A Review of the Genus Cloeon from Chinese Mainland (Ephemeroptera: Baetidae)

Simple Summary The mayflies of baetid genus Cloeon Leach, 1815 are common inhabitants of lentic environments, such as ponds, lakes, reservoirs, and even some temporary bodies of water near human communities. Therefore, they are frequently collected and comprehensively researched around the world. In China, however, although nine Cloeon species names have been reported, few of them are confirmed historically. In this study of over 1000 specimens from over 50 sites on the Chinese mainland, four species of Cloeon were identified, namely C. bicolor Kimmins, 1947, C. dipterum (Linnaeus, 1761), C. harveyi (Kimmins, 1947), and C. viridulum Navás, 1931. The record of C. bicolor is the first in China, and its nymphs are described for the first time. Three junior synonyms are proposed for the species C. viridulum. The present research is not only a revised review of the genus Cloeon for the Chinese mainland but also a solid base for future works on Chinese and Asian Cloeon taxonomy. Abstract The widely distributed and species diverse genus Cloeon Leach, 1815 has never been reviewed in China, although nine species names have been reported from this country. After checking types of two species (C. viridulum and C. apicatum) and newly collected materials from more than 50 sites, four species are recognized, compared, and photographed in this research. Among them, the species C. bicolor Kimmins, 1947 is first recorded in China, and its nymphs are described for the first time. The distribution in China of two other species (C. dipterum (Linnaeus, 1761) and C. harveyi (Kimmins, 1947)) are also confirmed. The two species established by Navás (C. apicatum Navás, 1933 = C. navasi Bruggen, 1957, C. pielinum Navás, 1933, syn. nov.) and the species C. micki named by Tong and Dudgeon in 2021 are synonymized with C. viridulum Navás, 1931—the fourth species in this study. Furthermore, the C. virens Klapálek, 1905 from the Chinese mainland, found by Ulmer in 1925, is also regarded as the last species. Among the four species, the C. dipterum and C. harveyi seem closer because of the similar female color patterns of their stigma and ventral abdomen, while the species C. viridulum and C. bicolor are more alike because they have neither pigmented stigma nor stripes on abdominal sterna.

Historically, nine Cloeon species names have been reported from China [4,17,18]. Among them, eight were described by Ulmer between 1910-1940. They describe subimagoes, females, or even undeclared sexes, but the types are lost or damaged [19,20] and the descriptions are usually extremely brief and often lack figures. Consequently, these
Distribution: China (south); Bengal, India. Descriptions: Adults (see Kimmins, 1947 [10]). Mature nymphs (first description): body length 5.0-7.0 mm, caudal filaments 3.0-4.0 mm, terminal filament slightly shorter than cerci ( Figure 1A,B). Body generally brown with various dark dots and markings dorsally (living nymphs usually greenish brown) ( Figure 1A), ventral part of body pale ( Figure 1B). Midline of dorsal body pale, one pair of brown stripes along with midline on vertex ( Figure 1A). Antennae 2.0-3.0 mm, scape and pedicle brown, flagella pale; scape and pedicle with tiny sparse hair-like setae on surface, flagella with tiny setae on articulations ( Figure 2D). Compound eyes and ocelli dark brown. Mouthparts: dorsal surface and free margins of labrum with hair-like setae, ventral surface with two tufts of hair-like setae near median emargination; leading margin of labrum slightly convex, median notch emarginated smoothly ( Figure 3A); mandibles as in figures ( Figure 2G,H), lateral margin with very sparse hair-like setae, prostheca strong, with a finger-like denticle and a tuft of spines. Maxillary palpi with very sparse hair-like setae on surface, second and third segments subequal in length, both of them shorter than first segment ( Figure 3E). First segment of labial palpi slightly longer than second, the latter longer than third, which is slightly expanded ( Figure 3I); all segments with sparse hair-like setae on surface, those of apical segment longer and denser than others; glossae and paraglossae with relatively long setae on free margins. Lingua of hypopharynx round, free margins of lingua and superlinguae with hair-like setae ( Figure 2I). Nota with irregular brown markings. Apical half of trochanter brown, femora of all legs pale but with subapical brown band and brown base, tibiae and tarsi with bands near bases (Figure 2A-C). Legs with hair-like setae on surface, inner and outer margins; besides those setae, femora of all legs with pectinate setae on inner margin, spatulate setae on dorsal surface and spine-like setae on outer margin; setae patterns of tibiae and tarsi similar to femora except without any spine-like setae on outer margins. Patellar-tibial fusion lines present on all legs; all legs subequal in length; length of femora: tibiae: tarsi of foreleg = 1.5:0.82:1.0, length of ratio of midleg = 1.8:1.0:1.0, length of ratio of hindleg = 1.83:1.0:1.0. All claws similar, with slightly expanded base and two rows of spine-like denticles at basal half, denticles progressively larger from base to apex ( Figure 3M). Nota with irregular brown markings. Apical half of trochanter brown, femora of all legs pale but with subapical brown band and brown base, tibiae and tarsi with bands near bases (Figure 2A-C). Legs with hair-like setae on surface, inner and outer margins; besides those setae, femora of all legs with pectinate setae on inner margin, spatulate setae on dorsal surface and spine-like setae on outer margin; setae patterns of tibiae and tarsi similar to femora except without any spine-like setae on outer margins. Patellar-tibial fusion lines present on all legs; all legs subequal in length; length of femora: tibiae: tarsi of foreleg = 1.5:0.82:1.0, length of ratio of midleg = 1.8:1.0:1.0, length of ratio of hindleg = 1.83:1.0:1.0. All claws similar, with slightly expanded base and two rows of spine-like denticles at basal half, denticles progressively larger from base to apex ( Figure 3M).
Besides pale midline of abdominal terga, each tergum with three pairs of pale dots ( Figure 1A): one pair near midline, one on anterolateral angles and one on posterolateral angles. Median pair round pale dots relatively regular but other two pairs irregular in size and shapes. Terga IV and VII paler than others. Lateral margins of posterior half tergum VIII and whole tergum IX with spines ( Figure 2F); posterior margins of terga with spines. Paraproct with spines on inner margin except 1/3 base ( Figure 2J). Gills I-VII with clear dark tracheae ( Figure 4A-G), gills I and II with reddish pigments at base (color very clear in living nymphs) ( Figure 4A,B); gills I-VI with double lamellae but gills VII single ( Figure 4A-G); ventral one of gills I-VI nearly round to heart-shaped, but dorsal lamellae of gill I elongated to leaf-like in shape ( Figure 4A), dorsal lamellae of gills II heart-shaped with expanded basal lobe ( Figure 4B), those of gills III-VI near round ( Figure 4D-F); ventral lamellae of gills II-VI with slightly sclerotized outer half margins; both inner and outer margins of gills VII slightly sclerotized ( Figure 4G), hard line of outer margin broader and longer than that of inner one ( Figure 4I,J); all free margins of all gills with very sparse tiny hair-like setae, but those of gills VII slightly denser and more numerous ( Figure 4H). Caudal filaments pale, every four segments with a brown ring ( Figure 1A,B and Figure 2E); basal 2/3 cerci with long hair-like setae on mesal margins but terminal filament with similar setae on both lateral margins; all articulations of caudal filaments with a ring of spines, those on outer margins of cerci longer and broader ( Figure 2E).
Diagnosis (based on specimens stored in alcohol): The male imagoes of this species can be identified by their transparent wings ( Figures 5A and 6A), almost chestnut to reddish terga ( Figures 5A and 7A), each tergum with a pale midline and a pair of triangular to round pale dots ( Figure 7A); pale portions are usually enlarged and emerged into a larger portion in each tergum, producing a wide longitudinal pale midline; lateral margins of terga also pigmented with short conspicuous reddish stripes ( Figure 7A,B); abdominal sterna washed with irregular light reddish pigments or stains at midlines, those of sterna VII-IX sometimes forming reddish dots ( Figure 7B); a round projection with a round tip between two forceps; segment 2 of forceps with slightly expanded base and apex; segment 3 tiny ( Figure 7C). The female imagoes can be differentiated by their almost reddish terga ( Figures 5B and 8A), each tergum with a pale midline and a pair of pale dots ( Figure 8A); lateral margins of terga also with reddish lines ( Figure 8A  Besides pale midline of abdominal terga, each tergum with three pairs of pale dots ( Figure 1A): one pair near midline, one on anterolateral angles and one on posterolateral angles. Median pair round pale dots relatively regular but other two pairs irregular in size and shapes. Terga IV and VII paler than others. Lateral margins of posterior half tergum    Remarks: Mature nymphs of this species are similar to C. harveyi ( Figure 1E,F) and C. viridulum ( Figure 1G,H) by having terga VIII and IX with lateral spines and caudal filaments without median dark bands ( Figure 1A,B) (in contrast, C. dipterum have lateral spines on terga VI-IX and dark markings on caudal filaments, Figure 1C,D). However, the nymphal stages of C. bicolor can be identified by pale abdominal sterna with median reddish dots on posterior margin only (those of C. viridulum are totally pale, abdominal sterna of C. harveyi have clear longitudinal stripes) (Figure 8A,E,G), the length of claws (longer than C. viridulum but shorter than C. harveyi) ( Figure 3M,O,P), and relatively regular pairs of pale dots on terga. The nymphs of this species are very similar to C. viridulum in color pattern and most structures (such as the claw, maxilla, lateral spines on terga VIII-IX), but they have relatively dark bodies (especially those terga II-III, V-VI) but lack independent dark dots on terga II and V ( Figure 1A,G).
The six indoor-reared nymphs were seen molting into subimagoes at 18:00-20:00 p.m. local time. Before 7:00 a.m. the next morning, they became imagoes. Remarks: Mature nymphs of this species are similar to C. harveyi ( Figure 1E,F) and C. viridulum ( Figure 1G,H) by having terga VIII and IX with lateral spines and caudal filaments without median dark bands ( Figure 1A,B) (in contrast, C. dipterum have lateral spines on terga VI-IX and dark markings on caudal filaments, Figure 1C,D). However, the nymphal stages of C. bicolor can be identified by pale abdominal sterna with median reddish dots on posterior margin only (those of C. viridulum are totally pale, abdominal sterna of C. harveyi have clear longitudinal stripes) (Figure 8A,E,G), the length of claws (longer than C. viridulum but shorter than C. harveyi) ( Figure 3M,O,P), and relatively regular pairs of pale dots on terga. The nymphs of this species are very similar to C. viridulum in color pattern and most structures (such as the claw, maxilla, lateral spines on terga VIII-IX), but they have relatively dark bodies (especially those terga II-III, V-VI) but lack independent dark dots on terga II and V ( Figure 1A,G).
Diagnosis (based on specimens stored in alcohol): The male imagoes of this species can be identified by their pairs of crescent to triangular reddish brown markings on abdominal terga ( Figure 7D); posterior half of terga I-III and midline of terga VI-IX are alsoreddish ( Figure 7D); lateral margins of terga are reddish ( Figure 7D); pairs of reddish near-rectangular markings present on abdominal sterna ( Figure 7E). Wings lacking any pigments except at the base of subcostal brace ( Figure 6C). A cone-like median projection presents between forceps or on penial bridge ( Figure 7F). The female imagoes can be differentiated by their similar marking patterns to males ( Figures 8C and 7D), the heavily pigmented C and Sc sections of wings with irregular pale blanks ( Figure 6D). The mature nymphs of this species can be recognized by their relatively short apical segments of maxillary palpi ( Figure 3F), spines at least present on lateral lines of terga VII-IX, base of claw slightly broadened and basal 1/2 of claw with two rows of denticles ( Figure 3N).
Remarks: Both Gattolliat et al. (2008) [3] and Bauernfeind and Soldán (2012) [6] stated clearly that the nymphs of European C. dipterum or complexes of this species have spines on terga V-IX and the basal 2/3 of tarsal claws. However, all our collections from China and several nymphs from Vladivostok (Russia) have spines on the lateral margins of terga VII-IX only, although some individuals have three spines on terga VI. In addition, the tarsal claws have denticles on the basal half ( Figure 3N). Nevertheless, considering that Russia, Korea, and Japan have this species [12,13,37], we treat these two differences as variations between populations.  [4]. Diagnosis (based on specimens stored in alcohol): The male imagoes of this species can be identified by their pale terga I-VII and abdominal sterna but terga VIII-IX reddish brown ( Figure 7J,K), wings transparent ( Figure 6G); round projection between forceps ( Figure 7L). The female imagoes of it can be differentiated by its orange to reddish dorsal terga, each tergum with a pair of short bent pale stripes dorsally, lateral margins of terga pigmented with reddish dots ( Figure 8G); pale abdominal sterna ( Figure 8H). Wings transparent but costal and subcostal sections yellowish and semi-hyaline ( Figure 6H). The mature nymphs of this species can be diagnosed by their three subequal segments of maxillary palpi ( Figure 3H), without an expanded apical segment of labial palpi ( Figure 3L), relatively short tarsal claw with denticles on basal 2/3 ( Figure 3P). Most nymphs have two conspicuous dark spots on terga II and V, respectively ( Figure 1G).
Remarks: This species can be found in most parts of the Chinese mainland, including some man-made habitats, such as ponds in school campuses, city gardens, or reservoirs.
Based on pictures of specimens studied by Navás in 1931 and 1933, the species C. apicutum is regarded as a junior synonym of C. viridulum. The types of both species were collected from the same region (Shanghai, Jiangsu to Zhejiang province, eastern China), in which we intensively collected specimens.
Although the types of C. micki Tong and Dudgeon have not been examined, the presence of two spots on abdominal terga II and V in all life stages (occasionally not obvious), the similar male genitalia, dorsal color pattern of males and females, and characteristics of the tarsal claws and mouthparts of the nymphs support the designation of C. micki as a junior synonym of C. viridulum.
Due to the wide distribution and frequent collection of C. viridulum, the species C. virens found by Ulmer (1925Ulmer ( , 1935Ulmer ( -1936 on the Chinese mainland is also recognized as this species here. We found no individuals similar to C. virens in this region. The type of the species C. pielinum is lost and without sex declaration. Based on its equal distribution (type from Shanghai) and similar description to the C. viridulum, it is also synymonized here.

Discussion
The males of four Chinese Cloeon species can be divided into two groups based on their abdominal sterna with or without reddish longitudinal stripes. The species C. harveyi ( Figure 7H) and C. dipterum ( Figure 7E) fall into the former category, but C. viridulum ( Figure 7K) and C. bicolor ( Figure 7B) belong to the latter. Similarly, the females of those four species can be divided into two groups due to their similar color patterns to males. In addition, the females of C. harveyi ( Figure 8F) and C. dipterum ( Figure 8D) have reddish brown stigmas, pigmented C and Sc sections of wings with clear pale dots or markings, while those of C. viridulum ( Figure 6H) and C. bicolor ( Figure 6B) have uniformly pigmented and semi-hyaline C and Sc sections only.
Besides color patterns on abdominal sterna, the nymphs of C. harveyi ( Figure 3G,O) and C. dipterum ( Figure 3F,N) have longer claws and maxillary palpi than those of C. viridulum ( Figure 3H,P) and C. bicolor ( Figure 3E,M).
Morphologically, four Cloeon species of the Chinese mainland can be identified easily. However, in most cases, the color patterns of the body and wings are used, and a few solid structures can be found in their tiny bodies for differentiation. Hopefully, molecular evidence can provide more information in future work, especially on the C. dipterum complex or other similar species, such as C. viridulum and C. ryogokuensis Gose, 1980 [52,53].
The Cloeon nymphs are typical lentic mayflies, and the imaginal stages of mayflies are thought to have weak flight abilities. However, all four Cloeon species found in China are also reported in other Asian countries, such as India and Japan, and C. dipterum is a cosmopolitan species. Some Pacific Islands are also Cloeon habitats. The dispersal or migration routes and biogeography of these species are interesting topics for future research.