Rhithrogeniella, Ulmer, 1939

Kluge, Nikita, Sivaruban, T., Srinivasan, Pandiarajan, Barathy, S. & Isack, Rajasekaran, 2023, Redescription of the subgenus Rhithrogeniella Ulmer 1939 (Ephemeroptera, Heptageniidae, genus Ecdyonurus) based on reared specimens from India and Thailand, Zootaxa 5319 (4), pp. 501-523 : 512

Status of Rhithrogeniella

Sartori (2014) regarded Rhithrogeniella as a genus distinct from Nixe or Paracinygmula based on the fibrillose portion of tergalius VI, the setation of larval caudalii and the structure of imaginal penis structure. He stated that ́nymphs of Nixe / Paracinygmula ... present gills with a weakly developed fibrillose part, either absent or reduced to a single filament in gill VI, which is not the case in Rhithrogeniella ...» ( Sartori 2014: 58). However, some species placed in Nixe have well developed fibrillose portion on tergalii I–VI ( Kluge 1980: figs 84, 95), and presence/ absence of the fibrillose portion on tergalius VI was regarded as the subgeneric character separating the subgenera Akkarion and Nixe in the genus Nixe ( Flowers 1980) . Among examined individuals of Ecdyonurus (Rhithrogeniella) ornatus , number of filaments or their branches on tergalii VI varies from 2 to 12.

Difference in caudalii setation was reported as: ́The genus can be distinguished from all relatives by the peculiar structure of the cerci and terminal filament which possess a row of stout setae in the proximal part and bunches of long and thin setae in the medial and distal parts» ( Sartori 2014: 48). However, caudalii of the Palaearctic species E. (Rh.) joernensis (placed by this author in Nixe or Paracinygmula ) have the same structure ( Figs 51–52 ).

Soldán and Braasch (1986) reported the following feature of the cerci as diagnostic for Rhithrogeniella : ́segments with large blunt spines and bristles on anterior margin regularly alternate with those with very fine bristles and individual scales on posterior margin» and ́(10) segments of cerci bearing stout spines regularly alternate with those without spines» ( Soldán and Braasch 1986: 203–204, 205, figs 14, 15). Among specimens examined, this alternation is expressed in some individuals on some portions of cerci, but not expressed on others; sometimes such alternation is expressed on ventral side of the cercus segments ( Fig. 47 ), but dorsal sides of these segments bear equally stout spine-like setae ( Fig. 48 ).

Difference in penis structure was reported as following: ́Contrary to Nixe / Paracinygmula , the male genitalia have a very different shape and lack well developed median titillators as well as basal sclerite spines» ( Sartori 2014: 58). However, general shape of penis is quite different in the type species of Nixe ( Burks 1953: fig. 372), the type species of Paracinygmula (Kluge 1983: fig. 2M) and other representatives of this taxon.As shown below, the median titillators are present in the type species of Rhithrogeniella . Thus, the single difference between the type species of Rhithrogeniella from one side, and the type species of Afghanurus , Paracinygmula , Nixe and Akkarion from other side, is the absence of the pair of spines on ventral side of the penis in the type species of Rhithrogeniella . Formerly ( Kluge 2004) the presence of these spines was regarded to be a key autapomorphy of the taxon Afghanurus /g1 (incl. Leucrocuta ). However, the same pair of spines occur in Compsoneuriella Ulmer 1939 which has the autapomorphy of the taxon Atopopus/g1 sensu Kluge 2004; this fact testifies that phylogenetic status of this character was wrongly estimated.

Figures

FIGURES 42–52. Larvae. 42–50, Ecdyonurus (Rhithrogeniella) ornatus, larval exuviae: 42–44, specimen from Agumbe (India), posterior margins of abdominal terga I, VIII and X; 45, specimen from Pai (Thailand), posterior margins of abdominal tergum VI; 46–48, cercus of specimen from Agumbe: 46, proximal portion; 47, middle portion in median-ventral view, to show bases of primary swimming setae on inner (median) side; 48, the same portion with focus on lateral-dorsal side, to show bases of secondary swimming setae on outer (lateral) side; 49–50, cercus of specimen from Kodaikanal (India). 51–52, Ecdyonurus (Rhithrogeniella) joernensis (Russia, Orenburg province, Tashla, 16.VIII.1986, coll. N. Kluge): two portions of cercus, dorsal view.