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A new species of Bessierus Thomas & Orth, formerly a monotypic genus, is described from Amapá State, Brazil. The type species of the genus, B. doloris Thomas & Orth, had only its nymphs described until recently, when its male imago was associated to nymphs mainly based on pigmentation pattern. Bessierus riobranco sp. n. here described has no significant difference on pigmentation pattern from B. doloris, thus the new species could be the male imago of B. doloris, and the putative imago of B. doloris a new species, or even both described imagoes could be new species. Still, a new male imago existence cannot be neglected and we opted to provide a properly description, diagnosis and illustration, hence contributing to the knowledge of mayfly neotropical diversity and future research on the genus. The generic concept of Bessierus and its type species diagnoses are altered to accommodate the new species, and a new record of B. doloris is provided.

Three unusual nymphs of Euthyplociidae, subfamily Euthyplociinae, are described from Ecuador. A new genus and new species are established to accommodate these individuals. The morphologically unique specimens are defined by the following characters: short, stout tusks lacking spine-like setae but densely covered with long, hair-like setae; head almost as long as wide; long, quadrate clypeus with acute apicolateral projections; apex of 3rd segment of labial palp acute; apex of 3rd segment of maxillary palp acute and narrow. The hind wing is small, similar to that of Mesoplocia. An ongoing study of the phylogeny of the family recovers Dasyplocia gen. nov. as closely related to Euthyplocia and Mesoplocia.

Polyplocia nebulosa sp. nov. is described based on male and female imagos from Malaysia, Sabah State. Nymphs provisionally assigned to this species are also described. This species can be readily separated from P. vitalisi by the color pattern of the wings, with light brown longitudinal veins, narrow dark brown clouds on cross veins and margins of wings tinged with brown. The male genitalia are distinct from those of other species of Polyplocia: penes are broad, T-shaped, with large lateral projecting lobes, which are apically rounded, and with a small dorsolateral spine on each lobe; the styliger plate is short and not projected posteriorly. Eggs of the genus are described for the first time using scanning electron microscopy. Eggs are 265-267 µm in length, 170-186 µm in width, barrel-shaped, without polar caps or other attachment structures, with one visible micropyle and with chorion forming an irregular mesh with raised ridges (mesh between 3.8-8 µm). A key to male adults of Polyplocia is provided and additional records of Polyplocia from Thailand are given.

Two new species of Ulmeritoides Traver, 1959 are described from Southeastern Brazil based on male and female imagos. Ulmeritoides angelus sp. nov. (Holotype male deposited in DZRJ: Minas Gerais State) differs from all other species of the genus by the following combination of characters: forewings hyaline, base dark brown and purplish; veins R1 light brown; dark brown spots on bullae of veins Sc and R2; apex of penis lobes somewhat rounded, each with one central spine. Ulmeritoides tamoio sp. nov. (Holotype male deposited in DZRJ: Sao Paulo State) differs from all other species of the genus by the following combination of characters: wings hyaline; veins C, Sc and R1 brownish; brownish spots on bullae of veins Sc and R2; apex of penis lobes straight, ending in rather acute apical and mesal projections. An updated key to the genera Ulmeritoides Traver, 1956 and Ulmeritus Traver, 1956 is also given.

A new species of Thraulodes Ulmer, 1920 is described from Rio de Janeiro. The species resembles the recently published Thraulodespelicanus Mariano and Froehlich, 2011 (in Mariano et al. 2011) due to the length and width of the penes. However, Thraulodes bonito sp. nov. differs from this and all other species of the genus by the wide penes, the triangular shape of the styliger plate with median projection short and apex rounded, the abdominal color pattern and the presence of 1-3 weakly marked cross veins basal to bullae. The new species is described based on male imagos, female subimagos and nymphs.

A new species of the recently erected genus Tortopsis is described from males and females imagos collected in Macaé river, Rio de Janeiro State. Tortopsis canum sp. nov. can be recognized by the color pattern of the head and pronotum, strongly shaded with black in both sexes, male genitalia with parastyli long and straight and female parastyli receptor “C” shaped, with receptors large, occupying nearly all extension of sternum VIII. This new species represents the first record of the genus Tortopsis in Brazil.

Cloeodes Traver, 1938 (Ephemeroptera, Baetidae) was established to include Baetidae specimens from Puerto Rico. Of the three species described by Traver (1938), only the type-species C. maculipes remains in this genus (Domínguez et al. 2006). A revision of Cloeodes by Waltz & McCafferty (1987) redefined its distinct characteristics and revealed the genus to be a widespread group of Neotropical origin with several species from South and Central America as well as from Southwestern North America and Southeast Asia.