The Century Old Taxonomic Confusion Surrounding Wiedemannia zetterstedti Fallén and Related Species Is Resolved (Diptera: Empididae): Revision of the W. zetterstedti Group

Simple Summary The aquatic dance fly species of the Wiedemannia zetterstedti group are studied and species definitions are re-evaluated. A new species is described from Corsica (France) and two species synonyms are proposed. The species concepts are also investigated using COI barcodes and a key to species is provided. Abstract The Wiedemannia zetterstedti species group is revised after examination of all available type specimens and includes one new species (W. ulrichi Ivković & Sinclair sp. nov.) and four redescribed species (W. czernyi (Bezzi), W. longipennis (Mik) stat. rev., W. rufipes (Oldenberg) stat. rev. and W. zetterstedti (Fallén)). The following new synonyms are proposed: W. (Roederella) ouedorum Vaillant, 1952 = W. czernyi (Bezzi, 1905); Paramesia riparia Robert, 1836 = W. zetterstedti (Fallén, 1826). Lectotypes are designated for the following species/subspecies: Atalanta hirtiloba Speiser, Brachystoma escheri Zetterstedt, Clinocera czernyi Bezzi, Clinocera longipennis Mik, Paramesia riparia Robert, and Roederia czernyi rufipes Oldenberg. In addition to morphological evidence, molecular species concepts were investigated using a molecular phylogenetic divergence-based species delimitation (bPTP) and results confirmed the morphological conclusions. A key to species is presented and geographic distributions are mapped.


Introduction
The aquatic dance fly genus Wiedemannia Zetterstedt is a common empidid in streams and small rivers of Europe [1][2][3][4]. Adult flies gather on emergent rocks, where mating and feeding occurs [5]. In Europe, some 78 species are recorded, which until very recently were assigned to a number of subgenera [6]. Species of Wiedemannia are readily identified on the basis of facial colouration, genal width, pterostigma shape, scutal chaetotaxy and male terminalia, with their uniquely shaped clasping cercus [7][8][9]. Although the species concepts are mostly well established, species related to W. zetterstedti (Fallén) have remained difficult for more than a century. There are two reasons for unstable taxonomy in this group: (1) variability in the shape of the clasping cercus; (2) reluctance of workers to carefully examine the type specimens of the species/subspecies involved.

Wiedemannia zetterstedti Group
Diagnosis. Gena broad, less than half as deep as height of eye; only a few small and fine, irregularly biserial presutural acrostichal setulae; fore coxae with 2-3 long, erect setae projecting beyond shorter proclinate setae ( Figure 6A); pterostigma narrow, elongate and pale to brownish ( Figure 2E); clasping cercus smaller than epandrium, apex bifurcated ( Figure 3A-H); phallus without swelling on distiphallus.    Remarks. This group of species was previously assigned to two different subgenera, Eucelidia Mik and Roederella Engel, and included five species/subspecies: W. czernyi (Bezzi), W. czernyi rufipes (Oldenberg), W. ouedorum Vaillant, W. pirata Mik, and W. zetterstedti (Fallén) [20]. Currently, the former subgenera of Wiedemannia are no longer recognized [6] and this group of species is now referred to as the W. zetterstedti group. Wiedemannia pirata was considered related to the W. zetterstedti group on the basis of erect preapical setae on the fore femur. The former species has been removed from this species group on the basis of five pairs of dorsocentral setae, with interspersed setae and distinct biserial acrostichal setae extending around the prescutellar depression. Additionally, the clasping cercus is much larger, twice the size of the epandrium, differently shaped than in all other species of this group, and the distiphallus bears a mid-length swelling. Remarks. This group of species was previously assigned to two different subgenera, Eucelidia Mik and Roederella Engel, and included five species/subspecies: W. czernyi (Bezzi), W. czernyi rufipes (Oldenberg), W. ouedorum Vaillant, W. pirata Mik, and W. zetterstedti (Fallén) [20]. Currently, the former subgenera of Wiedemannia are no longer recognized [6] and this group of species is now referred to as the W. zetterstedti group. Wiedemannia pirata was considered related to the W. zetterstedti group on the basis of erect preapical setae on the fore femur. The former species has been removed from this species group on the basis of five pairs of dorsocentral setae, with interspersed setae and distinct biserial acrostichal setae extending around the prescutellar depression. Additionally, the clasping cercus is much larger, twice the size of the epandrium, differently shaped than in all other species of this group, and the distiphallus bears a mid-length swelling. Group in Europe (based on dried,  pinned specimens) 1.

Key to Species of the Wiedemannia zetterstedti
Face brown similar to frons ( Figure 10A), concolourous with scutum, contrasting with blue pruinescent gena; wing with stout erect setae on costa starting near humeral crossvein ( Figure 10C); clasping cercus same colour as epandrium, bilobed, lobes not in same plane, with prominent anterior lobe projecting obliquely from posterior lobe ( Figure 10B), anterior lobe usually distinctly longer than posterior lobe ( Figure 3H      Notes on type material. Bezzi [21] stated that this species was collected in the central Italian mountains, from specimens initially identified as Roederia longipennis Mik in Bezzi [22]. In the latter publication, numerous specimens were collected along the streams "Fiastone, Acquasanta, Tennacola and Tronto above Aquata" in Macerata Province, Marche Region, Central Italy. Upon review of specimens received from Czerny that were reportedly R. longipennis, Bezzi [21] reconsidered his identification and described them as a new species. But in an extract of a letter to Engel, Czerny clarifies that he sent Bezzi specimens of Eucelidia escheri Zetterstedt [9] (p. 755), [23] (p. 79). This observation led Engel [23] to assume R. longipennis was conspecific with E. escheri, and consequently he synonymized the former with the latter, assigned Roederia in synonymy with Eucelidia and described a new genus, Roederella for C. czernyi. Collin [9] examined the types of Roederia longipennis and considered them conspecific with Roederella czernyi and that the description of the new genus was unjustified.
Unfortunately, the definition of W. czernyi is somewhat more complicated than outlined by Collin [9]. More than 20 syntypes have been identified in MSNM (Rigato 2020, pers. comm.), and we received six purported syntypes, representing a mixed series. Two syntypes represent W. czernyi in its current concept and three conspecific with W. longipennis based on comparison with type specimens of the latter species (sixth specimen was W. longicornis (Mik)). Interestingly, the illustration of the male terminalia of W. czernyi [21] (Figure 1) appears very similar to W. longipennis. Doubts on the definition of the species was further compounded by Engel [23] (Figures 26 and 27), who illustrated a dried and macerated specimen of "W. czernyi", but they appear to represent two different species, with the dried specimen ( Figure 26) matching the types of W. longipennis and the macerated specimen ( Figure 27) matching W. czernyi. Rather than synonymize these two species and describe a new species, we have chosen to maintain nomenclatural stability by selecting a lectotype specimen from the two representatives that most closely match the current concept of W. czernyi.
The private collection of F. Vaillant is now located in Musée cantonal de zoologie, Lausanne, Switzerland [24]. Unfortunately, only larval syntypes of W. ouedorum could be found and adults are presumed destroyed (Gattolliat pers. comm. 2020). The proposed new synonym is based on the illustrations by Vaillant [25]  Diagnosis. This species is distinguished from other species of the Wiedemannia zetterstedti group by the following combination of characters: face bluish pruinescent in both sexes, without brownish stripe; wing often longer than 5 mm; stout erect setae on costa beyond vein Sc; hind tibia with at least 2-3 erect spine-like posterodorsal setae apically; clasping cercus pale brown, mitten-shaped.
Redescription. Wing length 4.7-5.2 mm. Male ( Figure 2E). Head with gena, postgena and lower half of occiput with blue-grey pruinescence; gena less than half as deep as eye height. Face slightly wider than antennal sockets, with bluish pruinescence (Figure 2A); distinct carina on lower margin. Frons short, brown, broader than face; ocellar triangle and vertex brown. Pair of long ocellar setae; one pair of strong vertical setae; 5-6 distinct upper postocular setae, all similarly sized and black, with interspersed shorter setae; lower postocular setae much finer and paler, merging with pale, longer setae on middle and lower occiput and postgena; several dark setulae on vertex and between ocellar area and eye margin. Antenna brown; scape and pedicel subequal; scape with dorsal setulae; pedicel with circlet of apical setulae; postpedicel pointed ovate; arista-like stylus shorter than length of face.
Wing membrane infuscate, veins dark; basal costa seta not extending to humeral crossvein. Origin of veins M 1 and M 2 widely separated at end of cell dm. Vein CuA + CuP short, faint streak, or absent. Pterostigma narrow, elongate, faint to strong. Squama with pale fringes. Costal margin with short, strong erect setae beyond apex of Sc. Halter with dark knob and paler shaft.
Legs entirely dark brown, except coxae with pale grey pruinescence; uniformly covered with rows of small dark setulae, slightly longer on ventral side of fore femur; fore femur often with strong anterior preapical seta; fore coxa with 2-3 long erect setae; hind tibia with 2-3 erect, spine-like posterodorsal setae ( Figure 2D).
Abdomen concolourous with thorax, covered in short setae; lateral third of tergites and sternites with bluish pruinescence. Terminalia ( Figures 2B and 3A,B): Hypandrium slightly shorter than length of epandrium, bearing 2−3 setae with bluish pruinescence. Epandrium elongate, oval, covered with dark, long setae; rounded surstylus emerging from inner face apically. Clasping cercus pale brown, paler than epandrium; shorter than length of epandrium; bifurcated (bilobed), mitten-shaped with lobes in same plane; anterior lobe much narrower than broad posterior lobe, with long outer setae; posterior lobe with inner face with long, peg-like setae; anterior and posterior lobes variable in size. Phallus more or less linear, brownish; distiphallus straight, without swelling in middle.
Distribution. This species is confirmed from Algeria, France, Italy, Morocco, and Spain ( Figure 11).
Remarks. The size of the apical lobes of the clasping cercus is variable in size, confusing species identification ( Figures 2B and 3A). Additional features outlined in the key are needed to aid species identification. Notes on type specimens. This species was described on the basis of an undetermined number of male and female specimens, and Mik [26] stated that the specimens were from "Hungary" (Herkulesbad was in the Austro-Hungarian Empire, now Romania), presented to him by F. Kowarz. Mik [27] transferred the species to his new genus Roederia. Engel [23] (p. 79) wrote that the types in the "Wiener Hofmuseum" were either destroyed or in poor shrunken condition. Additional type specimens identified in the Hope Collection were from the Kowarz collection, which was purchased by Verrall in the 1890s [28], also mentioned in Collin [9]  Diagnosis. This species is distinguished from other species of the Wiedemannia zetterstedti group by the following combination of characters: face bluish pruinescent in both sexes, without brownish stripe; wing usually longer than 5 mm; stout erect setae on costa beyond vein Sc; hind tibia with at least 4-5 erect, spine-like posterodorsal setae apically; clasping cercus pale brown, narrow, with posterior lobe slightly broader than anterior lobe, thickened setae on inner face confined to posterior lobe.
Redescription. Wing length 5-5.5 mm. Male ( Figure 4F). Head with gena, postgena and lower half of occiput with blue-grey pruinescence; gena less than half as deep as eye height. Face slightly wider than antennal sockets, with bluish pruinescence ( Figure 4A); distinct carina on lower margin. Frons short, brown, broader than face; ocellar triangle and vertex brown. Pair of long ocellar setae; one pair of strong vertical setae; 5-6 distinct upper postocular setae, all similarly sized and black, with interspersed shorter setae; lower postocular setae much finer and paler, merging with pale, longer setae on middle and lower occiput and postgena; several dark setulae on vertex and between ocellar area and eye margin. Antenna brown; scape and pedicel subequal; scape with dorsal setulae; pedicel with circlet of apical setulae; postpedicel pointed ovate; arista-like stylus shorter than length of face.
Wing membrane infuscate, veins dark; basal costa seta not extending to humeral crossvein. Origin of veins M 1 and M 2 widely separated at end of cell dm. Vein CuA + CuP short, faint streak or absent. Pterostigma narrow, elongate, faint to strong. Squama with pale fringes. Costal margin with short, strong erect setae beyond apex of Sc. Halter with dark knob and paler shaft.
Legs entirely dark brown, except coxae with pale grey pruinescence; uniformly covered with rows of small dark setulae, slightly longer on ventral side of fore femur; fore femur often with strong anterior preapical seta; fore coxa with 2-3 long erect setae; hind tibia with 4-10 erect, spine-like posterodorsal setae ( Figure 4E).
Abdomen concolourous with thorax, covered in short setae; lateral third of tergites and sternites with bluish pruinescence. Terminalia ( Figure 3C,D and Figure 4B): Hypandrium nearly subequal in length with epandrium, bearing 1−2 setae with bluish pruinescence. Epandrium elongate, oval, covered with dark, long setae; rounded surstylus emerging from inner face apically. Clasping cercus pale brown, paler than epandrium; long and narrow, subequal to length of epandrium; bifurcated (bilobed), with lobes in same plane and posterior lobe slightly broader than anterior lobe; anterior lobe flatter than posterior lobe, with short outer setae; longer setae in area separating lobes; posterior lobe thumblike with short setae along margin; inner face with peg-like setae confined distinctly to posterior lobe. Phallus more or less linear, brownish; distiphallus straight, without swelling in middle.
Remarks. This species was synonymized with W. escheri (Zetterstedt) by Engel [23] (p. 80), and its concept remained as such until we re-examined the type specimens of W. longipennis. The long and narrow clasping cercus and numerous posterodorsal setae of the hind tibia readily distinguishes W. longipennis from all other species of the W. zetterstedti group. Notes on type material. Oldenberg [29] described the type locality of W. rufipes as "Seitenbach der Cserna eine Stunde oberhalb Herkulesbad" (side stream of the Cserna one hour above Hercules Bath). The 18 syntypes found in SDEI are all labelled "Mehadia", a town on the river Belareca, which joins the river Cserna downstream from Băile Herculane (Romania). It is likely that Oldenberg used the name Mehadia to describe this region Notes on type material. Oldenberg [29] described the type locality of W. rufipes as "Seitenbach der Cserna eine Stunde oberhalb Herkulesbad" (side stream of the Cserna one hour above Hercules Bath). The 18 syntypes found in SDEI are all labelled "Mehadia", a town on the river Belareca, which joins the river Cserna downstream from Băile Herculane (Romania). It is likely that Oldenberg used the name Mehadia to describe this region where the rivers meet. Diagnosis. This species is distinguished from other species of the Wiedemannia zetterstedti group by the following combination of characters: male face bluish pruinescent or female face brownish-blue, both with a brownish median stripe; wing often longer than 5 mm; stout erect setae on costa beyond vein Sc; legs often reddish brown; hind tibia with at least 2-3 erect spine-like posterodorsal setae apically; clasping cercus pale brown, mitten-shaped.
Redescription. Wing length 4.8-5.8 mm. Male. Head with gena, postgena and lower half of occiput with blue-grey pruinescence; gena less than half as deep as eye height. Face slightly wider than antennal sockets, with bluish pruinescence and brown, narrow median stripe ( Figure 5A); distinct carina on lower margin. Frons short, brown, broader than face; ocellar triangle and vertex brown. Pair of long ocellar setae; one pair of strong vertical setae; 5-6 distinct upper postocular setae, all similarly sized and black, with interspersed shorter setae; lower postocular setae much finer and paler, merging with pale, longer setae on middle and lower occiput and postgena; several dark setulae on vertex and between ocellar area and eye margin. Antenna brown; scape and pedicel subequal; scape with dorsal setulae; pedicel with circlet of apical setulae; postpedicel pointed ovate; arista-like stylus nearly subequal to length of face.
Wing membrane infuscate, veins dark; basal costa seta extending to humeral crossvein. Origin of veins M 1 and M 2 widely separated at end of cell dm. Vein CuA+CuP short, faint streak or absent. Pterostigma narrow, elongate, faint to strong. Squama with pale fringes. Costal margin with short, strong erect setae beyond apex of Sc. Halter with dark knob and paler shaft.
Legs often reddish brown, especially on ventral surface ( Figure 6A); coxae with pale grey pruinescence; uniformly covered with rows of small dark setulae, slightly longer on ventral side of fore femur; fore femur often with strong anterior preapical seta; fore coxa with 2-3 long erect setae; hind tibia with 2-3 erect, spine-like posterodorsal setae.
Abdomen concolourous with thorax, covered in short setae; lateral third of tergites and sternites with bluish pruinescence. Terminalia ( Figure 3E): Hypandrium slightly shorter than length of epandrium, bearing 1−2 setae with bluish pruinescence. Epandrium elongate, oval, covered with dark, long setae; rounded surstylus emerging from inner face apically. Clasping cercus pale brown (difficult to discern in air-dried older specimens)? Paler than epandrium; shorter than length of epandrium; bifurcated (bilobed), mittenshaped with lobes in same plane; anterior lobe much narrower than broad posterior lobe, with long outer setae; posterior lobe with long setae along outer posterior margin; inner face of posterior lobe with long, peg-like setae. Phallus more or less linear, brownish; distiphallus straight, without swelling in middle.
Female. Similar to male except, face brownish blue with faint median strip ( Figure 5B); terminalia with cercus short, cylindrical, and minutely pilose.
Distribution. This species is recorded from Georgia, Greece, and Romania ( Figure 11).
Remarks. This species was originally considered a subspecies of W. czernyi, distinguished from the latter species by reddish-coloured legs [29]. Leg colouration is often not a dependable character to distinguish species and can by modified during preservation of specimens. The features in the key help to further characterize this species. Diagnosis. This species is distinguished from other species of the Wiedemannia zetterstedti group by the following combination of characters: face blue pruinescent in both sexes, with complete central brownish stripe; wing length variable, sometimes longer than 5 mm; stout erect setae on costa beyond vein Sc; hind tibia without distinct erect spine-like posterodorsal setae apically; clasping cercus pale brown, mitten-shaped, shallowly notched with posterior lobe broader than anterior lobe.
Description. Wing length 4.6-5.5 mm. Male ( Figure 7E). Head with gena, postgena, and lower half of occiput with blue-grey pruinescence; gena less than half as deep as eye height. Face slightly wider than antennal sockets, with bluish pruinescence and brown median stripe ( Figure 7A); distinct carina on lower margin. Frons short, brown, broader than face; ocellar triangle and vertex brown. Pair of long ocellar setae; one pair of strong vertical setae; 5-6 distinct upper postocular setae, all similarly sized and black, with interspersed shorter setae; lower postocular setae much finer and paler, merging with pale, longer setae on middle and lower occiput and postgena; several dark setulae on vertex and between ocellar area and eye margin. Antenna brown; scape and pedicel subequal; scape with dorsal setulae; pedicel with circlet of apical setulae; postpedicel pointed ovate; arista-like stylus shorter than length of face.
Wing membrane infuscate, veins dark; basal costa seta extending to humeral crossvein. Origin of veins M1 and M2 widely separated at end of cell dm. Vein CuA + CuP Legs entirely dark brown, except coxae with pale grey pruinescence; uniformly covered with rows of small dark setulae, slightly longer on ventral side of fore femur; fore femur often with strong anterior preapical seta; fore coxa with 2-3 long erect setae; hind tibia without erect, spine-like posterodorsal setae ( Figure 7D).
Abdomen concolourous with thorax, covered in short setae; lateral third of tergites and sternites with bluish pruinescence. Terminalia (Figures 3F and 7B): Hypandrium slightly shorter than length of epandrium, bearing 1−2 setae with bluish pruinescence. Epandrium elongate, oval, covered with dark, long setae; rounded surstylus emerging from inner face apically. Clasping cercus pale brown, paler than epandrium; shorter than length of epandrium; bifurcated (bilobed), mitten-shaped with lobes in same plane, shallowly separated; anterior lobe much narrower than broad posterior lobe, with long outer marginal setae; posterior lobe without long outer setae; inner face of posterior lobe with long, peg-like setae. Phallus more or less linear, brownish; distiphallus straight, without swelling in middle.
Female. Similar to male except terminalia; cercus short, cylindrical, and minutely pilose. Distribution. This species appears to be endemic to Corsica (France) (Figure 11).

Remarks.
In a study of the aquatic Empididae of Corsica, Pusch [30] and Ivković et al. [31] identified this new species as Wiedemannia czernyi. Wiedemannia ulrichi sp. nov. is the most frequently encountered species of Clinocerinae, collected between 80 and 1700 m and found in sunny or shady sections of narrow to broad rivers [30].  Note on type specimens. Wiedemannia zetterstedti, W. escheri and W. hirtiloba were all described on the basis of an unspecified number of male and female specimens [34][35][36]. Although the lectotype of P. riparia is a female, it is confirmed as conspecific with W. zetterstedti on the basis of the brown-coloured face, wing shorter than 5 mm and erect costal setae appearing opposite the humeral crossvein. Images of the type specimens are available online: https://science.mnhn.fr/institution/mnhn/collection/ed/item/ed607 3?listIndex=1&listCount=6 (accessed on 7 February 2022).  Note on type specimens. Wiedemannia zetterstedti, W. escheri and W. hirtiloba were all described on the basis of an unspecified number of male and female specimens [34][35][36]. Although the lectotype of P. riparia is a female, it is confirmed as conspecific with W. zetterstedti on the basis of the brown-coloured face, wing shorter than 5 mm and erect costal setae appearing opposite the humeral crossvein. Images of the type specimens are available online: https://science.mnhn.fr/institution/mnhn/collection/ed/item/ed6073?listIn-dex=1&listCount=6. Diagnosis. This species is distinguished from other species of the Wiedemannia zetterstedti group by the following combination of characters: face brownish in both sexes, without stripe; wing usually shorter than 5 mm; stout erect setae on costa beyond humeral crossvein; clasping cercus brown, similar to epandrium, with lobes not in same plane and prominent anterior lobe.
Remarks. The length, width and setation of the apical lobes of the clasping cercus in W. zetterstedti are highly variable ( Figures 3F cf. 10B), often in specimens captured in the same collecting event [37]. Despite this variation, features of the wing, face, and leg setation are consistent among specimens.

Discussion
After a thorough morphological study of all available type material and additional material of all species that belong to the Wiedemannia zetterstedti group, as well as additional confirmation by molecular species concept, we can confirm with certainty that W. escheri and W. hirtiloba, as suggested by Niesiolowski [37], are synonyms of Wiedemannia Diagnosis. This species is distinguished from other species of the Wiedemannia zetterstedti group by the following combination of characters: face brownish in both sexes, without stripe; wing usually shorter than 5 mm; stout erect setae on costa beyond humeral crossvein; clasping cercus brown, similar to epandrium, with lobes not in same plane and prominent anterior lobe.
Redescription. Wing length 3.4-4.8 mm. Male ( Figure 10C). Head with gena, postgena and lower half of occiput with blue-grey pruinescence; gena less than half as deep as eye height. Face slightly wider than antennal sockets, dark, brownish ( Figure 10A); distinct carina on lower margin. Frons short, brown, broader than face; ocellar triangle and vertex brown. Pair of long ocellar setae; one pair of strong vertical setae; 5-7 distinct upper postocular setae, all similarly sized and black, with interspersed shorter setae; lower postocular setae much finer and paler, merging with pale, longer setae on middle and lower occiput and postgena; several dark setulae on vertex and between ocellar area and eye margin. Antenna brown; scape and pedicel subequal; scape with dorsal setulae; pedicel with circlet of apical setulae; postpedicel pointed ovate; arista-like stylus longer than length of face.
Wing membrane infuscate, veins dark; basal costa seta extending to humeral crossvein ( Figure 10C). Origin of veins M 1 and M 2 widely separated at end of cell dm. Vein CuA + CuP short, faint streak or absent. Pterostigma narrow, elongate, faint to strong. Squama with pale fringes. Costal margin with short, strong erect setae beyond humeral crossvein. Halter with dark knob and paler shaft.