I am fascinated by water striders. These insects are uniquely adapted for their life on the surface of the water. I filmed several of them in a creek. You can’t really see the actual insects very well in the video below, but you can see two things that give away their presence.
The first is that each insect generates two small round shadows and a larger round shadow on the surface of the water. These shadows are created by the way light is altered by the bending of the surface of the water by the strider’s rear (small shadows) and front (large shadow) legs. These insects exploit the surface tension of water to stay afloat coupled with adaptions in the tips of their legs that make the legs water repellent and also help them trap little bubbles of air resulting in high buoyancy.
The second is that these insects use their middle legs as oars generating an explosive movement that propels them forward giving rise to waves that radiate from the strider in a concentric pattern. A multitude of striders in one place can create quite a display of circular waves giving rise to cool interference patterns as shown in the video.
Water molecules are made up of one atom of oxygen bonded to two atoms of hydrogen (H2O). When atoms become bonded, they share electrons. However, the oxygen atom is so large compared to the hydrogen atoms that is tends to “hoard” these electrons to a greater extent. This hoarding of the electrons confers upon the oxygen atom a partial negative charge, and conversely, the electron deficit confers upon the hydrogen atoms a partial positive change. Thus water molecules tend to stick because the positive and negative charges attract each other. This creates the phenomenon known as surface tension. Surface tension is an important property of water that living things such as the insect known as water strider exploit.
Surface tension and the effects that soap has on it can be used to perform some fun experiments as Physics Girl shows in the video below.
Water strider image by Alexander used here under an Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0).