Overweight pond skaters stand no chance on the water
Being overweight isn't an issue in the world of Pond Skaters.
People who keep a garden pond or who go dabbling in wet places are well familiar with Pond Skaters, very common water insects that skate gracefully about on the surface of still water. How they manage to walk on the water is amazing.
Water is made up of units called water molecules. It is a fact of physical chemistry that each water molecule attracts its neighbours so there are six attractive forces at work for every water molecule in the middle of the pond. Each water molecule attracts its neighbour to the right, left, in front, behind, above and below. While that holds true for all of the water molecules in the middle of the pond, those at the surface are different.
Because they are at the surface, these water molecules exert only five forces: they attract their neighbour to the right, left, in front, behind and below but not above for the simple reason that there is no water above them.
The outcome is that the surface of the water is pulled down and sideways thereby forming a relatively tight skin. While we can easily pierce that skin with our fingers, it is tight enough to support the weight of a Pond Skater.
Just as a snow shoe or ski can support a person on soft snow because of the increased surface area, the Pond Skater has evolved to slide around on the side of its legs rather than standing up and walking on the soles on its feet.
The weights that the surface tension of water can support are in a pretty narrow range so Pond Skaters must have slim bodies and long legs to waltz around effectively. Fat ones puncture the surface film and fall prey to predators. Nature selects those who are fit to survive and selects out those who are unsuited.
The obese and overweight skaters perish as individuals but, more importantly from a survival perspective, their demise filters out of the gene poll whatever genes contribute to excess weight. The on-going process of evolution thereby refines the gene pool to produce slim, trim, long-legged individuals.
Even the slim ones are heavy enough to cause the surface skin of the water to sag under their insignificant weight as illustrated by the image above. Like a person skating on thin ice, they live a precarious life. But, hey, that's what life is all about for a Pond Skater.