Contributions to the knowledge of water bugs in Mindoro Island, Philippines, with a species checklist of Nepomorpha and Gerromorpha (Insecta, Hemiptera, Heteroptera)

Abstract Background This survey aims to provide an updated species checklist of aquatic and semi-aquatic bugs in the intra-Philippine biogeographic Region of Mindoro. An assessment survey of water bugs (Hemiptera, Heteroptera) was conducted mostly by manual collection in selected areas of Oriental Mindoro from 2017 to 2018, in which some of the collecting activities were undertaken by graduate students of Ateneo de Manila University. New information Twenty-nine aquatic and semi-aquatic heteropteran species were documented and some are known island-endemic species or subspecies, including Enithares martini mindoroensis Nieser & Zettel, 1999, Hydrotrephes stereoides mindoroensis Zettel, 2003, Aphelocheirus freitagi Zettel & Pangantihon, 2010, Rhagovelia mindoroensis Zettel, 1994, Rhagovelia raddai Zettel, 1994, Rhagovelia potamophila Zettel, 1996 and Strongylovelia mindoroensis Lansbury & Zettel, 1997, which were found in new areas in the Region. In addition, there are also new records for the Island that have already been documented in other parts of the Philippines, such as the Philippine-endemic Ochterus magnus Gapud & San Valentin, 1977 and Hebrus philippinus Zettel, 2006 and the widely-distributed backswimmers Anisops nigrolineatus Lundblad, 1933 and Anisops rhomboides Nieser & Chen, 1999. Several undescribed specimens and potentially new species are also discussed in this paper. Further surveys in the other parts of Mindoro and in the other regions of the Philippines, are encouraged to produce a comprehensive baseline data of heteropteran species richness in the country.


Introduction
The order Hemiptera constitutes a large part of the insect fauna, both in terrestrial and aquatic ecosystems in the Philippines (Gapud 1986). The majority of the aquatic forms belong to Nepomorpha and Gerromorpha (water bugs) under the suborder Heteroptera (true bugs or typical bugs). These water bugs have been relatively well-studied compared to other aquatic macroinvertebrate taxa, mainly due to the comprehensive Philippine Water Bug Inventory Project (Gapud and Zettel 1999). Most of the represented species are Philippine-endemic, many of which are island-endemics. More than 200 species are known from the country, although numerous undescribed, or even undiscovered species still await their formal description. Despite their abundance, Heteroptera were subject to only a few studies in Mindoro. In Lake Naujan, Mindoro, 12 heteropteran species, along with 49 coleopteran species, were documented (Freitag and Pangantihon 2010), while in Lake Manguao Catchment, Palawan, 21 heteropteran species were recorded .  documented 85 gerromorphan species from the Island of Luzon, while Pangantihon and Freitag (2016) recorded 31 new island records of coastal and marineassociated heteropteran species from central Visayas and Mindoro. In total, 43 species were recorded from Mindoro in these previous studies. Since then, no published papers dealt with similar faunistic surveys on aquatic and semi-aquatic Heteroptera in any region of the Philippines. Thus, this study aims to update the species list of Gerromorpha and Nepomorpha of Mindoro. The majority of the survey efforts were accomplished within the scope of a graduate course activity by the junior authors in selected areas of the Island for which a Gratuitous Permit was issued by the Philippine Bureau of Fisheries and Aquatic Resources (BFAR).
Along with field collection from the easily-accessible municipalities of Puerto Galera and Baco, special attention is given to the Baroc River basin in Roxas, which belongs partly to the Key Biodiversity Area (KBA) "69 Hinunduang Mt." with extremely high critical conservation priority ("EHc") and high socioeconomic pressure (Ong et al. 2002). This study aims to address the lack of biodiversity data from Mindoro Island. The comprehensive assessment project of the Baroc River basin by the Ateneo Biodiversity Laboratory has led already to the discovery of several interesting aquatic arthropod species , Mey and Freitag 2013, Komarek and Freitag 2014, Vidal et al. 2017, Garces et al. 2018, Komarek and Freitag 2020, Pelingen and Freitag 2020. These papers also provide more details on some of the collecting sites. The main sampling method used was handpicking with the use of a hand-held net to collect the aquatic bugs. Some of the specimens had also been collected using a black light trap (L) or in emergence traps (E) as described by Freitag (2004).

Materials and methods
In the label data of the material, the codes mentioned before for the collecting sites are followed by a single minor letter for the microhabitats (Fig. 3) listed and encoded in Table 2.

Handling of Material Collected
Specimens collected were preserved in 96% ethanol and stored (-20°C) prior to identification. Morphological examination was done using a dissecting microscope (LEICA EZ4) and a compound microscope (OLYMPUS SZ61). The habitus images were produced using a Canon EOS 6D with macro lens and a stack rack. This stacking of images was operated using Helicon Remote and Helicon Focus. The figures were generated then processed with Adobe Photoshop.
Identification keys and other relevant literature were used for the taxa as stated in the respective taxonomic sections. In some cases, loaned type material from the Natural History Museum Vienna, Austria (NHMW) was used for comparison. The dissected parts and actual specimen were glued on to entomological papers, while some were stored in the vial with ethanol. All material is labelled and kept at the collections of the Biodiversity Laboratory, Ateneo de Manila University (ADMU), National Museum of the Philippines,

Taxon discussion
This still unnamed new species (Fig. 4A) of the Ranatra gracilis Dallas, 1850 group (see Dallas 1850, Lansbury 1972 will be described and discussed further in an upcoming paper by Tran and Zettel (in prep.).

Habitat
Specimens were found in both flowing and calm littoral sections of shallow streams, such as in Fig. 1B. See Fig. 5 for current records. Microhabitats sampled with their respective label codes, as listed above. A. side pool with mineral deposits ("t"); B. littoral pool with mineral deposits ("b"); C. bottom gravel in running water ("c"); D. leaf packs trapped in riffles ("d"); E. leaf litter in rivulet ("k"); F. rock surface in riffle ("g"); G. root packs in running water ("h"); H. neustic in running water ("z").  Distribution of the collecting sites of Nepomorpha material treated in this study.
Contributions to the knowledge of water bugs in Mindoro Island, Philippines, ...

Habitat
The specimens were found on wet rocks.

Distribution
This species (Fig. 4B) is widely distributed in the Philippines. See Fig. 5 for our additional records.

Habitat
We collected specimens in emergence traps spanned over littoral portions of small streams, on hygropetric rocks and along small rivulets. The species is generally found in banks of streams, ponds, lakes, freshwater marshlands and in association with waterfalls. It is also found in substrates that are muddy or sandy.

Taxon discussion
The species cannot be identified yet because the examination of comparative type material is needed, which is currently inaccessible. Nieser and Chen (2003) described four species from the Philippines but the Mindoro specimens differ from those.

Habitat
All specimens were collected using a light trap, so their habitat cannot be accurately described. However, species of Micronecta Kirkaldy, 1897 are usually found in stagnant or shallow, slowly flowing waters (Nieser 1999). See Fig. 5 for current records, amongst them is site HR2 (Fig. 2B).

Distribution
This subspecies is endemic to Mindoro ). See Fig. 5 for new records.

Taxon discussion
For identification, refer to the key by , a habitus illustration is provided in Fig. 4C. The genus Asthenocoris is endemic to the Philippines ). All material treated in here is brachypterous.

Habitat
Asthenocoris luzonensis paradisianus Zetttel & Nieser, 2009 Fig. 4C is found in middlesized streams running through secondary rainforest (Fig. 2B), as well as large, fast flowing streams, partly in secondary vegetation (Fig. 1C), but then only downstream of forested areas . The specimens were retrieved in more or less fast flowing water from several substrates, foremost gravel, but also wood, leaf litter and root packs (Fig. 3G).
Here we present the first records from Baco and Roxas (Fig. 5).

Taxon discussion
For identification, refer to the key by Zettel (2003a) and the original description (Matsumura 1915).

Habitat
The single specimen was collected using a light trap not allowing for a specific microhabitat association. In general, representatives of this genus are found in isolated side pools (Fig. 3A) of streams and other stagnant water bodies (Zettel 2003a

Distribution
Anisops nigrolineatus Lundblad, 1933(see Lundblad 1933) is widely distributed from India, Myanmar, Thailand, Brunei, Java and up to the Philippines , with only a single previous record from Sibuyan Island (Nieser and Chen 1999). This is the first record from Mindoro Island (Fig. 5).

Taxon discussion
For identification, refer to the key by Zettel (2003a).

Habitat
The specimen was found in a small side pool near a slow-flowing stream.

Distribution
The species (Fig. 4F) is recorded from Brunei, Borneo, Sulawesi and the Philippines Chen 1999, Chen et al. 2005). Previous Philippine records refer to Leyte, Mindanao, Palawan and Tawi Tawi . This is the first record from Mindoro (Fig. 5).

Taxon discussion
For identification, refer to the key by Zettel (2003a). Its specific epithet, rhomboides, refers to the lozenge-shaped fossa on the tylus (Nieser and Chen 1999), which is a good character for identification.

Distribution
This species (Fig. 4G) is widely distributed in the Philippines, but the subspecies is endemic to Mindoro . See Fig. 5 for the new records.

Taxon discussion
For identification, refer to the key by .

Habitat
Enithares martini mindoroensis Zettel, 1999 (see Nieser and can be found in calm shores and connected and isolated pools on the banks of streams (Nieser and Zettel 1999, current study).

Distribution
The subspecies (Fig. 4H) is endemic to Mindoro Island. See Fig. 5 for new records.

Taxon discussion
For identification, refer to the key by Zettel (2003b).

Habitat
Hydrotrephes stereoides Zettel, 2003 is mainly associated with lentic sections of running waters, swimming actively at the edges of plant material, rarely benthic in running waters. We found most specimens attached to wood in clean mountain rivers.

Taxon discussion
For identification, see Zettel (2006). The parameres have long setae both in the lateral and apical portions. Most of the species have straight parameres and only few were directed mesally. In contrast to the Hebrus harrisi complex, this species has a less distinct endocorium with only a small and elongate white spot. All specimens examined were macropterous. Distribution of the collecting sites of Gerromorpha material (part 1) treated in this study.

Habitat
Hebrus philippinus Zettel, 2006(see Zettel 2006 is quite euryoecious and can also thrive in anthropologically-disturbed habitats. It is commonly found on the banks of running waters and more rarely in nearby stagnant waters. Unlike other Hebrus species, it is often found in sunny and dry areas. We obtained most records from hygropetric microhabitats.

Taxon discussion
The Hebrus harrisi complex is discussed by Zettel (2004a), Zettel (2006). Due to the lack of specimens for comparison, our material ( Fig. 6A) cannot be identified to species level. All specimens examined were macropterous.

Habitat
The specimens (Fig. 7) were found at the banks of more or less fast flowing sections of rivers of small to medium size.

Taxon discussion
For identification, refer to the key by Yang and Murphy (2011). More recently, Jehamalar et al. (2019) have shown that Mesovelia horvathi consists of a complex of closely-related species. At least two species of this complex occur in the Philippines. Their correct names remain uncertain. As a result, we refrain from concluding that the previous records from other areas in the Philippines and specimens in this study are indeed M. horvathi.

Habitat
Mesovelia horvathi Lundblad, 1933 is common in plains and mountains in stagnant, slow flowing and even in brackish water (Yang and Murphy 2011). We found the species in similar, partly identical habitats like Mesovelia vittigera, but so far, never syntopic with the former species.

Distribution
The species (Fig. 6B) is widely distributed in the tropics and subtropics of the Old World, including the Philippines . See Fig. 7 for additional records.

Taxon discussion
For identification, refer to the key by Yang and Murphy (2011). All specimens examined were apterous.

Habitat
Mesovelia species are commonly found amongst marginal vegetation in standing waters of ponds and streams. Mesovelia vittigera Horváth, 1895 (see Horváth 1895) can also be found in brackish-water habitats (Yang and Murphy 2011). We found several specimens on shaded, wet rocks and at the stream littoral with mineral substrates (Fig. 1B).

Taxon discussion
For identification, refer to the key by Gapud et al. (2003). All specimens examined were macropterous.

Habitat
The species is often found in large stagnant water bodies and rarely seen in running waters; however, all our samples are from stream banks.

Taxon discussion
For identification, refer to the key by Gapud et al. (2003).

Habitat
This species can be found in both stagnant waters and edges of streams and rivers (Gapud et al. 2003). We found the species in shaded hygropetric sites and at stream banks with mineral and organic substrates (Fig. 1B).

Distribution
This species (Fig. 6D), which was originally described from Japan (Scott 1874), is widely distributed in the Oriental, Australian and Melanesian Regions reaching eastwards to the remote islands of the West Pacific Region (Andersen and Weir 2003). See Fig. 7 for the additional records.

Taxon discussion
See Andersen et al. (2002) for identification. The species is rarely collected in the Island.

Habitat
The species was found on stream banks, specifically in side pools (Fig. 3B), such as at site BR3 ( Fig. 2A).

Distribution
The typical form is only known from the Mountain Province in northern Luzon (Hecher 2006). See Fig. 7 for the collecting sites of the slightly varying Mindoro material.

Taxon discussion
The specimens (Fig. 6E) closely resemble those of Pseudovelia curvata Hecher, 2006 following the key in Hecher (2006), displaying the following characters: first metatarsal segment devoid of a tuft of very long setae basally; first and second metatarsal segment with a row of long setae over entire length; and metatarsus about half as long as metatibia. However, the pygophore of males has long, bristle-like setae on its caudo-lateral margin like P. gapudi Hecher, 2006. Given the lack of any records, except for the specimens in hand and the type material from northern Luzon, it must remain unsolved if the material is conspecific or represents a new, but related species. This variation is only recognised in our specimens from Mindoro.

Habitat
The specimens were found at banks of creeks and rivers, both in calm and flowing sections.

Taxon discussion
For identification, refer to the key by Zettel (2003c).

Habitat
The species was found in slow flowing water and a residual pool with floating plants.

Habitat
Rhagovelia mindoroensis Zettel, 1994(see Zettel 1994 is usually found in secondary forests and in anthropogenic terrains close to the coast. They particularly inhabit still waters sections of streams (Zettel 1994). We also found it in small side pools of medium-sized rivers and creeks. The collection at site TDR4 (Fig. 2C) at 700 m altitude is surprising.

Habitat
The species is commonly found in moderately fast flowing creeks and lotic sections of the river (Zettel 1994), such as in our study.

Habitat
The specimens were found neustic on flowing water near root packs of a small river in a rural area (Fig. 1B).

Distribution
Rhagadotarsus ( Rhagadotarsus) kraepelini Breddin, 1905(see Breddin 1905) is a widespread species in southern and south-eastern Asia and Micronesia (Polhemus and Karunaratne 1993). In the Philippines, R. kraepelini is distributed throughout the country, but the limits of its distribution in the Philippines are still unclear . See Fig. 8 for the additional record.

Taxon discussion
For identification, refer to the key by Polhemus and Karunaratne (1993).

Habitat
The specimens were found amongst floating water plants in a residual pool of a driedup lowland creek in a rural area.

Distribution
The species (Fig. 9A) is widespread in the Philippines, Maldives, India, Sri Lanka, southern China and Indonesia (Damgaard et al. 2014), with the first records from Cambodia recently documented by Zettel et al. (2017). See Fig. 8 for our additional records.

Taxon discussion
Refer to Cheng et al. (2001) for the identification.

Habitat
In the Oriental realm, most species of Limnogonus Stål, 1868 prefer sheltered places in standing waters, which makes them somewhat gregarious. Limnogonus nitidus (Mayr, 1865)

Habitat
Limnometra nigripennis Mayr 1865 is amongst the most widespread and abundant Gerromorpha of Philippine running waters (Zettel 2004b. It exhibits higher tolerance to environmental disturbances in streams than other species. However, we recorded it predominantely from clean streams (Fig. 2C, D). Lentic sections of very small to medium-sized streams and pools associated with running water are its typical habitats

Taxon discussion
This genus is in need of revision . The specimens (Fig. 9C) probably belong to a new species related to Tenagogonus bergrothi Hungerford &Matsuda, 1958 (see Hungerford andMatsuda 1958), from Luzon. The genus is widespread throughout the Afrotropical, Oriental and Australian Regions, extending eastwards up to Fiji (Damgaard et al. 2014). Fifteen species are known in the Malesian Region, including the Philippines (Chen et al. 2005).

Habitat
The collected specimens were found in small forest streams, a common habitat of representatives of the genus.  (Fig. 9D) is endemic to the Philippines and has been recorded from Luzon, Mindanao, Mindoro (Freitag and Pangantihon 2010, see Fig. 8 for additional records), Negros and Panay (Chen and Nieser 2002). Some unpublished records from Catanduanes, Marinduque, Sibuyan, Ticao, Cebu, Samar, Leyte, Mindanao, Siquijor and Poro have also been recognised belonging to this species .

Taxon discussion
For identification, refer to Chen and Nieser (2002).

Habitat
The specimens were collected from the surface and littoral of calm and moderately fast flowing creeks and medium-sized rivers (Fig. 1A, B). In general, Rheumatogonus Kirkaldy, 1909 species inhabit mountain streams, creeks and waterfalls in the Oriental Region, specifically the shaded, steady slow-lotic sections of such watercourses (Chen et al. 2005).

Distribution
Metrocoris tenuicornis Esaki, 1926(see Esaki 1926 (Fig. 9E) is a widspread species in western India, southern China, mainland southeast Asia, Sumatra, Java and Borneo, but only known so far from Mindoro and Greater Palawan, in the Philippines Nieser 1993, Freitag and. Hydrometra lineata Eschscholtz, 1822 • As generally observed in the Philippines and adjacent areas, Veliidae (riffle bugs) and Gerridae (true water striders) are the most speciose families. With the two new records of Anisops, Notonectidae are also surprisingly diverse.
The biogeographic history of the Island is partly reflected by the species assemblages, especially in terms of a good number of island-endemic species. Unlike many other Philippine Islands of marine origin, Mindoro belongs to the so-called Palawan Microcontinental Block, a fragment of the Eurasian continental margin (Hall 2002). This is, however, not notably reflected in the current species composition of water bugs, since more recent dispersal and species radiation mechanisms might have had more impact on the current species distribution. Amongst the taxa treated here, only Metrocoris tenuicornis Esaki, 1926(see Esaki 1926) has a distinct Palawan-Mindoro distribution range in the Philippines (although it is widely distributed in southeast Asia). An unusual pattern is observed for some closely-related species/subspecies of the Rhagovelia papuensis Lundblad, 1936(see Lundblad 1936) group, of which two different subspecies of Rhagovelia kawakamii (Matsumura, 1913) (see Matsumura 1913) are distributed in either Borneo and Palawan or Taiwan and Luzon, respectively, but missing on Mindoro Island, where it is replaced by the closely-related R. mindoroensis (Zettel et al. 2020).
Despite their close vicinity to Luzon, the Islands remained always disconnected during the Quaternary (Hall 2002). Nevertheless, Mindoro and the eastern Philippine Islands share a large proportion of water bugs.
Amongst the species of the checklist that were unambiguously identified, 20% are endemic to Mindoro, another 28% Philippine-endemic, making almost half of all species endemic to the country. The island-endemism rate is slightly lower than in Palawan, with one-third endemism amongst aquatic and semi-aquatic bugs . This is likely due to the closer vicinity of Mindoro to other intra-Philippine biogeographic regionsnotably Luzon -enabling easier dispersal across sea barriers.
In this study, special emphasis was given to the collection in lotic systems, which might have led to an under-representation of typical pond-and lake-dwelling species. Nevertheless, stream-associated lentic microhabitats, such as side pools and residual pools, were sampled in most collecting sites. Representatives of the genera Anisops, Enithares, Hydrometra, Micronecta, Microvelia and Ranatra are typically found there, but also Rhagadotarsus kraepelini (which is usually a pond or lake dweller) (Freitag and Pangantihon 2010, current study).
Worth noting is that hygropetric microhabitats are an important habitat for several, partly rare and endemic species, foremost of these being Hebrus philippinus, Hydrometra mindoroensis, Mesovelia vittigera and Ochterus spp. Such habitas are particularly threatened by deforestation and land-use changes since they are prone to drying up when not continuously fed with water from the forested areas or when they are fully exposed to direct sunlight.
Fast flowing or even torrent waters, on the other hand, are typically inhabited by species of Aphelocheirus, Asthenocoris and Hydrotrephes, as well as Rhagovelia raddai amongst the taxa treated here.
A few nepomorphan taxa are particularly attracted to light. Micronecta sp. and Anisops kuroiwae were only retrieved by black light traps in this study. Emergence traps rarely yield aquatic and riparian Heteroptera. We caught only very few specimens of Ochterus polhemusi and Hebrus sp. in such traps.