How is it possible for insects and spiders to walk on water or on the walls?

Tiny insects can walk on water because of the phenomenon of surface tension.  The unbalanced intermolecular force makes the surface behave like a stretched membrane.  The classical demonstration of the carrying capacity of this membrane is to gently lay flat a shaving blade(to ensure that the weight per unit area is kept low)  upon the surface of still water;  the blade does not sink.  The blade is heavier than an equal volume of water and would surely sink if the force of surface tension were absent.  Thus,  one can easily understand why insects and larvae can float on the surface of water.  As regards the ability of insects to walk on walls,  several explanations are provided.  The most popular isthe hypothesis that such creatures have suction cups on their feet using which they can stick to walls and recent ceilings.  Some special investigations indicate a construction of the feet; thousands upon thousands tiny protruding hair-like projections stick to surfaces due to molecular forces.