Water striders , the genus Cylindrostethus Mayr ( Insecta : Heteroptera : Gerridae ) from India with a new record

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Journal of Threatened Taxa
For Focus, Scope, Aims, Policies and Guidelines visit http://threatenedtaxa.org/index.php/JoTT/about/editorialPolicies#custom-0 For Article Submission Guidelines visit http://threatenedtaxa.org/index.php/JoTT/about/submissions#onlineSubmissions For Policies against Scientific Misconduct visit http://threatenedtaxa.org/index.php/JoTT/about/editorialPolicies#custom-2 For reprints contact <info@threatenedtaxa.org> margin of pronotum, less prolonged eighth abdominal segment of the male, non-rotated abdominal segment VIII and IX of the male, simple postero-median margin of abdominal sternum VII in the female, and mostly visible abdominal sternum VIII of the female. The genus Cylindrostethus presently contains 18 species (including two nominotypical subspecies) and three subspecies, distributed in the Neotropical, Ethiopian and Oriental regions (see Hungerford & Matsuda 1962;Linnavuori 1981;Polhemus 1994;Floriano & Cavichioli 2013;Zettel et al. 2017). The Old World species of Cylindrostethus were revised by Hungerford & Matsuda (1962) and Polhemus (1994). The present paper provides the first record of C. costalis costalis from India (the Andaman Islands) and accompanied by photographs showing detailed morphological characteristics of the two species known from India to facilitate easy identification.

Material and Methods
The studied materials were collected from very cool and well shaded, slow flowing forest streams of India by using a D-frame aquatic insect net, and preserved in 75% ethanol. The photographs and measurements were taken through a Leica stereo zoom microscope (Leica M205A), using the software Leica application suite (LAS V3.8). All measurements are given in mm. All the studied specimens are deposited in National Zoological Collection, Hemiptera Section, Zoological Survey of India, Kolkata, India, except that the specimens collected from Madhya Pradesh are deposited in Central Zone Regional Centre, Zoological Survey of India, Jabalpur, Madhya Pradesh, India.

Taxonomy
Cylindrostethus Fieber, 1861 Notes: For a detailed description of the genus, see Hungerford &Matsuda (1962) andD. Polhemus (1994). Diagnosis: Cylindrostethus costalis costalis can readily be identified by the presence of a yellowish costal margin on the forewing (Image 1A,B,F), the absence of spinules on the mesosternum and the presence of spinules on the posterior region of the metasternum (Image 1E), and the presence of a broad longitudinal yellow stripe on the thorax, except on the metanotum; absence of spinules on the pygophore of the male (Image 1K, L); and the shape of the lateral process on the male proctiger (Image 1N).
Colour: Dorsum black and shiny, ventral yellow; head between eyes yellow to orange; yellow median line on thorax continues up to abdominal tergum IV, mostly absent on metanotum and indistinct on abdominal tergum II (Image 1C); antero-ventral side of fore femur one third from base with a broad black stripe and reaching up to one fourth distance from base and not confluent with ventral black apical ring of fore femur, black stripe on flexor region of fore femur confluent with black apical ring; tibia and tarsus of all legs black except hind tibia orange; mid and hind tibia outer margin with a longitudinal black stripe; dorsolateral region of meso-and metacetabula with black longitudinal stripe; sublateral region of abdominal sterna with a black longitudinal stripe; abdominal terga I-IV with median longitudinal yellow stripe interrupted between joints (indistinct in abdominal tergum II); dorsal connexival margin yellow, connexival spine basolateral region and subapex brown (Image 1G, H), lateral region of tergum VIII yellow.
Structural characters: Body venter clothed with minute silvery-white hairs; head length 1.54, head width 1.89, eyes convergent anteriorly near vertex, dorsal inner margin concave and slightly depressed, minimum interocular width 0.47, eye length 0.96, eye width 0.57; last antennal segment slightly curved (Image 1C), antennal segment lengths I-IV 2.41, 1.03, 0.77, 1.06; posterior region of metasternum to sternum VII clothed with black spinules, except sublateral regions; dorsal thorax length 5.97, pronotal length 1.21, pronotal width 1.91, mesonotal length 3.42, metanotal length 1.45, prosternal length 0.84, mesosternal length 4.13, metasternal length 2.09, metasternal scent channel prominent; flexor region of fore femur with a few dispersed setae, flexor region of fore femur subapically with a short blunt tooth; flexor region of fore tibia fringed with minute setae, fore tibia slightly inwardly curved apically, inner apical process of fore tibia blunt; all legs with prominent claws; mid and hind legs clothed with minute spines; inner margin of hind tibia and tarsus fringed with short curved setae. abdominal tip, length 1.48. Genitalia: pygophore suddenly acuminated near apex, spinules absent, ventrolateral region along midway fringed with setae (Image 1H, K, L); proctiger highly sclerotised, apical margin with eleven short stout spines, sub-basal region with lateral processes, left process broader than right process, base of lateral processes with a few long setae, proctiger produced and tongue-like below lateral processes (Image 1G, N); endosomal sheath highly sclerotised, dorsal and ventral endosomal sclerites united, ventral sclerite less sclerotised coiled and enclosed round central sclerite (cs-term newly proposed here), dorsal sclerite bent forwards but not extending one third distance from base of endosoma, lateral sclerites boat-shaped with apical region appearing as 'folded hands in prayer' shape in lateral aspect, single apical sclerite long and linear, placed ventrally, upper region of endosoma infront of dorsal sclerite with ring-like impression up to subapex (Image 1O); parameres slightly visible from outside, symmetrical and narrowed medially (Image 1M). Macropterous male (Image 1A, F): Similar to apterous male with following exceptions: body length 17.34-17.74, width 2.88-2.95, pronotal lobe length 5.06-5.32, width 2.20-2.24, hemelytral length 9.94-10.30, width 1.20-1.35. Median longitudinal yellow line confined to anterior pronotal lobe; lateral region of posterior pronotal lobe anteriorly with thin and posteriorly with thick yellow border; humeral angle slightly produced; wings dark brown to black, not exceeding posterior margin of tergum VII, two third from the base of hemelytral costal margin with yellow line (Image 1A).
Distribution: India (Andaman Islands, a first record for India); Cambodia, Laos, Myanmar, Thailand, Vietnam (Hungerford & Matsuda 1962;D. Polhemus 1994;Zettel & Chen 1996;Zettel et al. 2017   Diagnosis: Cylindrostethus productus can easily be identified by the absence of a yellow costal margin on the forewing (Image 2F), the presence of the spinules on the mesosternum, metasternum and the abdominal sterna II-VII; the presence of a pair of tubercles on the mesonotum of the apterous female (Image 2A); the presence of the spinules on the pygophore of the male (Image 2B, C); the unique shape of the lateral process on the male proctiger (Image 2D); the basally broad male paramere (Image 2E); the presence of a long stalk-like conjunctivum (ct) coupled with the absence of coiled ventral sclerite, (Image 2G); the shape of the wavy line on the process of second gonapophysis of the female genitalia and the proportion of the process of the second gonapophysis and its peglike setae (Image 2H).

Discussion
Prior to the present study, only two species of Cylindrostethinae, Cylindrostethus productus and C. scrutator, were known from India (Thirumalai 2002). Hungerford & Matsuda (1962) indicated that they had examined two apterous males and one apterous female of C. scrutator from India, but did not provide any specific locality data. These specimens were deposited in the British Museum. Polhemus (1994) suspected that the record of C. scrutator by Hungerford & Matsuda (1962), was probably from Assam. C. scrutator, however, has not been encountered in several surveys conducted by scientists from Zoological Survey of India over the past hundred years and therefore the record of C. scrutator from India is doubtful. In the old colonial period, "India" was a term used for a much larger region, including Myanmar. Cylindrostethus scrutator is relatively common on the Greater Sunda Islands (Sumatra, Java, and Borneo) and also occurs in southern Myanmar, central Thailand and southern Vietnam within continental Southeast Asia. It appears very unlikely that the species occurs in India (Dr. Herbert Zettel pers. info. 2014). The authors have seen photographs of the specimens of C. scrutator, sent by Mr. M.D. Webb, Curator of Hemiptera, Natural History Museum, London, which were examined by Hungerford and Matsuda. It seems one is an apterous male from Tavoy (Myanmar) and another from Tenasserim (Myanmar) and one specimen without locality. So the record of C. scrutator from India remains doubtful.
The present record of C. costalis costalis from the Andaman Islands adds one more species to the fauna of India. This record is quite interesting and is of zoogeographical importance, because the Andaman Islands are considered as the exposed part of the long Arakan Yoma Mountain range of Myanmar, in the Bay of Bengal. Cylindrostethus costalis costalis is widely distributed in Cambodia, Laos, Myanmar, Thailand and Vietnam, but not southern China or Peninsular Malaysia (D. Polhemus 1994;Zettel et al. 2017). Cylindrostethus productus is distributed only in Sri Lanka, Nepal and mainland India (D. Polhemus 1994;Thirumalai 2002). Both C. costalis costalis and C. productus belong to the same species group, but possess major differences in the male genitalia, particularly in the endosomal sclerites. The posterior region of the pygophore in the males of C. productus possess spinules, but such spinules are absent on the posterior region of the pygophore of C. costalis costalis. In C. productus, only the lateral sclerites are prominent and the other sclerites such as dorsal, ventral and apical sclerites are absent, however all these sclerites are present in C. costalis costalis. The absence of endosomal sclerites, except lateral sclerites, in C. productus may have evolutionary significance and needs phylogenetic study. The female genitalia of both the species are similar in appearance, but specific differences are seen between the species. The proportion of the peg-like seta and the second gonapophysis and the wavy line on the second gonapophysis (Image 1P, 2H) can be considered as the significant characters for identification of the females, if males are absent in collections. The evaporative channel or groove on the metasternum forms a short segment-like portion on the posterior region of the metasternum, which often creates confusion as to the position of abdominal sternum I, but the detailed study after removing the muscles lying in the metathorax shows that the part above abdominal sternum-II is a part of metasternum (Image 1E). www.threatenedtaxa.org The Journal of Threatened Taxa is dedicated to building evidence for conservation globally by publishing peer-reviewed articles online every month at a reasonably rapid rate at www.threatenedtaxa.org. All articles published in JoTT are registered under Creative Commons Attribution 4.0 International License unless otherwise mentioned. JoTT allows unrestricted use of articles in any medium, reproduction, and distribution by providing adequate credit to the authors and the source of publication.