A surprising variety of insects (including water striders, flies, and beetles), and some spiders, are able to “skate” across the surface of puddles, ponds, and even quiet pools in streams and rivers. How do they do it? Why do they do it? The answers lie in the anatomy of the insects and the unique properties of water itself.
At the boundary where air and water meet, liquid water molecules are more strongly attracted to each other than to gas molecules in the air. This phenomenon of attraction exerts an inward force that makes it appear as if the surface of water is composed of an elastic membrane, if only a thin one. If you have ever placed your hand onto the surface of water, you’ve felt this strange property.
This is termed “surface tension,” and water has a greater surface tension than most liquids thanks to the web of hydrogen bonds strongly linking water molecules together.
Sprawling Bug Legs
Certain insects like water striders, and spiders, take advantage of the surface tension of water. The creatures are light in weight, but that alone does not explain why they do not sink. Most water-walking insects have extremely long legs that allow them to spread their body weight over a much greater area than short-legged insects. It is the same principle of physics as snowshoes that spread your body weight over a greater area of snow.
Water Repellent Bodies & Legs
Insects and spiders have their skeletons on the outside of their bodies. This armor is coated in a material called chitin that is waterproof, or at least water-repellent. It keeps them from becoming dehydrated. It also works in reverse and makes it difficult for insects to drown. Many water-walking insects also have hydrophobic (water-repellent) hairs on their legs and feet that resist the attraction of water molecules so they don’t get sucked under.
Why Live on Top of the Water?
Most insects that live on top of the water are predators (like water striders) or scavengers. They thrive on non-aquatic insects that fall onto the water and cannot fly or crawl away. The water-walking lifestyle also makes it difficult for their predators to reach them without getting stuck in the water.
Try an Experiment
Try this experiment at home (your budding entomologist will love it!) to see just how strong the surface tension of water can be: Fill a cup or glass to the brim with water, then slowly and gently slide or lower a paperclip onto the surface.
Spoiler alert: It floats! Well, it is not actually floating: it is being supported solely by the surface tension of the water. (True flotation is due to the buoyancy of a large volume of water beneath an object at the surface.)
Ocean-going Water Striders?
Yes, there are some water striders in the genus Halobates that live on the surface of the open ocean. They are the only truly marine insects that can exist without contact with land. Bon voyage!