MadSci Network: Chemistry

Re: What exactly does a water bug have to do with cohesion?

Date: Thu Oct 7 11:54:58 1999
Posted By: Richard Kingsley, Science teacher
Area of science: Chemistry
ID: 937500472.Ch

Hi Bethany

One could write volumes here, but I am going to give you the watered down version (miserable pun intended). Cohesion is an important property of water. It means that water molecules like to stick to each other. Raindrops are groups of water molecules, which stick to each other.

This sticky property is so strong that when some insects are placed on the surface of a pond, the molecules hold together underneath and support the insect. This phenomenon is often called surface tension and results in a surface film, which operates whether the insect is above or beneath the surface. If you visit a pond in the summer, you will see water striders (In Europe, a.k.a. pond slaters) moving across the surface film. They have water repellent hairs to that prevent them from becoming submerged and flattened feet to help them spread the load. You can observe the depressions made by the flat feet by looking at their shadows. See this cool 3d animation of the water strider.

Also, look at this excellent page about water striders which shows the water repellent hairs on a magnified picture of the head.

The surface film is quite delicate and is disturbed by wind and rain. Thus, when a storm disrupts the surface film, water striders must take refuge on something solid like a plant stem or leaf to prevent drowning.

For other insects that fall into the water by mistake, the surface film is a death trap. Parts of their bodies will get trapped beneath the film upon impact making it difficult for them to escape. As they struggle, they send out vibrations along the surface of the water that alerts pond striders to their presence. The pond striders home in on the source of the vibrations and devour their prey. Other water bugs such as the backswimmers (Europe, a.k.a. Water Boatmen) also sense the vibrations of struggling insects, but from beneath the surface film. To learn more about these creatures click here.

Adult water bugs and many immature forms are air-breathers and during the summer, when they are active, they must return periodically to the surface to renew their supply of oxygen. For these creatures, the surface film is an obstacle that they must break through if they are going to breathe. They tend to have tubular devices (this includes hair) or pointed body parts that are easy to force through the film.

The surface film is an obstacle for water bugs to negotiate when they are moving between water and air. Some insects are able to move through the surface film to the air above quite easily because they have water repellent hair. Others use plants or other solid objects to give them a firm foothold for pushing through. Thus, water cohesion is important for water bugs and it provides some species with a unique way of life. This lifestyle is threatened by pollution from detergents, which lower surface tension.

Why not visit a pond next summer to see what I am talking about? If you sit silently and motionless at the side of a shallow pond for just five minutes, you should be able to observe most of the things mentioned above. You can even catch an unwanted housefly from your home and throw it into water to see the water striders in action. (You need to catch the fly without using chemicals - alternatively trying using a stick to create vibrations).

Richard Kingsley

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