We know that whether a solid object sinks or floats in a liquid depends on their relative densities. If the object has a higher density than the liquid, it will sink but if it has a lower density, it will float.
But I am sure that all of us have seen a counter-example, how it is possible to carefully place a needle on the surface of water and have it float. This is because of surface tension, in that the surface of a liquid can act like a membrane and as long as the membrane is not broken, it can support light objects. This is how some insects such as water striders seem to be able to ‘walk’ on the surface of water, because their legs have fine projections that prevent the surface from being broken.
I came across this variation of this phenomenon where a beetle was walking along the underside of a water surface.
An Australian beetle has been observed walking upside down along the surface of water – the first instance that such behaviour has been visually documented.
The tiny aquatic beetle, about 6mm to 8mm in length, has been recorded scuttling along the undersurface of a pool of water in New South Wales.
While the specific species of beetle hasn’t been identified, the researchers believe it belongs to the Hydrophilidae family of beetles, commonly known as water scavengers.
The beetle Gould observed appeared to have trapped a layer of air along its abdomen, giving it the buoyancy to push it up to the surface of the water.
“We also propose that the beetle has attachment organs on its legs that are trapping more air bubbles,” he said, which allows it to move without breaking the surface tension of the water.
In this way, the beetle is similar to water striders, which can travel across the top surface of water. “They also have hairlike projections along their legs that stop them from breaking surface tension,” Gould said.
The researchers concluded: “The water’s surface seems to be acting as a support for the beetle, with the force needed for forward movement generated by the beetle’s legs as each transition between the stance and swing phases of walking.”
Pretty cool, I thought.