﻿A new species of Siphlonurus Eaton, 1868 (Ephemeroptera, Siphlonuridae) from Yunnan, China

﻿Abstract Siphlonurusdongxi Li & Tong, sp. nov. from Shangri-La City, Yunnan Province, China, is described based on egg, nymph, and winged stages. The new species is closely related to S.davidi (Navás, 1932), and can be distinguished by the colour of the imago, the forking point of MP, the penis, posterolateral spines of tergum IX of imagoes, and first abdominal terga nymph, as well as the structure of the egg. The new species and S.davidi have the same morphological and structural characteristics, such as the long cubital area with many intercalaries, cross veins between C, Sc, RA, and RSa1 surrounded with distinct pigments, the strong curvature of vein CuP in the forewing, the broad expansion of the hindwing, the membranous penis lobes fused without teeth, supporting the proposition of a new species complex, the Siphlonurusdavidi group. The structures of the penis and the egg of the new species could help understand the origin and evolution of the genus Siphlonurus.


Introduction
The genus Siphlonurus Eaton, 1868 (Ephemeroptera, Siphlonuridae) is characterized by many plesiomorphies (Kluge et al. 1995). About 40 Siphlonurus species have been reported from the Nearctic and Palaearctic realms (Kluge 2004). A few species have been mentioned from China, but Kluge (2004) suggested that there was an "unknown group" of Siphlonurus in China. So far, only S. davidi (Navás, 1932) is distributed in China. It was initially described from a single male subimago (Zhou and Peters 2003), while the type specimen was redescribed later (Sartori and Peters 2004). Afterwards, the egg, nymph, and imago of S. davidi were described by Han et al. (2016). Consequently, the lack of research on adults of Siphlonurus might limit the understanding of this genus in China.
Siphlonurus davidi presents some plesiomorphies, indicating a close relationship with the ancestor of the Siphlonurus lineage, such as the forking point of MP subequal to that of the fusion point of MA and RS, the cubital area longer and with more intercalaries between CuA and the posterior margin of the wing, and the hindwings approximately half the length of the forewings, longer than in other Siphlonurus species (Sartori and Peters 2004;Han et al. 2016). During our recent survey of the mayfly fauna of the Hengduan Mountains area, at the eastern end of the Himalayas, a not yet described species of Siphlonurus similar to S. davidi was found in Shangri-La City, western Yunnan, China, at an altitude of more than 3000 m. Here, we describe this new Siphlonurus species based on imago, subimago, nymph, and egg stages.

Materials and methods
Siphlonurus nymphs were collected with a D-frame net from the floodplain habitats of the Dugang River in Shangri-La City, northwestern Yunnan, China. Following the guidelines from Li et al. (2022), the habitat photographs were taken using a Huawei Nova 8 mobile phone equipped with a Kase 40-75 mm macro lens. Some specimens were dissected under a stereomicroscope and were mounted on slides with Hoyer's solution for examination with a digital microscope. Slide-mounted specimens were examined and photographed with a Keyence VHX-S550E digital microscope. For scanning electron microscopy (SEM), eggs were dried, coated with gold, and observed with a VEGA3 SBU SEM (Tescan, Brno, Czech Republic). Measurements were taken using ImageJ image processing software. The final plates were prepared with Adobe Photoshop CC 2018.
All examined materials were deposited at the Museum of Biology, Institute of Eastern-Himalaya Biodiversity Research, Dali University, Dali, Yunnan, China (MBDU). Diagnoses. The new species is similar to S. davidi. It can be distinguished from S. davidi by the colour of the imago, the morphological structure of egg, the forking point of MP, the transversal sclerite of the penis with two dorsal elongations, the dorsal elongation of the penis basally expanded, the elongations of the ventral sclerite, the posterolateral spines of tergum IX of imagoes and the first abdominal terga of the nymph, as well as the structure of the egg.  Head: compound eyes contiguous (Fig. 1A), each of them spherical, upper portion and lower portion grey, without clear line between them (Fig. 1B).
Female subimago (in alcohol). Similar to male subimago except the tarsal segments of foreleg with more spines and usual sexual differences    Winged stages of Siphlonurus dongxi Li & Tong, sp. nov. (living) are shown in Fig. 9A-D. Eggs. Oval with irregular flat areas (Fig. 10A), length of 224-240 um, and width of 168-175 um. Chorion without obvious reticulation, with micropyle with or without reticulation (Fig. 10A-D). No accessory attachment structure apparent, but the egg surface has convex rough structures (Fig. 10A, B). The eggs were stuck tightly together, and the mass remained intact when placed in the water or ethanol.
Persistent mouthparts of winged stages. The new species presents persistent mouthparts in winged stages; in ventral view of head, the labial and maxillary are present and clearly visible (Fig. 11A-D, indicated by white arrow).
Final nymphal instar (in ethanol) light yellow with red and dark markings (Fig. 12A, B), body length 14.1-16.7 mm (excluding cerci); head width 2.6-2.8 mm, cerci lengths 6.7-7.7 mm, median filament 5.6-6.8 mm, antennae 1.8-2.1 mm. Morphology and structure of the head (Fig. 12A, B) and mouthparts ( Fig. 13A-H) of the new species similar to that of S. davidi. All legs similar (Fig. 14A), surface with short thick sparse setae, the apex of femora, tibiae, and tarsi with black spots or rings; femora broad, median marking black band; midleg with one clear patellar-tibial suture and hindleg with two ones on the tibiae (Fig. 14A, indicated by red arrow). Length of femur: tibia: tarsus of foreleg = 1.8: 1.0: 1.3, femur: tibia: tarsus of midleg = 1.9: 1.0: 1.3, and femur: tibia: tarsus of hindleg = 1.8: 1.0: 1.2. Claws simple, without teeth (Fig. 14A). Abdomen with each tergite with one pair of parallel stripes near median line and tracheae with distinct pigmentation (Fig. 12A), posterolateral spines present on terga II-IX, surface with short, thick, sparse setae (Fig. 15A). Abdominal sternum (Fig. 14C) light yellow with dark spots and oblique dashes similar to that of winged stages; surface with short thick sparse setae (Fig. 14D); posterior margin of sternum IX of male and female concave (Fig. 15B, C). Middle instars light yellow without pigmentation (Fig. 14B). Caudal filaments with dark bands at the top of each segment, each segment with whorls of short, thick setae apically and long, hair-like setae laterally (Fig. 15D). Gills double on segments I and II; dorsal lamella of gill I (Fig. 16A) triangle, small, anterior rim relatively short, posterior margin straight. Dorsal lamella of gill II (Fig. 16C) leaf-shaped, anterior rim relatively short, posterior margin round, apically pointed. Ventral lamellae of gills I (Fig. 16B) and II (Fig. 16D) heart-shaped and their posterior margin slightly notched. Gills III (Fig. 5E), single, leaf-shaped, anterior rim relatively long, posterior margin round, apically slightly pointed. Gills IV-VII ( Fig. 16F-J) single, oval, apically round, anterior rim relatively long; posterior margin round. Anterior rims of each gill with very short stout setae (Fig. 16J). Gill size gradually increased from gill I to gill III and decreasing from gill III to gill VII.
Etymology. The specific epithet dongxi, is named after the Chinese abbreviation of the Institute of Eastern-Himalaya Biodiversity Research, Dali University (https://www.eastern-himalaya.com.cn/contents/16/923.html). We hope that the Institute of Eastern-Himalaya Biodiversity Research can become an important platform for biodiversity research in the world. At the same time, we hope to cooperate with scientists from all countries through the International Centre of Biodiversity and Primate Conservation (http:// www.icbpc.org/index.html).

Distribution. China (Yunnan).
Ecology. Nymphs of this new species prefer to live in pools or slow current areas with aquatic plants close to the bank in clear, high-altitude, wide streams (Fig. 17A). The last instar nymphs molted at noon and stayed on the grass for a relatively long time (Fig. 17B). In the laboratory, the larvae showed a behaviour of tearing water grass and collecting sediment (Fig. 17C). The emergence time is consistent with that observed in the field, the mature larvae crawled out of the water, the subimago stopped for a short time to drain a little water from the end of the abdomen before taking off (Fig. 17D). The subimago stage persisted until the third night while the observed lifespan of imagoes was about 4 days.

Discussion
The identification key of three Asian Siphlonurus species, S. binotatus Eaton, 1892(Gose 1979 fig. 59), S. davidi (Han et al. 2016: figs 3-6), S. palaearcticus (Tshernova, 1930) (Kluge 1982 fig. 3) with coloured wings was provided by Sartori and Peters (2004) and Han et al. (2016). However, S. yoshinoensis Gose, 1979, with colourful wings (Takayanagi 2021, was ignored. In Asia, five species with colourful wings have been identified. Among them, the imagoes of S. binotatus and S. palaearcticus have ventral penis lobe with teeth, which can be differentiated from the other three species; S. davidi and S. dongxi Li & Tong, sp. nov. feature a fused penis lobe, while S. yoshinoensis has a penis lobe with a deep median incision. Siphlonurus dongxi Li & Tong, sp. nov. is closely related to S. davidi, whose adults share the markings of the wings, the strong curvature of vein CuP, the broad expansion of the hind wing, the longer cubital area in the forewing, and the membranous penis lobes fused without teeth. Siphlonurus dongxi Li & Tong, sp. nov. and S. davidi differ from all other described Siphlonurus species in these characters (Sartori and Peters 2004;Han et al. 2016), which offer support that a new species complex, the Siphlonurus davidi group, should be distinguished.     These numerous and significant differences between S. dongxi Li & Tong, sp. nov. and S. davidi suggest that the existence of a species bridging the gap between them is possible.
Obviously, the new species shows characteristics that fall somewhere between S. davidi and other ones. The discovery of this new species bridges the gap between S. davidi and other Siphlonurus species, and could help reveal the origin and evolution of the genus Siphlonurus.