Genus Orthetrum Newman, 1833
Type species: Libellula coerulescens Fabricius, 1798
The genus ranges widely across the Old World with over 80 species, of which almost half occur in Africa. The species are common and conspicuous at most freshwater habitats, especially when these are open. Many can be found almost anywhere in tropical Africa, although they tend to prefer (or are dominant in) certain habitats: O. abbotti (small marshy spots, like grassy puddles, seeps and streams), O. brachiale (open pools), O. chrysostigma (drier areas), O. guineense (open streams), O. hintzi (marsh in highland or near forest), O. icteromelas (larger marshes, like oxbows or lilypad lakes), O. julia (streams, especially shaded), O. stemmale (sheltered pools), and O. trinacria (larger standing waters). While O. julia and O. stemmale may predominate in the forested region from Uganda and northern Zambia to western Africa, additional species occur there: O. africanum (swampy rivers and streams), O. austeni (ponds, small lakes), O. saegeri (shaded swampy streams), and O. microstigma (any swamp or pool). O. agaricum and O. sagitta appear to replace O. saegeri and O. africanum in (parts of) western Africa, but the exact limits are unclear, while O. umbratum replaces O. saegeri at the darkest rainforest pools in Gabon. O. julia is also abundant at exposed habitats in south-western South Africa: this subspecies capicola is largely reddish brown with milky stripes when fresh and therefore often confused with O. caffrum, O. chrysostigma and O. rubens.
O. brevistylum, O. ransonnetii and O. sabina extend from Arabia into the arid periphery of north-eastern Africa. The first two seem real desert species, while the latter occurs at any open water in the warmer parts of Asia. O. caffrum, O. camerunense and O. kristenseni are characteristic of marshes and marshy streams in highlands. O. machadoi has a similar habitat, but is tied less to higher elevations. O. rubens has only recently been rediscovered at boggy runnels above 1000 m altitude in the Western Cape. The remaining Orthetrum species occur mostly in seasonal woodland, such as miombo and Guinea-Sudan savanna: O. angustiventre and O. monardi at pools and marshy spots, O. latihami and O. macrostigma at wet grassy depressions and marshy streams, and O. robustum at large open marshes. Two new species, O. kafwi and O. lusinga, were recently discovered at springs and bogs on high plateaus in Katanga.
While the size range, from small to very large (hindwing 22-50 mm), is greater than in any other African genus, and females and fresh males are yellow to brown with distinctive black markings, mature males appear very similar on account of their blue-grey pruinosity. This may cover the entire body and matches the characteristic bluish eyes. Distinguishing Orthetrum species is often considered difficult, but is worthwhile, as up to ten may be present in a single area. It is essential to check the male’s hamule to confirm identification, although its shape is difficult to describe. Be aware that hamules may have the hook flexed behind the lobe. Also important is whether the subcostal sections of Ax are dorsally dark (like the costal sections) or pale, as well as the number of doubled cells in the radial planate (count those cells touching Rspl, but not IR3). Many species differ by markings, but these get obscured by pruinosity. For example, the metepimeral carina (the ventral border of metepimeron) or the dorsal carina of abdomen can be marked with a black line. [Adapted from Dijkstra & Clausnitzer 2014]
Male of genus is similar to Nesciothemis by (a) size, Hw 22-50 mm; (b) frons flattened, resulting in 2 anterior ‘frontal shields’ demarcated by ridges, ventral ridge is strong and transverse, separating anterior border from remaining frons; (c) frons at most glossy black; (d) wings at most narrowly marked at tip or base; (e) Fw triangle of 1-4 cells, subtriangle of 3-5 cells [1-10]; (f) Fw discoidal field of 3-5 rows, if 2 at base then Fw supratriangle normally with 1-2 cross-veins; (g) 1-3 rows in radial planate (2 rows if Hw has 2 Cux and S7 marking larger); (h) anal loop usually long and boot-shaped with 10 or more cells; (i) 1 Cux in Hw; (j) hamule with prominent lobe than often dwarfs smaller hook; (k) genital lobe large or small, rounded, usually directed away from hamule; (l) if S7 marked, then this mark not or scarcely more prominent than that on S6. However, differs by (1) frontal shields roundly triangular rather than oval; (2) Fw arculus usually distal to Ax2; (3) Fw supratriangle with 1-2 cross-veins [0-2], rather than without; (4) anterior lamina and genital lobe about as high as hamule. [Adapted from Dijkstra & Clausnitzer 2014; this diagnosis not yet verified by author]
Map citation: Clausnitzer, V., K.-D.B. Dijkstra, R. Koch, J.-P. Boudot, W.R.T. Darwall, J. Kipping, B. Samraoui, M.J. Samways, J.P. Simaika & F. Suhling, 2012. Focus on African Freshwaters: hotspots of dragonfly diversity and conservation concern. Frontiers in Ecology and the Environment 10: 129-134.
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- Dijkstra, K.-D.B. & V. Clausnitzer, in prep. An annotated checklist of the dragonflies (Odonata) of Eastern Africa: with critical lists for Ethiopia, Kenya, Malawi, Tanzania and Uganda, new records and taxonomic notes. Zoologische Mededelingen. [PDF file]
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- Schouteden, H. (1934). Annales Musee Congo belge Zoologie 3 Section 2, 3, 1-84. [PDF file]
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- Schmidt, E. (1951). Libellen aus Portugiesisch Guinea, mit Bemerkungen über andere aethiopische Odonaten. Arquivos Museu Bocage, 20, 125-200. [PDF file]
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Citation: Dijkstra, K.-D.B (editor). African Dragonflies and Damselflies Online. http://addo.adu.org.za/ [2017-10-23].