Scientists have long struggled to study the floating gait of the water strider. How do the delicate insects walk on water?

Boats float by displacing water. As the Greek scientist Archimedes explained some 2,000 years ago, an upward floating force is equal to the weight of displaced water. Water striders don't displace water; tiny hairs on their legs expel water.

An updated Archimedes principle suggests the amount of water expelled by the strider's tiny hairs should equal the flotation force, but confirming the phenomenon experimentally has proven quite difficult.

Recently, researchers at Tsinghua University managed to test their arithmetic by tracing the shadows cast by the strider's legs.

The scientists placed a white sheet at the bottom of a small tank of water and a light source above. The curvature of the expelled water caused the leg shadows to be rounded. The shape of the shadows allowed the scientists to calculate the amount of expelled water, and thus, the upward flotation force.

"The sum of leg forces agreed well with the body weight measured with an accurate electronic balance, which verified updated Archimedes' principle at the arthropod level," scientists wrote in their paper on the research -- published this week in the journal Langmuir.

By monitoring the changing shadow shapes, researchers were also able to calculate the insect's shifting body weight and leg angles.