A comparison of the external morphology and functions of labial tip sensilla in semiaquatic bugs ( Hemiptera : Heteroptera : Gerromorpha )

The present study provides new data on the morphology and distribution of the labial tip sensilla of 41 species of 20 gerromorphan (sub)families (Heteroptera: Gerromorpha) obtained using a scanning electron microscope. There are eleven morphologically distinct types of sensilla on the tip of the labium: four types of basiconic uniporous sensilla, two types of plate sensilla, one type of peg uniporous sensilla, peg-in-pit sensilla, dome-shaped sensilla, placoid multiporous sensilla and elongated placoid multiporous subapical sensilla. Based on their external structure, it is likely that these sensilla are thermo-hygrosensitive, chemosensitive and mechanochemosensitive. There are three different designs of sensilla in the Gerromorpha: the basic design occurs in Mesoveliidae and Hebridae; the intermediate one is typical of Hydrometridae and Hermatobatidae, and the most specialized design in Macroveliidae, Veliidae and Gerridae. No new synapomorphies for Gerromorpha were identified in terms of the labial tip sensilla, multi-peg structures and shape of the labial tip, but eleven new diagnostic characters are recorded for clades currently recognized in this infraorder. One synapomorphy is recorded for Hydrometridae + Hermatobatidae + Macroveliidae + Veliidae + Gerridae; five for Macroveliidae + Veliidae + Gerridae; one for Veliidae + Gerridae; and one for Gerridae alone. Within Gerridae, one possible autapomorphy is recorded for Trepobatinae, and two possible autapomorphies for Rhagadotarsinae. We also record a possible autapomorphy for Cylindrostethinae, which indicates that this taxon is monophyletic, as is indicated by other morphological characters. Finally, we record one putative diagnostic character for Macroveliidae + Veliidae, two families that are otherwise not considered sister groups. This new set of characters based on features of the mouthparts of gerromorphan bugs might provide new insights into the cladogenesis of the infraorder.


IntroductIon
Mouthparts of Gerromorpha, or semiaquatic bugs, are of the same piercing and sucking type found in other hemipterous insects.The rostrum or beak consists of a foursegmented, sheath-like labium enclosing two pairs of long stylets, the outer mandibular and inner maxillary pair (Cranston & Sprague, 1961;Cheng, 1966;Andersen, 1982;Schuh & Slater, 1995).
In Gerromorpha and other taxa of Heteroptera, the apical segment of the labium is tripartite and consists of two lateral lobes and a middle lobe (apical plate) situated on the ventral side.There are only sensory structures on the tips of the lateral lobes (Cobben, 1978;Hatfield & Frazier, 1980).
In hemipteran insects the labial tip sensilla monitor the outer surface of the plant or animal food source, whereas the inner sensory organs (e.g., cibarial sensilla) come into direct contact with the food as it is ingested (Miles, 1958;Backus, 1985;Chapman, 1998).Consequently, the labial tip sensilla can apparently perform both chemosensory and mechanosensory functions while bugs explore the surface of plants or animals with their labium.
Little is known about the labial sensilla of Gerromorpha.In earlier studies it was revealed that the presence of sensory hairs on the labial tip is helpful in localizing suitable spots for penetration of the prey (Cheng, 1974;Andersen, 1976Andersen, , 1977)).Cobben (1978) indicates that in the gerrid Aqua rius najas (De Geer) the terminal labial sensilla (which he called nipple-like sensilla) are more or less spherical and can be inserted into solid cuticle.There is little data on the typical gustatory sensilla in Gerromorpha, and only in the gerrid Aquarius remigis (Say) is the epipharyngeal sense organ described as gustatory (Cranston & Sprague, 1961).We decided to conduct a morphological study of the labial tip sensilla of gerromorphan groups in order to determine their diversity and phylogenetic significance.Damgaard (2008) has proposed a new cladogenesis for the Gerromorpha and reclassified the superfamilies Ger-table 1. Description, terminology and definition of insects sensilla used in the present study with examples of sensilla on the labium of various hemipteran insects are also presented.Abbreviations used for gerromorphan taxa in the present study are explained in the last column.
Chemosensitive dendrites do not branch, extend to tip of cone, entirely enclosed within dendritic sheath.One mechanosensitive dendrite ending in tubular body at the base of the cone (Chapman, 1998).Hairs or various cones (peg and basiconic, short or tall) with single pore at or close to the tip.Sensilla are exposed and articulated in a flexible socket.
Dendrites extending into the lumen of the cone lack a sheath, are branched or unbranched, may approach the cuticle of pore plates (Altner & Prillinger, 1980;Chapman, 1998).
A non-typical intermediate (uniporous/multiporous) sensillum basiconicum D was described by Peregrine (1972) and Gaffal (1981) on the labial tip of Dysdercus fasciatus and D. intermedius (Heteroptera: Pentatomomorpha: Pyrrhocoridae).This sensillum is short (5-7 µm height) and differs in ultrastructure from the tall (20 µm) sensilla basiconica A (10 pairs).The basal two-thirds of the shaft is thick-walled due to the presence of a lamellated endocuticular layer and traversed by perforations to about the same extent as the surrounding labial cuticle.The apical third lacks an endocuticular layer and is more extensively perforated with a system of pore tubules extending into the shaft lumen.The sensillum is innervated by two dendrites, one of which has a six-fold greater diameter (1.2 µm) than the other (0.2 µm).Both dendrites extend to the tip of the sensillum and are surrounded by a dendritic sheath whose apical part is irregularly wrinkled.This folded part has thinner walls and opens into the lumen of the sensillum.Many tubules terminate immediately at the tip of the dendritic sheath.
Based on this description, the sensillum may represent an intermediate form between a single-walled type of wall-pore olfactory sensillum and a terminal-pore gustatory sensillum.Similar intermediate type of sensillum basiconicum has been described in other species of Pyrrhocoridae, Dysdercus fulvoniger and D. koenigii, by Schoonhoven & Henstra (1972).
roidea and Hydrometroidea, which previously were distinguished by Andersen (1982), Muraji & Tachikawa (2000), Andersen & Weir (2004) and Damgaard et al. (2005).He suggests a close relationship between Hydrometridae and Hermatobatidae, and indicates a sister-group relationship between Hermatobatidae + Hydrometridae and Hebridae (although this was poorly supported and there were no convincing diagnostic morphological synapomorphies).Moreover, he also points out that Veliidae are a non-monophyletic group and proposes that the subfamilies Ocelloveliinae, Veliinae, Rhagoveliinae, Perittopinae, and Microveliinae + Haloveliinae be raised to family rank.
Many unresolved questions and absence of a strong confirmation of relationships between some of the gerromorphan groups are good reasons for looking for a new set of characters for establishing the monophyly of groups and their close relationships.
The aim of this paper is to find a new set of characters in Gerromorpha.The objectives include: 1. Determine whether there are differences in structure, distribution and number of the labial tip sensilla in the various (sub)fami-lies.2. Attempt to identify the probable functions of the sensilla.3. Identify the characters of the labial tip sensilla in the different systematic groups of Gerromorpha, which might be useful in future phylogenetic studies.

MAterIAl And MetHods terminology used to describe apical sensilla
In this study, the terminology and classification of the apical labial sensilla is mainly based on the morphological criteria of sensilla established by Altner &Prillinger (1980) andZacharuk (1980).The proposed functions of the sensilla of gerromorphan species are based on the morphological and ultrastructural characteristics of the receptors of insects described by several authors (Slifer, 1970;Altner & Prillinger, 1980;Zacharuk, 1980;Chapman, 1998Chapman, , 2003)).The ultrastructural and general morphological characteristics of sensilla of insects, as well as some examples of sensilla in different hemipteran insects similar to the currently described shapes and functions of labial tip sensilla of the Gerromorpha, are presented in Table 1 (except for the unimodal no-pore mechanosensitive sensilla such as tactile hairs and campaniform sensilla which have not been identified on the labial tip).Some of the information on the structure of sensilla used to describe previously unrecorded labial tip sensilla such as sensillum basiconicum, double -(Tp-bds), sensillum basiconicum with a flat socket -(Tp-bfs), oval plate sensilla -(Tp-opls), triangular plate sensilla -(Tp-tpls), and elongated placoid sensilla (Wp-ples), came from the interpretation of insect sensilla by the aforementioned authors.The remaining types of sensilla mentioned in the present paper (Table 1) were previously described by the following authors: Altner & Prillinger (1980), Zacharuk (1980), Chapman (1998), Brożek & Chłond (2010).

taxa sampled
This study is based on dry material of 41 species belonging to 20 (sub)families (listed in Table 2).All material was dissected (i.e., all heads) and the basal part of the head with the labium was glued vertically onto the stage of a scanning microscope, coated The original phylogenetical tree of Damgaard ( 2008) is inserted in Fig. 75, in order to present the relationships of gerromorphan families proposed by this author.
Schemas of the shapes and number of labial tip sensilla in individual species are summarized in Table 2.
The two wide rows (Fig. 3     Np-pip -peg-in-pit sensillum, no-pore; Tp-bfs -basiconic sensillum with a terminal pore inserted in a flat socket; Tp-bhstall basiconic sensillum with a terminal pore; Tp-bss -short basiconic sensillum with a terminal pore; Tp-ds -dome-shaped sensillum with a terminal pore; Tp-opls -oval plate-like sensillum with a terminal pore; Tp-tpls -triangular plate-like sensillum with a terminal pore; Wp-pls -placoid sensillum, multiporous. usually located distant from the labial tip (Figs 5,71).The pegs of these structures are directed towards the end of the terminal segment and are connected with its inner side.

Morphological types, classification and functions of the labial tip sensilla
The labial tip sensilla are classified into four (I-IV) main morphological and functional categories, within which there are currently eleven types of sensilla (Np-pip, Tptpls, Tp-opls, Tp-ds, Tp-ps, Tp-bhs, Tp-bss, Tp-bfs, Tpbds, Wp-pls, Wp-ples) differentiated on the basis of their external appearance and location.The criteria for recognition of these sensilla are based on external morphological characters, identifying the types of sensilla by the system of pores (visible or not), the relative position of the sensilla with respect to the surface of the labium, whether short or tall (flexible or inflexible sockets), and the general shape of the sensilla.Their presumed functions correspond with external morphology (Table 1).
np-pip -peg-in-pit sensillum (poreless coeloconic sensillum) (Fig. 7).The short and smooth peg is concealed in a depression in the surface with the apex of the peg not projecting from the pit.
On the tip of the labium of Gerromorpha there are three morphological forms of sensilla: tp-tpls -triangular plate sensillum (Fig. 8).This is a flat triangular plate on the same level as the general surface.There is one pore visible in the centre of the plate.This type of sensillum is specific for Rhagadotarsinae, Rhaga dotarsus kraepelini (sensillum no. 7, Fig. 56) and Rheuma tobates crassifemur.
tp-opls -oval plate sensillum (Fig. 9).This flat oval sensillum has one pore in the centre of the plate.This sensillum slightly protrudes above the surface.
tp-ds -dome-shaped sensillum (Fig. 15).These peg sensilla have a slightly rounded tip and one pore situated apically.They are embedded in shallow open cavities and occur in one row at the back of the sensory field.The row is divided into two areas, with two sensilla in one area and three in the other.This type of sensillum occurs in all species of Macroveliidae, Veliidae and Gerridae examined .
tp-bhs -tall basiconic sensillum (Fig. 11).This coneshaped sensillum has a tapered tip with one apical pore.Cone and socket (hs) are tall and this type of sensillum is larger than the other labial tip sensilla.In most species of veliids and gerrids it is designated no. 3 .
tp-bss -short basiconic sensillum (Fig. 12).This sensillum is inserted in a socket (ls), which raises the sensory cone above the general surface of the cuticle.The sensory cone is blunt with one terminal pore.This type of sensillum is usually designated no.1-2, 4-6, and is pointed in species of macroveliids, veliids and gerrids .
tp-bds -double basiconic sensillum (Fig. 14).This sensillum consists of a tall cone with a single terminal pore and a short peg (pe).The peg is on the wall of the cone.This type of sensillum is embedded in a tall socket (hs).It is a very specific type of sensillum occurring only in Cylindrostethinae, Potamobates tridentatus (sensilla no.4-6, Fig. 62) and Cylindrostethus costalis.-multi-peg structures; Np-pip -peg-in-pit, no-pore sensillum; Tp-bds -double basiconic sensillum with terminal pore; Tp-bhs -tall basiconic sensillum with a terminal pore; Tp-bss -short basiconic sensillum with a terminal pore; Tp-ds -dome-shaped sensillum with a terminal pore; Tp-opls -oval plate-like sensillum with a terminal pore.
Two types of sensilla are identified that have the morphological characteristics connected with olfaction: Wp-pls -placoid sensillum (Fig. 15).Flat, oval plate raised above the surface the walls of which are covered with multipores.This type of sensillum (no.7) occurs in Trepobatinae, Naboandelus signatus (Fig. 54) and Pseudo halobates inobonto (Fig. 55).

dIscussIon
The present study provides a comparison of the morphological characters of the labial tip sensilla of 20 gerromorphan taxa belonging to different subfamilies and families.The tip of the labium bears a set of different types of sensilla, which presumably function as gustatory, tactile and gustatory, and olfactory (Table 1).In addition, the multi-peg structures situated on the ventral and internal sides of the labial tip are included in this study.
Plate-(triangular or oval) and dome-shaped sensilla are presumed to be gustatory sensilla in the species of Gerromorpha (Table 1B) because of the central/terminal pore; similar sensilla occur in many insects (Cook, 1972;Zacharuk, 1980Zacharuk, , 1985;;Altner & Prillinger, 1980;Mitchell et al., 1999).For instance, a gustatory plate-like (A1) sensillum occurs on the clypeolabrum of Locusta migratoria migra torioides (Reiche & Fairmaire) (Cook, 1972).Flattened plate-like papillae on the epipharyngeal organ of aphids are gustatory (Wensler & Filshie, 1969).Oval plate-like sensilla occur singly in the some species of gerromorpha studied (Table 2), whereas the triangular plate-like sensilla occur only in Rhagadotarsinae.There is no information on the shape of these plate-like labial sensilla in the literature on heteropterans.In the Macroveliidae, Veliidae and Gerridae the dome-shaped sensilla are always five in number and do not differ morphologically.
In hemipteran insects a labial tapping of host surfaces prior to feeding probably exposes the mechano-and contact chemoreceptive sensilla to tactile and chemical stimuli (Backus, 1988).According to the latter author, Heteroptera and Auchenorrhyncha, although rather diverse, are rather similar in their labial sensory system.
These peg (Tp-ps) and basiconic (Tp-bhs, Tp-bss, Tpbfs and Tp-bds) mechano-chemoreceptive sensilla (Table 1C) are rather numerous on the labial tip of Gerromorpha (Table 2).Peg sensilla are present mainly in basal groups of gerromorphan insects (Figs 39-44) and morphologically all peg sensilla are very similar with no clear differences in their structure.Basiconic sensilla occur at the tip of the labium in Macroveliidae (Fig. 45), Veliidae  and Gerridae .The dominant (i.e. the largest) sensillum, which usually distinctly protrudes from the labial tip, is the tall basiconic sensillum (Tp-bhs), which is characteristic of Veliidae and Gerridae.This position, together with its perpendicular orientation, probably results in the Tp-bhs sensillum being the first to come into direct contact with the surface.The basiconic sensilla (Tp-bss) identified in the present study are similar to those in Aquarius najas, which were called nipple-like sensilla and have the same function (Cobben, 1978).
Generally, all types of uniporous sensilla that respond to gustatory stimuli can also respond to strong odours (Zacharuk, 1980).An intermediate type of basiconic sensillum (D) described in Dysdercus fasciatus and D. intermedius has typical terminal pore and a multiporous wall, suggestive of gustatory and olfactory functions (Peregrine, 1972;Gaffal, 1981).Perhaps for these reasons, specialized olfactory sensilla seldom occur on the labium of heteropterans.
Generally, the labial sensilla of Gerromorpha need to be studied ultracturcturally to further improve and reinforce the conclusions on their function.

the types and distributions of labial tip sensilla in Gerromorpha
Based on the currently accepted phylogenies of Gerromorpha (reviewed by Damgaard, 2008) the families included in this study are: the Mesoveliidae, Hebridae, Hydrometridae + Hermatobatidae, Macroveliidae + Paraphrynoveliidae, Veliidae (includes Oceloveliidae) and Gerridae.The sensory endings on the labial tip and other labial structures of these families are analyzed below.
I. Mesoveliid and hebrid pattern.Four pairs of pegs scattered over the smooth apex and one pair of elongated placoid sensilla positioned sub-apically constitute the basal pattern of Mesoveliinae (Fig. 17).A very similar pattern to that occurring in mesoveliids, with four or five pegs and one pair of subapical sensilla occurs in both subfamilies of Hebridae, Hebrinae (Fig. 18) and Hyrcaninae (Fig. 19) (Table 2).II.Hydrometrid and hermatobatid pattern.This pattern consists of five (in Hydrometridae, Fig. 20) to ten pairs (in Hermatobatidae, Fig. 21) of identical peg sensilla.Surprisingly, in both families this one type of sensillum func-tions as a contact chemoreceptive sensillum and occurs in large numbers on the smooth apex of the labium.
III. Macroveliid, veliid and gerrid pattern.A comparison of the mesoveliid pattern with that of the evolution-ary advanced taxa Macroveliidae, Veliidae and Gerridae reveals significant differences.Most of all, these families display a much greater diversity of types and numbers of sensilla (Table 2).The labial tip is folded.Sensilla basiconica are inserted in distinct sockets.Usually, one plate-like sensillum is visible in the centre of the tip of the labium among several short basiconic sensilla.Another diagnostic feature is the presence of a row of five dome-shaped sensilla at the back of the tip.In general, the sensilla are numerous (10-15 pairs) and morphologically varied (peg-in-pit, plate-like, basiconic, dome-shaped, placoid sensilla).
The distribution of sensilla on the labial tip of Veliidae is fairly homogeneous (Table 2).Only in Paravelia basalis (the Veliinae) is a clearly distinguishable coeloconic sensillum present.A coeloconic sensillum is also present in Macroveliidae and several subfamilies of Gerridae [Charmatometrinae, Rhagadotarsinae, Gerrinae and eotrechinae (Table 2)].This sensillum is usually not clearly visible in most veliids and gerrids because it is probably in a deep hole in the folded surface of the labial apex.Due to the fact that a coeloconic sensillum occurs in Macroveliidae and several species of Veliidae and Gerridae this sensillum may also be present in the remaining subfamilies of these groups.In Gerridae the presence of significantly different types of basiconic sensilla is a new characteristic.Another significant feature is the presence of olfactory-placoid sensilla.There are morphological peculiarities in various subfamilies of Gerridae.A placoid sensillum is characteristic of Trepobatinae, a basiconic sensillum with wide sockets and a triangular plate sensillum are characteristic only of Rhagadotarsinae and a double basiconic characteristic of Cylindrostethinae.
The type, distribution and number of labial tip sensilla determines the pattern.The species of the same gerromorphan subfamilies have identical patterns (Table 2).The patterns vary slightly among the different (sub)families: pattern I, II and III.In subfamilies of Gerridae there are several different forms of sensilla and thus more variation than in other families of Gerromorpha.In some other heteropteran families there are also differences in distributions and shapes of labial tip sensilla; e.g., such differences occur in Reduviidae between Triatominae (Catalá, 1996) and Peiratinae (Brożek & Chłond, 2010), i.e., the sensilla are similar in species belonging to the same subfamily.

other structures on the apical labial segment
In addition to the labial sensilla, the SeM micrographs show multi-peg structures distributed in rows on the inner side of the apical segment of some Gerromorpha (67)(68)(69)(70)(71)(72)(73)(74).These multi-peg structures may have a sensory function.Cranston & Sprague (1961) have interpreted them as sense organs in Aquarius remigis.In contrast, Cobben (1978) describes them as comb-like and suggests they might have a mechanical function, recording friction between mandibles during sucking.
It is interesting that the special location of these structures is similar in the taxa studied.We identified a distinct difference between (1) the more basal taxa (Me soveliinae, Hebrinae, Hyrcaninae, Hydrometrinae, and Hermatobat-idae) in which the multi-peg structures do not occur on the ventral edge of the tip of the labium; (2) the Macroveliidae and Veliidae in which some multi-peg structures occur on the ventral edge; and (3) the Gerridae in which all structures occur on the ventral edge.
In terms of the surface of the tip the labium it is smooth in species of Mesoveliinae, Hebrinae, Hyrcaninae, Hydrometrinae and Hermatobatidae and folded in Macroveliidae, Veliidae and Gerridae.

structure of the sensilla and the phylogenetic history of semiaquatic bugs
The labial tip sensilla are of a similar type (peg sensilla) in Mesoveliidae, Hebridae, Hydrometridae, and Hermatobatidae and a similar but different type (basiconic short sensilla and dome-shape sensilla) in Macroveliidae, Veliidae and Gerridae.There is only a slight difference in the elongated placoid sensillum of Mesoveliidae and Hebridae.The elongated placoid sensillum in the first two families seems to be a plesiomorphic condition, which contrasts with the absence of this sensillum in the other taxa (synapomorphy for Hydrometridae, Hermatobatidae, Macroveliidae, Veliidae, and Gerridae).
In the Hydrometridae and Hermatobatidae the sensilla are of an intermediate type.The presence of peg sensilla (plesiomorphic character) places these taxa closer to Meso veliidae and Hebridae.In turn, the lack of elongated placoid sensilla in hydrometrids and hermatobatids places them closer to Macroveliidae, Veliidae and Gerridae (synapomorphy of these five families).Similarly, based on the mode of locomotion Perez-Goodwyn et al. (2009) indicate that the gait of hermatobatids is intermediate between that of Mesoveliidae, Hebridae, Macroveliidae, and Hydrometridae (plesiomorphic character of the gait) and the clade Gerridae + Veliidae, in which it is autapomorphic.
Moreover, there is strong evidence that the types and distribution patterns of the apical sensilla are essentially the same  in Macroveliidae, Veliidae, and Gerridae.The presence of basiconic short sensilla, oval plate-like and dome-shaped sensilla are synapomorphies for Macroveliidae, Veliidae, and Gerridae, but not Rhagado tarsinae (Gerridae), which have a triangular platelike sensillum that is homologous with an oval plate-like sensillum.This is interpreted as a new autapomorphic state of the sensillum in this subfamily.The process of differentiation of the apical sensilla into tall basiconic sensilla is evident in Veliidae and Gerridae, so for them this type is a syn apomorphy.
A trend towards a modified type of basiconic sensilla seems to have taken place in subfamilies of Gerridae.This variation in the shape of the sensilla is a new autapomorphic character; a basiconic sensillum with wide sockets is an autapomorphy in Rhagadotarsinae; and a double basiconic sensillum an autapomorphy in Cylindrostethinae.Another example of further differentiation of the apical sensilla can be found in Trepobatinae.A new type of sensillum (placoid sensillum with an olfactory function) is an autapomorphy for this subfamily.Probably, this sensillum is a modified form of the elongated placoid sensillum in Mesoveliidae and Hebridae, which occurs only in Trepobatinae; it seems likely that basiconic sensilla evolved from peg sensilla.
In addition, a pair of coeloconic sensilla occur only in several gerromorphan taxa (such as the genera Macrovelia and Paravelia or subfamilies Charmatometrinae, Rhagadotarsinae, Gerrinae, and eotrechinae).Moreover, there are two pairs of coeloconic sensilla in Limnometra nigripennis (Gerrinae).Consequently, the presence of such sensilla in advanced taxa can be regarded a putative synapomorphy; conversely, the absence of a coeloconic sensillum in other gerromophan groups should be interpreted as plesiomorphic.This type of sensillum probably evolved independently of other sensilla.
There are variations in the number of sensilla.The number of sensilla increases from four or five pairs in Mesoveliidae, Hebridae and Hydrometridae to ten pairs in Hermatobatidae and even more pairs, i.e., from 11 to 14, in other families of Gerromorpha.Moreover, in several groups of Heteroptera similar numbers occur, i.e., twelve sensilla occur in species of Pentatomomorpha (Shoonhoven & Henstra, 1972) and from 10 to 16 on the labial tip of species in nepomorphan families (Brożek, 2008(Brożek, , 2013)).Probably the lower number of sensilla is a characteristic of basal taxa such as in Gerromorpha.A similar situation occurs in the basal taxa of Nepomorpha: 9 pairs of sensilla in Nepidae and 16 pairs in the advanced taxon Naucoridae.
The distribution of several labial structures other than the labial tip sensilla vary in Gerromorpha.Multi-peg structures do not occur on the ventral edge of the labial tip and this character is specific to and a symplesiomorphy of Mesoveliinae, Hebrinae, Hyrcaninae, Hydrometrinae, and Hermatobatidae.Multi-peg structures that occur on the ventral edge are a synapomorphy of Macroveliidae and Veliidae and these structures occur on the ventral edge and are an autapomorphy of Gerridae.
Differences in the distributions of these multi-peg structures might indicate they are derived from a basal position in which they do not occur on the ventral edge, via an intermediate position as in macroveliids and veliids to the final state as in gerrids.Multi-peg structures are homologous in all gerromorphan taxa, however, these structures are polymorphic and can evolve into quite different shapes.
In addition, the smooth surface of the tip of the labium is probably a symplesiomorphy for Mesoveliidae, Hebridae, Hydrometidae and Hermatobatidae, however, the folded apex in Macroveliidae, Veliidae, and Gerridae is for them a synapomorphy.

conclusIons
The aforementioned new apomorphic characters should be included in a future cladistic analysis of Gerromorpha.There are several significant indications of the relationship of the families of Gerromorpha that can be pointed out based on their current characters (apomorphies).
1. Apparently, characteristics of those labial tip sensilla (tall basiconic sensilla) viewed as a synapomorphy in the present study provide support for the monophyly of Veliidae and Gerridae.This provides support for solutions pro-posed by Andersen (1982), Andersen & Weir (2004) and Perez-Goodwyn et al. (2009).
2. This study of labial tip sensilla has provided evidence of the monophyly of Cylindrostethinae, Trepobatinae, and Rhagadotarsinae.The autapomorphic character (double basiconic sensillum) is an especially important indicator of the monophyly of Cylindrostethinae as Damgaard (2008) evaluated Cylindrostethinae as paraphyletic, due to the lack of convincing apomorphic characters but stated that "within the Gerridae, the lack of monophyly in the 'Cylindrostethinae' is surprising, since this subfamily has been diagnosed previously and currently it is also supported by the 16S rRNA".The monophyly of the two subfamilies: Trepobatinae and Rhagadotarsinae, suggested in previous studies, is confirmed.
3. In the present study one synapomorphy (lack of an elongated placoid sensillum) is regarded as evidence that Hydrometridae, Hermatobatidae, Macroveliidae, Veliidae and Gerridae separated from the basal lineage of Mesoveliidae and Hebridae (unfortunately there is no synapomorphy in Gerromorpha).
4. Short sensilla basiconica, oval plate-like sensillum, dome-shaped sensilla and folded labial tip are four synapomorphies and coeloconic sensillum is a putative synapomorphy of Macroveliidae, Veliidae, and Gerridae 5.In this study, the autapomorphic character, multipeg structures, is thought to indicate that the Gerridae is a monophyletic group.The monophyly of Gerridae sug-Fig.75.A phylogenetic tree of the relationships among gerromorphan bugs, which is based on an analysis of both morphological and molecular data (Damgaard, 2008).
gested in many previous studies is confirmed (Andersen, 1982;Muraji & Tachikawa, 2000;Andersen & Weir, 2004;Damgaard et al., 2005;Damgaard, 2008).In addition, the position of the multi-peg structures may be a synapomorphy of the Macroveliidae + Veliidae.However, it is not supported by additional characters and might be shown to be a homoplasy in future studies.
even though the characteristics of labial tip sensilla and multi-peg structures and the surface of the labial tip were recognizable at the (sub-)family level in the present study, these characters do not provide support for the superfamilies identified by Andersen (1982), Muraji & Tachikawa (2000), Andersen & Weir (2004), Damgaard et al. (2005), andPerez-Goodwyn et al. (2009) or for the alternative cladogenetic solutions proposed by Damgaard (2008) (Fig. 75), who left unanswered several major questions regarding the future systematics of Gerromorpha.
This study revealed similarities in the labial tip sensilla in particular taxa of Gerromorpha.Currently, in terms of a higher classification, three groups can be clearly distinguished based on the pattern of types and distribution of apical sensilla: (1) Mesoveliidae and Hebridae; (2) Hydrometridae and Hermatobatidae; (3) Macroveliidae, Veliidae (including Ocelloveliidae sensu Damgaard, 2008) and Gerridae.In addition, the multi-peg structures provide evidence of a division between the lineage including Mesoveliinae, Hebrinae, Hyrcaninae, Hydrometrinae, and Hermatobatidae and that of Macroveliidae and Veliidae from the Gerridae.Slightly different arrangements of the families are obtained based on the type of surface of the labial tip; they are: (1) Mesoveliinae, Hebrinae, Hyrcaninae, Hydrometrinae and Hermatobatidae and (2) Macroveliidae, Veliidae and Gerridae.

Figs
Figs 1-2.General organization of the labium and the sensory field at the tip of the labium in Gerromorpha [the actual structures are those of Velia caprai (Veliinae)].I-IV, number of labial segments; DS -dorsal side; IS -internal sclerites; Lmd -left mandible; LLleft lobe; LR -right lobe; MS -multi-peg structures (arrows indicate their location) on inner side of lobes (LL and LR); Rmd -right mandible; SFl -left sensory field; SFr -right sensory field; V -ventral lobe.