It flooded where I live. I saw multiple spiders on the surface of the water trying to crawl to shore. How do they do that. Is is because they are so light?
It is because surface tension and also obviously they are very light. This short video is very explainatory: https://www.youtube.com/watch?v=E2unnSK7WTE
So, if an insect or the like had very pointy legs, like the hooks on a caterpillar, they would break the water tension?
It seemed from the video that the setae (tiny hairs) were contributing significantly to the success of the striders.
It depends on how much mass they have. Gravity is pulling the insect down against the surface tension. If the insect’s weight is too great, it will overcome the surface tension and become partially submerged. The insect may then also sink completely if its overall density is greater than the density of the water - although if it spreads itself out, it may still be able to float by distributing its weight over a larger area.
Some insects also have water-resistant surfaces (made up of tiny, waxy hairs), which prevent them becoming water-logged. This acts like a cushion of air which allows them to skate across the surface of the water. They control their movements by pushing against the dimples made in the surface by their legs.
It’s true; many terrestrial insects are poorly adapted to deal with surface tension and fall victim to aquatic predators. Specifically adapted invertebrates (water striders; adult mosquitoes) are often hydrophobic. Here’s some light reading on the topic: