The name Ephemeroptera refers to the short-lived nature of the winged adult stages of mayflies. Mayflies are one of the more widely known macroinvertebrate groups, mostly because of their importance to fish diets and thus fly fishermen. In some parts of the country certain families (i.e., Ephemeridae) are well-known because of their spectacular mass emergence. These coordinated emergences can clutter the bridges and roads with mayfly bodies and make a mess of a passing car's windshield.

Mayflies are among the most primitive insects and have no pupal stage (incomplete metamorphosis). They are unique in that they go through an intermediate adult stage called a sub-imago, in which they have wings but are not sexually mature. Sub-imagos can generally be identified by their cloudy wings and relatively bland color patterns. Typically, sub-imagos emerge from the water and rest on the surrounding vegetation for a day or two, then molt into an adult that lives from another day or two to a month. Neither the sub-imago or the adult feed.

Mayflies are important because of their prominence in fish diets, their importance as food to other aquatic predators, and their value in processing plant material. Nearly all mayflies survive by eating detritus, diatoms, and other algae, and thus play a crucial role in decomposition.

Mayflies inhabit a diversity of aquatic habitats, which is reflected in the diversity of body shapes, behaviors, feeding strategies, and movement seen in this order. Generally, mayflies are either sleek and torpedo-shaped, minnow-like (swimmers) or robust or flattened clingers. Slack waters such as lakes and wetlands are primarily inhabited by minnow mayflies, but all types live in flowing waters. Many of the clingers have evolved traits that help them hang on in fast water, such as gills or fine hairs for suction and extreme flattening of the body.

The abdominal gills of mayflies are one of the most distinguishing features of the group, and are usually prominent. All mayflies have flattened gills on their abdomen. These gills range from feathery to plate-like and are always present on at leat a few segments. You can watch some mayflies move their gills while they rest in a collection tray. The Ephemerellidae are noted for waving their round gills rapidly.

Most mayflies have three tails (cerci) and a single claw at the end of each leg. However, some of the most common species have only 2 tails, with the middle tail either very short or altogether absent.

In the wetlands of the Pacific Northwest, you are most likely to encounter the families Baetidae and Siphlonuridae.