Representantes do grupo plebejum são encontrados de Honduras até o sul da América do Sul e compreendem sete espécies de pequeno porte. Como resultado de um estudo de revisão do grupo, são descritas duas espécies novas: Belostoma estevezae Ribeiro & Alecrim, sp. nov., do Estado do Mato Grosso, Brasil, similar a B. nicaeum Estévez & Polhemus, 2007, em termos de carena prosternal, e B. nessimiani Ribeiro & Alecrim, sp. nov., do Estado do Amazonas, Brasil, sendo bastante similar a B. parvum Estévez & Polhemus, 2007, em termos de genitália masculina. Uma chave de identificação para as espécies do grupo plebejum com as espécies novas incluídas é fornecida.

Based on published information of about 200 references, 479 species of Gerromorpha and Nepomorpha have been recorded from Brazil and compiled here for the first time. Of these, 105 belong to the family Veliidae. Families Gerridae, Belostomatidae, Corixidae, Naucoridae, Nepidae, and Notonectidae contain between 44 and 61 species each. Because of their apparent inherent lower diversity or difficulty of collection and/or study, other families present at most 21 species. Of the recorded species, 47% occur solely in Brazil, 44% in Brazil and other South American countries, 7% are more distributed along the Neotropical Region, and only 2% are widely distributed in Americas. Species records from Brazil are more concentrated in the North and Southeast regions, and on Amazonas, Pará, Minas Gerais, and Rio de Janeiro states, not alone due to environmental conditions, but also because of historical factors. A high number of endemic species of Rhagovelia, Limnocoris, and Ranatra is found in the Southeast and South regions. Other groups are exclusive or more diverse in the Amazon Region, such as Lathriobatoides, Bacillometra, Cryptovelia, Darwinivelia, Euvelia, Stridulivelia, and Weberiella.

Eighteen species from seven genera are recorded from the Amazon River floodplain, Brazil. An identification key, diagnoses, drawings, and taxonomic and biological notes are given for these species. The main features used in the identification key and diagnoses are: body and leg segments proportions, coloration patterns, presence and distribution of setae, and modifications of the apical abdominal segments and external genitalia of the male. Based on material collected in the floodplain streams and lakes, Cylindrostethus bassleri Drake, 1952 is recorded for the first time from Brazil, and new municipality records are presented for Brachymetra lata Shaw, 1933, B. shawi Hungerford & Matsuda, 1957, C. erythropus (Herrich-Schäffer, 1850), C. linearis (Erichson, 1848), C. Palmaris Drake & Harris, 1934, C. regulus (White, 1879), Neogerris lotus (White, 1879), N. lubricus (White, 1879), N. visendus (Drake & Harris, 1934), Ovatametra obese Kenaga, 1942, Rheumatobates crassifemur esakii Schroeder, 1931, R. klagei Schroeder, 1931, and Trepobates taylori (Kirkaldy, 1899). Additional new records from the Brazilian Amazon are presented for B. lata and O. obesa

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