.4 mph.), and their colours blend in nicely with the mud and stones of the creek bed. And their shadows, usually somewhere off to the side, attract our attention first.

The shadow is distinctive; six circles or ovals with a canoe shape in the centre.

It gives the impression that the feet are round or oval pads, something like snowshoes or pontoons. This time we got some photos that dispelled that notion.

This one looks like a cheerleader with pom-poms.

Funny how the shadow looks so solid, while the disturbance in the water still lets light through.

Another strider; here the feet can be seen as a straight line, surrounded by a "bottle brush" of wavelets.

This one caught the light, creating a variegated pom-pom effect.

The last long segment of the strider's legs makes contact with the water. It is covered with tiny water-resistant hairs, angled off at about 20°, each one grooved to trap air, making a sort of cushion (or pontoon, after all) between the leg and the water. I assume that this pattern of hairs and grooves is what creates the radiating lines on the water.

The strider rows with the long middle legs, steers with the rear legs, and uses the front ones, resting on the water surface, to detect the motion of prey insects, much as a spider checks the vibrations of its web.

And here, the pattern changes:

A mating pair. Ten legs on the water. Two used to hold the female.

Cruising down the river on a Sunday afternoon,

With one you love, the sun above, waiting for the moon.

The old accordian playing a sentimental tune,

Cruising down the river on a Sunday afternoon.

The birds above all sing of love, a gentle sweet refrain;

The trees around all make a sound like softly falling rain.

Two of us together, we'll plan a honeymoon

Cruising down the river on a Sunday afternoon.

A tangle of legs in another pair.

About half of our photos showed mating striders. But it wasn't quite as peaceful as in the old song; female water striders are not easily convinced. The males want the honeymoon; guarding their mate to prevent other males from fertilizing their chosen female. But this endangers the females, opening them up to attacks from backswimmers, so they resist. (I wrote about this in "The Case of the Reluctant Brides".)

This couple was struggling.

"Don't go yet!"

In the next photo of the couple, she has broken away and is sitting on the rock, safe for now from further attentions.

Some interesting pages on striders:

Wikipedia, "Gerridae"

Bug of the Month, "Water Striders"

Flickr, by Tommok (photo plus a discussion of the leg structure)

National Geographic; "Robot Helps Show That Water Striders 'Row" on H2O"

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