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New Hampshire Department of Environmental Services

Common Stream Invertebrates

Why Study Bugs?
bugAquatic insects and similar organisms have a long taxonomic history. Ever since the first fly rod was cast, fishermen have sought to understand the cuisine of their quarry. Stream ecologists may occasionally wet a line too, but they seek to understand the organisms for other reasons. Invertebrate populations act as indicator organisms for the ecosystem. How so?

For one thing, they represent a low point on the food chain. That is, if you could trace back what each one had for lunch, you would find it was either plant matter or something else that just ate plant matter. Basically, they are not very far from the autotrophic introduction of energy into the ecosystem. As such, their response to ecosystem disturbance is quite immediate.

Compared to "higher" organisms, they have simpler physiology. When a pollutant enters its small domain, an invertebrate is capable of limited adjustments to survive. Generally, an organism's response to pollution can be to adapt, flee, or perish. Most aquatic invertebrates are capable of limited physical, spacial movement, so escape is often not an option.

These characteristics make invertebrates excellent indicator organisms. Some species have a "reputation" for being pollution intolerant. Their presence indicates minimal disturbance in its habitat. Some species, classified as tolerant are capable of withstanding stresses, more so than pollution-sensitive species. An over abundance of tolerant species indicates pollution.

Another consideration that makes invertebrates good candidates for ecosystem assessments is their size and ease of collection. It's not that stream ecologists are lazy, but logistics must be a consideration. A terrestrial comparison could best illustrate the advantage. To assess a prairie ecosystem, you could capture a representative sample of buffalo. Granted they would be relatively easy to key out to the species level, but darn tough to net and take back to the lab. Perhaps a representative sample of field mice would be more suited to your time schedule and budget.

What Are Invertebrates?
Invertebrates are those organisms without a backbone, representing roughly 95% of the species on earth. For the purposes of biomonitoring, it is usually only macroinvertebrates that are sought. Loosely defined, these are the invertebrates that can be seen with the naked eye. Populations of microscopic animals have a huge impact on aquatic ecology, but are not as easily recognized and studied. It's common to refer to macroinvertebrates as insects, but this is not technically accurate. Most of the organisms included in macroinvertebrate studies are insects, at varying stages of their life cycles. Also included however, are crustaceans, mollusks, aquatic worms, spiders, and others.

Body Adaptations
The types of organisms you can expect to find in stream ecosystems, while quite diverse, all have certain adaptations in common. Evolution has provided clever alterations in their morphology that make life in a current more tolerable.

Many of the alterations are designed, not to withstand current, but to avoid it. For example most lotic mayfly species are flattened dorso-ventrally (from top to bottom). This allows them to live in the boundary area right near the stream bottom where the flow is minimal. The flattened body also makes them less likely to sink in mucky bottoms. Other morphological adaptations include a comparatively small size, allowing them to scoot into tiny crevasses, safe from the dangerous current.

If current avoidance is not the goal, organisms may exhibit a streamlined body shape, which makes short, rapid bursts of swimming more successful. In some of the fastest-flowing streams, you may find species with various forms of suckers. They will literally suction-cup themselves to a rock and go about their business. All aquatic insects have tarsal claws, but they are better developed in lotic organisms. In this manner they cling to periphyton, submerged vegetation, or even an inorganic substrate.

Some organisms, like our friend the black fly, produce silky or sticky secretions to attach themselves to the substrate like glue. An ingenious feeding technique used by some stream macroinvertebrates is to extend a silken net into the current, wait for a drifter to become ensnared, and then reel them in. Some researchers have seen actual "pantries" where these organisms have stored net-captured food. Caddisflies are big silkers. Often they will avoid drifting away by creating a house out of rocks to withstand the current, and then use silken nets to capture dinner. The same ballast approach is used by snails and clams. These organisms are filter feeders who siphon ambient water, retaining the zooplankton load. Some caddisflies prefer light-weight plant debris for home building and excrete silk strands to tether themselves securely to the substrate.

Insect Life Cycles
A true distinction between terrestrial and aquatic insects cannot be made. Many insects inhabit aquatic areas only during brief stages of their ridiculously short life cycle. Also, a good stream biologist should be familiar with those organisms considered terrestrial, since there are frequent "drop ins" from the riparian canopy. You can also observe winged species, like a wayward wasp, trapped in the surface film of slow-moving waters. An understanding of the life cycles of macroinvertebrates is crucial for identification, since an organism can change drastically in appearance.

The life cycle for most aquatic insects, that is from egg stage to egg stage, is usually about one year, with many exceptions. Most start as an egg, with 10 billion brothers and sisters, in an egg sac of some sort. The individuals emerge either as larvae or nymphs. Nymphs are what you may view as "baby" bugs, since they very closely resemble the adults. These organisms pass through stages which are collectively considered an incomplete metamorphosis. They will outgrow and shed their exoskeleton, passing through 3-5 instars - still very much resembling the adult. Dragonflies, beetles, and true bugs, stoneflies, and mayflies all have this type of life cycle. The nymphs of dragon flies are called naiads.

Organisms that emerge from the egg in a larval stage are said to have a complete metamorphosis, and will pass through the larval stage, to form a pupa, and then form an adult. These early stages do not even remotely resemble the adults. The changes that lie ahead are really quite incredible. The larvae will often have affectionate names all their own - like maggots. These are the larvae of flies. The larvae of butterflies are called caterpillars. Among stream organisms, the true flies and caddisflies are examples of this type of life cycle.

Common Taxa
Listed below are common taxa you could expect to find in a stream macroinvertebrate study. Techniques for capturing these are included under the Invertebrates section. Remember, some of the organisms have only a brief aquatic stage before emerging as adults; others are aquatic throughout their lives.

Stoneflies (Plecoptera)
As adults these are winged terrestrial insects, usually not far from running water. The nymphs are recognized by having two tails, actually called cerca. They basically are quite a sluggish lot, occurring among mats of organic debris, and under stones. Their feeding habits range from herbivory to carnivory, and a glance at their mouth parts should allow you to place them at the correct deli counter. In many stonefly species, the mating is accompanied by drumming - although I wouldn't listen for it. The drumming is species-specific, and some dialects have actually been detected! In the New Hampshire Biomonitoring Program, the most commonly encountered family of stoneflies is Perlidae. Unlike some other insect orders, stoneflies are rarely referred to by any other common name - just all basically called stoneflies. I'm sure they all have nick-names for each other, which we are not privy to.

Mayflies (Ephemeroptera)
These too are terrestrial as adults, usually remaining near water. Initially the larvae can be confused with stoneflies, but a fairly constant distinguishing trait is the presence of three (not two) "tails". Mayflies also usually have gills on each abdominal segment. The larval organisms are extremely diverse with many specialized adaptations to habitat. Most species are scrapers and collectors, feeding on a variety of detritus and algae. The families most often encountered in the Biomonitoring Program are Baetidae and Heptageniidae.

Caddisflies (Trichoptera)
This is one of largest groups of aquatic insects. They are best known for their ingenious retreats and portable cases they construct. Some are saddle-case makers, others are tube makers, and still others create a silken purse. Basically when you observe a pile of debris slowly maneuvering along the bottom of a stream, it's probably a caddislfy larvae. The construction of the case is very significant in identifying the organism family. To speciate them, you usually must extricate them from their tiny homes. Be sure to cover them after you do this, as they are likely to get a chill. In New Hampshire streams, one of the most common families is Hydropsychidae. These are actually distinguishable by not building a case at all, but by creating an amorphous net case that is usually destroyed upon collection.

True bugs (Hemiptera)
Lethocerus is one of the meanest-looking critters you might encounter in aquatics. Fortunately they aren't common stream dwellers. Other members of this order have whimsical names like water striders and back swimmers. They all have mouth parts modified for piercing and sucking. Many of the true bugs are aquatic throughout their lives, leaving the water only to migrate. Hemipterans pass through five instars, all looking very similar, and very much like the adult. The predominant taxa in New Hampshire streams is extremely variable. Since these organisms frequently take advantage of surface films, they are not commonly found in riffle habitats. However, in stream backwaters, Belostomidae, the Giant Water Beetle, can usually be found.

Beetles (Coleoptera)
Most beetles encountered in stream surveys are aquatic throughout their lives. They are recognized by having leathery forewings covering their body. The metamorphosis is complete, with the larvae vaguely resembling the adults. After passing though 3-5 instars, the larvae excavate a terrestrial pupation site. Most transform into adults in about 3 weeks. The swift water habitat is not a favorite among beetles. However, one of the most common families is Psephenidae, the Water Penny. In the larval form this organism is greatly flattened and looks - well - like a penny. All instars look identical, with pupation occurring in the last larval skin. The adults are found under logs and rock, and do not feed. Bummer.

True flies (Diptera)
Larval flies live nearly everywhere, in the soil, in ponds and streams, in the sea. The order contains many familiar insects, like the black flies, midges, craneflies and mosquitoes. The Dipteran order actually contains very few predacious feeders; most are scrapers or grazers. As primary consumers, they are very significant indicator organisms. One of the most common genera is Chironomidae, the midges. The adults are non-biting but tend to swarm in enormous numbers to make them pesky. Speaking of pesky, how 'bout those blackflies? Simulidae are notorious biters - actually just the females bite. This is not merely bothersome, but they can also be disease carriers. The larvae are found in streams with particularly swift current, often in proliferation. Here again, ecological common sense (don't we all have this?) must prevail. As much as we love to hate them, black flies are a major food source, as larvae in the aquatic habits, and also as adults for terrestrial predators.

Dragonflies (Odonata)
Dragonfly nymphs and adults are active predators. Lazier nymphs take the sit-and-wait approach, while the over-achievers slowly stalk their prey. Nymphs have external wingpads, chewing mouthparts, and the labium (lower lip) modified into a shovel-like extendible structure. The order is divided into two suborders. Zygoptera are damselflies and Anisoptera are true dragonflies. And no, this is not a division of boys against the girls. Adult damselflies are smaller, more slender and delicate than the suborder Anisoptera. All odonate nymphs (naiads) are carnivorous, each with capturing schemes customized for their habitat. One of our most common families, Gomphidae will burrow into the mud or silt bottom so that only the eyes and rear are above the surface. Here they lie and wait, sometimes for days at a time, waiting for prey to come within reach. The human population would have a drastically different look if everyone waited to eat until food came within reach. Hmmm.

Dobsonflies and Alderflies (Megaloptera)
These are some of the largest aquatic organisms you might encounter in a stream survey. They are all predaceous (vicious?) and have a complete metamorphosis. The larvae exit water to pupate in the soil or leaf litter. The larvae can be recognized by having long spiracles and tracheal gills on the abdominal segments. The larval stage of some Megalopterans can last 2-3 years, which is considerable compared to most other aquatic insects. The dobsonfly, also called a hellgrammite, is a very popular live bait among fishermen. Some hellgrammites can grow to be 5 inches long and can deliver a nasty watch those toes!

There are others, you know
Although aquatic insects represent the majority of organisms collected during a stream survey, they form only a part of the invertebrate communities of most freshwater habitats. At a minimum, students of aquatic ecology should have a casual familiarity with other freshwater macroinvertebrates. One of the main reasons is that these other invertebrates are often intimately associated with the insects as food, predators, parasites, or competitors for available resources or space.

One of the most readily identifiable "non-insect" aquatic macroinvertebrates is the Aquatic Earthworm (Annelida, Oligochaeta). These segmented, cylindrical worms are well represented in freshwater habitats and may be found in silty substrates and among the debris and detritus of streams and rivers. Some are associated with algae mats in the slower reaches and pools. Some aquatic worms such as the tubificid worm tolerate low dissolved oxygen and are often found in large writhing communities in polluted streams and rivers.

Here's a frightening thought: there are over 63 species of freshwater leeches (Hirudinea)in North America. Although leeches have a reputation for being bloodsuckers, not all species are actually blood-feeding ectoparasites. The freshwater forms of leeches can be found on the substrate or attached to host animals in a wide variety of aquatic habitats. What do leeches do if they're not sucking on our toes? The food habits of other leeches include scavenging and preying on other invertebrates such as aquatic insects, mollusks, and other worms - and fellow leeches.

Snails, mussels and clams are also found in streams and rivers. Some snails feed exclusively on encrusted growths of algae while others are detrivores or omnivores. Mussels occur on or in the substrate and they generally feed by filtering planktonic (drifting) microorganisms out of the water. The soft body parts of snails, mussels and clams are eaten by a number of fish as well as some other animals such as muskrats.

Probably the most recognizable organism that doesn't belong in the aquatic insect community is the crayfish (Decapoda). Crayfish occur in a wide variety of shallow freshwater habitats from swamps, wetlands, ponds, streams and rivers. They are benthic and spend the daylight hours hiding under stones, logs and debris. They retreat backwards rapidly when disturbed and use their powerful front claws as defensive weapons. Crayfish may be herbivores, detrivores, omnivores or carnivores where they use their claws to cut or crush food. These claws can also be used to pinch fingers and toes so be careful not to back a crayfish into a corner!

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