The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed. http://funadvice.com/r/14knpflfo8l
I.e. the water molecules stick to each other and form like a surface, insects aren't heavy enough to penetrate that surface
What molecules are atrractied to eachother, spiders have really long legs so they're not heavy enough to break the water bonds. If human beings had, feet a mile and a hlaf long, I believe it is, we'd be able to walk on water too. :D
They do not really walk they float from the air bubbles caught in the microscopic hairs on the legs.
they are very weightless ? lol