Mayflies, stoneflies, and caddisflies are united by their importance as indicators of water quality in streams and rivers. The adults live on the edges of streams and have a lifespan as short as days or weeks. But, their larvae (naiads) hatch from eggs laid in the water and live up to several years. They are important food for fishes and other aquatic animals. Naiads are sensitive to water conditions such as chemistry, temperature, oxygen content (dissolved), current, and light availability. Human-caused changes, such as the addition of organic material (e.g. fertilizer runoff from agricultural), can make conditions unsuitable for the naiads. Because of the naiads' sensitivity, a healthy stream has a characteristic set of species of mayflies, stoneflies, and caddisflies. Changes to that set imply changes in water quality. A widely used index of water quality called the EPT (Ephemeroptera-Plecoptera-Trichoptera) Index is based on distributions of mayflies, stoneflies, and caddisflies.

Insects are by far the most diverse and abundant group of organisms on Earth. More than one million insect species have been identified, and estimates of how many species exist range into the tens of millions. Insects got their start way back, in the Paleozoic (about 500 million years ago) and have evolved in tandem with flowers (coevolution). Insects have become specialized on particular flowers, leading to complex sets of adaptations that couple them in a feeding and pollination relationship (a mutualism). Specialization allows for many types of insects to live in the same habitat, accommodating their exceptional biodiversity. In a backyard in the temperate zone, one might find several thousand species of insects. Specialization can occur within one plant species as well, with different insects using different parts of the plant. In just one species of tropical tree, Dr. Terry Erwin of Smithsonian's National Museum of Natural History found about a thousand species of beetles.