Friday, April 27, 2007
Biodiversity Corner Water strider
This is the time of year for intense activity in our ponds, streams and vernal pools. The peeping of the peepers has replaced the beeping of the fuel trucks backing up our driveways. The quacking of the wood frogs is telling us where the vernal pools are and telling the other frogs where the party is. The water striders, also known as pond skaters, are darting about on the surface of the water shouting "Look, Ma. No ice!"
Name. The water strider or pond skater, sometimes called the Jesus bug, is in the order of insects called Hemiptera. They are true bugs which means they have mouthparts designed for piercing and sucking. Not all insects that pierce and suck are Hemiptera. The ubiquitous mosquito is in the order Diptera — it is a type of fly. Water striders are in the family Gerridae. There are 47 species in North America. The common ones walking our waterways are probably in the genus Gerris.
When and where seen. Last Saturday, April 21, I saw lots of water striders on the still waters alongside the Tophet Trail and Heartbreak Ridge in Great Brook Farm State Park. They are easy to spot when the sun is shining because they cast a very distinctive shadow (see photo). The round parts of the shadow are from dimples in the water surface created by pressure from the feet. The narrow shadow is from the insect body. I like to watch them in the shallows at the pond edge where the bottom is often the tawny color of dead oak leaves. This makes a great contrast for the shadow and gives each footprint a golden outline. If the bottom of the pond is dark or the sun is not out you can spot them by their rapid darting movements.
Characteristics. The water strider looks at first glance like a rather large mosquito standing on the surface of a pond or slow-moving stream. If you see one, there will probably be many others nearby. The body is a dark color and about 1/2 inch long. The three pairs of legs are each highly adapted for three quite different functions. The first pair is used to capture food. These two legs are much shorter than the other four. The middle pair is used for "rowing" and the hind pair for steering and braking. The legs are covered with layers of microscopic hairs that trap air and allow the insect to float. If you want to measure the length of the hind leg femur, you will discover it is longer than the abdomen, a tell-tale sign that your aquatic bug is in the family Gerridae.
Walking on water. The motion is more sprinting than walking or striding. It reaches speeds of 100 times its body length in a second. This is equivalent to a six foot person swimming at 400 mph. An MIT team of applied mathematicians and mechanical engineers theorized that the water strider's feet created underwater currents (without breaking the surface of the water) that could propel the insect forward as if it were rowing. To prove their theory, they built a model called the Robotstrider and with dyed water and high speed video, they showed that the model did indeed mimic the water strider action.
Why we love the water strider. In nature's perpetual high-stakes game of rock-scissors-paper, the mosquito (which is paper, of course) is cut by the water strider scissors. Water striders are predators. They eat aquatic insects — like mosquito larvae when they approach the surface, or newly hatched mosquitoes — and terrestrial insects that inadvertently land on the water.
References. www.bugGuide.net has a good collection of water strider photos. You can send your own insect photos in for their experts to try and identify); Borror and DeLong's Introduction to the Study of Insects; study of Northern Virginia ecology at www.fcps.edu/islandcreekes/ecology/common_water_strider.htm; www. nationalgeographic.com (search on water striders).
© 2007 The Carlisle Mosquito