Megaloptera include some of the largest macroinvertebrate insects. The two families within the Megaloptera are similar in appearance and are active predators but live in different habitats in the Northwest. Dobsonflies and fishflies (Corydalidae) are very large and typically found in small, cold, mountain streams, but are not commonly found in wetlands. Alderflies (Sialidae) tend to be a little smaller and are more typical of still waters and lowland streams, but are still not very abundant in wetland macroinvertebrate samples.

Megaloptera are unusual because three of their four life stages are terrestrial. Eggs are laid on the underside of objects (leaves, rocks, bridge abutments, etc.) that hang over the aquatic larval habitat. The larvae then drop into the water after they hatch. They live and grow for 2 to 5 years, then crawl out of the water to pupate in the soil or in soft vegetation very close to their larval habitat. The terrestrial adults emerge in spring and the first half of summer; adults are nocturnal, live only a few days, and are awkward fliers (though some may fly long distances).

Megaloptera larvae are fairly large and distinctive. Their heads are thick and tough with noticeable mouthparts projecting forwards. A plate covers the top of each thoracic segment and there are long tapering filaments on the sides of each abdominal segment. The abdomen ends in either a single tapering point (Sialidae) or a pair of small clawed prolegs (Corydalidae). A few beetle larvae also have long filaments on the sides of the abdomen ( Gyrinidae and some Dytiscidae, Haliplidae, and Hydrophilidae). However, these beetles are generally smaller, lack large plates on top of each thoracic segment, and do not have abdominal prolegs.