Beetles, caddisflies, stoneflies, mayflies, hellgrammites, dragonflies, true flies, and some moths are among the groups of insects represented in streams. Insect life cycles can be grouped as either complete or incomplete metamorphoses. A complete metamorphosis includes a pupae stage. The adult and larva tend to look very different from each other. Incomplete metamorphosis lacks the pupae stage and the nymph and adult are more similar in appearance.
Most aquatic insects remain underwater in the immature stages and leave the stream only as adults.
The life cycle of macros ranges from a few weeks to several years. Aquatic Insects Order Ephemeroptera Mayflies Mayfly nymphs are adapted to aquatic habitats that range from ponds to mountain streams with the greatest diversity found in rocky-bottomed small streams. Both the immature and adult stages are an important food supply for fish, in particular trout. Many mayflies are intolerant of changes in water quality. This makes them good biotic indicators of water pollution.