Rhyacobates angustus, Tran, A. D. & Nguyen, X. Q., 2016

Tran, A. D. & Nguyen, X. Q., 2016, Three new species of the water strider genus Rhyacobates Esaki, 1923 (Hemiptera: Gerridae) from Vietnam, Zootaxa 4121 (5), pp. 501-516 : 502-503

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 yellow-brown at basal part, dark brown or black at distal part. Fore tibia yellowbrown and basal part, dark brown at distal part; other tibiae and all tarsi dark 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

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.

FIGURES 40 – 42. (40) Habitat of Rhyacobates angustus sp. n. and R. gongvo. (41) Apterous female of R. gongvo resting on the exposed rock in the middle of the stream. (42) School of adults and nymphs of R. gongvo on the exposed rock in the middle of the stream (40: photographed by Nguyen Thanh Son; 41, 42: photographed by Tran Anh Duc).