Invertebrates are animals that don't have backbones. Macroinvertebrates are invertebrates large enough to be seen with the naked eye.
Those that live in streams differ from those found in lakes and ponds. This is because the environmental conditions are very different. Most types of ponds offer low velocities (or even stagnant water), great fertility from rotting natural materials that collect on the bottom, microscopic floating diatoms and crustaceans, large amounts of algae and other plants, often warm temperatures, and occasionally low light (because of depth, algae, and plants). The bottom of the pond is usually very dark, and doesn't have much oxygen.
In contrast, streams are fast, cold, and often clear. Although plant material decays and diatoms cling to rocks to provide food, this food is only available to animals that can eat from the bottom (the benthos). For a macroinvertebrate to be a benthos feeder, it must be able to stay on the bottom when it wants to. Animals do that by clinging, gluing themselves down, or burying themselves in the sediments. Although certain crustaceans, snails, and worms can perform this feat, by far the most numerous of stream invertebrates are insects. Because of their easy availability, they figure strongly in stream research.
Stream macroinvertebrates develop marvelous strategies for utilizing niches. Each species has a body specially modified to take advantage of the many different foods and habitats available even in the smallest stream. If you take a walk along a stream, you will see that there are places where the water flows slow, fast, and inbetween. You'll see stream bottom that is covered with mud, sand, pebbles, rocks, or even boulders. You'll notice that parts of the stream are shaded, while others are always in the sun, and that some areas are deep while some are shallow. You'll see areas where debris has piled up, creating dark, shaded, deep areas as well as tiny waterfalls. There are macroinvertebrates that have adapted to each of these varied conditions--and more. Once you know how they have specialized, you'll be able to find them easily.
The pages below will introduce you to the most common aquatic insects. All of them begin their lives by hatching out of eggs on the stream bottom, underwater. While they live underwater, we call them nymphs, or in the case of the caddisflies and true flies, larvae. They all have gills that allow them to breathe oxygen directly from the water, like a fish or a clam does, although these gills aren't always easy to see. They can be little threads, plates, bushes, or combs, and may be almost anywhere on their bodies, though often they are on their abdomens.
These insects also have in common that they spend a great deal of time growing up in the water of a stream or river, while their lives as winged adults are brief; they only stay alive long enough to mate and lay their eggs back in the stream.
They are all members of the Kingdom Animalia, Phylum Arthropoda, and Class Insecta.
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More links about aquatic invertebrates.
For some in-depth study, head for The Zoobenthos of Freshwaters .
Wonderful, Wacky, Water Critters, a beautiful little guide for both kids and adults! (PDF)
Scientists and activists use stream invertebrates, because they are numerous and easy to collect, to help them gauge the health of streams. To learn more about aquatic insects, crustaceans, molluscs, and worms, check out the reading list, where you will find some excellent references listed. (You will be able to order them from here, too).
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1. Shredders - These have strong, sharp mouthparts that allow them to shred and chew live plants or decomposing fragments. These are common among true flies, caddisflies, and stoneflies.
2. Collectors - These gather the very finest suspended matter in the water. To do this, they often sieve the water through rows of tiny hairs. These sieves of hairs may be displayed in fans on their heads (blackfly larvae) or on their forelegs (some mayflies). Some caddisflies and midges spin nets, and catch their food in them as the water flows through.
3. Scrapers - These scrape the algae and diatoms off of surfaces of rocks and debris, using their mouthparts. Many mayflies, caddisflies, and true flies eat this way.
4. Piercers - These herbivores pierce plant tissues or cells and suck the fluids out. Some caddisflies do this.
5. Predators - Predators eat other living creatures. Some of these are engulfers; that is, they eat their prey whole or in parts. This is very common in stoneflies and dragonflies, as well as caddisflies. Others are piercers, which are like the herbivorous piercers except that they are eating live animal tissues.