Distribution extension of Oriental mayfly Clypeocaenis oligosetosa Soldán , 1983 ( Ephemeroptera : Caenidae )

Clypeocaenis Soldán, 1978 consists of 7 species, 3 of them are known from the Oriental region. Clypeocaenis oligosetosa Soldán, 1983 was described from Vietnam and until now was known only from 2 localities within this country. We found C. oligosetosa in Thailand for the first time. This record significantly extends the known distribution of the species. The main distinguishing characters of the larvae of C. oligosetosa are illustrated. We observed a few small morphological differences between Thai specimens, investigated paratypes of C. oligosetosa and its original description.

Clypeocaenis Soldán, 1978 (Caenidae: Clypeocaenini) is a rare genus consisting of 7 species distributed in the Oriental, Palaearctic, and Afrotropical regions (Soldán 1978, 1983, Soldán & Landa 1991, Malzacher 2013, Malzacher & Stanichek 2016).Three species of the genus are known from the Oriental region, and only one of them, Clypeocaenis oligosetosa Soldán, 1983 is distributed in the continental part of this region.The species was described from Thuan Hai Province in Vietnam (Soldán 1983).Later it was found in Nam Cat Tien National Park (Vietnam) in 1989 (Soldán 1991).According to Soldán (1983Soldán ( , 1991) ) C. oligosetosa was abundant in both sampling places.There were no other records of this species until now.We found C. oligosetosa in Thailand for the first time, what extends its known distribution.
All material used in this investigation was preserved in 80-95% EtOH; some larvae were mounted on slides with Canada balsam.The collected Thai specimens were identified as Clipeocaenis oligosetosa basing on the main distinguishing features of this species: shapes of labrum, maxillary palp and labial palp, setation of legs.These characters also distinguish C. oligosetosa from all other species of the genus.
The Thai specimens of C. oligosetosa almost fully correspond to the Vietnamese type specimens in the characters as illustrated in Figures 1-10.We observed one difference: specimens from the Chaem River had only 2 long hair-like setae on clypeal protrusion (Fig. 2) in contrast to larvae from the type locality with 4-6 (usually 4) long hair-like setae (Soldán 1983, original data).Additionally, larvae from Thailand and examined type specimens appeared to have pedicellus distinctly broader than the first segment of flagellum (Fig. 2), in contrast to the illustration of Soldán (1983: 198, fig. 4).
The first record of the genus Clypeocaenis from Thailand (Songkhla Province) was published by Sites et al. (2001), but as they only had immature larvae, the mayflies were only identified to genus (Fig. 11).They noted that the diagnostic character of the number of setae on the anterior margin of the clypeal protrusion showed intraspecific variation and suggested that a revision of the genus is required.
In Thailand, the larvae of C. oligosetosa were sampled from the Chaem River, Shan Highland (Fig. 12).The river here was 15-20 m wide, up to 1.5 m deep, the substrate was sand, pebble, silt, and detritus, and the current velocity was up to 1 m/s.Water temperature on the day of sampling was approximately 21 °C.Numerous settlements and agricultural lands along the river defined a very noticeable level of pollution (beta-mezosaprobic) and increased turbidity.Larvae were collected from submerged grass, detritus, and Phragmites sp.
Thus, ecological preferences of C. oligosetosa in North Thailand and South Vietnam are quite similar.The larvae of this species inhabit large, well-warmed plain or lowland  Our collection of C. oligosetosa in Thailand significantly expands the range of this species.This new record in Thailand is more than 1300 km northwest of the two previous records (Fig. 11).We predict that C. oligosetosa may inhabit similar waterbodies in Cambodia and Laos.It also might be found in adjacent regions of Shan Highland (including in Myanmar).Nevertheless, we did not find C. oligosetosa in the southern part of Cambodia during our investigation of the mayflies in this country in 2009 and 2015.Sites et al. 2001).12. Chaem River, site of new record of Clypeocaenis oligosetosa Soldán, 1983 in Thailand.

Comparative material. Paratypes
The material on C. oligosetosa from Thailand is stored in the collection in the National Museum of Natural History of the National Academy of Sciences of Ukraine (the inventory number of vial is Thai8Clysp, number of slide is 638)., 7 larvae (1 newly mounted in slide, no.659) and 1 larval skin (newly mounted in slide, no.660), Vietnam, Thuan Hai Province, Song Kinh-Dinh River, Nha-Ho, 15 km W of Phan Rang, 11°37ʹ58ʺ N, 108°52ʹ19ʺ E, 16.iv-5.v.1982, T. Soldán leg.