DSpace at VNU: Three new species of the water strider genus Rhyacobates Esaki, 1923 (Hemiptera: Gerridae) from Vietnam

DSpace at VNU: Three new species of the water strider genus Rhyacobates Esaki, 1923 (Hemiptera: Gerridae) from Vietnam t...

ZOOTAXA ISSN 1175-5326 (print edition)

ISSN 1175-5334 (online edition)

Copyright © 2016 Magnolia Press

Zootaxa 4121 (5): 501–516

http://www.mapress.com/j/zt/ Article

http://doi.org/10.11646/zootaxa.4121.5.1 http://zoobank.org/urn:lsid:zoobank.org:pub:BCADF85B-CB84-4694-963A-AF9F0E3F40D4

Three new species of the water strider genus Rhyacobates Esaki, 1923

(Hemiptera: Gerridae) from Vietnam

A.D TRAN & X.Q NGUYEN

Tran Anh Duc & Nguyen Xuan Quynh, Department of Invertebrate Zoology, Faculty of Biology, Hanoi University of Science (Vietnam National University, Hanoi), 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam E-mail: tran.anhduc@hus.edu.vn

Abstract

Three new species: Rhyacobates zetteli sp.n., R angustus sp.n., and R constrictus sp.n., all from mountainous areas of northern Vietnam, are described With these new taxa, a total of six species of Rhyacobates has been recorded from Vietnam A key to species of Rhyacobates occurring in Vietnam is provided

Key words: Gerridae, Rhyacobates, new species, Vietnam

Introduction

The genus Rhyacobates Esaki, 1923 has its distribution in China, Korea, Taiwan, northern Thailand, northern Myanmar, and northern Vietnam (Andersen & Chen 1995; Tran & Yang 2006) Species of Rhyacobates are strong skaters, occupying fast flowing parts of rivers and streams Although the distribution range of the genus is relatively wide, most species are only known from their type localities, except R takahashii Esaki, 1923 and R abdominalis Andersen & Chen, 1995 Rhyacobates takahashii is endemic to Taiwan but widespread within the island (Andersen & Chen 1995) Rhyacobates abdominalis is found in two disjunct general localities: eastern China (Guangdong Prov.) and northwestern Vietnam (Lao Cai Prov.) (Andersen & Chen 1995; Tran & Yang 2006) Only R malaisei Andersen & Chen, 1995 is found in three countries: China (Yunnan Prov.), Myanmar, and Thailand (Chiang Mai Prov.) (see Andersen & Chen 1995), but these areas are geographically close to each other This fact suggests that our knowledge on the distribution of species of Rhyacobates is still poor and there have not been adequate sampling efforts Andersen & Chen (1995), in their revision of this genus, recognised nine species, excluding Rhyacobates imadatei Miyamoto, 1967 Andersen & Chen (1995) believed that this species did not belong to Rhyacobates due to various morphological inconsistencies with their generic definition Subsequent studies by Polhemus & Zettel (1997), Zettel & Thirumalai (2001a, b), and Chen et al (2005) also acknowledged the incorrect placement of this species in Rhyacobates, but no taxonomic decision was made Eventually, Zettel (2009) placed Rhyacobates imadatei Miyamoto in a genus of its own, Celerobates Zettel, 2009

Tran & Yang (2006) provided the first records of Rhyacobates from Vietnam, including descriptions of two new species, R anderseni Tran & Yang, and R gongvo Tran & Yang The present paper reports three more species from Vietnam, all new to science and they are described herein Thus, to date, the total number of described species

of the genus has been raised to 14

Material and methods

Materials used in this study have been deposited in the following museums or collections:

NHMW Natural History Museum, Vienna (Austria)

ZMHU Zoological Collection of the Biological Museum, Hanoi University of Science (Vietnam)

ZRC Zoological Reference Collection, Lee Kong Chian Natural History Museum, National University of

Singapore (Singapore)

Material was studied with binocular microscopes Measurements were mainly made with a stage micrometre attached to oculars of the microscopes Lengths of body segments (thoracic or abdominal) are measured along the midline All measurements are given in millimetres Line drawings were made with the help of a camera lucida fixed to the microscopes Some habitus illustrations were taken with a digital SLR camera using macro lenses

Taxonomy

Rhyacobates Esaki, 1923

Rhyacobates Esaki, 1923: 387 (type species: Rhyacobates takahashii Esaki, 1923); Hungerford & Matsuda, 1960: 8 (key to genus); Matsuda, 1960: 273–276

Esakobates Lundblad, 1934: 22 (type species: Esakobates svenhedini Lundblad, 1934); (synonymized by Hungerford & Matsuda, 1959: 69)

Key to species of Rhyacobates occurring in Vietnam

1 Both sexes: mesonotum totally black, without yellow marking Female: abdominal sternum 7 without connexival process, connexival and posterolateral margins expanded, folded inwards and covering dorsum of genitalia (Figs 34–39) R constrictus sp.n.

- Both sexes: mesonotum with median yellow marking Female: abdominal sternum 7 with connexival process, connexival and posterolateral margins not modified as above 2

2 Male: mesonotum with slender yellow median stripe only on posterior half Female: posterior margin of metanotum with a conspicuous median process; abdomen short, ventral length of abdomen about 0.2 times body length R anderseni

- Male: mesonotum with yellow median marking distinctly longer than half the length of mesonotum Female: metanotum without median process; abdomen longer, ventral length of abdomen about 0.4–0.5 times body length 3

3 Body length of male at least 7.3 mm, of female at least 9.8 mm Female: tergum 7 with median process on posterior margin 4

- Body length of male at most 7.0 mm, of female at most 10.0 mm Female: tergum 7 with posterior margin straight, without median process 5

4 Male: pronotum chiefly yellow; mesonotum with broad, sub-oval yellow median marking; metanotum with broad crown-shaped yellow marking (Fig 12) Female: sternum 7 with short connexival processes; posterior margin of sternum 7 bilobate (Figs 21, 22) R zetteli sp.n.

- Male: both pronotum and mesonotum with slender yellow median marking; metanotum without yellow marking Female: sternum 7 with longer and sub-triangular connexival processes (Figs 8, 10); posterior margin of sternum 7 without lobe, broadly convex (Fig 9) R angustus sp.n.

5 Male: yellow stripe on mesonotum about three quarters of mesonotum length, not reaching anterior margin; paramere without hook-shaped apex Female: abdomen straight, abdominal segment 7 with two long dorsal processes, posterior ventral margin straight and without median projection R gongvo

- Male: yellow stripe on mesonotum almost as long as mesonotum length; paramere with hook-shaped apex Female: distal half

of abdomen curved upwards; abdominal segment 7 without long dorsal process, posterior ventral margin with small and pointed median projection R abdominalis

Rhyacobates angustus sp.n

(Figs 1–10)

Material examined Holotype (apterous female) and allotype (apterous male),VIETNAM, Lao Cai Prov., Sa Pa, Ban Ho, Ban Den, Nam Pu stream (feeder stream of Muong Hoa stream), 22°15.709’N 103°58.054’E, 416 m asl., coll Tran A.D et al., 29 May 2013, TAD1316 (ZMHU)

Paratypes: 2 males (apterous), same locality data as holotype (ZMHU)

Description Size: apterous morph, males, length 7.7–8.1 (allotype 8.0), width 2.26–2.40 (allotype 2.40); female (holotype), length 11.3, width 2.95

Colour: Dorsum of body mainly black, covered with silvery pubescence, especially on thoracic pleura, metacoxa and abdomen Dorsum of head yellow with median, longitudinal brown mark on anterior three quarters

Antennae mainly black, segment 4 with whitish area in distal two-fifths Pronotum mainly black with median yellow mark (in female, width of mark broad, about one third of pronotum mark; in male, yellow mark narrow) Mesonotum black with median yellow mark from anterior fifth to posterior margin; median mark of female broader than in male Metanotum black, in female with median yellow marking, in male without yellow mark Abdominal terga of male chiefly black or brown Abdominal terga of female mainly black or brown, with yellow connexiva Venter of body covered with silvery and golden pubescence Venter of female mainly light yellow, except mesosternum and mesopleuron black on anterior part, with sub-triangular yellow on posterior part of mesosternum Venter of male similar to female, but triangular yellow mark on mesosternum narrower, abdominal venter dark yellow All acetabulae, coxae and trochanters yellow Fore femur yellow with light brown dorsal stripe on distal part Middle and hind femora brown at basal part, dark brown or black at distal part Fore tibia yellow-brown and basal part, dark yellow-brown at distal part; other tibiae and all tarsi dark yellow-brown or black

Apterous female (holotype): Body long and relatively slender Head width 1.60, interocular width 0.65, eye length (dorsal view) 0.65 Lengths of antennal segments 1–4: 4.95: 1.39: 1.54: 1.16 Pronotum broader than long, length 0.80, width 1.71; mesonotum length 2.72, width 2.72; metanotum length 0.79 Thoracic pleura with numerous long erect bristle-like black hairs, especially on mesopleuron All coxae with rings of dark bristle-like hairs at apical margins Fore trochanter with long hairs on ventral side Lengths of leg segments (femur: tibia: tarsus 1: tarsus 2): fore leg: 5.10: 4.75: 2.38: 1.10; middle leg: 13.80: 7.90: 3.30: 0.47; hind leg: 13.60: 5.69: 0.18: 0.27 Fore femur slender (width 0.51), venter of femur with row of dark bristle-like hairs and scattered soft long hairs, sub-apical part slightly constricted with small tooth-like elevation (Fig 1) Abdomen prolonged, narrower than thorax, ventral length 5.38, gradually tapering towards apex (Figs 7, 8) Tergum 1 posteromedially swollen, forming a semi-circular tumescence (Fig 8) Connexival margin of segments 1–6 thickened Abdominal segment 7 prolonged, almost completely enclosing genital segments, tergum 7 medially produced on posterior margin; sternum 7 (length 1.99) about 1.8 times as long as tergum 7 (length 1.12); connexival margin of sternum 7 about two thirds ventral length of sternum 7; posterior margin of sternum 7 broadly convex and thickened; each posterolateral corner of sternum 7 with broad, sub-triangular process (Figs 8, 10)

Apterous male (allotype): Head width 1.44, interocular width 0.64, eye length 0.61 Lengths of antennal segments 1–4: 4.80: 1.33: 1.48: 1.10 Pronotum broader than long, length 0.77, width 1.64; mesonotum length 2.16; metanotum length 0.68 Thoracic pleura with numerous long erect bristle-like black hairs, especially on mesopleuron Lengths of leg segments: fore leg: 4.95: 4.35: 1.30: 0.81; middle leg: 13.50: 7.20: 3.00: 0.47; hind leg: 13.25: 4.60: 0.16: 0.23 Fore femur slender (width 0.50) with few scattered soft long hairs in venter, sub-apical part with small tooth-like elevation Middle trochanter without spines, middle femur with many spines, those at basal fifth of femur not arranged in distinct row and those at distal four fifths arranged in distinct row (Fig 2) Other leg structures similar to those of female Abdomen relatively short, ventral length 3.13 Length of sternum 7: 0.31, posterior margin slightly concave Genitalia: directed slightly downwards; venter of abdominal segment 8 length (in-situ) 0.20; proctiger laterally widened forming a rounded lobe on each side (Fig 3); pygophore of moderate size, apical margin rounded; paramere relatively long, curved at basal third, slightly thickened at middle, apex narrow and directed inwards, setae not distinct (Figs 4, 5); endosomal sclerites as in Fig 6

Macropterous morph unknown

Etymology The species epithet refers to the slender appearance of the female body

Remarks Rhyacobates angustus sp.n differs from all of its congeners in the combination of following characters: the body in dorsal view is generally slender, especially in the female; the abdomen of the female is straight (Figs 7, 8); abdominal segment 7 of the female is prolonged and in lateral view, tapers apically (Figs 8– 10), each posterolateral corner of sternum 7 is produced into a broad, sub-triangular process (Figs 8, 10), the posterior margin of sternum 7 is broadly convex and thickened, without a median process (Fig 9); the apex of the male paramere is curved and directed inwards (Fig 5) For comparison with its congeners, see Remarks for Rhyacobates zetteli sp.n

Habitats Specimens of R angustus sp.n were found in a open mountain stream at an elevation of above ca

400 m They were found living sympatrically with R gongvo, a more common species in the area (Fig 40) After several searches and additional field trips, we could not find further specimens of this new species Therefore, we are unable to determine whether this new species is uncommon in the area or if this was an atypical habitat and it is more common elsewhere

Distribution Vietnam: Lao Cai

FIGURES 1–10 Morphological features of Rhyacobates angustus sp.n (1) right fore leg of female (2) basal part of right middle leg of male, ventral view (3) proctiger of male (4, 5) left paramere, from different views (6) endosomal sclerites (7) body of female, lateral view (8) abdomen of female, dorsal view, (9, 10) apex of abdomen of female, ventral and lateral views

Rhyacobates zetteli sp.n.

(Figs 12–23)

Material examined Holotype (apterous female) and allotype (apterous male), Vietnam, Lao Cai Prov., Sa Pa, Hoang Lien N'Park, Nui Xe, Suoi Vang, 22°20.835’N 103°46.446’E, 1366 m asl., coll Tran A.D., 4 July 2004, TAD0417 (ZMHU)

Paratypes: VIETNAM: Lao Cai Prov.: 11 males, 13 females (apterous), same locality data as holotype (ZMHU, ZRC, NHMW); 1 female (macropterous, de-alated), Sa Pa, Hoang Lien N'Park, Nui Xe, a small water flow, 22°21.110’N 103°46.216’E, 1978 m asl., coll Tran A.D., 4 July 2004, TAD0418 (ZRC); 1 female (apterous),

Sa Pa, Nui Xe, Tram Ton area, feeder stream of Vang stream, 22°20.020’N 103°46.223’E, 1904 m als., coll Tran A.D et al., 25 October 2013, TAD1357 (ZMHU)

Description Size: males (apterous), length 7.30–7.70 (allotype 7.55), width 2.44–2.67 (allotype 2.54); females, length 9.8–10.6 (holotype 10.5), width 3.04–3.58 (holotype 3.41) (apterous), length 10.3, width 3.36 (macropterous, de-alated)

Colour (Figs 12, 13): Dorsum of body mainly black or brown, covered with golden and silvery pubescence Head yellow with 2 (male) or 3 (female) small black markings dorsally Antennae mainly black, segment 4 with whitish area in distal two-fifths Pronotum mainly yellow with black outer margin Pronotum in macropterous form (female) with black anterior margin, anterior part (before pronotal lobe) yellow, pronotal lobe black with thin longitudinal yellow stripe, lateral and posterior margins yellow Mesonotum in apterous form black with broad yellow median marking from anterior to posterior margins; median marking of females broader than in males Metanotum with expanded yellow marking (broader in females) Abdominal terga mainly black or brown, with yellow-brown connexiva Venter of female mainly light yellow, except anterior half of mesosterno-pleuron black or brown, with median light yellow marking confluent with light coloured posterior part Venter of male, mesosterno-pleuron mainly black or brown, with long light yellow triangular marking on posterior half, abdominal venter light yellow All coxae and trochanters yellow Fore femur yellow with light brown dorsal stripe Middle and hind femora yellow-brown at basal part, dark brown or black at distal part All tibiae and tarsi dark brown or black Apterous female (holotype): Head width 1.56, interocular width 0.76, eye length (dorsal view) 0.63 Lengths

of antennal segments 1–4: 4.02: 1.08: 1.49: 1.12 Pronotum broader than long, length 0.76, width 1.86; mesonotum length 1.51, metanotum length 0.56 All coxae with rings of dark bristle-like hairs at apical margins Fore trochanter with long hairs on ventral side Lengths of leg segments (femur: tibia: tarsus 1: tarsus 2): fore leg: 4.48: 4.07: 2.01: 0.98; middle leg: 12.30: 7.65: 4.17: 0.46; hind leg: 12.50: 6.75: 0.22: 0.32 Fore femur slender (width 0.44), with row of 11 long dark bristle-like hairs and scattered soft long hairs on venter, sub-apical part with small tooth-like elevation (Fig 14) Abdomen (Figs 20, 21) straight, prolonged, ventral length 5.10 Connexival margin narrow Each connexival corner of segment 6 with small blunt process (Fig 21) Abdominal segment 7 prolonged, almost completely enclosing genital segments, tergum 7 medially produced on posterior margin; sternum 7 (length 1.68) slightly more than 1.5 times as long as tergum 7 (length 1.02), posterior margin of sternum 7 bilobate; posterolateral corner of abdominal segment 7 with small blunt process (Figs 20, 21), sometimes bent inwards thus not visible in lateral view (Fig 23)

Apterous male (allotype): Head width 1.45, interocular width 0.70, eye length 0.60 Lengths of antennal segments 1–4: 4.02: 1.06: 1.56: 1.11 Pronotum broader than long, length 0.73, width 1.68; mesonotum length 2.08, metanotum length 0.69 Lengths of leg segments: fore leg: 4.41: 3.82: 1.27: 0.91; middle leg: 12.00: 7.10: 3.72: 0.46; hind leg: 11.70: 7.55: 0.21: 0.30 Fore femur with similar pubescence as in apterous female, sub-apical part also with small tooth, width of fore femur 0.51 Middle trochanter with 6–7 spines at distal part, middle femur with spines but not in distinct row (Fig 15) Other leg structures similar to those of female Abdomen relatively short, ventral length 2.74 Length of sternum 7: 0.43, posterior margin straight Genitalia: directed slightly downwards; venter of abdominal segment 8 length (in-situ) 0.24; proctiger relatively widened, with broadly angular lobes laterally (Fig 16); pygophore of moderate size, apical margin almost straight; paramere relatively long and slender, curved at distal two-thirds, setae not distinct (Figs 17, 18); endosomal sclerites as in Fig 19

De-alated macropterous female: Head width 1.52, interocular width 0.78, eye length 0.67 Lengths of antennal segments 1–4: 4.12: 1.11: 1.52: 1.22 Pronotum covering most of mesonotum, length 3.23, width (across humeri) 2.71; metanotum length 0.62 Lengths of leg segments: fore leg: 4.31: 4.02: 1.98: 1.03, width of fore femur 0.44; middle leg: 12.30: 7.60: 4.17: 0.48; hind leg: 12.40: 6.60: 0.21: 0.29 Other structural characteristics similar to apterous females

Macropterous male: unknown.

Etymology This species is dedicated to Dr Herbert Zettel (Natural History Museum Vienna) for his great contribution to the knowledge of aquatic bugs of Southeast Asia, and for enthusiastically supporting the first author

in the field of water bug research

FIGURES 11–13 (11) Habitat of type locality of Rhyacobates zetteli sp.n (12, 13) Habitus of R zetteli sp.n (12) apterous male (13) apterous female (photographed by Tran Anh Duc)

FIGURES 14–23 Morphological features of Rhyacobates zetteli sp.n (14) right fore leg of female (15) basal part of right middle leg of male, ventral view (16) proctiger (17, 18) left paramere, two different views (19) endosomal sclerites (20) body

of female, lateral view (21–23) apex of abdomen of female, dorsal, ventral, and lateral views respectively

Remarks Rhyacobates zetteli sp.n differs from all of its congeners in the combination of following characters: the unique dorsal colour pattern of the body (Figs 12, 13); the abdomen of the female is straight; each connexival corner of segment 6 has a small, blunt process (Fig 21); abdominal segment 7 of the female is prolonged and in lateral view, tapers apically (Figs 21–23), the connexival margin of sternum 7 is much shorter

than the ventral length (Figs 21, 23); the connexival corner of sternum 7 of the female is produced into a small blunt process (Fig 21), the posterior margin of sternum 7 is bilobate, but without a median process (Fig 22) The shape of the paramere of this new species is similar to that of R gongvo, but males of the latter can be separated from this new species by other characteristics, i.e., colouration, shape of the proctiger and endosomal structures Both new species, Rhyacobates angustus sp.n and R zetteli sp.n., are closely related to each other, as both taxa possess the prolonged and rather straight abdomen of the female, the posteromedian process on tergum 7 of the female, and the sub-apical tooth-like elevation on the fore femur of the female In the subfamily Ptilomerinae, similar modification on the posterior margin of female tergum 7 is previously known only in the genus Stridulobates Zettel & Thirumalai, 2001a (see Zettel & Thirumalai 2001a: Figs 15, 16) However, in species of Stridulobates, tergum 7 of the female is prolonged and covers most of tergum 8, whereas in R angustus sp.n and

R zetteli sp.n., tergum 7 is shorter and covers only a small anterior portion of tergum 8 Other characteristics, e.g., the absence of a metanotal process in females, the lack of a "stridulatory device" on the middle trochanter and sterna 2–6 in males, also suggests that R angustus sp.n and R zetteli sp.n do not belong to Stridulobates Additionally, males of R angustus sp.n and R zetteli sp.n clearly differ from males of Stridulobates in the shape

of the parameres and structure of endosoma (see Matsuda 1960: Figs 677, 679; Zettel & Thirumalai 2001a: Figs 9, 10)

Rhyacobates angustus sp.n and R zetteli sp.n can be easily separated from each other by the yellow median markings on the nota, the shape of the male proctiger and paramere, and the structures of female sternum 7 In R angustus sp.n., yellow markings on the pro- and mesonota are slender (the metanotum of the male is totally black and without yellow marking, the metanotum of the female is with very slender with a yellow median marking; the male proctiger has a rounded lobe on each side) (Fig 3); the apex of the male paramere is more abruptly narrowed (Figs 4, 5), sternum 7 of the female has large, sub-triangular connexival processes and a broadly-arched posterior margin (Figs 8–10) In R zetteli sp.n., yellow markings on the nota of both sexes are broad; the male proctiger has

a broadly angular lobe on each side; the male paramere gradually tapers apically (Figs 17, 18); sternum 7 of the female has small blunt connexival processes (Fig 21), the posterior margin of sternum 7 is bilobate (Fig 22) Habitats Most specimens of Rhyacobates zetteli sp.n were collected in a shaded and fast flowing mountain stream at an elevation of over 1800 m (Fig 11) They were found skating in schools of numerous individuals: adults and nymphs in a mixture of which the nymphs were predominant Seasonal fluctuation in population size and ratios of nymphs to adults are still unknown The individuals of this species were also found often resting on partially submerged rocks in the middle the stream, as do other species of Rhyacobates (Esaki 1923; Tran & Yang 2006) One macropterous specimen (with a de-alated wing) was found in an unshaded, tiny water flow (locality TAD0418), which continues underground and was ca 500 m from the main stream (locality TAD0417) None of the specimens found in the main stream were the macropterous form This observation suggests that this species has good flying ability (probably for dispersal), but upon migrating, the macropterous individuals were likely to have their wings broken off

Distribution Vietnam: Lao Cai

Rhyacobates constrictus sp.n

(Figs 25–39)

Material examined Holotype (apterous female) and allotype (apterous male), VIETNAM, Phu Tho Prov., Xuan Son N’Park, Lap stream, site 1, at Ngoc waterfall, 21°07.817’N 104°55.506’E, 390 m asl., coll Tran A.D et al., 5 June 2013, TAD1331 (ZMHU)

Paratypes: VIETNAM: Phu Tho Prov.: 8 males, 7 females (apterous), 1 female (macropterous, de-alated), same locality data as holotype (ZMHU, ZRC); 2 males, 3 females (apterous), Xuan Son N’Park, Lap stream, site 2, first concrete bridge from Ngoc waterfall, 21°08.008’N 104°55.768’E, 330 m asl., coll Tran A.D et al., 5 June

2013, TAD1332 (ZMHU); 1 female (apterous), Xuan Son N’Park, Lap stream, site 3, second concrete bridge from Ngoc waterfall, 21°08.299’N 104°56.26’E, 268 m asl., coll Tran A.D et al., 5 June 2013, TAD1333 (ZMHU); 1 male (apterous), Xuan Son N’Park, Kim Thuong, Tan Ong stream, site 2, ca 2km from Chin Tang waterfall, 21°04.201’N 104°56.919’E, 340 m asl., coll Tran A.D et al., 28 August 2013, TAD1341 (ZMHU); 2 females (apterous), Xuan Son N’Park, Lap stream, site 1, at Ngoc waterfall, coll Tran A.D et al., 30 August 2013,

TAD1352 (GPS data same as TAD1331) (ZMHU); 2 males, 2 females (apterous), Xuan Son N’Park, Lap stream, site 2, first concrete bridge from Ngoc waterfall, coll Tran A.D et al., 30 August 2013, TAD1353 (GPS data same

as TAD1332) (ZMHU); 5 males, 2 females (apterous), Xuan Son N’Park, Lap stream, site 3, second concrete bridge from Ngoc waterfall, coll Tran A.D et al., 30 August 2013, TAD1354 (GPS data same as TAD1333) (ZMHU, NHMW); 3 males, 1 female (apterous), Xuan Son N’Park, Lap stream, site 1, at Ngoc waterfall, coll Nguyen T.S et al., 27 August 2014, TS1409 (GPS data same as TAD1331) (ZMHU); 2 males, 2 females (apterous), Xuan Son N’Park, Lap stream, site 2, first concrete bridge from Ngoc waterfall, coll Nguyen T.S et al.,

27 August 2014, TS1410 (GPS data same as TAD1332) (ZMHU); 1 male, 2 females (apterous), Xuan Son N’Park, Lap stream, site 2, first concrete bridge from Ngoc waterfall, coll Nguyen T.S et al., 23 May 2015, TS1512 (GPS data same as TAD1332) (ZMHU); 2 males (apterous), Xuan Son N’Park, Lap stream, site 2, first concrete bridge from Ngoc waterfall, coll Nguyen T.S et al., 26 August 2015, TS8512 (GPS data same as TAD1332) (ZMHU) Description Size: apterous morph: males, length 6.90–7.10 (allotype 6.95), width 2.20–2.30 (allotype 2.24); females, length 8.7–9.4 (holotype 9.0), width 3.08–3.20 (holotype 3.20); macropterous morph (de-alated): female, length 9.10, width 3.23

Colour (Figs 25, 26): Dorsum of body mainly black, covered with silvery pubescence, especially dense on thoracic pleura, metacoxa and abdomen Dorsum of head yellow with median, longitudinal brown mark on anterior three quarters Antennae mainly dark brown, segment 4 with whitish area in distal two-fifths Pronotum mainly black with median yellow mark (in female, width of mark about one third of pronotum mark; in male, yellow mark narrower), anterolateral corners of pronotum yellow Mesonotum and metanotum black, without yellow mark Abdominal terga chiefly black, except abdominal apex yellowish and brown (in proctiger and pygophore of male; and in abdominal segment 7 and genitalia of female) In both sexes, venter of body covered with dense silvery pubescence Venter of female mostly yellow, except mesosternum, mesopleuron and anterior third of metasternum black Venter of male with mesosternum, mesopleuron and metasternum black, abdominal sternum 2 black or brown, the remaining parts yellow All acetabulae, coxae and trochanters yellow Fore femur yellow with brown longitudinal stripe dorsally Middle and hind femora yellow at basal part, dark brown at distal part Fore tibia and tarsus dark brown; other tibiae and tarsi yellowish brown

Apterous female (holotype): Head width 1.43, interocular width 0.57, eye length (dorsal view) 0.63 Lengths

of antennal segments 1–4: 4.30: 1.21: 1.50: 1.09 Pronotum broader than long, length 0.81, width 1.53; mesonotum length 2.56, metanotum length 1.07 Mesopleuron with scattered long erect bristle-like brown hairs All coxae with rings of dark bristle-like hairs at apical margins Fore trochanter with long hairs on ventral side Lengths of leg segments (femur: tibia: tarsus 1: tarsus 2): fore leg: 4.35: 3.55: 2.00: 0.94; middle leg: 11.50: 6.70: 4.10: 0.44; hind leg: 11.80: 5.19: 0.14: 0.22 Fore femur slender (width 0.42), venter of femur with row of 11 dark bristle-like hairs, with scattered soft long hairs, sub-apical part with small tooth-like elevation, sometimes indistinct (Fig 27) Abdomen relatively short, ventral length 3.85; abdominal apex directed dorsocaudad (Fig 34) Tergum 1 posteromedially swollen (Figs 36) Connexival margin of segments 1–5 narrow, of segment 6 thickened Abdominal segment 7 prolonged, completely enclosing genital segments, tergum 7 short (length 0.45), posterior part thickened, medially notched on posterior margin; sternum 7 (length 1.79) about 4.0 times as long as tergum 7; connexival and posterolateral margins expanded dorsad, folded inwards, meeting in middle of abdominal dorsum (holotype, Fig 36) or overlapping each other (Fig 38), thus covering most of genital segments; sternum 7 tapering caudad, posterior margin with narrow median process, apex of process narrowly rounded (Figs 36–39)

Apterous male (allotype): Head width 1.32, interocular width 0.49, eye length 0.65 Lengths of antennal segments 1–4: 3.65: 1.10: 1.50: 1.06 Pronotum broader than long, length 0.72, width 1.41; mesonotum length 2.13, metanotum length 0.79 Mesopleuron with scattered long erect bristle-like brown hairs Lengths of leg segments: fore leg: 3.90: 3.20: 0.98: 0.69; middle leg: 10.30: 5.81: 3.32: 0.40; hind leg: 10.40: 4.00: 0.12: 0.18 Fore femur slender (width 0.45), with row of seven bristle-like hairs and few scattered soft long hairs on venter, sub-apical part with small tooth-like elevation Middle trochanter without spines, middle femur with scattered small spines, not arranged in distinct row (Fig 28) Other leg structures similar to those of female Abdomen relatively short, ventral length 2.15 Length of sternum 7: 0.30, posterior margin slightly concave Genitalia: slightly directed downwards; venter of abdominal segment 8 length (in-situ) 0.15; proctiger laterally widened forming broad, angular lobe on each side (Fig 29); pygophore of moderate size, apical margin rounded; paramere relatively long, curved at basal fourth, strongly thickened at middle, then tapering towards narrow apex, apex slightly directed inwards, setae mostly on distal part (Figs 30–32); endosomal sclerites as in Fig 33

FIGURES 24–26 (24) Habitat of type locality of Rhyacobates constrictus sp.n (25) A copulating pair of R constrictus sp.n., skating on the fast current of the stream (26) A copulating pair of R constrictus sp.n., resting on the emerged rocks in the middle of the stream (photographed by Tran Anh Duc)

De-alated macropterous female: Head width 1.46, interocular width 0.56, eye length 0.67 Lengths of antennal segments 1–4: 4.25: 1.17: 1.42: 1.08 Pronotum covering most of mesonotum, length 3.35, width (across humeri) 2.56; metanotum length 0.83 Lengths of leg segments: fore leg: 4.35: 3.60: 1.91: 0.95; middle leg: 11.50: 6.80: 4.15: 0.46; hind leg: 12.10: 5.30: 0.14: 0.23; width of fore femur 0.44 Other structural characteristics similar to apterous females

Macropterous male: unknown

Etymology The species epithet constrictus refers to the posteriorly constricted abdominal sternum 7 of the female, which holds genital segments within