Fish flies are insects that belong to the subfamily Chauliodinae, which is part of the megalopteran family Corydalidae. They are most easily distinguished from their closest relatives, dobsonflies, by their jaws and antennae. Fishflies are quite large, with a wingspan of 2.5 to 3 inches (6 to 8 cm) and are usually light-colored (gray, tan, rusty) . Fishfly larvae are omnivorous or predatory, using their stout mouth pincers for grasping and chewing. They feed on algae and plants and live underwater for more than two years. Fishflies lay their eggs upon vegetation overhanging streams, whence the larvae, as soon as hatched, drop into the water, and go about preying upon aquatic animals. When ready to transform to pupae, they crawl out upon the bank and are then found in cavities under stones or even under the bark of trees.

Fish flies only emerge in places that have clean water, good oxygen levels, and not a lot of nutrient runoff. They are a sign that water quality is really good and are an indication of healthy lakes. Fish flies are also known as shadflies or mayflies. Adult fishflies do not eat, but they can pinch hard if mishandled. They live for only a few days and their entire lifespan is several years, but most of this time is spent as larvae. Fish flies are attracted to light, so turning off porch lights and keeping windows closed can reduce their numbers.