The presence of mayflies, stoneflies, and caddisflies is possibly the best indication that a stream is healthy. Stonefly nymphs have two tails and each leg ends with a double hook. Their color ranges from white, through yellow-brown to a glossy black. Stoneflies live in protected areas of debris, leaves, or under stones. They take oxygen from the water by way of fixed gills that cannot be moved.
Many species are voracious predators.
While they are found in lakes, stoneflies prefer clean, cold flowing waters where there is a good supply of oxygen. The nymphs crawl clear of the stream before emerging as adults. Mayfly nymphs are very similar in shape to the stoneflies. These creatures dart, crawl, cling, and sprawl. Mayflies inhabit all types of freshwater where the oxygen supply is good.