A new species of Onychotrechus Kirkaldy, 1903 (Hemiptera, Heteroptera, Gerridae) from Dooars, West Bengal, India, and a key to males of all species

A new species of hygropetric water strider, Onychotrechus dooarsicus sp. n. is described from Buxa Tiger Reserve, Dooars, West Bengal, India. Representatives of the new species were collected from a stream with boulders and rocks splashed by water, with slow current, situated at an altitude of 630 m a.s.l. This is the seventh species of Onychotrechus confirmed for India. The new species is closely related with O. robustus Andersen, 1980 from northwestern India and O. jaechi Zettel & Tran, 2007 from Bhutan. An identification key to males of all species of Onychotrechus species is also provided.


Introduction
The hygropetric water striders of the genus Onychotrechus Kirkaldy, 1903 are members of the subfamily Eotrechinae and chiefly distributed on the Indian subcontinent and Sri Lanka (11 species, including the new one), but two species inhabit southeastern Asia (Andersen 1980).They can be recognized by the small size (body length less than 7.4 mm); middle and hind legs with subequal lengths, first tarsomere shorter than second, long pre-apically inserted claws, and ventral row of stout spine-like setae; and genital segments of males small, with relatively simple structure.
Species of Onychotrechus are mostly found resting, walking, running or jumping on the wet seeping rock faces covered with algae or splashed by water of small mountainous streams.Both nymphs and adults, including copulating individuals, can be encountered in their typical habitats.Although their coloration appears vivid, it is cryptic and makes them difficult to recognize in the dark wet rocks.The hygropetric way of life is a secondary adaptation evolved from life on water surface in this group (Andersen 1980).
In a recent expedition to the Buxa Tiger Reserve, near the Indo-Bhutan border of West Bengal, the second author collected a specimen, that is herein described and named as Onychotrechus dooarsicus sp.n.

Material and method
Specimens were collected by a short-handled aquatic net and preserved in 70% ethyl alcohol (C 2 H 5 OH).Samples were identified using a binocular microscope Leica M205A and body parts were photographed.Male genitalia was dissected and kept in 10% KOH for 30 minutes to clear the hard sclerotized structure and drawings were prepared using a camera lucida attached to the Leica M205A binocular microscope.Identified specimens were deposited in the National Zoological Collections of Zoological Survey of India, Kolkata.All measurements are given in millimetres.Description of apterous male (Fig. 1).Size: Body length (from head tip to tip of proctiger) of apterous male holotype 3.30, male paratype 3.27; maximum width of body across mesoacetabula 1.12 (holotype), 1.07 (paratype).

Onychotrechus dooarsicus
Colour: Body dorsally black with yellow markings.Head with a median yellow longitudinal stripe, which ends in a blunt arrow head (Fig. 3); head anteriorly and along medial and posterior eye margins yellow.Antenna and leg segments yellowish brown.Pronotum with two broad yellow sub-lateral stripes, a thin median yellow stripe, and two yellow curved lateral stripes.Meso-and metanotum (Fig. 4) with two thin lateral stripes, two broad sub-lateral stripes and an indistinct short yellow median line.Abdominal tergites completely black (Figs 4,12).Forefemur yellow with one broad dark band on dorsal side and one very slender dark brown band on extensor side.Venter yellow.Mesosternum (Fig. 6) with paired black patches behind forecoxae and metasternum posteriorly with an almost W-shaped broad dark region.Abdominal sternites pale yellowish brown (Fig. 13).
Pronotum 0.84 mm in width; pronotum length 0.53, width 0.79.Combined length of mesonotum and meta-notum 1.12, width 0.98.Mesosternum (Fig. 6) modified, with narrow median groove slightly widened posteriorly; depression with black, scattered setae directed to its centre; posterior swelling with densely arranged long stout black setae covering base of mesosternum.Forefemur (Fig. 8) slender, widened basally, but almost evenly tapering apically; length of forefemur 1.17, width 0.25; forefemur apically with a few setae and with short dark stiff setae distributed throughout.Foretibia (Fig. 9) strongly curved, basally with patch of few short setae on flexor side; a soft spinous structure protruded outwards from base of curvature; apical region with two prominent black spines that diverge from almost touching bases (forming a 'V'), and with four black spines and a row of short setae towards extensor side.Mid and hind femora slender, each about 1.1 times as long as body; both with distinct rows of short spines arranged equidistantly.Foreclaws (Fig. 11) sharply bent and slightly longer than mid and hind claws.Measurements of leg segments provided in Table 1.
Abdomen (Figs 12,13) short, only about one third of body length.Length of abdominal sternites 1.01, width 0.90.Sterna II-VI visible as very narrow curved strips; sternum VII (Fig. 13) much longer; sterna II-VII without median groove.Segment VIII broad, with median depression, posterior part with several setae, bluntly ending.
Colour: Almost identical to male, with the following exceptions: posterior yellow mid-line lacking from metanotum (Fig. 5).Venter yellow, with two broad markings on mesosternum.Metasternum brownish yellow.Abdominal sterna II-VI yellowish brown, paler at mid-line and sides; sternum VII black anteriorly, yellowish posteriorly.
Macropterous male and female.Unknown.Comparative notes.Onychotrechus dooarsicus sp.n. is probably a close relative of O. jaechi, recently described from Bhutan.They share some common characters: males have almost identical pro-, meso-and metasternal markings, basally incrassate forefemora, similar mesosternal modifications and short abdominal segments.However, the male of O. dooarsicus sp.n. distinctly differs from O. jaechi by the following characteristics: 1) The foretibia of O. dooarsicus sp.n. bears a patch of few short setae basally on flexor side, and a soft spinous structure protruding outwards from the base of the curvature; its apical region bears two prominent black spines (forming a 'V'), four black spines and a row of short setae towards extensor side; whereas, in O. jaechi its base bears a patch of long hairs on the flexor side and the distal modifications are absent.2) The mesosternum of O. dooarsicus sp.n. has a median narrow groove that is slightly widened posteriorly, whereas O. jaechi has a strongly modified mesosternum, with a medial, posteriorly widened depression and a prominent transverse swelling  Figures 6-13.Onychotrechus dooarsicus sp.n. 6. Mesosternum of male, ventral view.7. Lateral view of female.8. Forefemur of male.9. Foretibia and tarsi of male.10.Foreleg of female.11.Foreleg claws in male.12. Abdominal tergites of male.13.Abdominal sternites with genital segments in male.
at its hind margin.3) In O. dooarsicus sp.n., the paramere has a notch at mid-length, that is absent from O. jaechi.
The female of O. jaechi is unknown.
Etymology.The specific epithet 'dooarsicus' comes from its place of occurrence, 'Dooars' of West Bengal.Dooars is a large region, forming the gateway from India to Bhutan and stretching from the plains of Darjeeling District, Jalpaiguri District, and the upper regions of Cooch Behar District of West Bengal to some parts of Assam.This part mainly consists of Himalayan foothills.
Habitat.Representatives of this species have been found in the shallow zone between the rocks, splashed by a slow-flowing stream (630 m), known as Buxa Jhora, located near Buxa fort of Buxa Tiger Reserve Range.

Key to Onychotrechus species (males)
Note: The authors were able to study male specimens of most species, but not of O. baijali and O. singalensis.For these species characters were taken from the original descriptions.

Discussion
The genus Onychotrechus shows its maximum diversity in southern India (Kerala, Karnataka, Tamilnadu) (Andersen 1980).In contrast, Onychotrechus dooarsicus sp.n. is the first species reported from north-eastern India.This new species was collected from the banks of a stream at an altitude of e 630 m in the Buxa Tiger Reserve Forest, which falls within the Jalpaiguri subregion of the Himalayas.It is expected that the Eastern Himalayan states of Indiaincluding the Darjeeling Himalayas and the Himalayan foot-hills -harbor further undiscovered species of Onychotrechus and other eotrechine water striders that could be described if an extensive faunal survey was carried out.Due to their cryptic habits, specialized collecting is necessary to find the hygropetric species of Onychotrechus or Eotrechus that are usually overlooked when doing general surveys of aquatic insects.
Species identification of eotrechine water striders is usually based on the strongly modified genitalia of the males (see, e.g., Andersen 1982 for Eotrechus and Chimarrhometra; Polhemus and Andersen 1984 for Amemboa and Amemboides), whereas the identification of females is often extremely difficult or -as in Amemboa -even impossible so far.In contrast with related genera, in males of Onychotrechus male genitalia is relatively simple and provide few differentiating characters (Andersen 1980).For Onychotrechus, Andersen (1980) provided one key to both sexes, although characteristics of males forelegs and abdomen are dominating.However, the starting couplet of his key using the relative lengths of claws and tarsal segments -proved to be problematic, because the differences are small and subject to intraspecific variation.For this reason, and also because some additional species were described afterwards (Gupta 1982;Zettel and Tran 2007; this paper), we present a new key only for males, but with the advantage of a clearer discrimination of the species.Special attention is recommended to the inexperienced researcher who wants to identify females not associated with males.

Table 1 .
Measurements (value in mm) of leg segments of apterous males and female of Onychotrechus dooarsicus sp.n.