Diversity of these animals declines rapidly at the first signs of human disturbance. Different species build distinct cases, but they often lose them when removed from a stream. Caddisflies are insects that emerge to mate as winged adults. All three of these groups leave the water to mate as winged adults. Large swarms of mating mayflies and caddisflies often occur when all the individuals of a single species emerge at the same time.
Stoneflies crawl out of the water and mate on the ground.
The females of all three groups fly upstream and drop their eggs onto the water or dive into the stream to attach them to rocks or leaves. After emergence, adults fly for a short time but return to the water to feed in the same habitat as the larvae. Both the larvae and adults are rather small, dark-colored, and tend to drift to the bottom of a sample so they may be hard to see. Riffle beetles collect and gather a variety of different foods. There are many species of true flies, but you are likely to recognize three main groups or families.