The character of a stream changes as it flows through the countryside, tumbling over boulders, rushing over rocky cobbles, or gliding over a bed of sand. Streams provide homes for fish and salamanders, while animals like mink, raccoons, and birds visit them in search of food. If you look closely, you’ll find that a surprising number of insects live underwater in brooks and streams, a seemingly inhospitable environment for these small creatures. How do they meet the challenges of life in fast-flowing water, and why does this environment attract such a wealth of insects?
Streams afford a variety of habitats, from noisy cascades to quiet pools, from smooth, straight runs to rough, rocky riffles. The composition of the streambed depends on the speed of the water. Fast-moving water carries away all but the largest rocks and boulders. As the water slows, smaller rocks settle to the bottom in order of size, from cobbles to pebbles to gravel to sand. The finest sediments, particles of silt and clay, settle out where the current is slowest. The nature of the streambed is a key factor determining what lives where in a stream.
The land on either side of the stream, the riparian zone, also affects the conditions within it. Forested banks keep the water shaded and cool and provide a constant supply of dead leaves, an important food source for a multitude of small animals. Branches and twigs snag floating debris and create underwater hiding places. Sunny open streams with treeless banks have less organic debris, warmer water, and more algae growth, a different environment altogether. Cold water holds more dissolved oxygen than warmer water, so a stream that is largely shaded makes a better habitat for trout and many other aquatic animals. Trees also prevent erosion, keeping sediments and pollution from washing into the water.
An abundance of small animals, including insects, worms, snails, mussels, and crayfish, collectively called benthic macroinvertebrates, live on the bottom of streams. The term “benthos” means “bottom-dwelling,” “macro” means big enough to see without a microscope, and “invertebrates” are animals without backbones. Many of these are the larvae of insects we know as winged adults, like dragonflies, black flies, and long-legged mosquito-like crane flies. They spend months, or even years, under water before they metamorphose into adults with wings. Each has a preferred habitat in the stream: black fly larvae live on rocks in fast current, crane fly larvae burrow in gravelly places, and dragonfly nymphs lurk in muddy-bottomed pools.
Insects that live in the streambed get their food in many different ways. Some are scrapers, feeding on the film of algae that grows on submerged rocks. Others are shredders, chewing up leaves and other dead organic material that fall into the stream and reducing them to fragments that become food for smaller insects downstream. Collectors eat tiny floating plants and animals or particles of organic matter suspended in the water or on the stream bottom. Some insects are predators, preying on smaller creatures in the stream. Benthic macroinvertebrates are an important food source for fish and other stream dwellers, and many of them play a key role as decomposers in the stream ecosystem, helping to break down organic material in the water.
Anywhere along a stream can be a good place to find insects, but the richest places are often in riffles, shallow rocky areas with swift, choppy water. Why would delicate insects prefer places with fast-moving water, where they face the constant challenge of hanging on? Most likely, it is because here, among the cobbles and pebbles, are countless nooks and crannies for hiding places, edges to snag leaves and catch floating organic debris, and surfaces on which algae can grow, and because the current delivers a continual supply of fresh, oxygenated water and floating food.
To keep in place, many insects hide under or between rocks where there is less current. Others have various means of hanging on. Mayflies and stoneflies have tiny hooks on their feet, water pennies stick to rocks like suction cups, and a number of insects use glue! Black fly larvae fasten a small pad of sticky silk to the upper surface of a rock, attaching themselves to it with a ring of tiny hooks on the tip of the abdomen. When dislodged from a rock they let out a strand of silk like a lifeline and then hoist themselves back up to their perch.
Some of the most common riffle residents are the larvae of mayflies, stoneflies, and caddisflies. Pick up any small rock in a riffle and you’re likely to find mayfly nymphs, their bodies flattened against the surface. They are easy to recognize: most have three tail-like filaments on the tip of the abdomen, and on their sides a row of fan-like gills that they undulate in waves. When swimming, mayflies wave their tails up and down like a butterfly kick. Some are algae scrapers while others are collectors, gathering tiny fragments of debris from the water. Most species of mayfly live for a year underwater and then emerge in large swarms as winged adults. Their adult life is brief – in some cases only a few hours – just enough time to look for mates and lay eggs before they die.
Another riffle creature is the stonefly, distinguished from mayflies by having only two tail filaments, and, instead of gills on the abdomen, they have fringe-like gills under their legs and thorax. When swimming, their jerky, side-to-side motion is distinctive. In still water they can often be seen to pump their bodies up and down, an action thought to circulate water over their gills and help get them more air. Many kinds of stoneflies prey on other stream insects, but some, like the giant stonefly, are shredders, feeding on decaying leaves. Clumps of leaves caught in the streambed provide both hiding place and food supply for this insect nymph.
Tiny, intricately constructed tubular cases of sand grains, sticks, or other debris can often be found stuck to rocks in riffles. A closer look may reveal a small animal poking its head and legs out of the case. These are the larvae of caddisflies, common inhabitants of cold-water streams. Fastened to the rocks with glue or carried on their backs, these cases provide camouflage and a means of holding on in the fast current. Some caddisflies are algae scrapers, but many are collectors with various ways of filtering food particles from the water. Some extend bristly legs into the current and periodically lick off food caught in the hairs. Others build a silken net to strain food out of the water.
The living things in an ecosystem interact in many ways with the nonliving parts of their habitat. The place where each stream insect lives depends on a number of factors. Some need rocks to hold onto, and some need sand or gravel in which to burrow. Some prefer tree-shaded places where there are dead leaves to eat, and others need sunny places where algae grows on the rocks. Many can only survive where the water is cold, fast, and well aerated. Some are highly sensitive to pollution, which lowers oxygen, and can only live in the cleanest water, while others are much more pollution-tolerant. Finding mostly leeches and midges in a stream is a clue to poor water quality or high temperature or both. Finding a large proportion of mayflies, stoneflies, and caddisflies is a sure sign of a healthy stream, as these insects can only live in clean, well-oxygenated water.
As a stream slips by sandy banks, tumbles over cobbles, or plunges over boulders, it provides a variety of habitats, oxygen in plenty, and an abundance of food. In spite of the challenges of life in fast-moving water, streambeds teem with tiny but tenacious insect life.
Reid, George K. and Herbert S. Zim. Pond Life. New York: Golden Press, 2001.
Voshell, J. Reese. A Guide to Common Freshwater Invertebrates of North America. Blacksburg, VA: McDonald and Woodward, 2002.