Why do many animals walk on water surfaces and walls as if they were walking on flat ground?

The most fundamental reason is that these animals are very small. Spiders rely on adhesion to climb rocks and walls, while water striders rely on surface tension and fluid resistance to walk on the water. These supporting forces are only related to the contact area between insects and water surfaces or spiders and walls; their reaction force - gravity - is only related to the mass of these animals. Therefore, in general, large animals are more controlled by gravity and inertia; small animals are more controlled by surface forces such as adhesion and fluid tension.

However, the two phenomena of rock climbing and water walking are completely different. When a fly walks on a vertical glass plate, the adhesion between its tarsal pads and the glass plate is enough to prevent it from sliding or falling off. However, once the volume of the fly increases by 10 times, its volume-to-weight ratio increases by about 1,000 times, while its contact surface area with the glass plate increases by only about 100 times. In this case, the fly will fall, because although the adhesion has increased, the gravity has increased even more, and the adhesion can no longer compete with the gravity; moreover, the wings of the fly are too small to fly at all.

The legs of water striders have a waxy hydrophobic surface, which is both hydrophobic and not wetted by water. Therefore, unless the gravity (the weight of the water strider) exceeds the vertical reaction force of the water surface tension, the water strider will not drown. In addition, because the water strider's legs create pressure on the water surface, it can swim leisurely in this frictionless environment.

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