Perhaps you've noticed on a lazy day at the lake an insect walking on the water and had to look twice to make sure your eyes were not deceiving you. It's true. Some insects actually can walk on water. In fact, the water strider---dubbed the Jesus bug---doesn't just walk on it, it can skip on the surface of water without ever sinking.
It was long thought that the strider and other insects that can walk on water secrete a wax from their legs that, when coming into contact with the water's surface tension, created an odd shield--kind of like plastic wrap--that helped them stay afloat. This has been found not to be the case, however.
The water strider's legs are covered with tiny hairs visible only under a microscope. These hairs trap tiny air bubbles. It's these air bubbles that allow the insect to float on the water's surface and prevent them from actually getting wet.
The strider is uniquely designed for the task. However, other insects, including spiders and beetles, that walk on water can displace their body weight using extremities that are spread widely apart. Thus, they essentially make themselves lighter than the water they are walking on. Obviously, only very light insects can do this.
Biology is only part of the story. It also depends on surface tension, a force occurring at the surface of a liquid. It can be visually explained by looking at a glass of water that has been filled just past the brim. It's the surface tension that keeps that little bit of water rising past the brim from spilling.
It's one thing to read about a bug that can walk on water and another entirely to see it actually happen in front of you. Watching an insect walking on water can help you understand how biology and science interact to create this cool effect. Find a pond or lake and settle in near a woodsy part of the shore. Sooner or later, you will spot an insect walking on the water's surface.