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.

An insect has a brain, connected to bundles of nerves (ganglia) in each of its three body segments. Like us, they have sensory nerves that receive information from their environment and send it to the brain. Information comes from many types of touch receptors. Touch-sensitive hairs all over an insect's body are sensitive enough to detect vibrations in the air, such as from an approaching predator. Insect hairs also do chemical detection (chemoreception). Pores at the end of chemoreceptors on mouthparts or other body parts allow odors to reach the nervous system. Insect antennae may have thousands of chemoreceptors, used to detect chemicals in the air (pheromones) emitted by members of the opposite sex. An insect, like us, has a pair of eyes on its head. But, insect eyes are compound, with hundreds or thousands of lenses, together making an image consisting of spots of light, like pixels. An insect's sound receptors are on its legs.