﻿A review of Microdytes J. Balfour-Browne, 1946 from Thailand, Laos, and Cambodia with descriptions of five new species and new records (Coleoptera, Dytiscidae)

﻿Abstract The diving beetle genus Microdytes J. Balfour-Browne, 1946 in Thailand, Laos, and Cambodia is reviewed, and five new species are described: Microdyteseliasi Wewalka & Okada, sp. nov. (Thailand, Cambodia), M.jeenthongi Okada & Wewalka, sp. nov. (Thailand), M.maximiliani Wewalka & Okada, sp. nov. (Laos, China), M.sekaensis Okada & Wewalka, sp. nov. (Thailand, Laos), M.ubonensis Okada & Wewalka, sp. nov. (Thailand, Laos). Two species are the first country records: M.balkei Wewalka, 1997 (Laos, Cambodia) and M.wewalkai Bian & Ji, 2009 (Laos). For 12 and 8 species, the first provincial records from Thailand and Laos, respectively, are given. A checklist, a key to the 25 known Microdytes species from these countries, and habitus images and illustrations of diagnostic characters are provided. Distribution maps of the recorded species are presented, and species distribution patterns are also briefly discussed.


Introduction
The dytiscid genus Microdytes J. Balfour-Browne, 1946 contains 47 described species classified in the tribe Hyphydrini of the subfamily Hydroporinae (Nilsson and Hájek image stacking software Helicon Focus (Helicon Soft Ltd., Kharkov), and subsequently edited with Adobe Photoshop elements (2008) (Adobe Systems Inc., USA) where necessary.
The terminology to denote the orientation of the genitalia follows Miller and Nilsson (2003); the style of the descriptive notes follows Wewalka (1997Wewalka ( , 2011 and Miller and Wewalka (2010).
Body measurements were made with a compound microscope equipped with a micrometer eyepiece. The abbreviations of measurements used in this study are as follows: TL (total body length), TL-H (body length without head), MW (maximum width of body). The ratio TL/MW was also calculated. Measurements of holotypes are added between round brackets. Label data of holotype specimens are cited between quotation marks. A backslash (\) indicates separate labels. Our comments are given between square brackets.
All localities where Microdytes species were recorded in Thailand, Laos, and Cambodia are shown in Fig. 1. Literature records (Wewalka 1997(Wewalka , 2011 and additional For those records where specific information of province was not reported, the location was mapped by dotted lines. Ecoregion classification is given following Olson et al. (2001) records in this study are symbolized separately. For interpretation of distribution patterns of Microdytes, we mapped the recorded localities onto terrestrial ecoregions as defined by Olson et al. (2001). The main ecoregions for the distributions of species in Thailand, Laos, and Cambodia are abbreviated as follows:   12°57'38"N, 101°46'30"E, 26.VI.2021, R. Okada leg. (CRO). All paratypes are provided with printed red paratype labels.

Microdytes eliasi
Diagnosis. Microdytes eliasi sp. nov. very closely resembles M. maculatus (Motschulsky, 1860) (Figs 25,41) in size and coloration but differs from this species by the more regularly oval habitus, the laterally expanded median lobe at apex and constricted tips of paramere (Figs 12, 42C, 43C) (see also comments in M. maculatus). It is also similar to M. feryi Wewalka, 2011 (Fig. 21) in habitus and coloration, but it is smaller and can be distinguished by male genitalia [compared with a male paratype from Myanmar, "Tenasserim, Birma Coll. V. Helfer National Museum Prague" (CGW)].
Coloration. Head reddish brown. Pronotum reddish brown, narrowly dark brown along anterior and posterior margins. Elytron dark brown with yellowish brown markings forming a distinct transverse band near base not reaching suture connected along lateral margin with a post-median transverse lateral band, with a small post-median spot near suture, and a triangular spot near apex (Figs 2, 7). Ventral surface of head, prothorax and elytral epipleuron yellowish brown; thorax and abdominal ventrites reddish brown to dark brown. Legs, antennae and palpi yellowish brown.
Sculpture and structure. Head finely, sparsely, and relatively regularly punctured; anterior half to two-thirds finely microreticulate; clypeus not bordered. Pronotum quite regularly, sparsely, and fairly strongly punctured, with coarser punctures along posterior margin; without microreticulation; lateral margins finely bordered, regularly rounded. Elytron quite regularly, moderately densely and fairly strongly punctured, progressively finer and sparser towards lateral margin; without longitudinal rows of stronger punctures; highly polished and shining; without microreticulation. Ventral surface: metacoxae and metasternum strongly but sparsely punctured, abdomen finely and sparsely punctured; without microreticulation.
Male. The two parts of the median lobe expanded laterally at apex in ventral aspect (Figs 12A, 42A); slightly curved in lateral aspect (Fig. 12B). The tips of parameres twisted in lateral aspect (Fig. 12C, D); constricted in ventral aspect (Fig. 43C).
Female. Without secondary sexual characters. Sclerotized spermatheca not found. Variation. Variation of markings is shown in Fig. 7.
Etymology. This species is dedicated to Elias Bramböck, Vienna, Austria. The species epithet is a name in the genitive singular.
Habitat. In Thailand, this species was collected in small streams at low altitude lower than 200 m (Fig. 45).

Microdytes jeenthongi
Coloration. Head reddish brown. Pronotum reddish brown, along the anterior margin narrowly and along the posterior margin more widely dark brown, especially medially. Elytron dark brown with a distinct transverse band at the base not reaching suture, continuing along the margin to a post-median transverse band and a triangular spot near the apex, and a small round post-median spot near suture (Figs 3,8). Ventral surface of head and prothorax yellowish brown; thorax, abdominal ventrites and elytral epipleuron reddish brown to dark brown. Legs, antennae and palpi yellowish brown.
Sculpture and structure. Head finely and sparsely, slightly irregularly punctured; anterior forth finely microreticulate; clypeus not bordered. Pronotum punctured moderately irregularly, finely, and sparsely, very coarsely along posterior margin; without microreticulation; lateral margins finely bordered, regularly rounded. Elytron very sparsely and finely punctured, progressively finer and sparser towards lateral margin; one longitudinal row of punctures distinct; highly polished and shining; without microreticulation. Ventral surface: metasternum and metacoxae very finely, sparsely, and irregularly punctured; abdomen almost without punctures; without microreticulation.
Male. The two parts of the median lobe expanded and pointed at apex forming the unique arrowhead shape in ventral aspect (Fig. 13A); almost straight and slightly expanded downwards at apex in lateral aspect (Fig. 13B). Parameres moderate triangular in lateral aspect (Figs 13C,D).
Female. Unknown. Variation. Variation of markings is shown in Fig. 8. Etymology. This species is dedicated to Tadsanai Jeenthong, collection manager of National Science Museum, Thailand. The species epithet is a name in the genitive singular.
Habitat. This species was collected in a small, shallow stream with gravel bottom flowing through a small valley. In this stream, specimens were collected from a restricted point where tree roots were exposed along the river bottom (Fig. 46 Wewalka, 1997 in coloration of pronotum and elytra but differs from this species by darker head, larger size, a distinct impression on lateral side in anterior third of elytron ( Fig. 38) and male genitalia. It is similar to M. paoloi Wewalka, 2011 in size and coloration of pronotum but can be distinguished by elytral markings, the impression on lateral side of elytron and male genitalia. The shape of median lobe of aedeagus of M. maximiliani is very similar to that of M. schoenmanni Wewalka, 1997, but this species is much smaller and differs in coloration, punctation and a missing impression on lateral side of elytron. Coloration. Head dark reddish brown, darker on vertex and along eyes. Pronotum dark brown. Elytron dark brown with yellowish brown markings forming a distinct transverse band near base not reaching suture and lateral margin, a post-median lateral spot often connected with a triangular spot near apex (Figs 4,9). Elytral epipleuron and ventral surface dark brown to black. Legs, antennae and palpi reddish brown.
Sculpture and structure. Head finely and sparsely punctured, stronger punctures along eyes and on vertex; with fine microreticulation missing on vertex; clypeus not bordered. Pronotum irregularly punctured with sparse fine punctures and strong punctures concentrated along posterior margin and sparsely on disc; without microreticulation; lateral margins finely bordered, scarcely rounded. Elytron quite regularly, moderately densely and fairly strongly punctured, with traces of two longitudinal rows of punctures; highly polished and shining; without microreticulation; with a distinct impression on lateral side in anterior third (Fig. 38). Ventral surface: metacoxae, metasternum and abdomen moderately strongly but very sparsely punctured; without microreticulation.
Female. Without secondary sexual characters. Sclerotized spermatheca not found. Variation. Variation of markings is shown in Fig. 9. Etymology. This species is dedicated to Maximilian Bramböck, Vienna, Austria. The species epithet is a name in the genitive singular.
Habitat. The species was collected in Yunnan in a 1-2-m wide stream flowing through dense primary forest (Jäch and Ji 2003).
Distribution. Laos: Louang Namtha Province; China: Yunnan Province.  Wewalka, 1997 (Figs 34, 40B) in habitus and size but differs by elytral markings and clypeus not bordered. Size and coloration of M. sekaensis sp. nov. is also similar to those of M. gabrielae Wewalka, 1997 (Fig. 23), but it is distinguishable from the latter by the more rounded-oval body and finely and sparsely punctured ventral surface.

Microdytes sekaensis
Description Coloration. Head yellowish brown. Pronotum yellowish brown, along posterior margin narrowly dark brown. Elytron reddish brown to dark brown with two yellowish brown spots: one basal spot not reaching base, one anterior lateral spot continuing along the lateral margin to a post-median lateral spot and a transverse spot near apex, also with a round median spot near suture (Figs 5, 10). Ventral surface predominantly yellowish brown. Legs, antennae and palpi yellowish brown.
Sculpture and structure. Head very finely and sparsely, slightly irregularly punctured; entirely and distinctly microreticulate; clypeus not bordered. Pronotum quite irregularly, finely, and sparsely punctured on the disc, additionally with coarser punctures at medial part of posterior margin; without microreticulation; lateral margins finely bordered, regularly rounded. Elytron fairly finely, regularly, and moderately densely punctured, progressively finer and sparser towards lateral margin; two longitudinal rows of punctures moderately distinct; highly polished and shining; without microreticulation. Ventral surface: metacoxae and metasternum finely and sparsely punctured, epipleura and abdomen almost without punctures; without microreticulation.
Male. Median lobe of aedeagus in ventral aspect slightly tapered from base to apex and narrowly constricted before apex, apex spoon shaped (Fig. 15A); in lateral aspect slightly curved with an arrowhead shaped apical expansion (Fig. 15B). Paramere narrowly trapezoid in lateral aspect (Fig. 15C, D).
Female. Without secondary sexual characters. Sclerotized spermatheca not found. Variation. Variation of markings is shown in Fig. 10.
Etymology. This species is named after the district name of the type locality.
Habitat. This species was collected in the remaining water pools in a dried-up stream which had a sandy bottom with rich leaf deposits (Fig. 47).

Microdytes ubonensis
Coloration. Head yellowish brown. Pronotum dark brown, indistinctly yellowish brown at lateral sides. Elytron dark brown with yellowish brown markings forming a distinct transverse band at base not reaching suture, with a post-median lateral spot and a triangular spot near apex and often with a distinct longitudinal post-median spot near suture (Figs 6,11). Ventral surface of head and prothorax yellowish brown; thorax, abdominal ventrites and elytral epipleuron reddish brown to dark brown. Legs, antennae and palpi yellowish brown.
Sculpture and structure. Head finely, sparsely, and relatively regularly punctured; almost entirely microreticulate; clypeus not bordered (Fig. 40A). Pronotum quite regularly, sparsely, and fairly strongly punctured, coarser punctures along the posterior margin; without microreticulation; lateral margins very finely bordered, regularly rounded. Elytron moderately regularly and densely punctured, progressively finer and sparser towards lateral margin; longitudinal rows of stronger punctures not distinct; highly polished and shining; without microreticulation. Ventral surface: metacoxae and metasternum finely and sparsely punctured, abdomen without punctures; without microreticulation.
Male. Median lobe of aedeagus in ventral aspect equally tapered from base to apex, with a small swelling at apex (Fig. 16A); in lateral aspect tapered from base to apex, with a small ventral hook at apex (Fig. 16B). Paramere narrow triangular in lateral aspect (Fig. 16C, D).
Female. Without secondary sexual characters. Sclerotized spermatheca not found. Variation. Variation of markings is shown in Fig. 11. Etymology. This species is named after the popular provincial name of the type locality.
Habitat. This species was collected in a variety of habitats. At the type locality, specimens were collected from a small stream on a more or less deforested bedrock ( Fig. 48). At other localities, specimens were collected in small pit holes near small forest streams with rich leaf deposits.

New faunistic records from Thailand, Laos, and Cambodia
Microdytes akitai Wewalka, 1997 Fig. 18 Microdytes akitai Wewalka, 1997: 17;Wewalka 2011: 37;Nilsson and Hájek 2023: 211.  Comments. Specimens from Bolikhamsay have more distinct elytral markings and darker head color than in the specimens from the type locality but no other differences have been observed, in particular, in the male genitalia.
Distribution. Laos: Vientiane and Bolikhamsay (first record) provinces.  (23) Comments. Since the figure of the median lobe of this species was not depicted clearly in Wewalka (1997), we present a new figure of the aedeagus including parameres (Fig. 17).

Microdytes dimorphus
Comments. Microdytes dimorphus was described based on a single male specimen from Khao Yai National Park and no additional records are known so far. Therefore, this is the second record of this species. Wewalka (1997) suggested M. dimorphus is more closely related to M. menopausis Wewalka, 1997 (Fig. 27) but differs by having coarser punctures on the elytra similar in size, a punctured abdomen, a more produced clypeus, and larger second segment of antennae. The specimen examined in this study corresponds with M. dimorphus due to the status of punctures on the elytra and abdomen, although the characteristics of clypeus and antenna are intermediate between M. dimorphus and M. menopausis. This species can be distinguished from M. menopausis in having a more slender body shape and indistinct elytral markings at both shoulders and post-median part (Fig. 20).

Microdytes franzi Wewalka & Wang, 1998
Comments. Male specimens have distinct setae on the metacoxae (Fig. 39A)   Comments. The lectotype of Microdytes maculatus (Fig. 41) has a rhomboid-like habitus similar to that of most specimens from Myanmar, northern Thailand, and China, while most specimens from northeastern Thailand and Laos have a more regularly rounded habitus, but this is not a discriminant character.
There is also a geographic variation in the shapes of the median lobe and the apical part of the parameres. In specimens from Myanmar, northern Thailand, and China, the two parts of the median lobe have obtuse apical medial angles (Fig. 42A), and the parameres have narrow tips (Fig. 43A), while in those of northeastern Thailand and western Laos the two parts of the median lobe have almost right apical medial angles (Fig. 42B) and the parameres have rounded tips (Fig. 43B). Because some specimens show an intermediate status and no other distinguishing character can be found, they Distribution. Thailand: Trat, Nakhon Phanom (first record), Sakon Nakhon (first record), and Ubon Ratchathani (first record) provinces, Khao Yai NP [Nakhon Ratchasima or Nakhon Nayok Province]; Laos: Khammouan, Savannakhet (first record) and Sekong provinces. Wewalka, 2011 Fig. 28 Microdytes paoloi Wewalka, 2011: 24;Nilsson and (Fig. 28).
Distribution. Thailand: Phetchabun and Uttaradit (first record) provinces. Comments. Some specimens from Laos have less distinct elytral markings but no other differences from typical specimens have been observed.

Distribution patterns
The discovery of five new species with three first country records and 40 first regional records from Thailand, Laos, and Cambodia shows how poorly the Dytiscidae fauna of these countries is known. The data on species diversity of Microdytes species are summarized in Table 1 and Fig. 44. A total of 25 Microdytes species were recorded from these three countries. Among the recorded species, 20 are known from Thailand, 17 from Laos, and two from Cambodia; 16 species (64%) are known to occur only from those three countries, whereas nine species (36%) are also recorded in adjacent countries. This result makes Thailand and Laos the most and second-most fauna-rich countries for this genus.
In view of species diversity by ecoregions, LUA has the highest number of species (13 species, 52% of total fauna). This region comprises four species which occur only in one ecoregion. NAN has the second highest number of species (10 species, 40%), including the two new species M. sekaensis sp. nov. and M. ubonensis sp. nov. KAY, NIN, SEI, and CIN also have high numbers of species (8 species, 32%), including M. jeenthongi sp. nov., M. maximiliani sp. nov., M. ubonensis sp. nov., and M. eliasi sp. nov., respectively.
The distribution pattern of Microdytes species recorded from the three countries represents four types: 1) widespread type, occurring throughout continental Southeast Asia to adjacent countries (5 species, 20%); 2) northern type, distributed mainly from northern Thailand and Laos to adjacent countries (9 species, 36%); 3) central type, recorded only from northeastern Thailand to southern Laos (6 species, 24%); 4) eastern type, occurring in northeastern and eastern Thailand and southern Laos (5 species, 20%). The conclusions made above are not definitive because there are still many unexplored areas.

Notes.
Ecoregions where only 1 or 2 Microdytes species were recorded are excluded in the list (i.e., northern Thailand-Laos moist deciduous forests, southern Annamites montane rain forests, and Chao Phraya lowland moist deciduous forests).
Most of the Microdytes specimens examined in this study were collected at lotic habitats associated with running water flowing under primary or secondary forests, and many species were sympatric. At one locality of Tha Pha, northern Thailand, located at Kayah-Karen montane rain forests, four species were collected from the same small stream (Fig. 46). In this stream, M. maculatus, which shows a wide distribution, was found at several sites: at the edge of a water body, among roots of trees in the gaps between stone and gravel. The other species, however, were collected only in restricted areas: M. jeenthongi sp. nov. from a shallow, relatively fast-flowing area where tree roots were exposed at bottom of the river, M. zetteli from calm pool areas associated with slowly running water, and M. pasiricus among gravel under big stones in fast streams. Although our field surveys were conducted only two times at this site, it is worth noting that M. jeenthongi sp. nov. and M. zetteli, which morphologically resemble each other, were collected each time at the same places which are only one meter from each other. This result suggests that microhabitat preferences for some Microdytes species are very strict, and it may lead to the discovery of the richest fauna in this area.
Unlike the northern region of Thailand, where diving beetle surveys have been carried out relatively many times (e.g., by Dr. William D. Shepard, Dr. Manfred A. Jäch, Dr. Herbert Zettel; pers. comm.), a large part of this country remained poorly explored, especially Isan (northeastern region) and the eastern regions. Our study detected three new Microdytes species from these unexplored areas. From the southern region no Microdytes species have been reported so far, although two species (Microdytes elgae Hendrich, Balke & Wewalka, 1995 and M. pasiricus) were recorded from Singapore, situated at the southern end of the Malay Peninsula (Wewalka 1997;Hendrich et al. 2004). Further new species can be expected from Thailand, particularly by collecting in unexplored provinces and repeated sampling at the microhabitat levels.