Mayflies (Order Ephemeroptera) are classified into families. There are 22 families of mayflies in Wisconsin and neighboring Midwest states. These families are divided into genera. Several families contain only one genus, while others are more diverse. Within those genera are Wisconsin's 157 species of mayflies with published records. For a complete list of Wisconsin's mayfly fauna, see Mayfly Larvae of Wisconsin.

Family Arthropleidae. This family of mayflies has just one species from Wisconsin. It resembles species from the Heptageniidae, but with elongate palpi and very large gills.

Family Baetidae. This is the most diverse family of mayflies in Wisconsin, with nearly 50 species reported from the state. Nymphs have minnow-shaped bodies and are active swimmers.

Family Baetiscidae. This family includes only one genus and three species in Wisconsin. Nymphs have a turtle-like carapace formed by the thorax. Despite their robust appearance, they are good swimmers. Baetiscids are indicators of good water quality.

Family Behningiidae. This odd and rarely encountered family includes only one species in Wisconsin. They have a cockroach-like appearance and have dense patches of long hairs on the head and thorax.

Family Caenidae. This family contains mayflies with gill opercula (plates) formed by gills on the second abdominal segment. The superficially resemble tricorythids, but have operculate gills that are more quadrate. There are five genera known from Wisconsin, with Caenis being the most diverse with eight species.

Family Ephemerellidae. This is a diverse family in Wisconsin, with seven genera and 19 species reported from the state. Nymphs lack gills on at least the first two abdominal segments. Most species of ephemerellids are indicators of good to excellent water quality. Consequently, they are far more diverse and abundant in the northern half of the state.

Family Ephemeridae. Members of this family are large mayflies. Nymphs have mandibular tusks and burrow into sediments, features they share with a couple other families. Adults include those that form dense mating swarms along large, sediment-rich rivers. There are three genera and six species of ephemerids known from Wisconsin.

Family Heptageniidae. The flat-headed mayflies form the second-most diverse family of mayflies in Wisconsin, after the Baetidae. Heptageniids have flattened bodies, most evident in the head, to avoid current. The family includes some of our most ubiquitous mayflies as well as one of our two endangered species in Wisconsin, Spinadis simplex. There are ten genera and at least 25 species known from the state.

Family Isonychiidae. Wisconsin isonychiids include only one genus and four species known in the state. Nymphs have long filter-feeding hairs on front legs and large size.

Family Leptohyphidae. The leptohyphids are the most taxonomically challenging mayflies in Wisconsin. They have been my targets of several years of research. There is only one genus in the state, with five previously reported species. All of them are very small as both nymphs and adults.

Family Leptophlebiidae. This family includes five genera and ten species previously reported from Wisconsin. Most species emerge from streams and rivers early in spring. They easily are damaged during collecting and preservation, losing gills and legs, leading to a large number of unidentifiable individuals.

Family Oligoneuriidae. This odd family includes only one species in Wisconsin. Nymphs burrow in loose sand in medium to large rivers. It has been eliminated from many river systems due to changes in flow and sedimentation.

Family Polymitarcyidae. Like ephemerids, mayflies from the Polymitarcyidae burrow in substrates as nymphs. They have large mandibular tusks as well. Adults are less frequently encountered than those from the Ephemeridae and usually are pale. The only polymitarcyids known from Wisconsin are two species of Ephoron. Tortopsis primus, shown below, may eventually be found in the state.

Family Potamanthidae. This family includes one genus and two species. Nymphs are very large, with long mandibular tusks. Though nymphs from several other families also have such tusks, potamanthids differ in not burying in sediment and having a more sclerotized, flattened body. They occur in riffles and rapids, associated with rocks.

Family Siphlonuridae. Siphlonurids are medium-sized mayflies, usually with large gills as nymphs. Most avoid areas of strong current, being found in slack waters with leaf litter. There are only two genera and five species known from Wisconsin.