Walking on Water

Sometimes a common term can create very different images depending on how you view the world.
So what picture does the term “pond skater” bring to mind?
An iconic work of art or an aquatic insect?

Reverend Robert Walker (1755-1808) Skating on Duddingston Lock, 1795. Sir Henry Raeburn. National Galleries Scotland

The serene figure of the Reverend Robert Walker, minister of the Canongate Kirk and a member of the Edinburgh Skating Society, skating on Duddingston Loch near Edinburgh in 1795. Poised and elelgant, perfectly balanced he glides effortless across the ice demonstrating his prowess and skill.

Reverend Robert Walker (1755 – 1808).
Sir Henry Raeburn 1756 – 1823.
National Galleries of Scotland

 

Pondskater Gerris thoracicus

The champion pond skaters of the natural world, the water measurers (Hydrometridae), pond skaters (Gerridae) and water crickets (Veliidae), are no less accomplished than the Reverend Walker. Their legs are equally long and elegant, although they have six rather than two, their skill is a combination of natural selection and physics.

Gerris thoracicus. Photograph © Christine Johnson

There are ten species of pond skater in Britain, but only five have been recorded in the Outer Hebrides. Often seen in large groups, they ‘skate’ on the surface of ponds, lochs, ditches, pools and puddles, feeding on smaller insects which they stab with their sharp mouthparts or ‘beaks’. Depending on the species, adult pond skaters are between 6.5 -18 mm long with spindly legs, brownish-grey bodies sometimes with some orange hues, and small heads with large eyes.

They are agile insects and active predators. Sensitive hairs on the legs and body can detect movement or ripples on the surface of water which can indicate the presence of dead or dying insects that have fallen into the water. Pond skaters move across the water surface by using the middle pair of legs to propel them forwards. The back legs act as rudders while the short front legs seize the insect prey. They can move at great speed, something you will discover if you try to catch one!

So how do they “walk on water”?  The legs of pond skaters are long and slender, allowing the weight of the body to be distributed over a large surface area. Their strong and flexible legs enable them to keep their weight evenly distributed so that they can maintain their balance as the water moves. However, the real secret is the presence of  thousands of microscopic, water-repellent hairs on the body surface. These hairs have tiny groves which trap air and increase the bouyancy of the insect.

Water crickets (Velia species)  are smaller (6 – 8.5 mm long) and chunkier than pond skaters and more likely to be found where there is running water.  They can also walk on the surface of the water, and may use their middle legs to “row” like their larger relatives. However, Velia caprai, the most common species found in these islands, also has an interesting behavioural trick called “expansion skating”.  Saliva is ejected from the insect’s beak onto the surface of the water, this lowers the surface tension enabling the insect to travel at up to twice its normal speed!

Juvenile water cricket Velia caprai
Juvenile water cricket (Velia caprai) showing perfect balance with the legs forming a tiny depression in the surface film of the water. Photograph © Robin Sutton.

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