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What do we know about the phylogeny of the semi-aquatic bugs (Hemiptera: heteroptera: gerromorpha)?

Abstract--The present study summarizes knowledge about phylogenetic relationships of the heteropteran infraorder Gerromorpha. A phylogeny for all families and subfamilies, and for all genera but those assigned to the two most diverse families, Veliidae and Gerridae, is compiled from the many studies by the late Dr. Nils Moller Andersen. Comparisons with recently published studies, including DNA sequence data show that the superfamilies Hydrometroidea and Gerroidea, the family Veliidae, the subfamily Mesoveliinae, and the genera Mesovelia and Microvelia are probably not monophyletic, and that Paraphrynoveliidae, Gerridae, Madeoveliinae, Ocelloveliinae, Veliinae, Haloveliinae and Gerrinae are without convincing diagnostic morphological characters. In Gerridae, phylogenetic hypotheses are available for most subfamilies, and are evaluated against more recent studies indicating that the tribes Metrocorini and Metrobatini, and even well-known genera, such as Aquarius, Limnometra, Tenagogonus and Halobates, are not monophyletic. As taxonomic classifications should be based on observable morphological characters, and at the same time reflect phylogenetic relationships, a considerable task lays ahead in obtaining material of key taxa for DNA sequencing, and in identifying and redescribing clades based on new combinations of diagnostic characters.

Key words: Water striders, parsimony analyses, historical biogeography, ecological phylogenetics.

INTRODUCTION

The Gerromorpha, comprising waterstriders and their allies, is a relatively small yet strikingly diverse heteropteran infraorder in terms of morphology, ecology, and life history adaptations (Andersen, 1982). The size of gerromorphan bugs ranges from tiny representatives of Austrovelia Malipatil and Monteith, 1983 and Cryptovelia Andersen and J. T. Polhemus, 1980, with a body length of less than 1.5 mm, to the gigantic water strider Gigantometra gigas (China, 1925) with legs spanning more than 30 cm. Gerromorphan bugs are known to utilize an exceptionally broad range of habitats ranging from moist litter and mosses in rainforests to all kinds of freshwater habitats, such as rivers, streams, lakes, ponds, and even treeholes, bamboo internodes, and bromeliads, and several lineages have adapted to marine environments (Andersen, 1982; J. T. and D. A. Polhemus, 1991; J. T. Polhemus, 1999). Many species of Gerromorpha are robust organisms of considerable size, and live their entire life on the surface of water, making them relatively easy to observe and collect, and accordingly suitable for studies of locomotion and mating-, feeding- and anti-predatory strategies (Rowe et al., 1994; Spence and Andersen, 1994; Svensson et al., 2002; Perez-Goodwyn et al., 2009; Khila et al., 2012).

Five superfamilies, eight families, and 21 subfamilies of Gerromorpha are currently recognized (Andersen, 1982; Chen et al., 2005; genera and species numbers in families according to J. T. Polhemus and D. A. Polhemus, 2008a): Mesoveloidea: Mesoveliidae: Madeoveliinae, Mesoveliinae (12 genera: 46 species), Hebroidea: Hebridae: Hebrinae, Hyrcaninae (7 genera: 126 species), Hydrometroidea: Hydrometridae: Heterocleptinae, Hydrometrinae, Limnobatodinae (7 genera: 126 species), Paraphrynoveliidae (1 genus: 2 species), Macroveliidae (3 genera: 3 species), Gerroidea: Hermatobatidae (1 genus: 10 species), Veliidae: Haloveliinae, Microveliinae, Ocelloveliinae, Perittopinae, Rhagoveliinae, Veliinae (61 genera: 962 species), and Gerridae: Charmatometrinae, Cylindrostethinae, Eotrechinae, Gerrinae, Halobatinae, Ptilomerinae, Rhagadotarsinae, Trepobatinae (67 genera: 751 species). The relatively depauperate Holarctic faunas are the best known overall (Froeschner, 1988; J. Polhemus Polhemus, 1988b; Smith, 1988a, b, c, d; Andersen, 1995a; J. T. Polhemus and D. A. Polhemus, 1988a, 2008b; Smith, 1988a, b, c, d), but recent studies have addressed the more diverse Australian and Malesian faunas (Andersen and Weir, 2004a; Chen et al., 2005), and faunal surveys of neighboring islands, such as New Guinea (J. T. Polhemus and D. A. Polhemus, 2006a), New Zealand (Lariviere and Larochelle, 2004), Vanuatu (Nieser and Chen, 2005), Fiji (J. T. Polhemus and D. A. Polhemus, 2006b; Zettel, 2007), and New Caledonia (Damgaard and Zettel, in press) provide new evidence for the historical biogeography of the western Pacific Region. Outside this area, recent studies have addressed the diverse faunas of India (Thirumalai, 2002), Myanmar (Zettel, 2011), Ecuador (Buzzetti and Cianferoni, 2011), and Brazil (Moreira et al., 2011).

Gerromorpha has a large and well-documented fossil history dating back to the Early Mesozoic (Andersen, 1998a). Besides providing minimum ages for clades, fossils may provide valuable information on the origin of adaptive traits, e.g., mate-guarding (Andersen and Poinar, 1992), but assignments of fossils to taxa of a quite generalized heteropteran appearance, such as Mesoveliidae and Macroveliidae, remain dubious (Damgaard, 2008a). Dr. Nils Moller Andersen from the Zoological Museum in Copenhagen, Denmark, pioneered studies of the evolution of Gerromorpha by incorporating cladistic reasoning into revisionary studies and in tests of biogeographic and evolutionary scenarios, and his hypotheses of gerromorphan relationships are still the most comprehensive comparative studies across all taxa (obituary and list of Andersen's publications by Spence and Damgaard, 2008). The present study is an attempt to assemble, update, and evaluate phylogenetic relationships of gerromorphan families, subfamilies, tribes and genera of Gerromorpha, and to point out directions for future taxonomic revisions.

PHYLOGENETIC RELATIONSHIPS OF GERROMORPHA

In his book The Semiaquatic Bugs, Andersen (1982) established the phylogeny of Gerromorpha, and diagnosed the clade on presence of three pairs of cephalic trichobothria inserted in deep cuticular pits; quadrangular mandibular levers; pretarsi with one dorsal and one ventral arolium, and the female genital tract with a gynatrial complex, including a long tubular spermatheca and a secondary fecundation canal. Monophyly of Gerromorpha has been confirmed by later studies (e.g., Damgaard, 2008b), whereas relationships with other infraorders remain disputed (Wheeler et al., 1993; Mahner, 1993; Shcherbakov and Popov, 2002; Xie et al., 2008). Andersen's original interpretation of relationships among gerromorphan families and subfamilies is summarized in Fig. 1, and expanded with his hypotheses for relationships within the Hydrometridae (Andersen, 1977), Macroveliidae (Andersen, 1978), Hebridae (Andersen, 1981) and Mesoveliidae (Andersen, 1999a), and with more recently described genera assigned to these families tentatively placed among their presumed closest relatives (Zettel, 1999; Andersen and D. A. Polhemus, 2003; Andersen and Weir, 2004b; J. T. Polhemus and D. A. Polhemus, 2010). During the last few decades, systematics has undergone a remarkable revolution in implementing computer based analyses of DNA sequence data providing large amounts of data applicable that has greatly improved the possibility to build robust phylogenetic reconstructions at all taxonomic levels (Andersen, 1999b; Damgaard, 2006a). A series of these studies have addressed phylogenetic relationships among the major gerromorphan lineages (Muraji and Tachikawa, 2000; Andersen and Weir, 2004b; Damgaard et al., 2005), and have been summarized by Damgaard (2008b) in a parsimony analysis of 64 morphological characters and 2.5 kb of DNA sequence data from the mitochondrial genes cytochrome c oxidase subunit I + II (COI + II) and large mitochondrial ribosomal subunit (16S rRNA) and the nuclear gene large nuclear ribosomal subunit (28S rRNA) obtained from representatives of all families and most subfamilies (Fig. 2). This study confirmed Mesoveliidae as being the sister group to all other families of Gerromorpha and also the close relationship between the Gerridae and Veliidae, but suggested that the superfamilies Gerroidea (Hermatobatidae + Gerridae + Veliidae) and Hydrometroidea (Paraphrynoveliidae + Macroveliidae + Hydrometridae), the family Veliidae, and the gerrid subfamily Cylindrostethinae were paraphyletic entities, and that Gerrinae, Haloveliinae, Ocelloveliinae, Paraphrynoveliidae, and Veliinae lacked convincing morphological diagnostic characters. More recently, Damgaard et al. (in press) studied the phylogeny of Mesoveliidae based on the same compartment of genes, and found that the subfamily Mesoveliinae was also paraphyletic.

STATUS OF THE MAJOR GLADES OF GERROMORPHA

As outlined above there are reasons to be concerned with the current classification of Gerromorpha at all taxonomic levels if this classification is to follow phylogenetic relationships. First of all, the use of superfamilies should be abandoned, since these are either paraphyletic ("Gerroidea" and "Hydrometroidea") or includes a single family ("Mesoveloidea" and "Hebroidea"), which has already been sufficiently well-diagnosed, thus leaving the assignment to a superfamily without any additional content. On the other hand, the Gerromorpha contain six well-supported and/or well-diagnosed clades: Mesoveliidae, Hebridae, Hermatobatidae, Hydrometridae, Macroveliidae + Paraphrynoveliidae and Veliidae + Gerridae, five of which were already recognized by Andersen (1982). In the following, the six clades will be discussed in terms of content of taxa and diagnostic character combinations.

MESOVELIIDAE

This clade is diagnosed on presence of an ejaculatory bulb and pump of the male genital tract, and lack of an embryonic egg-burster, but females are also characterized by a well-developed ovipositor, even though this is probably a plesiomorphic state and has been achieved independently in the gerrid subfamily Rhagadotarsinae (Damgaard, 2008b). Andersen (1982) recognized two subfamilies, Madeoveliinae comprising Madeovelia guineensis Poisson, 1959 from West Africa and Mesoveloidea Hungerford, 1929 comprising two species from the Neotropical Region, and Mesoveliinae containing the majority of species, including those assigned to the cosmopolitan Mesovelia Mulsant and Rey, 1852 and to a number of smaller genera (see Fig. 1). Andersen (1999a) diagnosed Madeoveliinae on the basis of the head being deflected in front of eyes; adults always macropterous and without ocelli; preapically inserted tarsi, and three pairs of cephalic trichobotria forming a more or less irregular pattern on the dorsal surface of the head. He diagnosed Mesoveliinae on absence of a prothoracic suture separating tergum and sternopleuron, and interpreted presence of such sutures in Madeovelia and Mesoveloidea as plesiomorphic, whereas their presence in Cryptovelia was considered a character reversal. Other diagnostic characters included the ejaculatory bulb of the male with a broad pump flange and the anterior end of the egg with a complete pseudopercular rim, even though these characters are unknown in several genera. Damgaard et al. (in press) found that Mesoveloidea williamsi Hungerford, 1929 was the sister species to Mniovelia kuscheli Andersen and J. T. Polhemus, 1980 and that Austrovelia caledonica Malipatil and Monteith, 1983 was the sister species to all other Mesoveliidae, thus making the Mesoveliinae a paraphyletic entity. Mesovelia was also found to be paraphyletic, since an undescribed relative of M. indica Horvath, 1915 from Laos was sister to Phrynovelia, whereas M. amoena Uhler, 1894 was sister to Speovelia maritima Esaki, 1929 and the remaining congeneric species. The paraphyly of Mesoveliinae and the lack of convincing diagnostic characters for both this subfamily and Madeoveliinae, obviously a call for additional studies and perhaps a reclassification of the entire family.

HEBRIDAE

This clade is diagnosed by the bucculae being elevated posteriorly to form a pair of ventral plates; the fourth labial segment being about half as long as the third labial segment; two-segmented tarsi of middle and hind legs (fusion of primitive second and third segments), and genital segments inserted ventrally and slightly before apex of pregenital abdomen (Damgaard, 2008b). Andersen (1982) recognized two subfamilies, Hyrcaninae comprising Hyrcanus Distant, 1911 from the Oriental Region and Nieserius Zettel, 1999 with three species living submerged in fast flowing streams and rivers in East and Southeast Asia, and the cosmopolitan Hebrinae, comprising several genera, including Hebrus Curtis, 1833 with more than a hundred species assigned to several subgenera (see Fig. 1). Andersen diagnosed the Hyrcaninae by the eyes being distinctly removed from the base of the head and the anterior margin of prothorax; the antennae being subequal in length to, or shorter than the head; the third labial segment not surpassing the hind margin of the head capsule, and the dorsal arolium distinctly shorter than the ventral arolium. He diagnosed the Hebrinae by the fourth antennal segment bearing a preapical cluster of modified (blunt) hairs and the female gynatrial complex with a tubular basal thickening of the fecundation canal. Austrohebrus apterus Andersen and Weir, 2004 from Western Australia differs from most other members of the family in having no traces of wings (Andersen and Weir, 2004b). Due to the scarcity of Austrohebrus (only the female holotype and a female paratype are known), the structure of the gynatrial complex has not been investigated, but the presence of a "false joint" on the last antennal segment points to a relationship with the genera Timasius Distant, 1909, Neotimasius Andersen, 1981, Hebrometra Cobben, 1982, and Hebrus Curtis, 1833.

HYDROMETRIDAE

This clade is diagnosed by the elongate postocular part of the head; the unique insertion of the posterior pair of cephalic trichobothria on tubercles; the fourth antennal segment with an apical invagination with sensorial structures, and the vertical inclination of the meso- and metacoxal axes (Damgaard, 2008b). Andersen (1982) recognized three subfamilies, Heterocleptinae, comprising Veliometra schuhi Andersen, 1977 from Brazil and four species of Heterocleptes Villiers, 1948 from the Afrotropical and Oriental regions; the monotypic Limnobatodinae from South America; and Hydrometrinae comprising the majority of species, most of which are assigned to the cosmopolitan Hydrometra Latreille, 1796. Andersen diagnosed the Heterocleptinae by the basis of a very long posterior pair of cephalic trichobothria inserted on prominent rounded elevations and the preapical articulation between the first and second antennal segments; the Limnobatodinae on claws being inserted slightly before tarsal apex (the single species, Limnobatodes paradoxus Hussey, 1925 has a unique, lateral projection of the basal part of the last antennal segment; Andersen, 1982, fig. 193), and Hydrometrinae by the very elongated anteocular part of the head; the length of the first antennal segment subequal to, or shorter than, that of the second segment, and the absence of metasternal glands. He further diagnosed the sister group relationships of Hydrometrinae and Limnobatodinae based on the absence of ocelli, the body with spinous macrohairs, pronotal lobe with almost parallel sides; the forewing venation reduced to two longitudinal veins and 1-2 crossveins; the abdominal tergum without paired, longitudinal ridges; the absence of an abdominal scent gland, and the egg shell with an open air-filled inner layer and outer plastron. Andersen (2003) performed a cladistic analysis of 18 morphological characters obtained from all extant and fossil taxa known to him and found the same relationships for extant taxa as in his 1977 paper, but added important minimum ages for the origin of the various subclades. J. T. Polhemus and D. A. Polhemus (2010) described a new genus, Bacillometroides, for three species previously assigned to Bacillometra Esaki, 1927 leaving the genus monotypic for B. ventralis Esaki, 1927.

HERMATOBATIDAE

The Hermatobatidae are diagnosed by the extremely short and broad head and the lack of a fecundation canal of the female gynatrial

complex. There is a relatively strongly supported sister group relationship between Hydrometridae and Hermatobatidae, which was diagnosed on the basis of upright egg deposition and a distinctly prolonged mesothorax (Damgaard, 2008b). In a comparative study of locomotion gait PerezGoodwyn et al. (2009) found that the doublebipod gait in Hermatobates Carpenter, 1892 differs fundamentally from the syncronous rowing in members of the Halobatinae and Haloveliinae, which further separates the family from the Veliidae + Gerridae clade. The 10 currently recognized species of Herrnatobates are found in the Indian and Pacific Oceans, except H. bredini Herring, 1965, which lives in the Caribbean Sea. Nothing is known about the phylogeny of the contained species that explains this disjunct geographical distribution.

MACROVELIIDAE + PARAPHRYNOVELIIDAE

The Macroveliidae and Paraphrynoveliidae are the smallest of all currently recognized families, the former, consisting only of Macrovelia hornii Uhler, 1872; Chepuvelia usingeri China, 1963 and Oravelia pege Drake and Chapman, 1963, and Paraphrynovelia brinckii, 1957 and P. slateri Andersen, 1978, respectively. Andersen (1978) diagnosed Paraphrynoveliidae by the fourth antennal segment being subdivided and the abdominal tergum of the apterous form lacking paired, longitudinal ridges. He furthermore diagnosed Macroveliidae by the eggs having several micropyles; Macrovelia by the flightless, brachypterous form; Oravelia by the first antennal segment being longer than the head width, and Chepuvelia by the abdominal scent orifice being situated on a prominent tubercle. The sister group relationship between Maerovelia and Oravelia was diagnosed based by the metasternal scent orifice being situated on a prominent elevation, but Andersen (p. 221) stated that the two species are so closely related that they "perhaps do not deserve separate generic status."

No macropterous form is known for any of the two species of Paraphrynovelia, and Damgaard (2008b) could not find any convincing morphological characters to diagnose Paraphrynoveliidae, whereas Macroveliidae was diagnosed by the presence of paired, longitudinal ridges of the basal abdominal tergites and two characters also found in Hydrometridae: Eyes distinctly removed from the anterior margin of prothorax and the dorsally placed metathoracic spiracle. The lack of diagnostic characters for Paraphrynoveliidae and the strong branch support for its sister relationships with Macroveliidae diagnosed by welldeveloped bucculae and the presence of a mesepisternal process, led Damgaard (2008b) to suggest that the two families are synonymous.

VELIIDAE + GERRIDAE

This is by far the largest gerromorphan clade comprising more than 75% of all described species. The clade is strongly supported and diagnosed with convincing synapomorphies, such as the salivary pump being laterally inflated from behind, the preapically inserted pretarsus, and the asymmetrical of reduced parempodia (Damgaard, 2008b). Andersen (1982) placed Veliinae and Perittopinae as sister groups, but was unable to provide convincing diagnostic characters for this clade. He also placed Rhagoveliinae as sister group to this clade based on the second gonapophyses with feathered outgrowths and the gynatrial complex with a fecundation groove and, thus, assigned Haloveliinae + Microveliinae as sister groups. He diagnosed this clade on presence of a distinct dark sclerite in the dorsal wall of the salivary pump, the two segmented middle and hind tarsi (basal segment formed by fusion of the primitive first and second segments), and the fecundation canal with a pump. Finally, he diagnosed all subfamilies but Ocelloveliinae by the lack of ocelli, the forewing with less than six closed cells, and the second gonoeoxae fused medially.

Damgaard (2008b) found strong support for Gerridae, but the lack of diagnostic characters for the clade led him to question whether Haloveliinae and Microveliinae should be included as subfamilies in the already quite diverse family Gerridae. Several synapomorphies suggest this, such as the presence of a female fecundation pump (although the structure of the pump differs between clades), the two-segmented fore-tarsi (further reduced to one segment in Microveliinae), the middle and hind leg tarsi composed of primitive first and second segments, and the gynatrial complex with a fecundation canal. The most important argument against including Haloveliinae and Microveliinae in Gerridae is the poor branch support for the entire clade compared with the strong support for Gerridae alone, and the unresolved relationships of the remaining veliid subfamilies. Damgaard (2008b) found Veliinae, Rhagoveliinae, Perittopinae, and Ocelloveliinae as succeeding sister taxa to the Gerridae + Haloveliinae + Microveliinae clade, but with poor support and diagnostic character combinations, except for Gerridae + Veliidae less Ocelloveliinae, which foremost could be diagnosed by the lack of ocelli in the macropterous morphs, although this character is found convergently in Madeoveliinae and Hydrometrinae + Limnobatodinae.

The Veliidae + Gerridae clade thus comprises several subclades, which qualifies for--or potentially may qualify for--the status as families or subfamilies, but in comparison with other gerromorphan taxa of similar status, very few deeper subclades are at the same time well supported and convincingly diagnosed. If paraphyly of Veliidae stands the test against inclusion of more data both in terms of taxa and characters, and if convincing character combinations are found for diagnosing Gerridae, one has to accept that one or more, or perhaps all, of the subfamilies currently assigned to Veliidae should be given a similar status. Damgaard (2008b) found that in Gerridae, no relationships among subfamilies were strongly supported, but several were diagnosed with convincing morphological character combinations. Halobatinae + Ptilomerinae were diagnosed by the distal process of the fore tibia, the bilobed apices of the second female gonapophyses, and reduced evaporative channels of the metathoracic scent apparatus. Rhagadotarsinae + Trepobatinae were diagnosed on the absence of a metathoracic scent apparatus; and Eotrechinae + Gerrinae on the sclerotized inner lobes of the first female gonapophyses, divided glandular areas of the female gynatrical sac, and the reduced evaporative channels of the metathoracic scent apparatus. Eotrechinae + Gerrinae + Charmatometrinae + Cylindrostethinae was monophyletic, but without any diagnostic character combinations. The sister group relationship between this clade and the Rhagadotarsinae + Trepobatinae clade were diagnosed by the presence of four pairs of cephalic trichobothriae, possibly a strong synapomorphy for a more narrowly defined Gerridae, but this would leave Halobatinae + Ptilomerinae without a family assignment. While the family assignments of various clades in the Veliidae + Gerridae clade remain highly uncertain, there is relatively convincing support for the 14 contained subfamilies, which are evaluated below.

OCELLOVELIINAE

This subfamily comprises two species and one subspecies of Ocellovelia China and Usinger, 1949 from South Africa. Andersen (1982) placed Ocelloveliinae as sister group to all other Veliidae and diagnosed it by the egg possessing more than four micropyles, the female with laterotergites 8 fused with laterotergites 7, and the phallobase closed dorsally, but Damgaard (2008b) did not recognize any convincing synapomorphies for the group.

VELIINAE

This subfamily comprises the Old World genera Angilia Sffd, 1865 Angilovelia Andersen, 1981, and Velia Latreille, 1804 and the New World Oiovelia Drake and Maldonado-Capriles, 1952, Paravelia Breddin, 1898 and Platyvelia J. T. Polhemus and D. A. Polhemus, 1993, Steinovelia J. T. Polhemus and D. A. Polhemus, 1993, Stridulivelia Hungerford, 1929, and Veloidea Gould, 1934. Andersen (1982) diagnosed the Veliinae by the female fore tibia bearing a grasping comb, but this character was not used by Andersen and Weir (2004b). Damgaard (2008b) included representatives of PIatyvelia, Steinovelia, Stridulivela, and Velia, but found poor branch support and no convincing diagnostic characters for Veliinae, and suggested that the monophyly of the subfamily should be tested by inclusion of more taxa.

RHAGOVELIINAE

This subfamily includes the well-known riffle-bugs (Rhagovelia Mayr, 1865) comprising almost 300 species in all parts of the world, and with the subgenus Neorhagovelia Matsuda, 1956 from the Neotropical Region. Rhagoveliinae contains two additional genera from the Oriental region: Tetraripis Lundblad, 1936 with approximately 10 species and the monotypic Chenevelia Zettel, 1996. D. A. Polhemus (1997) analyzed the phylogeny of Neotropical members of Rhagovelia s.str, and assigned the members to presumably monophyletic species complexes and species groups. He found Tetraripis to share several characters with members of the Veliinae, and suggested a reclassification of the two subfamilies. This was disputed by Andersen (2000), who maintained that the microstructure of the middle tarsal swimming fan was homologous. Damgaard (2008b) included Chenevelia stridulans Zettel, 1996 and several representatives of Rhagovelia, and found relatively strong support for the group. Due to its great diversity and enormous distribution, a better understanding of the phylogeny of Rhagovelia species will undoubtedly provide important information for studies of systematics, biogeography, conservation biology, and ecology (J. T. Polhemus and D. A. Polhemus, 1988a, 1988b; Zettel and Duc, 2004; D. A. Polhemus and Andersen, 2010)

PERITTOPINAE

This subfamily comprises Perittopus Fieber, 1861 from the Oriental Region, and is convincingly diagnosed by their two-segmented fore tarsi (fusion of second and third segment) and the apical veins of the fore wing strongly reduced (0-1 closed cells in the wing) (Andersen, 1982; Damgaard, 2008b), but nothing is known about the relationships among the more than 10 currently known species.

HALOVELIINAE

This subfamily consists of two freshwater genera, Entomovelia Esaki, 1930 and Strongylovelia Esaki, 1924, from the Oriental and Malesian regions, and Halovelia Bergroth, 1893, Haloveloides Andersen, 1992; Ocheovelia J. T. Polhemus and D. A. Polhemus, 2006, and Xenobates Esaki, 1927 from marine habitats in the Indo-Pacific Region. Andersen (1989) investigated the phylogeny of 30 species of Halovelia and outgroup taxa from Entomovelia, Strongylovelia, and Xenobates on the basis of 46 morphological characters, and divided the genus into a number of monophyletic species groups, but could not find convincing evidence for relationships among the different genera. Damgaard (2008b) included representatives of Halovelia, Strongylovelia, and Xenobates and found a strong sister group relationship between the two marine genera, Halovelia and Xenobates, but no synapomorphies for the Haloveliinae, even though the clade had strong branch support.

MICROVELIINAE

This is the largest of all subfamilies of Gerromorpha with approximately 40 genera and about 350 described species (Chert et al., 2005; J. T. Polhemus and D. A. Polhemus, 2008b), and hundreds of species still awaiting formal description (J. T. Polhemus and D. A. Polhemus, 2008a). The Microveliinae are convincingly diagnosed based on their one-segmented fore-tarsi (Damgaard, 2008b). Stys (1976) established three tribes, Hebroveliini for the Afrotropical Hebrovelia Lundblad, 1939 from tropical Africa, Velohebrini for Velohebria antennalis Stys, 1976 from Papua New Guinea, and Microveliini for the remaining species, but Andersen (1982) and Andersen and Weir (2001) questioned whether this tribal subdivision could be upheld, since Hebrovelia and Velohebria may have descended from members of the Microveliini. Approximately 150 species are recognized in the genus Microvelia Westwood, 1834, but this genus is chiefly defined by the absence of structural specializations found in other microveliine genera and is not monophyletic (Damgaard, 2008b). Andersen and Weir (2003) performed a cladistic analysis of the Australasian genera of Microveliinae and separated Microvelia into a series of subgenera and presumably monophyletic species groups.

RHAGADOTARSINAE

This gerrid subfamily is diagnosed by a unique combination of characters, although all of them are either homoplasies, such as thornlike microstructures on the cuticulum (also found in Veliidae) and well-developed bucculae (also found in Hebridae, Hydrometridae and Macroveliidae) or reversals, such as the well-developed female ovipositor, the presence of second gonocoxae, the sclerotization of second gonapophyses, the absence of glandular areas of the female gynatrial sac, and the absence of a fecundation pump of the female gynatrial complex (Damgaard, 2008b). Only two genera are known, Rhagadotarsus Breddin, 1905 with a monotypic Afrotropical subgenus Caprivia China, 1931 for R. (C.) hutchinsoni China, 1931 and Rhagadotarsus s.str. with four species in East Asia and Australia and Rheumatobates Bergroth, 1892 with more than 30 species from the New World, whose phylogeny was examined by a parsimony analysis of 102 morphological characters (Westlake et al., 2000).

TREPOBATINAE

This clade is diagnosed on the short epipharyngeal projection and the shortened middle femora (Damgaard, 2008b). Four tribes are currently recognized: Metrocorini, Naboandelini and Trepobatini from freshwater habitats from most parts of the world and Stenobatini from marine environments in Malesia and Australia. Trepobatini includes Cryptobates Esaki, 1929; Gnomobates J. T. Polhemus and D. A. Polhemus, 1995; Lathriobatoides J. T. Polhemus, 1994; Cryptobatoides J. T. Polhemus, 1991; Halobatopsis Bianchi, 1896; Ovatometra Kenaga, 1942; Telmatometra Bergroth, 1908; Telmatometroides J. T. Polhemus, 1991; Trepobates Uhler, 1894, and Trepobatoides Hungerford and Matsuda, 1958, all found in Asia and in the New World. Metrobatini includes Metrobates Uhler, 1871 from the New World and Rheumatometra Kirkaldy, 1902; Metrobatopsis Esaki, 1926; Andersenella J. T. Polhemus and D. A. Polhemus, 1993; Ciliometra J. T. Polhemus and D. A. Polhemus, 1993; Metrobatoides J. T. Polhemus and D. A. Polhemus, 1993; Iobates J. T. Polhemus and D. A. Polhemus, 1993; Stygiobates J. T. Polhemus and D. A. Polhemus, 1993 and Talaudia J. T. Polhemus and D. A. Polhemus, 2002) from New Guinea and Australia. Naboandelini includes Naboandelus Distant, 1910; Hynesionella Poisson, 1949, and Calyptobates J. T. Polhemus and D. A. Polhemus, 1994 from the Old World tropics, and Stenobatini includes Stenobates Esaki, 1927; Rheumatometroides Hungerford and Matsuda, 1958; Pseudohalobates J. T. Polhemus and D. A. Polhemus, 1996; Thetibates J. T. Polhemus and D. A. Polhemus, 1996 and Talaudia J. T. Polhemus and D. A. Polhemus, 1996 from the Malesian and Australian Regions. J. T. Polhemus and D. A. Polhemus (2002) investigated the relationships of Trepobatinae in a parsimony analysis of 39 morphological characters, and found a sister group relationship between Stenobatini and Naboandelini, and with Trepobatini as sister group to this clade when characters were treated as ordered, whereas the relationships between Metrobatini and Trepobatini were unresolved when characters were treated as unordered. Damgaard (2008b) included a few species of Trepobatinae representing all tribes, and found Metrobatini to be paraphyletic since Metrobates was sister group to Trepobates, whereas Rheumatometra was the sister group to Hynesionella and Rheumatometra. The sister group relationship of Stenobatini and Naboandelini was also rejected since Rheumatometroides was sister group to the Hynesionella + Rheumatometra clade. While the relationships of Trepobatinae should be investigated with an expanded taxon sample, the sister group relationship between the New World genera Trepobates and Metrobates makes good sense from a biogeographical perspective compared with the geographically disjunct relationship between Metrobates and the remaining Australian and Malesian members of Metrobatini.

PTILOMERINAE

This clade is diagnosed by the dorsoventrally flattened fourth antennal segment and lack of sclerotization on the accessory scent gland (a reversal shared with Eotrechinae). The clade comprises a number of taxa from central and tropical Asia and New Guinea (Ptilomera Amyot and Serville, 1843; Heterobates Bianchi, 1896; Potamometra Bianchi, 1896; Rheumatogonus Kirkaldy, 1909; Jucundus Distant, 1910; Rhyacobates Esaki, 1923; Pleciobates Esaki, 1930; Potamometropsis Lundblad, 1933; Andersenius Zettel and Chen, 1996; Stridulobates Zettel and Thirumalai, 2000; Pleciogonus Chen, Nieser and Wattanachaiyingcharoen, 2002; Archaeoptilomera Zettel, 2009; Celerobates Zettel, 2009, and Ptilomerella Zettel, 2009) and one genus, Potamometroides Hungerford, 1951, from Madagascar. Many species of Ptilomerinae are only known from apterous morphs; macropterous forms are rare and often dealate (Chen et al., 2005). This condition limits the number of characters for phylogenetic analyses, but is compensated by great variation in the relative lengths of antennal segments, in the patterns of hairs on the middle legs facilitating skating on fast flowing waters, and the many modifications in the antennae and male genital segments that possibly play important roles in courtship. Despite these promising character systems, surprisingly little is known about the phylogeny of Ptilomerinae. Damgaard (2008b) found a sister group relationship between Potamometropsis and Potamometroides and between Ptilomera and Rheumatogonus. The nesting of Potamometroides from Madagascar within a clade of otherwise Asian and Malesian taxa calls for attention from a biogeographical perspective.

HALOBATINAE

This subfamily includes the famous sea-skaters, Halobates, Eschscholtz, 1822, with more than 40 species of the subgenus Halobates s.str, and three species in the subgenus Hiliella China, 1958. Most species inhabit estuaries, mangroves, and coral reef flats in tropical areas, but five species, unique among insects, have colonized the open ocean (Andersen and Cheng, 2004). One species of each subgenus is found in lotic freshwater habitats, which is the same type of habitat occupied by their presumed closest relative, Austrobates rivularis Andersen and Weir, 1994 (Andersen and Weir, 2004a). Aslepios Distant, 1915, with four species in marine habitats of East and Southeast Asia, and Austrobates and Halobates together comprises the tribe Halobatini, whereas the remaining genera are assigned to the tribe Metrocorini occuring in limnic habitats in the Old World tropics and subtropics. Damgaard et al. (2000a) analysed the relationships of Halobatinae in a combined analysis of 780 bp of COI and 64 morphological characters modified from Andersen (1991) and Andersen and Weir (1994), and confirmed the monophyly of Halobatini and its contained genera, but found considerable incongruence between the molecular and morphological data sets. Analyses of COI alone showed that both Austrobates and Hiliella were nested within Halobates s.str. Damgaard (2008b) confirmed the sister group relationships between Asclepios and Halobates, but found Ventidius Distant, 1910 to be more closely related to the Halobatini than to Metrocoris Mayr, 1865, thus implying a paraphyletic Metrocorini. The intraspecific variation in the five open- ocean species of Halobates was investigated by Andersen et al. (2000).

CHARMATOMETRINAE

This clade is diagnosed by the distinct lateral intersegmental suture between meso- and metathorax, a reversal also found in Rhagadotarsinae + Trepobatinae (Damgaard, 2008b). The clade comprises three genera from the Neotropics: Charmatometra Kirkaldy, 1899 with a single species, C. bakeri (Kirkaldy, 1898); Eobates Drake and Harris, 1934 with a single species, E. vittatus (Shaw, 1933), and Brachymetra Mayr, 1865 with less than 10 species.

CYLINDROSTETHINAE

This clade includes the circumtropical Cylindrostethus Fieber, 1861 with less than 20 species, and the Neotropical Platygerris White, 1883 with three species, and Potamobates Champion, 1898 with almost 20 species. Damgaard (2008b) included two species of Cylindrostethus and one species of Potamobates and found that the latter was sister group to a clade comprising representatives of Charmatometrinae, Eotrechinae and Gerrinae, thus making Cylindrostethinae paraphyletic. This relationship was poorly supported and quite surprising since Cylindrostethinae is diagnosed by the heavily sclerotized second female gonapophyses, which is a seemingly convincing diagnostic character, and the subfamily is therefore recognized until a more convincing relationship among its contained genera is provided. Species phylogenies have been constructed for both Cylindrostethus (D. A. Polhemus, 1994) and Potamobates (J. T. Polhemus and D. A. Polhemus, 1995; Cognato, 1998; Buzzetti, 2006; Padilla-Gil and Damgaard, 2011) on basis of 10-11 morphological characters.

EOTRECHINAE

Eotrechinae from the Oriental Region includes the only hygropetric and terrestrial species of Gerridae known, and is therefore an interesting example of a reversal to the type of habitat considered to be ancestral for the infraorder (Andersen, 1982). The clade comprises two monotypic genera, Chimarrhometra Bianchi, 1896 with C. orientalis (Distant, 1879) and Tarsotrechus Andersen, 1980 for T. polhemi Andersen, 1980, and three genera with a higher diversity, Eotrechus Kirkaldy, 1902, Amemboa Esaki, 1925 (includes the subgenus Amemboides Andersen, 1984), and Onychotrechus Kirkaldy, 1903. Andersen (1984) constructed a hypothesis for the relationships among genera (Fig. 3) and Andersen (1998b) provided a species phylogeny for Eotrechus on basis of 19 morphological characters.

GERRINAE

Members of this subfamily are among the most frequently encountered water striders throughout the world, and include the three principal northern temperate genera Gerris Fabricius, 1794 (incl. subgenera Gerris s.str., Gerriselloides Hungerford and Matsuda, 1958 and Macrogerris Andersen, 1993), Aquarius Schellenberg, 1800, and Limnoporus Stal, 1868, which are among the most studied taxa in terms of adaptation and ecology (e.g., Spence and Andersen, 1994). Andersen (1995b) analysed the phylogenetic relationships of the gerrine genera, and confirmed the two tribes Tachygerrini comprising Eurygerris Hungerford and Matsuda, 1958 and Tachygerris Drake, 1957 from the Neotropical Region and Gerrini comprising Aquarius, Limnoporus, Gerris, Gigantometra Hungerford and Matsuda, 1958, Tenagogerris Hungerford and Matsuda, 1958 from Australia; Gerrisella Poisson, 1940, Tenagometra Poisson, 1949 and Tenagometrella from the Afrotropical Region; Gigantometra; Limnometra Mayr, 1865 and Tenagogonus, Stal, 1853 from the Old World tropics and Limnogonus, Stall, 1868 and Neogerris Matsumura, 1913 each with a pan-tropical distribution. Andersen (1995b) diagnosed Tachygerrini by the location and structure of the metasternal scent apparatus, the venation of the forewing, and the retainment of pretarsal aroliae, even though this character is is probably plesiomorphic. He diagnosed Gerrini by the absence of a distinct lateral, intersegmental suture between meso- and metanotum, and the loss of pretarsal aroliae in the adult stage, even though he earlier had described this characters for Aquarius chilensis (Berg, 1888) (Andersen, 1990). Andersen (1995b) found a close relationship between Limnometra, Tenagogonus, and Tenagometra, and diagnosed the clade by the dark, median longitudinal stripe of pronotum and the membraneous inner lobes of the first gonapophyses. Chen et al. (2005: 355) stated that Limnometra and Tenagogonus are morphologically very similar, but differently shaped, with Limnometra having an elongated abdomen and long connexival spines and Tenagogonus with a short and robust abdomen and short connexival spines. Andersen (1995b) diagnosed the relationship between Limnometra and Tenagogonus based on the relatively long fourth antennal segment, the predominantly pale pronotum, and the convex plate-shaped accessory scent gland sclerite, while Andersen and Weir (1997) distinguished between Limnometra + Tenagogonus and the Australian genus Tenagogerris on basis of the relatively short rostrum in Tenagogerris. Aquarius, Gerris and Limnoporus is another clade of closely related genera diagnosed by the anterior margin of pronotum being elevated behind the eyes, the presence of a low. median carina on the pronotum, the length of first segment of middle tarsus being more than 2.5 (but less than 4) times that of the second segment; the mesosternum less than twice length of metasternum, and the short, conate parameres, whereas the sister group relationship between Gerris and Aquarius is diagnosed by the basis of the first antennal segment being shorter than half the body length; the absence of a pale median stripe on the pronotal lobe, the thickened fore femora of the female, and the incrassate fore femora of the male (Damgaard and Cognato, 2005). The phylogeny of the three genera has been studied intensively (Andersen, 1990, 1993, 1995b; Andersen and Spence, 1992; Sperling et al., 1997; Damgaard et al., 2000b; Damgaard and Sperling, 2001; Damgaard and Cognato, 2003, 2005). These studies were summarized by Damgaard (2008c) in a parsimony analysis of 66 morphological characters and almost 2.3 kb og DNA and with inclusion of all species of Aquarius and Limnoporus and with a dense sampling of Gerris species representing all subgenera and species groups according to Andersen (1993) and Damgaard and Cognato (2005). The resulting phylogeny confirmed many of the species groups from earlier studies, but more importantly showed that Aquarius was polyphyletic since A. remigis (Say 1832) and its closest relatives were found to be the sister group to Gerris, whereas A. chilensis (Berg, 1881) was placed unresolved at the root of the tree. The great availability of material from representatives of this clade has allowed detailed studies of genetic and morphological diversity and phylogeography (Abe et al., 2005; Damgaard, 2005, 2006, 2008c; Damgaard and Zettel, 2004; Gagnon and Turgeon, 2010). According to Andersen (1995b), the sister group to Limnoporus + Aquarius + Gerris is Gigantometra gigas and the relationship is diagnosed by the close position of the metathoracic spiracle in relation to wing base, the presence of apical and vesical sclerites, and a relative short proximal part of the fecundation canal. Neogerris (as Limnogonellus Hungerford and Matsuda, 1959) was treated as a subgenus of Limnogonus by Matusuda (1960), but was raised to generic rank by Andersen (1975), who found several diagnostic differences between these taxa. According to Andersen (1995b), Limogonus and Tenagometrella are sister groups diagnosed by a conical turbercular scent area surrounding metasternal scent orifice. Poisson (1965) separated some African and Mascarene Limnogonus as a distinct subgenus, Limnogonoides, but Andersen (1995b) was not able to define the subgenus on the basis of any convincing characters, and Damgaard et al. (2010) found that neither Limnogonus s.str, nor Limnogonoides was monophyletic, and suggested that the two subgenera should be synonymized. Limnogonus and Tenagometrella have the same bilobate scent reservoir as Neogerris, and the same circular structure of the scent orifices as this genus and Gerrisella. Andersen considered these structures to be independently derived and placed Neogerris as sister group to Gerrisella on basis of the relatively short and broad head, the absence of claws on the middle tarsus, and other structures in the metasternal scent apparatus. While the close relationship between Eotrechinae and Gerrinae has never been seriously challenged, the relationships within Gerrinae are in urgent need of a revision based on phylogenetic relationships. Fig. 3 synthesizes Andersen's idea of phylogenetic relationships within the two subfamilies.

CONCLUSION

According to Weirauch and Schuh (2011: 492) "Gerromorpha, or semiaquatic bugs, have achieved model status in Heteroptera systematics thanks to the early (from 1980's onward) and rigorous efforts of Nils Moller Andersen."

The many para- and polyphyletic groups revealed by the present review may seem to contradict this statement, but most of the families and subfamilies recognized by Andersen have in fact been either confirmed as monophyletic by subsequent studies, or are diagnosed on seemingly convincing synapomorphies, even though they still await to be tested against DNA sequence data. Poly- and paraphyly has also been demonstrated in a number of genera and may be a real challenge in the systematics of Geromorpha, since few studies of higher taxonomic levels include more than one or a few species of each genus. Another unfortunate tendency is "over-splitting" of taxa, for instance in the Microveliinae, where every specially adapted clade has been described as a new genus, and many clades of Microvelia as new subgenera, but without any knowledge about the underlying phylogenetic relationships. Andersen fully understood the importance of a quantitative approach both in comparison of taxa and inclusion of characters, when he pioneered computer based analyses, and when he later fully embraced the revolution brought forward by molecular systematics. He also knew that Gerromorpha is a difficult group to work with because adaptation to the strong environmental and physiological constraints associated with life on the surface of water leading to an abundance of convergent evolution. Since species may be the only "real thing" in systematics, everything else taxonomically being human abstractions, many phylogenetic studies have addressed particular genera, often those that can be used to investigate important evolutionary or biogeographical scenarios. While such an approach certainly can provide new insight and inspire new testable hypotheses, it avoids the difficult albeit crucial task of evaluating just how well the genera are diagnosed. In order to do this, one has to obtain the relevant in- and outgroup taxa for DNA sequencing, something that often necessitates many long and expensive collecting trips to remote parts of the World. The relatively few genetic loci broadly applied for gerromorphans have provided relatively well-corroborated and well-supported phylogenies and, since they exist in many copies per cell and are without introns, they can be amplified and sequenced using standard molecular techniques on both pinned and ethanol-preserved material. Entire mitochondrial genomes are now available for representatives of the Hydrometridae and Gerridae greatly improving the availability of new primers for PCR-amplification and sequencing (Hua et al., 2009). Whole genomes of representatives of many insect orders are now available through Genbank, which will further facilitate the use of molecular systematics. In conclusion, even with the dwindling number of active researchers working with Gerromorpha, the statement by Weirauch and Schuh may still hold true because the availability of testable hypotheses in Gerromorpha is unparalled in the Heteroptera.

ACKNOWLEDGEMENTS

This paper is dedicated to Toby Schuh in a recognition of his long and distinguished career in studying heteropteran systematics and inspiring and bringing together people from all over the world to collaborate on solving the mysteries of this fascinating clade of insects. I express my gratitude to the VILLUM KANN RASMUSSEN FOUNDATION and everybody at the Entomological Department and the Laboratory for Molecular Systematics at the Natural History Museum of Denmark for supporting me over the years, and especially to my late supervisor Nils Moller Andersen for providing a unique opportunity to begin my own independent research career. I also thank Christine Johnson and Thomas J. Henry for inviting me to participate in this Festschrift and Thomas J. Henry and Dan A. Polhemus for providing valuable comments on an earlier version of this manuscript.

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JAKOB DAMGAARD

Laboratory of Molecular Systematics, Natural History Museum of Denmark, Solvgade 83 Opg. S. 1307 Copenhagen K, Denmark, e-mail: jdamgaard@snm.ku.dk

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