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Perithemis mooma, Kirby, 1889 and Perithemis icteroptera (Selys in Sagra, 1857) live in sympatry from southern Brazil to central Argentina. The taxonomy of the genus Perithemis Hagen, 1861 has been hampered by the use of characters that are highly variable or show slight differences among species. Our objective was to assess the efficiency of traditional morphometrics (TM) and geometric morphometrics (GM) to discriminate between these species using wing size and shape and vulvar lamina contour, and to analyze the presence of sexual dimorphism in wing size and shape in both species. The TM and landmark-GM methods were applied on the fore and hind wings, while the outline-based GM method was applied on the vulvar lamina. GM allowed species delimitation using shape variables of either wing. The wing and vulvar lamina shapes were confirmed to be good diagnostic characters to separate these species and appear to be promising tools for distinguishing among other species of this genus. Centroid size failed to achieve species separation. Both species exhibited sexual size dimorphism (SSD). In contrast to what would be expected for Perithemis whose males are strongly territorial, P. icteroptera and P. mooma showed female-biased SSD suggesting a common pattern in Perithemis.

Psaironeura jeronimoi sp. nov. is described based on seven males (Holotype: ♂ (LABECO, N° 10002717), BRAZIL, Amazonas, Alto Maués Ecological Station, (-5.9852, -59.3182, 114 m asl, 12.v.2019) collected in the state of Amazonas (near Alto Maués Ecological Station) and Pará (municipality of Belterra). This new species fits into the tenuissima group but presents differences in color pattern and shape of the apex cerci. Main morphological characters are figured and compared with those of P. tenuissima (Selys, 1886) and P. bifurcata (Sjöstedt, 1918). Key to males and comments on genital ligula of the species of the group are provided. Finally, comments on the conservation status of the species are presented.

A new coenagrionid genus, Negragrion, is described for N. sagma sp.n. found in Argentina and Brazil (Holotype and allotype, pair in tandem. Argentina: Corrientes: Santo Tomé, arroyo Ita Cuá sobre RP 94, 28°26'48.30"S 56°00'33.11"W, 24.ii.2003, Muzón & Pessacq coll., MLP). The new genus is characterized by the shape of male cerci (decumbent from base; saddle-shaped; in lateral view with an acute apophysis directed dorsally located at 0.3 from base). The presence of a vulvar spine on S8 of females places this genus within Ischnurinae. A cladistics analysis using morphological data was carried out to determine its phylogenetic position. Negragrion gen.n. is recovered within the clade 4 as the sister group of the clade (Acanthallagma Acanthagrion (Oxyagrion, Fluminagrion)).

The American genus Argia, with more than 100 species described, is the most speciose genus of Odonata in the world. In this contribution, the final stadium larva of Argia serva, the southernmost distributed species of Argia, is described and diagnosed based on reared material from Martín García island, Buenos Aires, Argentina. The larva of this species can be easily separated from the other Argentinean Argia by the following combination of characters: sternum of S8 covered with spines (bare in A. translata); antennal segment 3 longer than 1+2 (equal to or shorter than 1+2 in A. joergenseni and A. jujuya); palpal setae absent (present in A. croceipennis). A key to the known larvae of Argia of the Southern Cone is provided.

The final instar larva of Limnetron antarcticum Förster is described and illustrated for the first time based on one specimen collected in Misiones Province, Argentina. It is compared with L. debile (Karsch). Color pattern and ovipositor morphology of the female imago are described.

Patagonia is a vast landmass with a distinctive environmental and biotic heterogeneity. Its Odonata biodiversity is the best known of South America, and it is composed of 36 species, of which more than 50% are endemic. We summarize the main taxonomic, distributional and biological information including illustrated keys for adults and known larvae, and distributional maps.