Aquatic Insects:

A Breath of Air

By Terry Hellekson

    Aquatic insects are important to man in many more ways than one might think. The scientific world has long since determined that they make good indicators for water quality. As aquatic insect populations decline, so does the water quality in most instances. There have been endless studies made in most regions around the world. Particular emphasis has been given to the respiratory systems of many species and how they breathe.

    When compared to terrestrial insects that live on land, aquatic insects have special alterations for breathing in water without drowning. The most common group are those who can breathe effectively underwater by using the oxygen that is dissolved in the water rather than oxygen that is a gas in the air above. Many aquatic insects, particularly during their immature stages, have gills similar to fish that enable them to obtain dissolved oxygen in the water. The gills on aquatic insects are located on the outside of their bodies in various locations. Gills come in various shapes, but many are flat oval plates or tufts of small filaments.

    Other aquatic insects have a soft flexible external skeleton that allows dissolved oxygen to pass from the water into their bodies all over their body surface. Some kinds still use the holes in their bodies to get oxygen from the air on the surface. They just keep the holes shut while they are underwater and only open them when they come to the surface.

    Some kinds take a bubble of air underwater and breathe out of the bubble, a bit like scuba diving, which allows them to stay underwater longer. Water boatman, backswimmers,  and diving beetles are some of the better known and most prevalent aquatic insects that carry a bubble of air with them.

    Water Boatman (Sugara atropodonta). These aquatic insects are easily identified by their oarlike hind legs that are flattened and fringed with hair which they use to propel themselves rapidly through the water, hence their name. They are about 1/4 inch long with a somewhat flattened oval body that is a brownish-gray or mottled. Their legs and underbody parts are yellow. They have special hairs on their bodies that hold the air bubble. Their life cycle takes about six weeks from egg to adult. To give you some idea of their abundance, they are collected, dried and sold for bird and fish food.

    Backswimmer (Notonecta undulata). Often mistaken for water boatman, but can be distinguished by the fact that they swim upside down on their backs. Adults are about 1/2 inch long with kidney shaped black and white bodies. They are often seen on the water surface with their long hind legs held straight out  and pointed forward, poised for a fast start.

    Diving Beetle (Dytiscus verticalis). Adults are about 1/4 to 1-3/16 inches long with oval, smooth, shinny black bodies with hairy, paddle-shaped hind legs. There is a cavity under their wings that holds an air supply. There is one generation annually and they can live several years.       

    A few kinds of aquatic insects have their spiracles (small openings) on the end of a long tube at the end of their abdomen. They keep their body underwater and just stick their breathing tube up to the surface to get air, a little like snorkeling. Many larva belonging to the Diptera family inhabit standing water and breathe through spiracles that they bring in contact with air on the surface at regular intervals.  While this restricts the depths at which these larva can live it also allows them to live in extremely polluted and anoxic conditions. One of the more notable species is the mosquito larva.

    The three major categories of aquatic insects that fly fishers tend to gravitate to are mayflies, stoneflies and caddis flies. Like trout, all three need well oxygenated water. It is interesting to know that colder water contains more dissolved oxygen than warm water. Caddisfly larva in cases have an advantage besides the protection their cases provides them. Their cases essentially help them breathe. They move their bodies back and forth, up and down inside their cases, and this makes a current that brings them fresh oxygen. The less oxygen there is in the water, the faster they move. This is somewhat like  pumping out the old water so fresh water can come in, and not unlike what we  do in our homes when we occasionally open one of the doors for awhile to let in fresh air.  It is believed that caddisfly larva in their cases get more oxygen than those out of cases, and this is why it is thought that caddisfly larva can often be found even in still waters where dissolved oxygen levels are low, as apposed to stonefly and mayfly nymphs.