The family Baetidae, which belongs to the order Ephemeroptera, was first described by Leach in 1815. Since then, almost 100 genera and 900 species have been described. Although diverse, this family is relatively homogeneous. The adults are extremely similar to one another, the wings vary little and the penes are membranous, features that significantly reduce differentiation among taxa. In contrast, the larvae have more conspicuous differences. Most are collector–gatherers, but a few are carnivorous or filter feeders. In South America, although knowledge concerning the 27 genera and 132 species of Baetidae described for this region has improved in the last three decades, phylogenetic relationships remain unknown. The present study, the first cladistic analysis of Baetidae in South America, included 70 species (55 are Neotropical) and 126 morphological characters. The matrix was analysed using tnt, under implied weights. Although the monophyly of the family Baetidae was obtained with good support, the subfamilies proposed originally (Baetinae, Cloeoninae and Callibaetinae) were recovered as paraphyletic. The Baetodes complex, as well as the relationships between genera, is discussed. The validity of some structures or characters as support of different groupings is also discussed.

A new species of Apobaetis is described based on nymphs and male imago, and the male imago of A. fiuzai. Specimens were collected in the state of Amazonas, northern region of Brazil. Apobaetis hamadae sp. n. can be easily distinguished from the other congeners by the following combination of characteristics: male imago: (1) abdominal colour pattern; (2) area between forceps not excavated medially. Nymphs: (1) distal margin of labrum with shallow medial emargination and with four to five spatulated setae near midline; (2) maxillary palp 1.4× longer than galea-lacinia; (3) tarsal claws 1.2× times longer than tarsus. The male imago of A. fiuzai can be easily distinguished by the following characteristics: (1) abdominal colour pattern; (2) area between segments I of forceps with a deep V emargination.

The extant global Ephemeroptera fauna is represented by over 3,000 described species in 42 families and more than 400 genera. The highest generic diversity occurs in the Neotropics, with a correspondingly high species diversity, while the Palaearctic has the lowest generic diversity, but a high species diversity. Such distribution patterns may relate to how long evolutionary processes have been carrying on in isolation in a bioregion. Over an extended period, there may be extinction of species, but evolution of more genera. Dramatic extinction events such as the K-T mass extinction have affected current mayfly diversity and distribution. Climatic history plays an important role in the rate of speciation in an area, with regions which have been climatically stable over long periods having fewer species per genus, when compared to regions subjected to climatic stresses, such as glaciation. A total of 13 families are endemic to specific bioregions, with eight among them being monospecific. Most of these have restricted distributions which may be the result of them being the relict of a previously more diverse, but presently almost completely extinct family, or may be the consequence of vicariance events, resulting from evolution due to long-term isolation.