It's a stream bug's life
Before we can talk about where stream bugs live, it is useful to understand some development basics. Because most insects hatch from eggs, much of their early (embryonic) development occurs inside the egg. Once the insect emerges from the egg, it begins its postembryonic development—its metamorphosis.
Once outside the egg, insects molt their exoskeletons as they develop. This shedding of the old exoskeleton allows them to change and grow to adulthood. They may molt up to forty times, but generally they molt five or six times. Each molt has a specific duration and increase in size of the insect. Each time they molt, the insects are a little different, and during each of these many stages before adulthood, the insect is known as an "instar." So for example, during the period of development from hatching to the first molt, the individual insect is called the first instar.
As insects go through their numerous developmental stages, they either exhibit dramatic changes in physical appearance during each instar or they look very similar to the adult form and just grow get bigger. These two types of life cycles are referred to as complete metamorphosis and simple metamorphosis, respectively. Whether an insect goes through complete or simple metamorphosis depends on the species.
With complete metamorphosis, each instar looks very different from the adult. These insects also go through a non-feeding pupal stage just before they emerge as an adult. The most widely known example of this life cycle is the butterfly. Aquatic insect examples include flies, beetles, and caddisflies.
In simple metamorphosis, which may also be called incomplete or gradual metamorphosis, the larval stages (also called "nymphs") look very similar to the adult stage and there is no pupal stage. Stoneflies, mayflies, and dragonflies are some types of aquatic insects that under simple metamorphosis.
Aquatic Insects. When thinking about aquatic macroinvertebrates, it is important to know that the majority of their lives are spent in the larval stages in the water. Usually, when aquatic insects reach adulthood, they fly the coop—they have wings and no longer live in the stream (though they usually live nearby so that when times comes to deposit their own eggs, they can do so in the same water). Caddisflies and mayflies are excellent examples, as are mosquitoes.
Other Invertebrates. Other benthic macroinvertebrates such as snails, clams, and crustaceans spend their entire lives in the water from egg to adult.
Stream Habitats. Now that it is obvious how many different life styles benthic macroinvertebrates have, we can start to think about where different species spend their time in the streams. Streams are composed of different habitat types such as riffles, runs, glides, and pools. Different bugs are adapted to the different microhabitats each of these represent. Some bugs need the clear, cold, fast water found near the headwaters of streams. For example, bugs such as Mayflies have a very flat profile and are able to cling tightly to rocks. They live in riffles where the water is fast and turbulent. Other bugs are more commonly found in the deep, slow, warmer reaches lower down in the system. Aquatic worms are often found in the bottom of pools where fine sediments accumulate and dissolved oxygen levels are lower.
Stream bugs and food
Benthic macroinvertebrates are not only good indicators for biologists who are studying stream health, but they also actively play a very important role in the stream ecosystem. In order to understand the role benthic macroinvertebrates play in the stream ecosystem, it is helpful to understand more about food chains.
Producers and consumers. We can think in general terms of a food chain consisting of three main groupings, or trophic levels: plants, herbivores, and carnivores. Plants are at the "bottom" of the food chain—they are able to make their own food by using the energy from the sun (they are autotrophic). Because they produce their own food, they are called Producers. All other living things that can't make their own food must consumer the producers, so they are called consumers (and are heterotrophic). Consumers are higher up the food chain (at a higher trophic level) and are either herbivores (they directly consume the producers) or carnivores (they eat other consumers). Macroinvertebrates are heterotrophic and may be both herbivores and carnivores.
Herbivores. Because benthic macroinvertebrates eat and are eaten, they are smack-dab in the middle of the food chain, a position that is critical in a healthy stream ecosystem. Herbivorous stream bugs consume plants and algae (these bugs are the scrapers and shredders).
Carnivores. The benthic macroinvertebrates that eat small fish, small amphibians, and other stream bugs are the carnivores. These predaceous macroinvertebrates are the predators.
Detritivores. To confuse matters slightly, there is another group in addition to the herbivores and carnivores. Detritivores are sort of a combination of herbivores and carnivores. Detritivores shred and eat leaves and other organic matter that contain nourishing fungi, algae, and bacteria. They absorb the nutrients as they break down the material into smaller sizes that other bugs utilize. Detritivores are the shredders and collectors.
Nutrient cycle. Stream bugs may get eaten by other macroinvertebrates and fish that depend upon them for food. For example, both juvenile and adult salmon eat macroinvertebrates. And not only do other consumers need them for food, but when the bugs die, they decay and leave nutrients behind that flow back into the food chain.
So without the stream bugs, organic materials won't be processed, nutrients won't get recycled back into the system, and many consumers won't have any food—in short, the system degrades until it is unhealthy or dead, then most bugs couldn't live there even if they wanted to.
If you have read the section above on stream bug food chains, you know that benthic macroinvertebrates are part of an integrated feeding relationship with plants, animals, and each other. Each type of eater has a specific and important role in the food chain. Each bug also has a specific place in the stream ecosystem, and this is largely governed by the way it feeds. The mouth parts of benthic macroinvertebrates vary among the different species and determine how the bugs get their food (and therefore what kinds of food they can eat). Scientists group bugs with similar feeding strategies into functional feeding groups.
Because different types of food are found in distinct areas of a stream, bugs with similar feeding techniques will likely be found together in the appropriate area. For instance, in forested areas where debris from the trees enters the water, there will likely be more shredders because there are more things for them to shred! Common kinds of functional feeding groups include the following:
Scrapers obtain food by scraping algae and diatoms off rocks and other hard substrates in the stream channel. Scrapers are most commonly found in parts of streams where sunlight can reach the bottom to allow algal growth. Often, sunny parts of streams are faster parts, so many scrapers have evolved mechanisms that allow them to stay attached to rocks in fast currents. It is also helpful if the layer of algae is not so thick that the bugs can't get a grip on the surface. Scrapers may include snails, certain mayfly larvae, and limpets.
Shredders feed on leaves, twigs, and other pieces of organic matter that fall into a stream. They need lots of trees overhead (a dense canopy cover) to supply the materials, and the stream must be slow-moving so the materials don't get washed away. Shredders chew up the debris into smaller particles, which can then be eaten by Collectors. Shredders include stonefly larvae and scuds. The caddisfly larvae pictured above is another example of a shredder.
Collectors eat fine organic matter such as leaf fragments, bacteria, stream bed deposits, and waste products from other organisms. There are two categories of collectors: filtering and gathering. Filtering collectors feed by filtering food out of passing water with hair-like fans or by spinning a fine net. Gathering collectors feed by gathering food from the bottom. Collectors may be found in all types of stream reaches but are typically most common in lower reaches because that is where fine sediments generally accumulate. Collectors include worms, blackfly larvae like the one pictured, and many mayfly and caddisfly larvae and midges.
Predators eat other invertebrates and even fish. Predators may actually be found anywhere in the stream ecosystem, but they are less numerous compared to other feeder types. There must be enough prey critters to support predators, or they will not be present at all. Predatory organisms include the larvae of dragonflies, damselflies, dobsonflies, adult beetles and some beetle larvae, some midge larvae, and some crane flies.