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 the only animals without backbones (invertebrates) that can fly. Wings of insects are flat and paper-thin, supported by a network of veins. They flap their wings at incredible rates, up to about two hundred times per second. Most insects have two sets of wings that work in tandem because they are coupled by a fold, a hook, or other structure that catches the back wing as the front wing beats. Wings are made of two layers of cuticle for strength. The front wing is often hardened to serve as a protective cover, leaving the back wing to provide most of the flying power. Many insects walk around instead of, or in addition to, flying. The good walkers tend to touch three legs to the ground at a time, alternating with other sets of three (tripedal gait). The stable triangles formed by the legs allows them to move quickly without falling over. Some insects "walk" on water or swim.