﻿A detailed comparison of two species in the genus Potamanthus Pictet, 1843 from China (Ephemeroptera, Potamanthidae)

﻿Abstract Photographs and details of structures of two Potamanthus species, P.huoshanensis Wu, 1987 and P.luteus (Linnaeus, 1767), are presented for the first time. Here, based upon Chinese specimens of those species, all external structures are illustrated digitally and compared. The results and photos clearly show that the adults of the two species are different in wing color and genitalia shape, and that their nymphs have different mandibular tusks and forelegs. Specifically, P.luteus has a more colorful body and wings, longer penes and nymphal mandibular tusks but shorter foretarsi than those of P.huoshanensis. This comparison not only confirms the differences between these two similar species, but also supports the updated generic delineations of Potamanthus and Potamanthodes.


Introduction
The Palearctic genus Potamanthus Pictet, 1843 comprises only two species and one subspecies (Bae and McCafferty 1991;Kluge 2004;Li and Zhou 2022). The first one, P. luteus (Linnaeus, 1767), is widely distributed from northern Africa and Europe to northeastern Asia, and its morphology has been described and mentioned by a long series of researchers (see Bauernfeind and Soldán 2012 and references therein). However, only Bae and McCafferty (1991) provided photos of this species, but no comprehensive, detailed photographs had so far been presented to show its exact characters. Further, P. luteus was divided into two subspecies by Bae and McCafferty (1991) using few structures, such as the shape of the anterolateral corners of the nymphal pronotum, the vestigial apical spines on the forefemora and forking point of the medius anterior (MA) in the imaginal forewings.
In contrast to P. luteus, the second species in the genus, P. huoshanensis Wu, 1987, has a very narrow distribution. Up until now, it has been found only at one site in China and two sites in Japan (Wu 1987a(Wu , 1987bIshiwata 2001). Only the wings and a drawing of the nymphal habitus of this species had been provided so far, by Bae and McCafferty (1991). In addition, Wu (1987a) and Bae and McCafferty (1991) regarded this species as very similar to P. luteus, the latter authors even identifying Japanese Potamanthus materials as belonging to P. luteus. Thus, proper photographic documentation of P. huoshanensis would not only reveal the real characters of this species but also show differences with the similar P. luteus.
The generic circumscription and phylogeny of the genus Potamanthus has been changing. Bae and McCafferty (1991) downgraded the taxon Potamanthodes to a subgenus of Potamanthus. Differently, Kluge (2004) placed it as a member of another genus, Rhoenanthus Eaton, 1881. Recently, Li andZhou (2022) reinstated the taxon Potamanthodes as an independent genus. With details of the two species in the genus Potamanthus and other recent related reports (Han et al. 2021;Kwanboon et al. 2021), differences among these three taxa will be clarified.

Methods
The nymphs of two species studied in the present paper were collected by hand net, whereas most adults were collected by light trap (using LED and mercury lamps). Some adults were reared from nymphs in the field. The materials are stored in ethanol (about 85%).
All specimens were examined under a stereomicroscope (MshOt MZ81) and photographed with a digital camera coupled to the microscope (Nikon Eclipse 50i). Some small structures, such as gills, mouthparts, terga and legs, were observed and photographed with a microscope camera on temporary slides. All specimens used in this study are deposited in the mayfly collection of the College of Life Sciences, Nanjing Normal University, China.
Description. see Wu (1987b) and Bae and McCafferty (1991). Diagnosis. This species resembles Potamanthus luteus in the main characters of both the adults and the nymphs, which can be differentiated only by very fine structures (Table 1). In the nymph, the labrum of P. huoshanensis is slightly narrower than that of P. luteus (Fig. 3A, B); the mandibular tusks are indistinctly shorter than in P. luteus, and this can be seen in nymphal dorsal views (Figs 2A, B, E, F, 3E-H); the maxillary palpi of both species are similar but different in their length ratio: the ratio in the former species is 1.0: 0.6: 1.0, whereas that of the latter species is 1.0: 0.7: 1.3 (Fig. 3I-L). The two species have a very similar hypopharynx and labia (Fig. 3C, D, M, N). Although the color pattern of examined P. huoshanensis has fainted and is pale, the leg lengths are different in the two species: ratio of forefemora: tibiae: tarsi = 1.0: 0.7: 0.6 in P. huoshanensis and 1.0: 0.8: 0.6 in P. luteus, the former having slightly shorter forelegs and tibiae (Fig. 2I, L). But the midlegs, hindlegs and their claws are very similar (Fig. 2D, H, J, K, M, N).
Males of the two species can be easily separated: (1) the pigments of the crossveins of the forewings of P. huoshanensis are almost invisible, but they are clear on the forewings of P. luteus (Figs 4A, C, 5E, G); (2) the costal projection of the hindwings are slightly blunter in P. huoshanensis than in P. luteus (Fig. 5F, H); (3) the compound eyes of P. huoshanensis are almost contiguous but they are clearly separated in P. luteus (Fig. 5A, C); (4) both the lateral and inner extended lobes of the penis of P. huoshanensis are slightly smaller than those of P. luteus (Fig. 6C-E, H-J); (5) the penes of P. huoshanensis are slightly shorter than those of P. luteus: the subgenital plate of P. huoshanensis almost covers the base of the penial lobes but the penes of P. luteus are longer, with the whole penes completely visible in ventral view (Fig. 6A, B, F, G); (6) the subgenital plate of P. huoshanensis has a shallow median emargination, whereas that of P. luteus has a clear V-shaped cleft (Fig. 6A-D, F-I); (7) the forking point of the MA in the P. huoshanensis forewings is more distal than that of P. luteus, with the ratio of MA: MA 1 = 1.0: 0.7 in the former species and 1.0: 0.9 in the latter (Fig. 5E, G); (8) the foretibiae of P. huoshanensis are shorter than in P. luteus, with the ratio forefemora: tibiae: tarsi = 1.0: 1.3: 1.6 in P. huoshanensis and 1.0: 1.6: 1.5 in P. luteus (Fig. 4A, C).
The females of the two species can differentiated by their wing color and the shape of the hindwings, like in the males (Fig. 4B, D). The compound eyes of female P. luteus are slightly smaller than those of P. huoshanensis (Fig. 5B, D), but the subgenital plates are very similar (Fig. 7).
Although the color of the P. huoshanensis material is not clear, the original description of Wu (1987b) and our specimens clearly show that the males, females and nymphs of this species do not have dots on their abdominal terga. In contrast, all stages of P. luteus have a pair of dark dots on the abdominal terga ( Fig. 4C-D). In addition, P. luteus has a longitudinal median reddish band on the abdomen (Fig. 4C-D).
Remarks. Bae and McCafferty (1991) mentioned that the nymphs of the subspecies Potamanthus luteus oriens have very pointed anterolateral projections of the pronotal and vestigial spine-row on the forefemora. In our material, the former character is distinct, and the transverse spine-row was not recognizable, which is consistent with the description of European P. luteus provided by Bauernfeind and Soldán (2012). However, we do not know whether this variation is just at the population level or representative of different subspecies or geographical populations, because we have no material from abroad for comparison.
In the present comparison and photos, we can see clearly that P. huoshanensis and P. luteus oriens are extremely similar in both nymphal and imaginal structures. The differences between them are very slight . Therefore, it is not surprising that Bae and McCafferty (1991) recognized Japanese materials of P. huoshanensis as P. luteus oriens, which was later corrected by Ishiwata (2001).
The distribution of P. luteus is wide, from Africa to Japan. In contrast, P. huoshanensis was reported from three allopatric sites in Japan and China. Biogeographic and genetic studies at the population level are required for these species.
At the generic level, the definitions of the genera Potamanthus and Potamanthodes were updated by Li and Zhou (2022) and confirmed by the characters presented in this study.  Mandibular tusks short (not protruding beyond labrum in dorsal view) ( Fig.  2A, B, 3E, G); apical segment of maxillary palpi subequal to or shorter than basal one (Fig. 3K)