The description provided in this paper is based on one instar III larva collected at the Iberá wetlands, Corrientes province, Argentina, in December 2019. For a picture of the habitat, see Michat et al. (2024). The specimen was cleared in lactic acid, dissected, and mounted on glass slides in polyvinyl-lacto-glycerol. Observations and drawings were made using an Olympus CX31 microscope equipped with a drawing tube. Drawings were inked digitally with a One by Wacom pen tablet.

The methods and terms used in the morphometric and chaetotaxic analyses follow those employed in previous papers dealing with Hydrovatini larvae (Michat 2006; Michat and Alarie 2021). Despite being an instar III, the primary sensilla of Queda hydrovatoides were tentatively identified by comparison with the Hydrovatini ground plan wherever possible. In these cases, homologies were recognized using the criterion of similarity of position (Wiley 1981).

Habitus photographs of the adult were taken with a Nikon D800 digital camera, whereas that of the larva was taken with a Samsung Galaxy S9 cell phone.

For the study of the phylogenetic relationships of Hydrovatini we used a cladistic approach considering the character set provided by the larval morphology and chaetotaxy. A broad taxon sampling of the subfamily Hydroporinae was selected as ingroup, with the purpose of allowing the target taxa to move freely, thus testing relationships in the broadest possible context. Forty-four species were included representing all hydroporine tribes, most of them with multiple genera (except for those that are monotypic) (Table S1). Data for all species were scored directly from the observation of the specimens. Members of all dytiscid subfamilies (except Hydrodytinae which larva is unknown) were selected as the outgroup, rooting the tree in Agabinae, which is considered an ancestral dytiscid lineage and clearly distantly related to the Hydroporinae (e.g., Miller and Bergsten 2014). During character codification, the same instars of each species were compared to avoid the problem of comparing different semaphoronts (Meier and Lim 2009). Exceptions to this rule were Hydrovatus crassulus Sharp, 1882 and Queda hydrovatoides, for which only instars II–III and Instar III were available, respectively.

The phylogenetic analyses were performed with the program TNT (Goloboff and Morales 2023). All characters were treated as equally weighted, and multistate characters were treated as non-additive. Three search strategies were implemented to find the most parsimonious trees, previously setting “hold 20000” in all cases. Analysis 1: a common heuristic search under “Analize/Traditional search”, using “tree bisection reconnection” (TBR) as algorithm with 200 replicates and saving 100 trees per replication. Analysis 2: involving additional heuristics with 100 random addition sequences and TBR followed by “tree drifting”, using the command “mult 100 =tbr drift ;”. Analysis 3: again 100 random addition sequences and TBR but followed by “ratchet”, using the command “mult 100 = tbr ratchet ;”. Bremer and bootstrap (as implemented in TNT) were used as measures of support for clades. Bremer values were calculated by repeating the commands “hold 20000”, “sub n” and “bsupport”, increasing n (= the length of the suboptimal cladograms) by one step until 10. Values higher than 10 are indicated by the symbol +10. Bootstrap values were calculated using the following parameters: “standard (sample with replacement)”, 1000 replicates.

In total, 147 characters derived from larval morphology and chaetotaxy were included, of which 112 were coded as binary and 35 as multistate. They are listed in Table S2. All included characters were informative and, therefore, useful to infer relationships of the studied taxa, except characters 20, 28 and 71 which are autapomorphies.

The data matrix is shown in Table S3. The searches with TNT using different parameters (Analyses 1–3, see above) converged to 31 most parsimonious trees of 485 steps (CI = 0.40; RI = 0.78). The strict consensus (of similar topology in the three analyses) is well resolved and informative, except for a large polytomy that involves several genera of Hygrotini and Hydroporini and the tribes Vatellini, Hyphydrini, and Bidessini (Fig. 2). Within a strongly supported Hydroporinae, the tribe Methlini is sister to the remainder of the subfamily, in agreement with some previous analyses based on larval morphology and molecular data (Michat et al. 2017; Vasilikopoulos et al. 2021) but not with others based on adults or molecules (e.g., Wolfe 1985; Miller 2001; Miller and Bergsten 2014). The remaining tribes are separated into two large sister clades, one of them comprising the monophyletic Vatellini, Hyphydrini, and Bidessini, the first two strongly supported and the third one showing weak support, and members of Hygrotini and Hydroporini, which are not recovered as monophyletic groups. The other large clade involves the tribes Laccornini, Laccornellini, Pachydrini and Hydrovatini, all resolved as monophyletic with moderate to high support, except for the weakly supported Laccornellini. Of this grouping, Laccornini is sister to the rest, and Laccornellini is sister to Pachydrini and Hydrovatini, which are resolved as sister groups with low support. The tribe Hydrovatini, with its two genera Queda and Hydrovatus, is robustly supported and well defined by several characters (see below). Characters were ACCTRAN optimized in one of the most parsimonious trees and character state changes were mapped (Figs 3, 4).

Figure 2. 

Strict consensus cladogram with Bremer support values indicated above branches and bootstrap values higher than 50 indicated below branches. Clade Hydrovatini indicated in red.

Figure 3. 

Portion of one of the most parsimonious cladograms with selected character state changes mapped for each clade. Clade Hydrovatini indicated in red. Green numbers indicate character state changes that are unique within Dytiscidae; red numbers indicate character state changes that are unique within the subfamily Hydroporinae.

Figure 4. 

Portion of one of the most parsimonious cladograms with selected character state changes mapped for each clade. Green numbers indicate character state changes that are unique within Dytiscidae; red numbers indicate character state changes that are unique within the subfamily Hydroporinae.

Within Hydroporinae, the larva of Queda is characterized by the combination of the following characters: cephalic capsule pear-shaped (Fig. 5A); nasale very broad, semicircular, without lateral branches (Fig. 5A, B); coronal line very short (Fig. 5A); parietal lacking occipital suture (Fig. 5A); occipital foramen somewhat reduced (Fig. 5B); A3’ arising from base of A4 (Fig. 6B); mandible very slender (Fig. 6C); cardo partially fused to stipes (Fig. 7B); galea present, relatively well developed (Fig. 7A); prementum longer than broad (Fig. 8A, B); abdominal segments V–VIII completely sclerotized, ring-like (Fig. 1D, E); cephalic capsule and abdominal segment VIII bearing setiferous tubercles (Figs 5A, B, 10A); seta MX4 present (Fig. 7A); setae LA2 and UR8 absent (Figs 8B, 11).

Figure 5. 

Queda hydrovatoides Zimmermann, 1921, instar III, cephalic capsule: A dorsal aspect (stemmata not represented); B ventral aspect. Lowercase letters indicate primary pores. Abbreviations: FR, frontoclypeus; LC, lamellae clypeales; PA, parietal; TP, tentorial pit.

Figure 6. 

Queda hydrovatoides Zimmermann, 1921, instar III. A right antenna, dorsal aspect; B left antenna, ventral aspect; C right mandible, dorsal aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Abbreviations: A3’, apical lateroventral process of antennomere 3; AN, antenna; MN, mandible.

Figure 7. 

Queda hydrovatoides Zimmermann, 1921, instar III. A left maxilla, dorsal aspect; B right maxilla, ventral aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Abbreviations: MX, maxilla.

Figure 8. 

Queda hydrovatoides Zimmermann, 1921, instar III. A labium, dorsal aspect; B labium, ventral aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Asterisk indicates presumably secondary pore. Abbreviations: LA, labium.

Color (Fig. 1D, E): Cephalic capsule light brown; antenna, maxilla and labium testaceous; mandible brown; protergite light brown laterally and on posterior third, brown on anterior two thirds; mesotergite light brown; metatergite and abdominal tergites I–II brown; abdominal tergite III brown except central portion light brown with brown macula in middle; abdominal sternite III brown; abdominal sclerites IV–VII light brown with dorsal brown macula in middle; abdominal sclerite VIII brown except siphon light brown; legs testaceous; urogomphus light brown.

Body (Fig. 1D, E): Broad, fusiform; measurements and ratios that characterize body shape are shown in Table S4.

Head (Fig. 5A, B): Cephalic capsule pear-shaped, longer than broad, maximum width posterior to stemmata; ecdysial line well marked but diffuse on anterior portion, coronal line very short, weakly marked; occipital foramen somewhat reduced, broadly emarginate ventrally; frontoclypeus very elongate, convex, lateral margins sinuate; nasale very broad, semicircular, sides converging anteriorly to rounded apex, without lateral branches; parietal lacking neck constriction and occipital suture, with six subcircular stemmata on each side, arranged in two parallel vertical rows; lateroventral surface with reticulation; posterior tentorial pits visible ventrally on central portion. Antenna (Fig. 6A, B). Shorter than head width, composed of four antennomeres; A3 longest, without ventroapical spinula, A2 slightly shorter than A3, A1 and A4 shortest, subequal in length; A3’ elongate, arising from base of A4. Mandible (Fig. 6C). Prominent, very slender, oriented obliquely, distal half strongly curved inwards and upwards, apex sharp; mandibular channel present. Maxilla (Fig. 7A, B). Cardo partially fused to stipes; stipes short, broad, incompletely sclerotized; galea elongate, subcylindrical; lacinia absent; MP elongate, composed of three palpomeres, MP1 and MP2 longest, subequal in length, MP3 shortest. Labium (Fig. 8A, B). Prementum large, subrectangular, 1.5 times longer than broad, anterior margin slightly indented medially; LP relatively short, composed of two palpomeres, LP2 longer than LP1.

Thorax: Terga convex, pronotum somewhat shorter than meso- and metanotum combined, meso- and metanotum subequal; protergite trapezoid-shaped in dorsal view, more developed than meso- and metatergite; meso- and metatergite transverse, with anterotransverse carina; sagittal line visible on three tergites; venter membranous except for a small central plate on prothorax; spiracles present on mesothorax. Legs (Fig. 9A, B). Long, six-segmented, L1 shortest, L3 longest; CO robust, elongate, TR divided into two parts by a transverse annulus, FE, TI and TA slender, subcylindrical, PT with two long, slender, slightly curved claws; posterior claw shorter than anterior claw on L1 and L2, claws subequal in length on L3; FE with a short row of comb-like spinulae (pecten) on anteroventral surface; TA and to a lesser extent TI with a ventral row of bifid or trifid elongate spinulae, more developed on L1.

Figure 9. 

Queda hydrovatoides Zimmermann, 1921, instar III. A left prothoracic leg, anterior aspect; B right prothoracic leg, posterior aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Asterisks indicate secondary setae. Abbreviations: CO, coxa; FE, femur; PT, pretarsus; TA, tarsus; TI, tibia; TR, trochanter.

Abdomen: Eight-segmented, with segments progressively narrower and longer to apex; sagittal line visible on segments I–IV; segments I–II sclerotized dorsally, membranous ventrally; segments III–IV sclerotized both dorsally and ventrally, with ventral sclerite independent from dorsal sclerite; segments V–VIII completely sclerotized, ring-like; all sclerites with anterotransverse carina; spiracles present lateroventrally on segments I–VII, terminally on segment VIII; segment VIII (= LAS) the longest, cone-shaped; siphon elongate, sharp apically (Fig. 10A, B). Urogomphus (Fig. 11). Short, two-segmented; U1 robust, not extending beyond siphon; U2 very slender, setiform, twice longer than U1.

Figure 10. 

Queda hydrovatoides Zimmermann, 1921, instar III. A abdominal segment VIII, dorsal aspect; B abdominal segment VIII, ventral aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Abbreviations: AB, abdominal segment VIII.

Figure 11. 

Queda hydrovatoides Zimmermann, 1921, instar III, left urogomphus, dorsal aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Abbreviations: UR, urogomphus.

Chaetotaxy (Figs 5–11): Cephalic capsule with several hair-like secondary setae of different lengths (Fig. 5A, B); cephalic capsule also with setiferous tubercles (i.e., small bulges bearing a minute seta) sparsely distributed on dorsal surface of frontoclypeus and parietal (Fig. 5A), and more densely distributed on ventral surface of nasale (anteriorly to antennal bases) and on anteroventral surface of parietal (Fig. 5B); anteroventral margin of nasale with a half circle of 49 lamellae clypeales (Fig. 5B); MN with one hair-like secondary seta on basoexternal margin (Fig. 6C); apical sensillum of MN with the shape of a pore (labelled MNd) (Fig. 6C); prementum with one secondary pore on each side of ventral surface (Fig. 8B); thoracic tergites with several secondary setae; secondary leg setation detailed in Table S5; abdominal sclerites I–VII with several secondary setae on posterior half, these setae changing from hair-like on anterior segments to spine-like on posterior segments; LAS with numerous hair-like secondary setae, mostly on dorsal surface, and with setiferous tubercles (as those present on cephalic capsule) sparsely distributed over dorsal surface (Fig. 10A); we were unable to find pore URc, but we cannot determine if it is really absent.

According to Michat et al. (2017) the tribe Hydrovatini is diagnosed by the proximal position of the egg bursters on the frontoclypeus, the absence of the primary pores PAj and PAo on the ventral surface of the parietal, the hair-like appearance of the mandibular sensillum MN2, the presence of basoventral spinulae on the claws in instar I, and the sclerotized ventral surface of the abdominal segments II–V in instar III. At that time, however, only the larvae of selected species of Hydrovatus were known. The study of the larvae of Queda, the only other genus in the tribe, allows us to revise this diagnosis in light of the new evidence.

Despite being a third instar, primary chaetotaxy is (with few exceptions) evident in the studied larva of Queda. In particular, the parietal pores PAj and PAo are absent, two conditions also found in Hydrovatus (Michat 2006; Michat and Alarie 2021). Within Hydroporinae, PAj is also absent in the tribes Bidessini and Hyphydrini, whereas the absence of PAo is unique. Therefore, the absence of these two pores is a good diagnostic feature for the tribe Hydrovatini.

An interesting character exhibited by Hydrovatini larvae is the position of the so-called “apical lateroventral process of antennomere 3” (generally abbreviated as A3’). This process is variously shaped in Dytiscidae, from a rounded and not protruding structure (such as in some Agabinae and Dytiscinae, and in many Colymbetinae) to an elongate and subcylindrical process that ranges from much shorter to almost as long as antennomere 4 (like in most dytiscids) (Alarie et al. 2011, 2024; Michat et al. 2017). In all cases, however, it arises from the apical portion of the antennomere 3 (as its name indicates). In the larvae of Queda and Hydrovatus, A3’ is somewhat transposed from its typical position and arises from the base of the antennomere 4 (Fig. 6B). This characteristic is rather difficult to see, which likely explains why in previous descriptions of Hydrovatus larvae (Michat 2006; Michat and Alarie 2021) A3’ is drawn in its usual position. A close inspection of these larvae, however, allowed us to corroborate that A3’ originates in the same position as in Queda larva.

The small sensillum near the apex of the mandible is generally described as a pore or minute seta in Dytiscidae. In Hydrovatus, however, it has the shape of a short hair-like seta (Michat 2006; Michat and Alarie 2021). This sensillum is generally labelled MNd when it has a pore-like appearance, or MN2 if it is a seta, either minute or more elongate. In Queda larva this sensillum has the shape of a pore, similar to the condition found in all hydroporines except Hydrovatus. Therefore, this character is not useful as diagnostic for Hydrovatini, although it remains a good character for distinguishing genera in this tribe.

A secondary pore is usually present on each side of the ventral surface of the prementum in non-hydroporine Dytiscidae (exceptions: Matinae, some Laccophilinae). Within Hydroporinae, however, the presence of these pores is a rare feature restricted to some ancestral lineages such as Methlini and Laccornellus Roughley & Wolfe, 1987. The larvae of both Queda and Hydrovatus have these secondary pores (Fig. 8B), which are therefore a useful diagnostic character for the tribe Hydrovatini.

The degree of sclerotization of the ventral surface of the abdominal segments varies greatly in Dytiscidae. In general, segments I–VI are ventrally membranous, with some exceptions in the subfamilies Laccophilinae and Hydroporinae (Michat et al. 2017). The larva of Queda studied here has the segment II membranous ventrally, thus differing from Hydrovatus where this part is sclerotized (Michat 2006; Michat and Alarie 2021). Segments III–V, on their part, are similarly sclerotized in both genera, which represents a good diagnostic character for the tribe as it is relatively easily seen under the microscope.

The urogomphal primary seta UR8 exhibits a considerable variation in position among diving-beetle larvae. It is generally inserted at the apex of the urogomphomere 1 in none-hydroporine dytiscids, but within Hydroporinae this seta is inserted on the urogomphomere 2 and varies from apical in Methlini and Vatellini, distal in Bidessini and Hydroporini, submedial in Hygrotini and Pachydrini, to proximal in Hyphydrini, and in a few taxa it is absent, such as members of the Laccornellini. Like in this last tribe, the larvae of both Queda and Hydrovatus lack seta UR8 (Fig. 11), differing in this way from most hydroporines.

With all this information at hand, the diagnosis of the larvae of the tribe Hydrovatini can be updated as follows: primary pore PAj absent on ventral surface of parietal (shared with Bidessini and Hyphydrini); primary pore PAo absent on ventral surface of parietal; A3’ arising from base of antennomere 4; secondary pores present on ventral surface of prementum (shared with Methlini and Laccornellus); ventral surface of abdominal segments III–V sclerotized in instar III (shared with Hyphydrini); primary seta UR8 absent (shared with Laccornellini). The position of the egg bursters on the frontoclypeus and the presence/absence of basoventral spinulae on the claws, also mentioned as diagnostic for Hydrovatini in Michat et al. (2017), are characters restricted to the instar I and therefore could not be evaluated in Queda. They remain as potential diagnostic characters for the tribe but need to be corroborated when the first instar of Queda is discovered.

Despite sharing several characters, the larvae of Queda and Hydrovatus also present some conspicuous differences, at least in the third instar (compare this study with Michat 2006 and Michat and Alarie 2021). As mentioned above, both genera differ in the shape of the sensillum placed at the apex of the mandible (MNd/MN2) and in the sclerotization of the ventral surface of the abdominal segment II. Apart from these, other significant features can be mentioned. In Queda the nasale is broad and progressively narrows to the apex (Fig. 5A), whereas in Hydrovatus it is much narrower and parallel sided. The primary setae AN1 and AN2 are shaped as short spines in Queda (Fig. 6A, B), whereas in Hydrovatus they are elongate and slender. The cardo is completely fused to the stipes in Hydrovatus but only partially fused to the stipes in Queda (Fig. 7B). The galea is somewhat more developed in Queda (Fig. 7A) than in Hydrovatus. The prementum is small, subquadrate, and does not bear secondary setae in Hydrovatus, whereas in Queda it is large, clearly longer than broad, and bears several very long, hair-like, most likely secondary setae (Fig. 8A). The abdominal spiracles in Queda larva are all well developed and located in the typical position found in Dytiscidae, i.e. laterally on segments I–VII and terminally on segment VIII. In Hydrovatus, however, the segments I–III and VIII bear well developed spiracles but the segments IV–VII have the spiracles obliterated yet still visible (erroneously considered as well developed in Michat 2006). Finally, the cephalic capsule and the abdominal segment VIII of Queda larva bear setiferous tubercles (Figs 5A, B, 10A) which are absent in those of Hydrovatus.

Our study supports a monophyletic Hydrovatini including both Queda and Hydrovatus, with high Bremer and bootstrap values (Fig. 2). The tribe is unambiguously supported by the absence of the primary pore PAo on the parietal (character 30.1) and by the process A3’ arising from the base of the antennomere 4 (character 35.1) (Fig. 6B). Other characters sustaining this relationship are homoplasious and include the absence of the primary pore PAj on the parietal (character 27.1, also absent in Bidessini and Hyphydrini), the presence of a secondary pore on each side of the ventral surface of the prementum (character 90.1, shared with Methlini and Laccornellus) (Fig. 8B), and the sclerotized ventral surface of the abdominal segments III–V in instar III (characters 114.0, 115.0 and 116.0, convergent with Hyphydrini) (Fig. 3). This result agrees with the original concept of the tribe introduced by Sharp (1882) and supported by Biström (1990, 1997), Miller (2001), Miller et al. (2006), and Miller and Bergsten (2014).

Our study also supports the inclusion of Hydrovatini in a clade of taxa generally considered ancestral within the large subfamily Hydroporinae, such as the tribes Laccornini, Laccornellini and Pachydrini (Figs 2, 3). Within this clade, the sister group of the Hydrovatini was found to be the tribe Pachydrini, in agreement with Miller and Bergsten (2014) and Vasilikopoulos et al. (2021). Although the support for this relationship was low, larvae of both tribes share a unique feature within Hydroporinae, the absence of an occipital suture in instars II–III (character 19.0; Fig. 5A). Other characters supporting this grouping are homoplasious, among which are the absence of the pore ANh on the second antennomere (character 38.0) and the sclerotized ventral surface of the abdominal segment VI in instar III (character 118.0), both shared with the tribe Hyphydrini.

In our analysis, the larvae of Queda are characteristic within Dytiscidae in the broad and semicircular shape of the nasale (character 3.1; Fig. 5A) and in the cardo partially fused to the stipes (character 48.1; Fig. 7B). Also, they are unique within Hydroporinae in the presence of setiferous tubercles on the cephalic capsule and on the abdominal segment VIII (character 121.1; Figs 5A, B, 10A) and in having a subcylindrical and relatively well developed galea (character 51.0; Fig. 7A). According to Michat et al. (2017), the presence of a galea is characteristic of ancestral hydroporines, which have retained this structure as a plesiomorphic condition. The galea found in Queda larva therefore reinforces the hypothesis of an ancestral condition of the Hydrovatini within Hydroporinae. Interestingly, the galea of Queda larva is somewhat more developed than those exhibited by other genera of Hydroporinae (i.e., Celina Aubé, 1837, Laccornis Gozis, 1914, Laccornellus and Hydrovatus).