Which property of water allows some lightweight insects to walk on top of a pond and not fall in

Light weight insects can walk on water due to the surface tension of water.

Explanation:

Surface tension is the tension in the surface layer of liquid, caused due to the attractive forces on surface molecules by surrounding molecules. The molecules in side the liquid try to attract surface molecules toward the water center. Due to which surface molecules don't interact with with other forces less than cohesive forces. That is why surface tension allow light weight insects to walk on water without sinking.

Surface tension is the property of water which allows for some lightweight insects to walk on top of a pond and not fall in. The molecules of water are held together by cohesive forces. The molecules of water present below the surface experience this cohesive force from all the directions, whereas the water molecules present on the surface experience a downward pull, as the force is not balanced. This leads to the concaving of the water surface inside a small tube.