(Figs. 20a–e)
[Key locator: Key to Group B species, couplet 15 (14)]
Heteragrion calendulum: Williamson 1919: 36–39 (key to species, couplet f1; description of ♁ from Colombia; diagnosis and comparison with H. mitratum; figs. 46, 94, 159);— Garrison et al. 2010: 88 (list of Heteragrion species);— Bota-Sierra & Novelo-Gutiérrez 2017: 556 (key to species, ♁ in couplet 5, ♀ in couplet 3; taxonomic history, remarks and additions to ♀ description; figs. 2, 3i–k, 4b–c, h–i, 5b, 6a–b, i–j, 8f, 9d);— Stand-Pérez et al. 2019: 92–93 (key to species, ♁ in couplet 14’, ♀ on couplet 10; list of Heteragrion species from Colombia, altitude range, type depository, IUCN status; figs. 5a–b, 8g –i, 13g –i, 18).
Heteragrion atrolineatum: Donnelly 1992: 60–62 (key to species, both sexes on couplet 5’; description of ♁ and ♀ from Panama and Costa Rica, comparison with H. alienum, H. chrysops, and H. tricellulare; figs. 5.4, 5.7, 5.11, 5.16);— Ramírez et al. 2000: 247–254 (listed among species found in Costa Rica);— Ramírez & Gutiérrez-Fonseca 2013: 99–100 (description of the F0 larvae; key to Heteragrion larvae known from Costa Rica, couplet 4’; figs. 2b, 3b);— Garrison & von Ellenrieder 2013: 14, 23 (record from Costa Rica; fig. 19);— Molinar et al. 2015: 87 (list of species from Panama).
Material examined. 1♁ (HOLOTYPE) COLOMBIA, Department of Antioquia, Municipality of Puerto Berrio, La Cristalina Township, 17.ii.1917, J.H. & E.B. Williamson leg., UMMZ; 1♁ Department Risaralda, Municipality of Pueblo Rico, Santa Cecilia Township, Alto Amurrupá Reserve, Ranas de Cristal stream, 15.ii.2017, (5.3203, -76.1735, 620m asl), J. Sandoval & C. Bota leg., CEUA; 6♁♁ PANAMA, Darién, Perresénico River, Estación Rancho Frío, 17.xi.2014, (8.0198, -77.7325, 80m asl), D. Delgado, A. Ramírez & R. Novelo leg., IEXA.
Synonyms. Heteragrion atrolineatum Donnelly, 1992 (Bota-Sierra & Novelo-Gutiérrez 2017).
Known distribution. Northwestern Colombia, Costa Rica and Panama.
Diagnosis and remarks. Due to its limited distribution and the significant loss of its natural habitats, H. calendulum is classified as Endangered (EN) on the IUCN Red List (Bota-Sierra et al. 2016). H. calendulum is placed in close proximity to H. alienum and H. valgum, although it can be distinguished from these and the remainder of Group B species based on the following combination of characters: BP the longest cercus portion (Figs. 20d–e); ridge above ML not partially covering it in dorsal and dorsolateral views, bearing strong teeth (Figs. 20d–e); AP small, subequal to MP (Fig. 20e).