The type material of Epeorella borneonia Ulmer, 1939, the sole species of the genus Epeorella Ulmer, 1939 is reinvestigated and a lectotype (male imago) is designated. Based on several morphological structures, the synonymy with Epeorus Eaton, 1881 (Rhithrogeninae) is rejected. Epeorella stat. prop., known only at the winged stages, belongs to the subfamily Ecdyonurinae, and is a probable endemic of the island of Borneo. The newly erected genus Darthus Webb & McCafferty, 2007, also endemic to Borneo and only known by one species at the nymphal stage, is shown to be a junior subjective synonym of Epeorella. The new combination Epeorella vadora (Webb & McCafferty, 2007) is proposed for the species. The distribution of known heptageniid species from the Sunda Islands is discussed.

Epeorella borneonia , Darthus vadorus , Borneo, lectotype, new synonym, new status, new combination

In his major work devoted to the Ephemeroptera of the Sunda Islands, Ulmer (1939) described three new genera in the family Heptageniidae. He gave them names which recalled names of those allied genera he assumed were closely related: Compsoneuriella Ulmer, 1939 and Compsoneuria Eaton, 1881; Rhithrogeniella Ulmer, 1939 and Rhithrogena Eaton, 1881; Epeorella Ulmer, 1939 and Epeorus Eaton, 1881.

The Afrotropical genus Notonurus Crass, 1947 was put in synonymy with Compsoneuriella (type species C. thienemanni Ulmer, 1939, known from winged and nymphal stages) by Gillies (1963; 1984), which in turn was put in synonymy with Compsoneuria (Braasch and Soldán 1986b; Webb et al. 2006). Recent studies, however, have shown the three genera constitute monophyletic clades supported by synapomorphies (Sartori 2014b; Vuataz et al. 2013).

The genus Rhithrogeniella (type species Rh. ornata Ulmer, 1939, based on winged stages only) had an enigmatic position for a long time, until another species from Vietnam (Rh. tonkinensis Soldán & Braasch, 1986) was described, with the first reference to the nymphal stage. Based on these descriptions, Wang and McCafferty (2004) indicated that Rhithrogeniella was a synonym of Rhithrogena and Rh. ornata was a species of Rhithrogena; the species Rhithrogeniella tonkinensis was transferred to the genus Ecdyonurus. Recently, the nymph of Rh. ornata was described for the first time, and the generic status of Rhithrogeniella revalidated (Sartori 2014a) as a member of Ecdyonurinae.

The monotypic genus Epeorella (type species E. borneonia Ulmer, 1939, known only from the winged stages) was synonymized with Epeorus (Wang and McCafferty 2004) on the basis of similarities in several characters which will be discussed below.

The family Heptageniidae is now divided into three subfamilies which can be broadly characterized as following (Kluge 1989; Webb and McCafferty 2008):

• Rhithrogeninae: nymph with a row of setae on the ventral surface of maxillae, with dorsal process of the forefemora projected and narrower than the ventral process, some genera with vestigial paracercus; winged stages with the median depression of the mesothoracic furcasternum convergent anteriorly, and prosternum lacking transverse and longitudinal ridges.

• Heptageniinae: nymph with a row of setae on the ventral surface of maxillae, with forefemora without a dorsal projection; winged stages with the median depression of the mesothoracic furcasternum convergent anteriorly, and prosternum with distinct transverse and longitudinal ridges.

• Ecdyonurinae: nymph with scattered setae on the ventral side of maxillae; winged stages with the median depression of the mesothoracic furcasternum parallel sided or divergent anteriorly, and prosternum generally lacking transverse and longitudinal ridges.

This study concludes the re-investigation of Ulmer’s Heptageniidae from Southeast Asia deposited in the Zoologisches Museum of Hamburg University (ZMH) (Sartori 2014a; b; c; d). The type material of Epeorella borneonia Ulmer, 1939 is described, some morphological structures are clarified, the subfamily position is established and a new synonymy is proposed.

The studied material is composed of three pinned specimens. The female imago was rehydrated in a solution of trisodic phosphate 0.35% and then put in alcohol. Pictures were taken with a Visionary Digital Passport II in ZMH, and figs were assembled in Adobe Photoshop CS6.

The synonymy of Epeorella with Epeorus was proposed by Wang and McCafferty (2004) based on the following assertions: i) male genitalia and forelegs are similar to those in Epeorus; ii) the vestiges associated with adults indicate that the larvae were two-tailed, and iii) the presence of median tubercles on abdominal segments VI–VIII can also be found in some Epeorus species.

The male genitalia greatly vary in shape among Epeorus species (see Webb and McCafferty 2008, figs 150–154) but are never as found in Epeorella; also, the forelegs of Epeorella are missing (see Ulmer 1939, p. 578) and therefore cannot be compared to Epeorus. The statement that, based on the vestigial paracercus, the nymph was two-tailed (hence comparable to Epeorus) is puzzling. “All Heptageniidae have the same vestigial paracercus, which does not allow to distinguish those with two-tailed and three-tailed larvae” (N. Kluge in litt.); the median ridge may be present on some Epeorus species which is true, but the genus Epeorus is absent from most of the Sunda Islands (Edmunds and Polhemus 1990, M. Sartori unpubl. data) as well as the Philippines. It is poorly diversified in Borneo where it is represented by a single species (Epeorus boonsoongi Braasch, 2011) with a large nymph of ca. 15 mm body length, and bifid tergal spines.

The male imago reinvestigated here presents all characteristics of the subfamily Ecdyonurinae, in peculiar the median depression of the mesothoracic furcasternum is not convergent anteriorly. Moreover, the presence of a clear transverse suture on the mesonotum, excludes it from the genus Epeorus. As already suggested by Braasch (2011) the synonymy proposed by Wang and McCafferty (2004) is incorrect and Epeorella is reinstated as Epeorella Ulmer, 1939, stat. prop.

One interesting character of Wang and McCafferty (2004) is the presence of remains of median tubercles visible at least on abdominal terga VI–VIII of the female subimago. It has already been demonstrated that the subimaginal stage may retain some nymphal structures, such as gill sockets (Sartori et al. 2008), which may help to link nymphal and winged stages. The presence of vestigial tubercles on the terga thus indicates that the nymph possesses a median single ridge on the abdomen. Among Ecdyonurinae, two genera are known to hold such structures, Notacanthurus Tshernova 1974 known from East Palaearctic, the Himalaya region and Southeast Asia, and Darthus Webb and McCafferty 2007, only known from Borneo (Figs 7–8). Male imagos of Notacanthurus possess the anterior margin of the head distinctly produced (Braasch 1986; Webb and McCafferty 2008), and genitalia possess clearly visible apical and lateral sclerites. Darthus is only known from the nymphal stage from the same island as Epeorella. Although distant by ca 500 km, the type locality of Epeorella borneonia and that of Darthus vadorus Webb & McCafferty, 2007 belong to the Dipterocarpaceae forest of lowland altitudes (Sartori et al. 2003). Moreover, the size of the mature nymphs of D. vadorus (5.5–8.5 mm) is compatible with the adult size of E. borneonia (5.0–5.5 mm) knowing that alate stages are generally smaller than mature nymphs. Therefore, it is likely that Darthus represents in fact the nymphal stage of Epeorella, and is considered as a subjective junior synonym of Epeorella syn. n. The species Epeorella vadora comb. n. is retained as a valid species because it exhibits a different colour pattern of the abdomen.

Figures 7–8. 

Epeorella vadora (Webb & McCafferty, 2007), comb. n. 7 Nymph paratype in dorsal view 8 Nymph paratype in lateral view with median abdominal ridge (arrows).

My sincere thanks to Kai Schütte and Hossein Rajaei (ZMH) for allowing me to study Ulmer’s collection in Hamburg, Prof. Hans Strümpel (ZMH) for his advices, and Nikita Kluge (St-Petersburg) for exchanging morphological considerations. Discussions with Jean-Luc Gattolliat (MZL) and comments by two anonymous reviewers helped to improve this paper. Michael Balke (Munich) is warmly thanked for placing his Indonesian collections at my disposal.