Acisoma Rambur, 1842

Mens, Lotte P., Schütte, Kai, Stokvis, Frank R. & Dijkstra, Klaas-Douwe B., 2016, Six, not two, species of Acisoma pintail dragonfly (Odonata: Libellulidae), Zootaxa 4109 (2), pp. 153-172 : 158

Genus Acisoma Rambur, 1842

With over a thousand species, Libellulidae is the largest family of Anisoptera ( Dijkstra et al. 2013). Acisoma belongs to the morphologically rather distinctive Erythemis -group ( Pinto 2013), which also has strong molecular support (K.-D.B. Dijkstra unpublished). The group is largely tropical and has most species in the New World genera Erythemis Hagen, 1861 , Rhodopygia Kirby, 1889 , and Carajathemis Machado, 2012. Acisoma overlaps with the superficially very different genera Cyanothemis Ris, 1915 , and Porpax Karsch, 1896 , in Africa, Viridithemis Fraser, 1960 , in Madagascar, and Rhodothemis Ris, 1909 , in Asia. All genera have similar secondary genitalia and share the enlarged occipital triangle with rather straight borders, as a result of which the eyes touch at most over a distance less than half the triangle’s length. Their coloration is also unusual, frequently including bright white, green or blue pigments that are otherwise rare in the family. These colours are often notably concentrated dorsally, e.g. the African genera have the frons and vertex ventrally black with contrasting dorsal markings.

Acisoma is easily recognised by the combination of its (1) small size, Hw 16–25 mm; (2) brown to black body with contrasting and fragmented (bluish) white markings; (3) open venation with only 6–9½ Ax in Fw and normally 1 cell in Fw triangle and 1 cell-row in all radial planates; (4) Hw with at most a small dark patch at their extreme base; (5) strongly swollen abdomen at base, narrowing abruptly between S5 and S7, with slender tip; and (6) S4 with transverse ridge of similar strength as that on S3 and the lateral carina of S4.