Why Can Mosquitoes Walk on Water?

They beat even the water striders

  Close up of the air pockets that help mosquitoes walk on water

Nature invented the most annoying insects: mosquitoes. They carry diseases, suck blood and what's worst, they are practically everywhere in the world, from tropics to poles.

Still, they have some amazing abilities (apart from detecting us no matter how the weather is): they can both walk on water and cling to walls and a new research shows exactly how they do this.

Other insect species, like flies, can also walk on walls and upside-down surfaces, but they sink in the water. Other insects, called water striders (a group related to bugs), can walk on water with ease, but if attempting to climb a wall, they'd fall on their backs.

But mosquitoes use walls and ceilings which are a good place to avoid predators, while they stay on the water bodies when laying their eggs.

"Many insects evolved from the water, but mosquitoes continue to spend part of their life cycle in the water. They're born underwater, and they have a larval stage, so they have to lay eggs on the water surface," MIT mathematician David Hu, not involved in this research but who has investigated water walkers extensively told LiveScience.

But how can a mosquito walk on such different surfaces? A new research made by physicists from Dalian University in China and Simon Fraser University in Canada has explained this.

Mosquitoes present special foot pads, similar to those of a fly, which have setae (stiff bristle-like structures), that help it adhere to vertical and upside-down surfaces with ease. But they are useless on water.

Water striders have tiny hairs wrapping their legs and making it difficult for water to penetrate. Instead of this, mosquitoes do have small grooves with air pockets covering their legs. The surface tension of water stops the water from seeping into the grooves, keeping the mosquitoes high and dry.

"The smaller the grooves are, the more difficult it is for water to penetrate," Hu said.

A single mosquito leg was found to support 23 times the mosquito's body weight. In this case, the water strides are humiliated just 15 times. When water striders or mosquitoes stand on water, their feet induce a dimple in the water and the surface tension of the water maintains them afloat.

"[The water] tries to minimize its surface contact area with the air. That means it tends to act like a trampoline, curving downward and supporting the weight of the insect." said Hu.

The insects' legs create a superhydrophobic (extremely water repelling) effect allowing them to stand on water surfaces. But these forces cannot be applied to large animals, like humans. With the increasing size, the surface gets smaller, and less surface available cannot take advantage of the surface tension of water to stand its weight.

"If you calculate the size of the feet you need to have to support your weight on water, it's something like a kilometer," Hu said.

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By    19 Jul 2007, 07:45 GMT