APRIL
1998

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

04-23-98

Water Striders Use Tension to Stay Afloat

Water striders are appropriately named. These insects are able to glide over the surface of the water with the greatest of ease. Being able to walk on water is a miracle of sorts in the animal world - so much so that some people actually refer to water striders as "Jesus Bugs".

Not many creatures in the animal world actually live on the water surface as do these insect striders. Some ducks and geese spend much of their life on water; so do some seagulls and terns. These birds are floaters. They are animal boats. Waterproof feathers form the bird equivalent of a hull. The creature displaces enough water to keep it on the surface.

Water striders, though, are different. They manage to stay on the surface by using the surface tension of the water to hold them up. Remember in grammar school how, with great care, students were able to float a needle on the surface of the water in a glass? The secret was to not break the surface tension of the water. Otherwise, the needle did a miniature version of the Titanic and headed to the bottom of the glass.

So how does this insect strider manage to pull off this miracle? It has the right stuff. Its insect equivalent of toes, called tarsae, are covered with very fine hairs that are difficult to wet. These nonwetting hairs do not break the tension of the water, so the insect floats, or skates, on the surface.

Those who have observed water striders closely notice that the insect appears to have only four legs. All adult insects have six legs, and the water strider is no different. However, this insect uses its front legs, which are smaller than the four used as pontoons, for capturing prey in much the same way that praying mantids use their front legs. The front legs do not contact the water, so only four legs are obvious to the casual observer.

Water striders float over the surface of the water in search of their food, which is small insects. Any hapless insect that falls into the water and stays afloat for a moment is potential food for these predators.

In general, water striders are found in protected areas of freshwater ponds or streams where the water is quiet. However, they can be seen riding the waves in fast-moving streams as well. One species lives on the surface of the ocean and can be found many miles from shore. It is one of the few insects that has any association with ocean water.

These insects lay their eggs on floating plant material. The newly hatched young skate over the water like their parents. Neither young nor adults can live under water, and if they sink they drown. Occasionally this happens. When the tiny hairs on their tarsae become wet, the insect cannot float and must climb up off the water to dry out.

Winged and wingless individuals occur in most species of water striders. It is the winged individuals that are found in unexpected places at times. For instance, on the birdbath.

What happens to water striders when the water where they are living dries up? Well, they might fly away or, in the case of a larger spring or pond, they just crawl under a rock or in the mud and hibernate until the water returns. After all, it's hard to stride on the water if there is none!  

 

Writer: Tom Turpin
Editor: Andrea McCann