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Water striders from the family Gerridae are infamous animals for their ability to skate on the waters surface, making them pleuston (surface-living) animals. Their entire bodies are waterproofed by hydrophobic hairs (hydrofuge) that keep water from sticking to them and create small pockets of air on their long feet. Thanks to this and their middle legs like oars, they can stride on the waters surface or jump off it like a trampoline. They mainly hunt waterlogged animals, impaling them with their proboscis and ingesting the insides.

The question is what if they were bigger? About horse sized or perhaps smaller? Small animals often use weak-forces to their advantage, for them physics are different so not all their adaptations translate well to larger sizes. Hydrofuge would be useless for such a large animal, so I take it they'll use a different tactic. Either large dense tufts of waterproof fur on their feet or balloon like organs? They would still use a pair of legs for paddling but could they do so with long thin legs, or should they be shorter and stubbier? Could they jump off the water?

How would they efficiently skate on the waters surface?

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  • $\begingroup$ The water's surface is solid ice. $\endgroup$
    – causative
    Aug 7 '21 at 18:40
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    $\begingroup$ @causative I'm not making a penguin. $\endgroup$ Aug 7 '21 at 18:51
  • $\begingroup$ The water striders are made of an aerogel and weigh almost nothing. $\endgroup$
    – causative
    Aug 7 '21 at 18:53
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Let us try to calculate what is needed to support this creature.

enter image description here

Here is a fine page that walks thru surface tension and how to calculate how much force a given line of surface can exert and so how much force from gravity acting on a mass surface tension can oppose.

http://scipp.ucsc.edu/~haber/ph5B/bubble.pdf

In the example, you are calculating the maximum weight of a needle that can be supported by surface tension - I was tempted to say "float" but it is not the same thing as floating.

showed work

Here W is the maximum weight of the needle. 0.032m is the length of the needle. λ is the surface tension of water which is an intrinsic property and is 0.073 N/m. Cos of 0 degrees is 1. Weight in N factors in mass and acceleration by earth gravity; divide 0.0047 N by gravity (9.8) = 0.00479 kg or 4.7 grams is the maximum weight of the needle given its length.

Now let us start with the weight and solve for L the length of the... support structure on this giant water strider. We will say the weight is 500 kg= 4903 newtons.

W = 2(λL)cos0° 4903 N = 2 (0.073 N/m)(L)1 4903 N = 0.146 N/m (L) 33582m = L

That is pretty long. 33 km. I am terrible with those orders of magnitude and it will not hurt my feelings if someone checks my math and finds my error.

But 33582m seems plausible. One could perhaps this by having a foot that ramifies into multiple small support structures to spread the needed length over many small lengths. If you had the legs of this creatures ramify into many hairs each could contribute to the total length needed to support its weight.

Which is how the water striders do it.

water strider leg https://phys.org/news/2015-07-legs-striders-repel.html

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The answer is that they wouldn't, because surface tension just isn't that strong. It's that square-cube law rearing its head again.

Now what you could have is something akin to the plesiosaurs https://en.wikipedia.org/wiki/Plesiosaurus that propelled themselves with paddle-like flippers. Make them light enough, and they could stay on the surface and reach fairly high speeds, like rowing sculls: https://en.wikipedia.org/wiki/Sculling

Now why they'd evolve to stay on the surface, instead of seeking the more abundant food supplies just below, is beyond me.

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  • $\begingroup$ They honestly wouldn't stay at the surface, because drag at the surface is usually a lot worse than drag if the entire animal is underwater. This is why whales don't swim at the surface all the time, and birds predominantly rest on the surface of the water and fly or dive when they really want to move. I think Steve Vogel's Life in Moving Fluids discusses this in detail. Water striders do because they expend little energy doing so. $\endgroup$ Aug 8 '21 at 3:28

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