Tri-State Neighbor: South Dakota Farm Newspaper

We often equate walking on water with the miraculous, and this expression of biblical origin has entered our vernacular as a description of superhuman qualities.

Walking on water is indeed a feat for humans, but the physics behind this process dictate that the smaller you are, the easier walking on water gets.

A familiar and eye-catching sight of summer is the insects that make a living on the surfaces of ponds, quiet streams and wetlands - the water striders. Although commonly taken for granted as mundane, what most people don't know is that these little critters are helping scientists to rethink the physics behind both water repellency and animal movement.

At first glance water striders resemble spiders, but a closer look reveals that their bodies are suspended on six legs, not eight. Water striders are in the order of true bugs, and reside in the family Gerridae.

Being the lumbering humans that we are, the surface tension of water often goes overlooked. This tension is what holds droplets together, and forms a significant barrier to anything too small to crack it (sometimes gases like oxygen also have trouble breaking the surface tension of water, which is an important function for that line of bubbles in aquariums). Water striders have learned to exploit the surface tension of water in order to make a living.

A close examination of just how water striders perform their little trick has revealed some fascinating adaptations. First, legs of water striders are hydrophobic (water repellent). This comes from the fact that they also have downward-facing hairs with very fine sculpturing that holds teensy air pockets.

A recent study published in one of the top scientific journals found that the water repellency of each gerrid leg can allow 15 times the insect's body weight to stay on top of the water. Understanding the structure of gerrid legs will likely help industries develop more efficient miniature aquatic devices and non-wetting materials.

Anyone who has tried to catch a water strider knows that these little suckers can really shake a leg. Rather than rowing like an oar, gerrid legs move in a circle and generate a minute “tornado” in the water that propels them forward, moving at maximum of 3.5 feet per second. This is yet another way that gerrid physiology has led scientists to rethink our preconceptions about animal movement.

Gerrids are hunters, skimming the surface of their habitats for potential meals. For the most part, these predators feed on insects that get captured in the surface tension of the water. They feed on hapless victims using piercing-sucking mouthparts functionally similar to those of mosquitoes.

Finally, living on the surface of the water has helped a unique form of communication to evolve in gerrids. They “speak” to each other using waves. Males defending a territory make surface waves at a certain frequency, those attracting females send out a different frequency of waves. In this way, their water-surface home becomes both their lunchbox and a telephone system.

Most insects have reason to fear the water surface because of the certain death that accompanies it. But water striders have come to integrate the surface of the water into every aspect of their lives, and will readily tell you that living with tension isn't so bad after all.

Comments »

Comment on this story

Comments will be approved within 48 hours