Most people are familiar with the larger animals that are commonly found in or near a stream - fish, frogs, snakes, raccoons, otter and a variety of birds - to name a few. But those animals are not always around when you are. On the other hand, you are almost guarnateed to find aquatic insects in any stream you visit. These small creatures provide a window into the fascinating and complex world of an aquatic ecosystem.
Did you know that most of the insects flying around in the air began life underwater? It's true. In fact, insects outnumber most other forms of aquatic life in an aquatic ecosystem. That's a good thing because aquatic insects are an extremely important part of the food web. They are the food for many larger animals including fish, frogs and birds.
Aquatic insects are invertebrates. Unlike fish, people and birds, invertebrates lack a backbone. Instead of a spine or backbone invertebrates may have a hardened body, like a beetle, or a shell, like a snail or mussel. Some invertebrates have soft bodies, like earthworms. Spiders and crayfish are other types of invertebrates that may be found in a stream. Invertebrates come in an amazing variety of shapes and sizes.
The life of an aquatic insect begins when the parent lays eggs in the water. Immature aquatic insects become adults after undergoing metamorphosis, a process which can radically change the insect's appearance. A caterpillar goes through metamorphosis to become a butterfly. Immature aquatic insects are called either larvae or nymphs. A nymph looks somewhat like the adult stage of the insect. Larvae look very different from the adult. Following metamorphosis many species of adult insects remain in the water, spending their entire lives there. Many others leave the water environment and become land based. Mosquitos, dragonflies and black flies are three common insects that are born underwater.
As with all forms of life in a stream, aquatic insects have developed certain unique adaptations that allow them to breathe, move, find food and otherwise survive. All insects need oxygen. Like fish, some insect species get oxygen from the water. Others breathe air just like people. What is fascinating is how some of these creatures do this. Water scorpions and giant water bugs have breathing tubes that reach to the surface of the water. The diving beetle actually carries a bubble of air as it swims underwater.
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Different insect species eat different types of food. Herbivores eat nothing but plants. Algae, the "slippery green stuff" that grows on rocks at the bottom of streams, is a favorite food of many herbivores. Predators eat other animals. Predatory aquatic insects eat other insects, fish and even small frogs. Omnivores feed on plants and animals. Scavengers eat dead plant and animal matter.
Aquatic insects move around in the water in a variety of ways. Some simply crawl along the rocks and mud on the stream bottom. Dragonfly nymphs move around by forcing water out through their abdomen, just like a jet engine. Water boatmen use their long hairy legs to paddle through the water, much like a rowboat. The waxy surface of the water strider's legs allow them to "skate" across the surface of the water. Insects lacking the ability to move themselves through the water simply drift with the current.
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Camouflage is an adaptation that provides protection from predators. The body color and pattern of many aquatic insects allows them to blend in with their surroundings. If a predator can't see them, it won't eat them! Camouflage is an important adaptation for many stream creatures, including salmon and trout. Atlantic salmon even have the ability to change color to match their background.
Aquatic insects can reveal a great deal about the health of a stream, pond, river or lake. Many species are very sensitive to water quality. They are particularly sensitive to the amount of oxygen in the water. A healthy stream generally has a large variety of aquatic insect species. Only the hardiest of insects can survive in a polluted aquatic environment.
Water quality monitoring is a popular envrionmental conservation activity. One aspect of testing for water quality generally involves determining the variety and number of aquatic insects and other invertebrates in a body of water. If you have been wondering about the health of a nearby water body you might begin to get some answers by doing a survey of the insects that live there. Perhaps this is something you can do with your class. It can be a lot of fun.