Why do many animals walk on the surface of the water and wall

The most fundamental reason is that the bodies of these animals are very small. Spiders climb the wall by adhesion, while water Striders walk on the water by surface tension and fluid resistance. These supporting forces are only related to the contact area of insect water surface or spider wall; Their reaction force – gravity – is only related to the mass of these animals. Therefore, generally speaking, 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 quite different. When a fly walks on a vertical glass plate, the adhesion between its tarsal pad and the glass plate is enough to prevent it from sliding or falling. However, once the fly’s volume increases by 10 times, its volume weight ratio will increase by about 1000 times, while its contact surface area with the glass plate will only increase by about 100 times. In this case, the fly will fall down, because although the adhesion has increased, the increase of gravity is greater, and the adhesion can no longer compete with gravity; Moreover, the wings of the fly were too small to fly at all.

The legs of water strider have a waxy hydrophobic surface, which not only has hydrophobic function, but also will not be wet by water. Therefore, unless the gravity (the weight of the water strider) exceeds the vertical reaction of the water surface tension, the water strider will not drown. In addition, because the water strider’s legs generate pressure on the water, it can relax in this environment with almost zero friction.

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