A review of Bornean Micronectidae (Hemiptera, Heteroptera, Nepomorpha) with descriptions of two new species from Sabah, Malaysia

Abstract Previous research of Bornean Micronectidae Jaczewski, 1924 (pygmy water boatmen) is summarized based on the data from the literature and recent work. All the Bornean micronectids belong to the genus Micronecta Kirkaldy, 1897. Descriptions or redescriptions and a key to the eight species, which have so far been found in Borneo are presented, namely Micronecta decorata Lundblad, 1933, Micronecta ludibunda Breddin, 1905, Micronecta liewi sp. n., Micronecta lakimi sp. n., Micronecta lumutensis Chen, Nieser & Lansbury, 2008, Micronecta skutalis Nieser & Chen, 1999, Micronecta kymatista Nieser & Chen, 1999) and Micronecta quadristrigata Breddin, 1905. The synonyms are indicated under each species. To facilitate identification, illustrations and habitus photos are provided. The faunistic components of Micronectidae in Borneo are discussed from a zoogeographic point of view.


Introduction
The Scientific Expedition to Mount Kinabalu-Crocker Range in September 2012 (http://kinabalu-expedition.blogspot.nl/), organized jointly by Sabah Parks, Malaysia and the Naturalis Biodiversity Centre (NBC), The Netherlands, offered us an opportunity to collect water bugs at several substations in the Parks. The result has led to a better understanding of the water bug fauna in the area, including the discovery of several undescribed species. As a result of the expedition, a review of the Sabah Micronectidae is presented. For locations and the ground plan of the Sabah Parks, see Figs 1-4.
The Micronectidae (pygmy water boatmen) belong to the superfamily Corixoidea (Leach, 1815), which is in the infraorder Nepomorpha Popov, 1971. Most species in the Nepomorpha live in water and are characterized by the antennae implanted under the head. In the most obligate aquatic species, their antennae are shorter than the head and not visible in dorsal view. Within Nepomorpha, the Corixoidea are recognized by the broadly triangular, unsegmented rostrum, although transverse grooves are present in most species (Fig. 5). The abdominal structure in males is strongly modified in Micronectidae as in other Corixoidea taxa, with segments V-VIII asymmetrical (Figs 10,11). The male genitalic structures (Figs 11,12) are similar to those of Sigara Fabricius, 1775, in the Corixidae: Corixinae. The females have a unique spermatheca (Fig. 13) by having a large distal seminal receptacle among water bugs (Larsen 1938, and Pluot-Sigwalt, personal communication).
Micronectids are small bugs with a body length less than 5 mm. The Bornean species are all less than 3.5 mm long. Most species of Micronectidae occur in the tropics and subtropics, with only a few found in temperate or cold climates of the Palaearctic Region. The Micronectidae can be easily separated from Corixidae (Leach, 1915) by the following characteristics: scutellum exposed, not covered by the pronotum, and the absence of ocelli. Micronectids are usually found in shallow stagnant or virtually stagnant habitats. Most species seem to prefer an open sandy or clayey bottom with little or no plant debris. In our experience, they can be especially numerous in shallow edges of ponds with sandy bottoms in temperate regions, and in open shallow pools of stream beds with sandy bottoms in tropical areas (Figs 98,99).

The history of Bornean Micronectidae
Although the history of studying of micronectids can be traced back to Linnaeus more than 200 years ago, the Bornean fauna of Micronectidae remains poorly known. Wróblewski (1968) speculated that Micronecta decorata Lundblad, 1933 might be present in Borneo. Only the recent expeditions to Borneo by NCB Naturalis have led to the first confirm records of micronectids on the island. Three species were found in 1999 (M. ludibunda Breddin, 1905;M. kymatista Nieser & Chen, 1999; M. skutalis Nieser & Chen, 1999); and later M. lumutensis Chen et al., 2008 was described from Kalimantan. The last expedition in 2012 exploring the mountainous areas of the Sabah Parks added two species new to science, one species new to Borneo, and the confirmation of M. decorata on the island.  Kitaura et al. 2003), with indications the localities of the samples. Credit: Sabah Parks.

Material and methods
Mount Kinabalu gives rise to five catchments (Wong and Philipps 1996). Sungai Silau-Silau and its tributary, the small stream of Carson Falls, originate in the Headquarters (Figs 2-3) area and flow into the Sungai Liwagu, which originates on the south slope near Headquarters and discharges into the Labuk River, which flows eastward into Labuk Bay north of Sandakan. Likewise, the Sungai Kipungit at Poring (Figs 2-3) ultimately discharges into Sungai Labuk. Our samples CN1268, CN1270, CN1271, CN1272 and CN1274 are from this catchment. The Sungai Kadamaian originates up-mountain from Kampong Kiau, and its tributary Sungai Kematis up-mountain from Kampong Sayap (Figs 2-4). The Sungai Kadamaian flows northwestward past Kota Belud into the South China Sea. Samples CN1262, CN1263, CN1264, and CN1275 are from the Sungai Kadamaian catchment area. Finally Sungai Kibambang and Sungai Mahua, which originate in the Crocker Range (Fig. 2) flows into Sungai Pegalan which joins Sungai Padas before draining into Brunei Bay. Samples CN1277, CN1278, CN1279, CN1281, CN1283, CN1285, CN1286, CN1288, and CN1289 are from the Sungai Pegalan catchment area.
The specimens obtained in Sabah were collected with a hand net, unless otherwise indicated in the material examined sections. The number of net sweeping or the time spent one locality was not standardized. We usually collected in a given locality until three subsequent netting hauls did not yield any additional species. When unusual specimens were collected, an additional effort was made to collect a longer series. Most  studied specimens are preserved in 96% ethanol, but some were mounted on carton labels or on microscopic slides.
To facilitate working with the key and better understanding the descriptions, three diagrammatic figures (Figs 6, 7, 10) and a photograph (Fig. 11) of the male genitalic structures of Micronecta sp. are provided. Anatomical abbreviations and terms used in species descriptions are indicated in Figs 5-13. Specimens were studied by using a binocular (Zeiss Stemi 2000) and a compound microscope (Olympus BX51). Measurements are in mm, based on five specimens of each sex from the series (including the holotype, if available) and presented as a size range. Ocular index is 2S/ (D-S). Photographs were taken with Zeiss Discovery V12 SteRIO, lens Zeiss Plan Apo S 1.0×, FWD 60 mm, and, if necessary, were further processed using Adobe Photoshop CS6. Line illustrations were made using a binocular Zeiss Stemi 2000 with a camera lucida.
The studied specimens from several museum collections were mainly caught at light. The holotypes of the newly described species are placed in the Naturalis Biodiversity Centre (RMNH); the remaining material collected in Sabah is divided over NCTN, NMPC, RMNH, and ZCSM.
The following acronyms of museum collections are used:

Key to species of Micronecta occurring in Borneo (mainly applicable to males)
1 Corium with four solid longitudinal, darker stripes with variation from weak to distinct rings; pronotum typically with a pair of oval rings, varying from virtually absent to distinct (Fig. 12); left paramere with a laterally compressed tip (Fig. 77)

Micronecta (Dichaetonecta) decorata
Pronotum. About two and a half times as wide as long (W/L 0.87/0.36), dorsally convex with lateral margins straight and more or less truncate (Fig. 14). Hemelytra smooth, with four shallow, longitudinal grooves on corium, densely beset with small spinules, notably on corium. The right membrane texture same as corium, smooth without grooves or spines. Spines laterally on abdominal segments: V with two short and one longer stout spine; VI with three short and one long spine; VII with two or three short and one long stout spine; VIII with four or five short and one long, stout spine or sometimes without a long spine and two long hair-like bristles.
Female. Fore femur with the same general arrangement of pegs and setae as in male. The seminal capsule of spermatheca clavate (Fig. 90).

Habitat.
We have taken this species several times in Chiang Mai and other northern provinces in Thailand, where it is apparently quite common. Sample C0220 was taken from shallow virtually stagnant water in a wide unshaded river bed with a sandy bottom.
Colour. Frons and vertex sordid yellow, eyes castaneous. Pronotum yellowish brown, disk typically with a pair of darker oval rings, varying from nearly absent via fragmented rings to complete; posterior margin with a distinct yellowish stripe. Hemelytra yellowish brown; clavus with a darker, V-shaped, medium-brown stripe; corium typically with four longitudinal, medium-brown, uninterrupted stripes (Figs 15,22,25); embolium yellowish with four or five brown spots; right membrane poorly delimited from the corium, with the same colour and texture but without darker stripes; left membrane more distinctly separated from corium, hyaline, and more membranous than corium. Venter, abdomen, thorax, and legs pale yellow.  Breddin, 1905, syntype, brachypterous female, body length 1.80 mm 23 Micronecta quadristrigata Breddin, 1905, syntype, macropterous female, body length 2.85 mm.

23
Pronotum short (Fig. 15), about four times as wide as long (W/L 0.79/0.20); in brachypterous specimens dorsally flat, in macropterous specimens dorsally somewhat convex. Hemelytra smooth, sparsely beset with small spinules, notably on corium. Spines laterally on abdominal segments: V with two short and one longer stout spine; VI with two short, and one long spine; VII with two or three short, and one long stout spine; VIII with five short and one long stout spine, and one long hair-like bristle.
Male. Fore femur (Figs 32, 33) with a pair of pegs on proximal third and a pair of small pegs distally; pala with three long dorsal hairs. Claw slender and clavate, apex mucronate. Dorsum of abdomen: prestrigilar lobe (Fig. 44) sub-triangular, with a short, truncate apex; strigil ( Fig. 52) small, suboval, comb with about 55 comparatively distinct teeth; free lobe of left part of tergite VIII (Fig. 67) with a slightly expanded apex and 10-15 apical bristles. Left paramere (Fig. 77) with a narrow, more or less parallel-sided shaft, apex laterally compressed, flag-like; right paramere in lateral view ( Fig. 76) with an evenly curved shaft and tapering apex, basal lobe not distinctly differentiated from basal part of paramere, with over 50 stridulatory ridges. Mediocaudal lobe of sternite VII (Fig. 59) long, with apical part elongate and obtusely rounded to pointed apically, with or without one to two larger bristles.
Female. Fore femur with the same general arrangement of pegs and setae as in male. The seminal capsule of spermatheca mushroom-shaped (Fig. 91).
Comparative notes. Within Bornean Micronecta, this species is easily recognized in both sexes by its distinct linear pattern on the hemelytra (Figs 15, 22, 25).
Colour. Vertex sordid yellow, the frons yellowish with a brown spot, eyes grey, rostrum yellowish with dark brown transverse grooves. Pronotum yellowish brown, disk unmarked, posterior and lateral margins with a yellowish stripe. Hemelytra yellowish brown, apex of clavus darker brown, corium with three interrupted longitudinal brown stripes (Fig. 16), right membrane poorly delimited from the corium, with the same colour and texture as corium but without darker stripes, left membrane more distinctly separated from its corium, hyaline, and more membranous than the corium. Venter of abdomen and thorax grayish, legs yellowish.
Pronotum (Fig. 16) convex dorsally, about two and half times as wide as long (W/L male 0.56/2.1, female 0.57/0.23). Hemelytra smooth, beset with small spinules, notably on corium, arranged in longitudinal rows, and along the membranal suture. Spines laterally on abdominal segments: V with two short and one longer stout spines; VI with two or three short and one intermediate spine; VII with three or four short, one intermediate, and one or two long, stout spines; VIII with five short spines and two long hair-like bristles.
Legs Male. Fore femur (Fig. 34) with a pair of pegs on proximal third, a subdistal peg dorsally, and one or two small pegs distally; pala with three long, dorsal hairs. Claw slender, clavate. Dorsum of abdomen: prestrigilar flap (Fig. 45) with a short, acute apex; strigil small, suboval, at a magnification of 400×, no separate teeth observable;  Breddin, 1905. free lobe of left part of tergite VIII (Fig. 68) more or less parallel-sided, softly curved, with a rounded apex and 9-10 apical bristles. Mediocaudal lobe of sternite VII (Fig.  60) short, acute, with three or four larger bristles. Left paramere (Fig. 79) apically slightly dilated, with an apical impression; right paramere in lateral view (Fig. 78) gradually widened toward apex, basal lobe with about eight stridulatory ridges.
Female. Fore femur with the same general arrangement of pegs and setae as in male. Seminal capsule of spermatheca mushroom-shaped (Fig. 92).
Comparative notes. The small size, with a body length of about 1.5 mm, separates this species from other Bornean species except M. skutalis. Males of M. lumutensis and M. skutalis can be separated by the characters of parameres as given in the key (84)(85). In addition, the seminal capsule of M. lumutensis is mushroom-shaped (Fig. 92), whereas that of M. skutalis is egg-or urn-shaped (Fig. 95). Females can be indentified only by their association with males.
Habitat. The type specimens were collected at light in a mountainous area. Distribution. Indonesia: Kalimantan Timur (Chen et al. 2008 Colour. Frons and vertex sordid yellow, eyes dark castaneous. Pronotum and hemelytra sordid yellow to light brown, the hemelytra with a broad transverse medium to dark brown band at middle (Fig. 26), left membrane medium to dark brown. Disk of pronotum unmarked. Venter and thorax sordid yellow, laterally infuscate, abdomen grayish brown, medially variably lighter. Legs pale yellow.
Head slightly narrower than pronotum, synthlipsis 1.7-1.8 times as wide as the posterior margin of an eye.
Pronotum well developed, dorsally convex with lateral margins distinctly straight, and more or less truncate (Fig. 17), slightly over 2.5 times as wide as long (W/L male 0.71/0.26, female 0.72/0.28). Hemelytra (Fig. 26) smooth, beset with extremely small unobtrusive spinules. Spines laterally on abdominal segments: V with one short and one long spine, VI with two short, and two long spines; VII with three or four short and one long spine; VIII with four or five medium long spines and two long hair-like bristles.
Male. Fore femur (Fig. 35) with a pair of pegs in proximal third, one peg dorsally at distal third and two pegs dorsodistally; tibia without dorsoapical peg; pala with three comparatively short dorsal hairs, 10-12 short bristles in upper row, distal bristle of upper row much stouter and longer than other upper bristles, and 14 to 16 longer bristles in lower row. Claw simple, elongate (Fig. 36). Dorsum of abdomen: prestrigilar lobe (Fig. 46) with a pointed apex, strigil small and narrow (Fig. 53), comb with about 75 teeth, free lobe of left part of tergite VIII (Fig. 69)   ( Fig. 83) with a wide shaft, apex with short longitudinal grooves; right paramere ( Fig.  82) with a medium-sized shaft and a slightly expanded apex, basal lobe strongly developed, stridulatory ridges not observed. Mediocaudal lobe of sternite VII (Fig. 61) with four bristles.
Female. Fore femur with the same general arrangement of pegs and setae as in male. Seminal capsule of spermatheca urn-shaped (Fig. 93).
Comparative notes. The hemelytral pattern is diagnostic among the Melanesian Micronecta fauna. Micronecta liewi is similar to M. melanopardala melanopardala Nieser & Chen, 2003 described from the Philippines by having a similar transverse band midway along the hemelytra, but it differs from M. melanopardala melanopardala by lacking a dark patch on the clavi as in M. melanopardala. In general, M. liewi has more distinct dark markings than in M. melanopardala adiaphana Nieser & Chen, 2003. Furthermore, in both subspecies of M. melanopardala, the shafts of the right parameres are more slender than M. liewi, and the apex of the right paramere of M. melanopardala is not expanded.
The strongly developed distal bristle of the upper row on the male pala (Figs 35, 36) gives impression of an additional claw as in the subgenus Unguinecta Nieser, Chen & Yang, 2005 from southern continental Asia. However, the four bristles on mediocaudal lobe of sternite VII of the male, the shape of the parameres, and the shape of the free lobe on the left part of tergite VIII of the male will all allow placement in the subgenus Micronecta.
Etymology. This species is named in honor of Dr. Thor Seng Liew (NBC Naturalis and Sabah University, Malaysia), for his outstanding contributions to the study of the biodiversity of Sabah, and his invaluable help with our work on water bugs in Borneo.
Habitat. The type series was collected in a small, virtually stagnant bay on the downstream side of Kibambangan waterfall (Fig. 98).
Pronotum well developed, dorsally convex with lateral margins distinctly truncate (Fig. 18), about 2.5 times as wide as long (W/L male 0.85/0.34, female 0.87/0.36). Hemelytra (Fig. 27) smooth, beset with small, distinct spinules, most notably on corium. Spines laterally on abdominal segments: V with three short and one long spine; VI with two short and two long spines; VII with two short and two long spines; VIII with four or five short spines and two long hair-like bristles.
Female. General arrangement of bristles on fore femur is the same as in male. The seminal capsule of spermatheca mushroom-shaped (Fig. 94).
Comparative notes. The right paramere is apically somewhat similar to that of M. ornitheia Nieser et al., 2005 from Yunnan, China. However, the shaft of the right paramere of M. orniteia is narrower, the left paramere is apically truncate; and it is a smaller species; body length of M. orniteia is 1.7-1.9, body length of M. lakimi is 2.1-2.2.
Etymology. The species is named after Dr. Maklarin Lakim for his great service organizing the joint expedition to Sabah Parks in 2012, and his various activities in support of biodiversity exploration in Sabah Parks.
Habitat. The type series was collected downstream of Mahua waterfall, at the edge of the stream with a slow current (Fig. 99).
Colour. Frons and vertex sordid yellow, eyes grayish. Pronotum and hemelytra sordid yellow to light brown; hemelytra with an often indistinct, transverse medium to dark brown band at middle (Fig. 28), left membrane medium to dark brown. Venter: thorax sordid yellow, laterally infuscate; abdomen grayish brown, medially variably lighter. Legs pale yellow.
Pronotum dorsally convex, 2-2.5 times as wide as long (W/L 2.1-2.7, Fig. 19). Hemelytra beset with spinules arranged in longitudinal rows and along membranal suture. Spines laterally on abdominal segments: V with two short and one long stout spine; VI with two short and one or two longer spines; VII with two short and two long stout spines; VIII with five short spines and two long hair-like bristles.
Legs Male. Fore femur (Fig. 39) with a pair of pegs on proximal third, and two or three small pegs distally; tibia with two to three small spines near dorsal margin; pala ( Fig.  40) with three long dorsal hairs. Claw simple, clavate. Dorsum of abdomen: prestrigilar flap (Fig. 48) with a short apex; strigil (Fig. 55) small, sub-oval, one comb with about 60 teeth; free lobe of left part of tergite VIII (Fig. 71) more or less parallel-sided, softly curved, with a rounded apex and about10 apical bristles. Mediocaudal lobe of sternite VII (Fig. 63) short, acute, with four larger bristles. Left paramere (Fig. 85) parallel- sided, apically rounded, with an indentation at the base of the shaft; right paramere in lateral view (Fig. 84) apically dilated, basal lobe with about eight stridulatory ridges.
Female. Fore femur with the same general arrangement of pegs and setae as in male. Seminal capsule of spermatheca ovate (Fig. 95).
Comparative notes. Its small size separates this species from other Bornean species of Micronecta, except for M. lumutensis (see that species).
Habitat. The specimens all have been collected at light near a stream. Distribution. Malaysia: Sabah (Nieser and Chen 1999).
Colour. Frons and vertex sordid yellow, eyes grayish. Pronotum light to medium brown, disk unmarked, posterior margin with a distinct yellowish stripe. Hemelytra sordid yellow to light brown, clavus with a darker medium-brown stripe along the suture between clavus and corium suture, corium typically with four fragmented longitudinal medium-brown stripes (Fig. 20), embolium yellowish with three or four indistinct brownish spots; right membrane poorly delimited from the corium, with the same colour and texture but without darker stripes; left membrane more distinctly separated from corium, hyaline to somewhat smoky and more membranous than the corium. Venter, abdomen, thorax, and legs pale yellow. Pronotum well developed, dorsally convex with lateral margins straight or more or less truncate (Fig. 20), about three times as wide as long (W/L male 1.00/0.34, female 1.05/0.36). Hemelytra smooth, beset with numerous small but distinct spinules. Spines laterally on abdominal segments: V with two short and one longer stout spine; VI with two or three short and one long spine; VII with two or three short and one long stout spine; VIII with five or six short and one longer, stout spine and two long hair-like bristles.
Male. Fore femur with a pair of pegs on proximal third, and a pair of small pegs distally; tibia with a dorsoapical peg. Pala (Fig. 41) with three long dorsal hairs, the apical bristles in lower row distinctly thicker than the bristles of lower row. Claw broadly clavate, gradually dilated from base to apex, without ventral notch. Dorsum of abdomen: prestrigilar lobe (Fig. 49) difficult to observe, strigil ( Fig. 56) with one, relatively broad comb with about 50 elongate teeth. Median lobe of sternite VII (Fig. 64) apically narrow with a rounded apex, without obvious longer bristles. Free lobe of left part of tergite VIII (Fig. 72) sigmoid with about 12 apical bristles. Medial margin of right lobe of tergite VIII with 28-35 bristles caudally, placed in a double to triple row on caudal half (Fig. 8). Left paramere (Fig. 87) with a comparatively narrow shaft and a sickle-shaped apex; right paramere, in lateral view (Fig. 86), with an evenly curved, more or less parallel-sided, apically tapering shaft, basal lobe with about 40 stridulatory ridges on the pars stridens.
Female. Fore leg with the same general arrangement of pegs and setae as in male. Seminal capsule of spermatheca elongate-clavate (Fig. 96).
Comparative notes. This species is similar to M. quadristrigata, which is smaller on average and has fewer bristles on the caudal half of inner margin of right part of tergite VIII in males (see key and Figs 8-9).
Habitat. This species has been found in ponds and sluggish streams mostly in less disturbed areas.
Colour. Frons and vertex sordid yellow, eyes grayish. Pronotum light brown, virtually unmarked in most specimens, in some specimens, with two indistinct, usually interrupted transverse stripes, posterior margin with a poorly defined yellowish stripe. Hemelytra sordid yellow to light brown, clavus with a darker medium-brown stripe along the claval suture, and a smaller medium-brown streak near the inner angle; corium typically with four interrupted, longitudinal, medium-brown stripes (Figs 21,23,29), embolium with four black spots; right membrane poorly delimited from the corium, with the same colour and texture but without darker stripes; left membrane more distinctly separated from corium, hyaline and more membranous than the corium. Venter, thorax, and legs pale yellow, abdomen yellowish to light brown.
Pronotum well developed, dorsally convex with lateral margins straight or more or less truncate (Fig. 21), slightly over 2.5 times as wide as long (W/L male 0.88/0.34, female 0.98/0.37). Hemelytra smooth, beset with numerous small but distinct spinules. Spines laterally on abdominal segments: V with three short and one longer stout spine; VI with two short and two long spines; VII four short and one long stout spine; VIII with six short to longer, stout spines and one long hair-like bristle.
Leg Male. Fore femur (Fig. 42) with a pair of pegs in proximal third, two (in some specimens only one) small pegs about midway dorsally and a small peg dorsodistally; tibia with a larger peg subventrally on apical third and two small dorsoapical pegs; pala with four long dorsal hairs, distal bristle of lower row much stouter and longer than other lower bristles. Claw plump, clavate. Dorsum of abdomen: prestrigilar lobe with a short, broadly rounded apex, strigil (Fig. 57) sub-oval, comb with about 25 long teeth, free lobe of left part of tergite VIII (Fig. 73) sigmoid-shaped. Left paramere (Fig. 89) with a wide shaft, apex sickle-shaped; right paramere in lateral view (Fig. 88) with an evenly curved shaft, basal lobe strongly developed with about 50 stridulatory ridges; in dorsolateral view, the shaft is somewhat sinuous. Mediocaudal lobe of sternite VII (Fig. 65), long, with apical part elongate and obtusely rounded apically, without larger bristles.
Female. Fore femur with the same general arrangement of pegs and setae as in male. The seminal capsule of spermatheca elongate-clavate (Fig. 97).
Notes. M. quadristrigata might have an even broader range of size variation. Wróblewski (1960a) reported that females had a length up to 3.3 mm from Hong Kong, and Leong (1966)  Habitat. Various stagnant and slowly flowing waters, especially in agricultural fields, including rice fields.

Discussion of faunistic components in Borneo
All Bornean species of Micronectidae belong to the dominant genus Micronecta Kirkaldy, 1897, which contains about 130 described species. The present knowledge of the Bornean fauna (and the Malesian fauna as a whole) of Micronectidae is still insufficient to discuss its proper biogeographical affinities. Judging from the literature, lowland species, such as M. decorata, M. ludibunda, M. quadristrigata, tend to be more widespread than species from hilly areas, such as M. lakimi, M. liewi, and M. lumutensis. This conclusion might be partly artificial because most taxa of Micronectidae are collected at light. They easily escape from the casual collector in the field due to their small size. The shallow stagnant waters in lowland ponds and marginal bays of streams are less stable than the stagnant waters in hills or mountains, such as a pond at the foot of a waterfall. Moreover, lowland species were found several times in very high densities, whereas species from hilly areas were always found in moderate to low densities. We hypothesize that lowland species more often colonize new habitats and therefore fly more often.
Of the eight species of Micronecta known from Borneo, three are so far endemic to the island: M. lakimi, M. liewi, and M. lumutensis. Their localities are all in mountainous areas. It is unclear which species are closely related to M. lakimi and M. liewi. But Micronecta lumutensis apparently is closely related to M. skutalis, which was described from Sabah and also has been found on Palawan in the Philippines (Nieser and Chen 2003). Another species related to M. lumutensis and M. skutalis is M. abra Nieser & Chen, 2003 described from Palawan. These three species apparently constitute a species-group by having small body size, and each of them has limited distributional range around Borneo.
Micronecta quadristrigata is a widespread species. In the west, it reaches the United Arab Emirates and southern Iran (Linnavuori et al. 2011, Wróblewski 1960. The area around the Gulf of Persia and the Gulf of Oman is considered to belong to the Palaearctic Region (Aukema and Rieger 1995), but for water bugs it has a strong Oriental element as well as some species of African origin (Linnavuori et al. 2011). Besides the water bugs, the water beetles also show Oriental elements in Arabian Peninsula. Hájek and Wewalka (2009) stated: "Our study further reiterates that the Arabian Peninsula is a typical transition area between the neighboring major zoogeographical regions". We agree with their observations based on the recent studies by different authors of insect fauna in the Arabian Peninsula, which has emphasized an interesting point from a zoogeographical point of view.
Eastward, M. quadristrigata reaches New Guinea and northern Australia (Tinerella 2013), indicating that this species probably originated in the Oriental origin. The open and shallow man-made waters, such as rice fields, provide conditions that have allowed micronectid to spread westward and eastward.
Micronecta kymatista is closely related to M. quadristrigata, but according to the locality information from Sulawesi and Borneo, it seems to prefer habitats somewhat less influenced by human activities. This might also explain the wide distribution of the other two lowland species M. decorata and M. ludibunda. It is clear that M. decorata is an Oriental element ranging from northern Thailand to Borneo, Java and Sumatra (Lundblad 1933). Micronecta ludibunda is also widespread, ranging from India and Sri Lanka to New Guinea and the Solomon Islands (Tinerella 2008). As its closest relative, M. albifrons (Motschulsky, 1863), known from India and Sri Lanka (Wróblewski 1968), is also considered a species of Oriental origin which has spread eastward. The four "lowland species" occurring in Borneo belong to the common Oriental elements. Choi (1996) has pointed out that "the sedimentary basin of Mt. Kinabalu was sinuated between three crustal or tectonic plates -The South China Sea Plate to the north, the Sulu Sea Plate to the east, and the main Eurasian Plate to the west". The uplifting of the Crocker-Trus Madi area began 40 million years ago with its collision with these other plates. The movement slowed down about 10 million years ago, although Mt. Kinabalu is said to be still pushing up at a rate of 5 mm per year, the Crocker-Trus Madi area has been pushed up into mountain ranges. According to radiometric age determinations, Mt. Kinabalu is somewhat younger, with the cooling of its magma taking place 10-4 million years ago. Micronecta lakimi and M. liewi are both from the Crocker Range and not closely related to the other species of Micronecta collected so far in Borneo. It is, therefore, possible that the origin of these two newly described species coincided with the rising of the Crocker Range.
In view of the endemism of various organisms in Mt. Kinabalu (Wong and Phillipps 1996), these two newly described species might also be endemic to this area. However, this point of view needs to be proved by further explorations in Sabah and Borneo, notably the confirmation whether M. lakimi and M. liewi are endemic in the area of Crocker Range.