Revision of fossil Metretopodidae (Insecta, Ephemeroptera) in Baltic amber – Part 4: Description of two new species of Siphloplecton Clemens, 1915, with notes on the new S. jaegeri species group and with key to fossil male adults of Siphloplecton

Abstract The Siphloplecton jaegeri species group is established here for three extinct species, namely for the earlier described Siphloplecton jaegeri Demoulin, 1968, and for two new species from Eocene Baltic amber, Siphloplecton landoltisp. nov. and Siphloplecton studemannaesp. nov. Based on the well-preserved specimens of these species, a diagnosis is provided for the newly established species group. Representatives of the S. jaegeri species group are characterized by the presence of large, medially contiguous eyes, stout pointed setae along the outer margin of the foretibia, three intercalaries in the cubital field of the forewing, and elongated penis lobes, which are apically triangular or rounded, medially contiguous, and with a V-shaped cleft apically. Further new specimens of the S. jaegeri species group are documented that cannot be attributed to species level due to their poor preservation. Finally, a key to male adults of fossil species of Siphloplecton is given.

Drawings were made with a camera lucida on an Olympus SZX7, a Leica S8 APO and a Leica M205 C stereo microscopes. Multiple photographs with different depth of field were taken through a Leica Z16 APO Macroscope using Leica Application Suite v. 3.1.8. Photo stacks were processed with Helicon Focus Pro 6.4.1 to obtain combined photographs with extended depth of field, and subsequently enhanced with Adobe Photoshop CS3.

Siphloplecton jaegeri
Material examined. Male imago in Baltic amber (Eocene), MNB, MB.I 7370, specimen labelled as: "6. Pseudoneuroptera III Ephemeridae"; "Museum für Naturkunde Berlin"; "Paläontologisches Museum"; "Slg.: Künow Inv. Nr.: Nr. 268-294 nur noch  side of head and prosternum not visible, view obstructed by resin influx and cracks in stone. Foremargin and distal part of left forewing and entire left hind wing dirty brownish coloured; several dark spots on remaining part of left forewing. Such irregular pigmentation is a side effect of the specific conditions of fossilization, and must not be confused with the natural pigmentation of Siphloplecton wings (the right wings of the same specimen are colourless and translucent). Right fore-and left middle legs lost. Cerci partly damaged.
For measurements see Table 1. Male imago in Baltic amber (Eocene), MNHN, 4655 BA (Figs 3−4). The specimen is visible in dorsoventral and, partly, lateral aspect. Head and thorax ventrally with "Verlumung". Right fore-and hind wings fully preserved; left pair of wings partly twisted; details of cubital field not discernible. Cerci lost.
Colour yellow to yellowish-brown with darker thorax, but generally paler than all other known specimens of S. jaegeri. Wings hyaline, translucent, without any pigmentation.
For measurements see Table 1. Description of specimens. General colouration from pale (yellow to yellowishbrown), to dark brown (yellowish-brown to intensively brown); details of wing colouration are described above.
Wings hyaline. Pterostigma with at least 6 anastomosed veins. Cubital field of right forewing with well-developed intercalary vein (iCu1) close to CuA and basally directly connected to it, followed by one pair of intercalaries (iCu2, iCu3) basally connected to each other and connected to CuA (in MNHN specimen also to CuP) by a short crossvein (Figs 1D, 2, 4A). Hind wings with triads RS, MA and MP (Fig. 4A); preserved part of hind wings 0.39× forewing length (MNB specimen). Hind wings of MNHN specimen with triads RS, MA and MP, poorly visible distally; preserved part of hind wings 0.35× forewing length. Costal process small. Legs brownish; tibiae and tarsi darker than femora; structure and proportions of leg segments similar to those of holotype of S. jaegeri; outer margin of foretibia with pointed setae; measurements of leg segments in Table 1.
Abdominal segments well preserved, paler than thorax. Shape of styliger and penis lobes (Figs 1E, 3D, 4B) similar to those of S. jaegeri holotype (Demoulin 1968: 252, figs 18a, c, Staniczek andGodunko 2012: 74, fig. 11c). Styliger plate angulate, deeply incised with three prominent projections; medial projection markedly broad; basal segment of forceps basally (a) markedly narrower than adjoining apical part of styliger plate (b) (a/b = 0.57); forceps 4-segmented; segment 2 longest, segment 4 approximately 2.47 to 2.65 times as long as wide; length ratio of segment 3 to segment 4 approximately 0.9:1. Penis lobes elongated, medially incised, triangular, with relatively inconspicuous incision between lateral and medial penis sclerites. Surface details of penis lobes not visible. Paracercus vestigial, 5-segmented. Comments. Some minor differences regarding the proportions of the fore/hind wings and forceps segments between the holotype of S. jaegeri and the specimens described above are present. However, we attribute these specimens to S. jaegeri due to the presence of 1+2 intercalaries in the cubital field, pointed setae along outer margin of foretibia, and the shape of styliger plate and penis lobes. For measurements see Table 1.
Wings translucent, hyaline, not pigmented, venation well visible; pterostigma translucent, without pigmentation, with simple veins. Cubital field of forewing well visible on right wing only (twisted on left wing): one distinct pair of intercalary veins (iCu2 and iCu3) situated close to CuP and connected with CuP and CuA; one long intercalary vein (iCu1) connected with CuA and also with iCu2 by crossveins; additional short intercalaries present, ending at hind margin of wing (one of them between iCu1 and iCu2, basally joining both veins) (Fig. 5C). Hind wings with triads RS, MA and MP, 0.31× forewing length; costal process bluntly pointed (Fig. 5D).
Legs damaged; both right fore-and hind legs lost; tarsus of left foreleg only partly preserved (for details see Table 1). Tibia of left foreleg paler than femur and tarsus. Left foretibia with strong, sharply pointed setae along outer margin. Tibia of left middle and hind legs black; preserved part of first tarsomere of left middle leg black. Tibia of middle and hind legs with trace of tibiopatellar suture; first tarsomere of middle and hind legs fused with tibia. Tarsal claws lost (Fig. 5A-B).
Comments. The new species can be placed within the jaegeri species group based on the characteristic shape of the penis lobes, the arrangement of cubital intercalaries of the forewing and the sharply pointed setae along the outer margin of the foretibia. Siphloplecton landolti sp. nov. can be separated from the closely related S. jaegeri by (1) shape of styliger with relatively small and apically rounded medial projection; (2) relatively wide base of basal forceps segment compared to adjoining apical part of styliger plate; (3) proportions of last forceps segments; (4) ellipsoidal shape of penis lobes with medial sclerite rounded at tip.
The body measurements of S. landolti sp. nov. are comparable to other representatives of the S. jaegeri species group, but the ratio of hind/forewing length is lower in comparison with S. jaegeri itself.
Etymology. Following our tradition of naming new fossil species of Siphloplecton after Swiss ephemeropterists, this species is named after Peter Landolt, Lausanne, to honour his contributions to the knowledge of Swiss mayflies.  Table 1 Material examined. Holotype. Female imago in Baltic amber (Eocene), SMNS BB-2626. Well preserved specimen in clear amber, well visible in dorsoventral aspect. Right foreleg lost; hind wings (especially left wing) twisted. Head and thorax covered by "Verlumung". Additionally, piece of amber with numerous cracks, thus details of thoracic terga invisible, and thorax only partly visible from ventral side. For measurements see Table 1.
Paratype. Female imago in Baltic amber (Eocene), MNHK, MP/1626. Partly damaged specimen, visible in ventral aspect. View on body hampered by resin influxes, numerous cracks, and considerable "Verlumung", so thoracic sutures are hardly visible. Head lost. Pronotum damaged, its structure invisible. Distal part of mesonotum, metanotum and abdominal segments I−IV dorsally covered by plant tissue. Right forewing twisted along its length; left forewing lacks its basal part and distal end. Right foreleg, right middle leg, left middle tarsus, and both hind legs lost. Cerci only partly preserved. For measurements see Table 1.
Description of holotype. General colour of body pale, yellow to yellowish-brown. Ventral side of body slightly darker than dorsal side. Ocelli well preserved. Eyes medially separated, but approximated (Fig. 8A). Distance between eyes 0.22× of head width. Antennae complete, slightly longer than head, pale.  Wings translucent, hyaline, not pigmented. Wing venation well visible only from ventral side; basal part of wings covered by numerous cracks from dorsal side. Cubital field of both forewings with one pair of intercalary veins (iCu2, iCu3) towards CuP and one additional vein (iCu1) near CuA 9). Right hind wing hardly visible, twisted; left hind wing completely preserved with three pairs of triads, 0.34× of right forewing length. Costal process bluntly pointed apically and small (Fig. 8E).
Legs relatively well preserved, except forelegs (right is lost; left incomplete). Measurements of leg segments in Table 1. Tibiae of middle and hind legs each with trace of tibiopatellar suture; first tarsomere of middle and hind legs fused with tibia (Fig. 8F). Tarsi with 5 tarsomeres; tarsal claws dissimilar: one hooked and one blunt (Fig. 8G).
Description of paratype. Body colouration light, yellowish-brown to brown. Irregular brown, dirty brown to black maculation over body, especially on forewings and legs (Fig. 10A).  Details of mesonotum hardly visible; mesonotal suture typical for Siphloplecton, original lateroparapsidal suture colouration invisible due to resin influx. Furcasternal protuberances contiguous, distinctly brown; other parts of mesosternum paler.
Pterostigma with anastomosed crossveins. Cubital field of right forewing well preserved; iCu1 basally connected to CuA and iCu2 by crossveins, 2 additional CuA-iCu1 crossveins present. Pair of intercalaries iCu2 and iCu3 basally connected to each other; further crossveins connect to CuA and CuP (Figs 10B, 11A). Hind wings relatively long, approximately 0.40× of length of preserved right forewing; costal processes hardly visible. Hind wings with triads RS, MA and MP; costal process (visible only on right hind wing) small and blunt (Fig. 10C).
Left foreleg intensely brown to blackish distally; middle legs paler, yellow to yellowish-brown. Preserved foreclaw dissimilar (one claw hooked, one claw blunt).
Comments. We allocate Siphloplecton studemannae sp. nov. within the S. jaegeri species group based on the presence of three distinctive intercalaries (grouped in one pair and one additional intercalary vein) in the cubital field of both forewings. The same arrangement of intercalary veins was described and figured for the holotype of S. jaegeri by Staniczek and Godunko (2012: 73, 74, fig. 10b). Other diagnostic characters of this species group, namely the presence of sharply pointed setae at the outer margin of the foretibia, cannot be observed in the type specimens of S. studemannae sp. nov. So it cannot be excluded that this character is present in the females of this fossil species, similarly to the different occurrence of this character in the two sexes of extant species (Berner 1978).
The new species can be characterized by the presence of the following characters: (1) eyes separated, but close-set; (2) cubital field of forewings with one pair of intercalaries and one intercalary vein connected with CuA; (3) subgenital plate relatively broad with width/length ratio 1.80−2.00. This combination of characters definitely separates the described females of S. studemannae sp. nov. from all other previously known Recent and fossil taxa. Siphloplecton studemannae sp. nov. is currently the only species of the genus Siphloplecton described from female specimens that can also be attributed to a certain species group. This is only possible due to the unique arrangement of cubital intercalaries, which is characteristic for the S. jaegeri species group. The previously described S. barabani and S. hageni, also only known from female specimens, do not provide any clear distinguishing characters that would allow their placement in one of the other fossil species groups defined for Siphloplecton, which can only be grouped based on similarities in the male genitalia.
Etymology. Following our tradition of naming new fossil species of Siphloplecton after Swiss ephemeropterists, this species is named after Denise Studemann, Lausanne, to honour her contributions to the knowledge of Swiss mayflies.

Siphloplecton spp. (jaegeri species group)
Siphloplecton sp. 5 Figures 12-13; Table 1 Material examined. Male imago in Baltic amber (Eocene), CCHH, BaB Nr. 1159/5. A generally completely preserved specimen, well visible in dorsoventral aspect in translucent amber (Fig. 12A-B). Some resin influxes around specimen. Right foreleg and right cercus lost. Dorsal side of body only with several very small spots of "Verlumung". Ventral side of head, thorax and, partly, abdominal sterna intensively covered with "Verlumung". For this reason, some aspects of mesosternum and the shape of the genitalia are poorly visible. One female nonbiting midge (Diptera: Chironomidae) is embedded in the same stone.
Relatively pale specimen, yellowish to brown. Wings with artificial, irregular dark spots; distal portions of forewings and right hind wing with distinct concentration of such spots (Fig. 12E). Legs uniformly coloured. For measurements see Table 1.
Description. Head light brown. Eyes large, slightly flattened, pale, dirty yellowish. Several brownish spots on eye surface. Ocelli and antennae of same colouration as head, completely preserved; antennae slightly longer than head (Fig. 12C, E).
Thorax with traces of brownish pigmentation on dorsal and ventral sides, brown. Pronotum well preserved. Arrangement of thoracic sutures on dorsal and ventral sides of mesothorax typical for Siphloplecton (Figs 12C-D, 13). Most of head and thorax ventrally covered with "Verlumung". Lateral aspect of thorax hardly visible.
Wings basally translucent, hyaline, with well visible longitudinal and transversal venation. Pterostigmatic area hyaline, opaque, with several anastomosed veins. Cubital field of forewings with well visible pair of intercalary veins connected with CuA, and one additional, well developed vein also directed toward CuA (Figs 12E-G, 13). Hind wings with triads RS, MA and MP, 0.36× forewing length; costal process bluntly pointed and small (Figs 12E, 13). Legs yellowish to light brown. Several pointed spines at outer margin of foretibia. Characteristics of legs similar to those of other representatives of Siphloplecton. Measurements of leg segments in Table 1.
Styliger plate only partly visible, most of styliger base covered with "Verlumung". Thus, shape of styliger projections not clearly visible (Fig. 12H). Basal segment of forceps basally not widened, distinctly narrower than adjoining apical part of styliger plate (character well visible only in right clasper); forceps 4-segmented, segment 4 approximately 3.20 times longer than wide; length ratio of segment 3 to segment 4 approximately 1:1 (Fig. 12H).
Paracercus vestigial, at least 3-segmented; left cercus slightly damaged; right cercus lost. Comments. This species is closely related to S. landolti sp. nov. due to the similar proportions of the forceps segments and shape of the penis lobes, especially the apical portion of the medial sclerite, and the presence of 2+1 intercalary veins in the cubital field of the forewings. This last character also confirms the inclusion of Siphloplecton sp. 5 within the S. jaegeri species group. At the same time, we could not confirm its conspecificy with Siphloplecton landolti sp. nov. or other species, since the details of the male genitalia are poorly visible.
Completely preserved specimen, well visible in lateral aspect. Details of head hardly visible due to resin influxes and small cracks in stone. Left forewing arcuated at half length; left hind wing twisted. Terga IX-X and, partly, genitalia covered by "Verlumung". For measurements see Table 1.
Description. General colouration of body brown to dark brown. Head brown with paler antennae; eyes large, medially contiguous, uniformly brown coloured.
Wings opaque. Forewings with several irregular dark spots (probably an artefact of fossilisation). Pterostigmatic area with 7-8 anastomosed veins. In cubital field of forewing one pair of intercalaries close to CuP (connected with CuP and CuA); one additional intercalary vein close to CuA. Hind wings with triads RS, MA and MP, 0.32× forewing length; costal process bluntly pointed (Fig. 14).
Legs typical for the genus Siphloplecton. Abdominal segments completely preserved; terga darker than sterna. Shape of styliger plate poorly visible in ventral view, but generally close to the one of S. landolti (deeply incised mediocaudally, angulate, with relatively small medial projection). Shape of penis lobes hardly visible because of "Verlumung". Visible part of penis lobes with apical sclerites rounded at tip.
Cerci partly damaged; paracercus vestigial. Comments. Due to the general shape of the styliger and penis lobes, and proportions of the fore/hind wings, this specimen belongs with high probability to S. landolti sp. nov. Some of the observed differences might be related to differences between subimago and imago, or may be due to the bad preservation of the genitalia. However, in order to avoid any taxonomic confusion, we refrain from attributing Siphloplecton sp. 6 to a certain species within Siphloplecton.

Discussion
The first classification of extant representatives of the genus Siphloplecton into species groups was undertaken by Berner (1978) in his work dedicated to the taxonomy of Metretopodidae. That grouping was based on wing pigmentation, the shape of the setae along the outer margin of the foretibia, and penis shape (Berner 1978: 96; Staniczek and Godunko 2012: 59-61, figs 1-2). Staniczek and Godunko (2012), for the first time, added fossil Siphloplecton species from Eocene Baltic amber to Recent species groups. In this contribution, S. picteti and S. demoulini were attributed to the S. basale species group established by Berner (1978). At the same time, the remaining investigated taxa, S. barabani and S. hageni, were neither attributed to the basale species group nor to the interlineatum species group. This was due to these species having only been described from female imagines, and no traces of wing pigmentation were preserved. Siphloplecton jaegeri was also excluded from previously known species groups for peculiarities in the genitalia and for the presence of pointed setae along the outer margin of the foretibia (Staniczek and Godunko 2012). In fact, these characters pointed to a relatively isolated position of S. jaegeri within both fossil and recent species of Siphloplecton.
Based on the investigation of further male specimens, Staniczek and Godunko (2016) established another two Siphloplecton species groups for four fossil species: S. picteti and S. sartorii were attributed to the S. picteti species group, and S. demoulini and S. gattolliati to the S. demoulini species group. At the same time, due to the poor preservation of the re-discovered lectotype of S. macrops, Staniczek and Godunko (2016) concluded that it is neither possible to define any distinguishing characters for this species nor to attribute it to any of the species groups proposed earlier. They also refrained from placing female specimens in the S. sartorii or demoulini species groups.
Representatives of the genus Siphloplecton are relatively abundant components of the mayfly fauna from Eocene Baltic amber. Judging from the museum and private collections investigated by us, this genus is not less abundant than Paraleptophlebia Lestage, 1917 (Leptophlebiidae), which is another commonly found mayfly genus in Baltic amber. Taxonomically, the genus Siphloplecton, with ten fossil and nine contemporary Nearctic species, is rather diverse.