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What Time is it in Nature: Water Boatman

December 27, 2013

Due to the cold weather, we’ve had a dearth of insects at Prairie Ridge recently.  However, as we covered in a recent What Time is it in Nature, many of the insects in our pond and stream will remain active throughout the winter.  One insect you can find swimming in the pond currently is the water boatman (Hesperocorixa sp), a spectacular aquatic insect with some surprising characteristics.

Water boatman

Water boatman

Water boatmen are insects in the true bug family Corixidae.  They are characterized by a few unique structures, including robust triangular mouthparts, long and oar-like hind legs (hence water boatman!), and short forelegs that are modified for scooping.  Water boatmen swim right side up (which helps you distinguish them from another insect found in the same areas of many pond, the backswimmers) and use their hind legs like oars to propel themselves through the water.  Most water boatmen species have intricate black patterns of wavy stripes on their wings, including the Hesperocorixa species depicted in the image above.

Unlike most other aquatic true bugs, not all water boatmen species are predators.  The species in our pond is a herbivore, so our water boatmen use their scoop-like forelegs to gather algae from the bottom of the pond or rocks.  They will then hold the algae near their faces so that they can use their stout, triangular “beaks” (their piercing-sucking mouthparts) to inject digestive chemicals into the plants.  Once the plant tissues have dissolved, the bugs will suck up the juices through their mouthparts like a straw.  Other species of carnivorous water boatmen will capture and eat prey, though the actual piercing-dissolving-sucking of prey is quite similar to the plant feeding of their vegetable loving relatives.

Water boatmen are one of many groups of aquatic insects that rely on atmospheric air to breathe.  This means that they must go to the surface periodically to gather a bubble of air that they then carry with them to breathe underwater.  The bubbles function similarly to a SCUBA tank for a human, allowing the bugs to breathe the oxygen from the bubble while they are completely submerged.  Unlike the SCUBA tank, the air bubble of a water boatman is held in place against the body by a special pad of hairs and is completely exposed to the water.  This allows the bugs to take advantage of a nifty trick of physics where, as oxygen is consumed by the bug, more oxygen is absorbed into the bubble from the water!  The bubbles shrink slowly over time and water boatmen must return to the surface to gather more air now and then.  However, water boatmen can extend the life of their bubbles by stirring the water around them with their legs.  This behavior, called ventilation, increases the absorption of oxygen into the bubble and can allow the bugs to remain underwater much longer.  Staying underwater longer means fewer trips to the surface (where they are more vulnerable to predators) and more time to feed, mate, and otherwise live in underwater, so ventilation is a very important behavior for water boatmen.

You can often see the Prairie Ridge water boatmen from the shore of the pond!  Just look out into the open, but shallow, areas of water near the shores.  You’ll occasionally see a water boatmen dash to the surface for a new air bubble, but you’ll find most of the water boatmen near the bottom of the pond, foraging for food.  They can be hard to see if the water is murky, but on a clear water day you might see hundreds of water boatmen going about their business in the water.  Be sure to take a look the next time you visit Prairie Ridge!

What Time is it in Nature is a weekly feature highlighting the current plants, animals, and other wildlife at the Musuem’s public outdoor facility, Prairie Ridge Ecostation.  Find out more about the natural happenings at Prairie Ridge at our What Time is it in Nature Archive!

(Photo by Chris Goforth)

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