The larvae of the Australian endemic species Barretthydrus tibialis Lea, 1927 and Barretthydrus geminatus Lea, 1927 are described and illustrated for the first time, with detailed morphometric and chaetotaxic analyses of the cephalic capsule, head appendages, legs, last abdominal segment, and urogomphi. A parsimony analysis based on 118 informative larval characteristics of 34 species in all 10 tribes of the subfamily Hydroporinae was conducted using the program TNT. No clear larval morphological synapomorphies support the monophyletic origin of the tribe Hydroporini. Compared to other known larvae of Hydroporini, Barretthydrus Lea is postulated to share a closer phylogenetic relationship with Antiporus Sharp, which reinforces their inclusion in the subtribe Sternopriscina.
Barretthydrus Lea, 1927 is an Australian endemic genus comprised of three species: the more common B. tibialis Lea, 1927 and B. geminatus Lea, 1927, and the rarely collected B. stepheni Watts, 1978. These dyticids are small (adult body length varying from 4.2 mm to 4.5 mm) and mainly live in clear mountain streams in the Great Dividing Range in Victoria and New South Wales (Watts 1978; Miller and Bergsten 2016).
Barretthydrus is included in the tribe Hydroporini (Dytiscidae: Hydroporinae) that consists of a large and diverse group of water beetles mostly distributed in boreal and temperate regions of the northern hemisphere. Worldwide, Hydroporini currently comprises more than 685 species classified in 38 genera (Nilsson 2017). Within Hydroporini, Barretthydrus is postulated to belong to the strictly Australian hydroporine radiation Sternopriscina along with Antiporus Sharp, 1882, Brancuporus Hendrich, Toussaint, and Balke, 2014, Carabhydrus Watts, 1978, Chostonectes Sharp, 1880, Megaporus Brinck, 1943 Necterosoma MacLeay, 1871, Paroster Sharp, 1882, Sekaliporus Watts, 1997, Sternopriscus Sharp, 1880, and Tiporus Watts, 1985 (Miller and Bergsten 2016).
The larvae of Antiporus and Paroster were described in much detail recently (Alarie and Watts 2004; Alarie et al. 2009). The larval morphology of the other members of the subtribe Sternopriscina are only poorly known, but late instars can be identified to genus using the illustrated keys in Watts (2002). The discovery of the larvae of both B. tibialis and B. geminatus provided the impetus for this study, which has the following three goals: (1) to describe, in detail, for the first time the larvae of B. tibialis and B. geminatus; (2) to compare the ground plan of larval features of Barretthydrus with those of other associated Sternopriscina genera for which the larvae have been described comprehensively; and (3) to study the phylogenetic relationships of Barretthydrus within the tribe Hydroporini based on larval characters.
Material and Methods
Larvae Examined. The descriptions of the larval stages and the taxonomic conclusions reported in this paper are based on the examination of larvae cooccurring with adults. Larvae of each species were identified to species either by rearing some larvae collected in the field (B. geminatus) or by collecting in situations where they could unequivocally be associated to either species. The localities from which the specimens were obtained are provided under the individual species descriptions.
Preparation. Larvae were disarticulated and mounted on standard glass slides in Hoyer's medium. Microscopic examination at magnifications of 80–800X was done using an Olympus BX50 compound microscope equipped with Nomarsky differential interference optics. Figures were prepared through use of a drawing tube attached to the microscope. Drawings were scanned and digitally inked using an Intuos 4 professional pen tablet (Wacom Co., Ltd. Kazo, Saitama, Japan). Voucher specimens are deposited in the larval collection of Y. Alarie (Department of Biology, Laurentian University, Canada).
Measurements. All measurements were made with a compound microscope equipped with a micrometer eyepiece. The part to be measured was adjusted so that it was, as nearly as possible, parallel to the plane of the objectives. We employed, with minimal modifications and additions, the terms used in previous papers dealing with larval morphology of Hydroporini (e.g., Alarie and Watts 2004; Alarie et al. 2009). The following variables were measured: Head length (HL) = total head length including the frontoclypeus, measured medially along epicranial stem; head width (HW) = maximum head width; length of frontoclypeus (FRL) = from apex of nasale to posterior margin of ecdysial suture; occipital foramen width (OCW) = maximum width measured along dorsal margin of occipital foramen; length of mandible (MN) = measured from laterobasal angle to apex; width of mandible= maximum width measured at base. Lengths of the antenna (A) and maxillary (MP) and labial (LP) palpi were derived by adding the lengths of the individual segments; each segment is denoted by the corresponding letter(s) followed by a number (e.g., A1 = first antennomere). A3′ is used as an abbreviation for the apical lateroventral process of the third antennomere. Length of the leg (L) including the longest claw was derived by adding the lengths of the individual segments; each leg is denoted by the letter L followed by a number (e.g., L1 = prothoracic leg). Length of trochanter includes only the proximal portion; length of the distal portion is included in the femoral length. Dorsal length of the last abdominal segment (LAS) was measured along the midline from the anterior to the posterior margin. Length of urogomphus (U) was derived by adding the lengths of the individual segments; each segment is denoted by the letter U followed by a number (e.g., U1 = first urogomphomere). These measurements were used to calculate several ratios that characterize the body shape.
Chaetotaxic Analysis. Primary (observed in instar I) and secondary (added throughout ontogenetic development) setae and pores were distinguished on the head capsule, head appendages, legs, last abdominal segment, and urogomphus. The setae and pores were coded according to the system proposed by Alarie (1991) and Alarie and Michat (2007a) for the cephalic capsule and head appendages, Alarie et al. (1990) for the legs, and Alarie and Harper (1990) for the last abdominal segment and urogomphi. Setae are coded by two capital letters corresponding to the first two letters of the name of the structure on which the seta is located (AB = abdominal segment VIII; AN = antenna; CO = coxa; FE = femur; FR = frontoclypeus; LA = labium; MN = mandible; MX = maxilla; PA = parietale; TA = tarsus; TI = tibia; TR = trochanter; UR=urogomphus) and a number. Pores are coded in a similar manner except that the number is replaced by a lowercase letter. The position of the sensilla is described by adding the following abbreviations: A = anterior; AD = anterodorsal; AV=anteroventral; D=dorsal; PD=posterodorsal; Pr = proximal; PV = posteroventral.
Color. Description of color is given for all species based on ethanol-preserved specimens.
Cladistic Analysis. To examine the phylogenetic signal of the larval characters of Barretthydrus and to test the relationships of this genus with other Hydroporini, a cladistic analyses of 16 species of Hydroporini (nine genera) and 18 species of all other nine tribes of the Hydroporinae was conducted. The genera Laccornis Gozis, 1914 (tribe Laccornini), Laccornellus Roughley and Wolfe, 1987 and Canthyporus Zimmermann, 1919 (tribe Laccornellini), Celina Aubé, 1837 (tribe Methlini), Pachydrus Sharp, 1882 (Pachydrini), and Hydrovatus Motshulsky, 1853 (Hydrovatini), which are generally recognized as basal lineages within the subfamily Hydroporinae based on adults (Roughley and Wolfe 1987; Miller et al. 2006), larvae (Alarie and Michat 2007b), and molecules (Miller and Bergsten 2014), and several taxa of Hyphydrini, Bidessini, Vatellini, and Hygrotini were used as outgroups rooting the tree with Laccornis. The analysis was performed using the program TNT (Goloboff et al. 2008). A heuristic search was implemented using “tree bisection reconnection” as the algorithm, with 200 replicates and saving 100 trees per replication (previously setting “hold 20000″). Bremer support values were calculated using the commands ‘hold 20000′, 'sub n' and ‘bsupport', where ‘n' is the number of extra steps allowed. The process was repeated increasing the length of the suboptimal cladograms by one step, until all Bremer values were obtained (Kitching et al. 1998). Bootstrap values were calculated using the following parameters: “standard (sample with replacement)”; 2000 replicates.
Barretthydrus Lea, 1927
Diagnosis. Instar III of Barretthydrus can be distinguished from those of other genera of Australian Hydroporini that have been well studied (i.e., Antiporus and Paroster) by the following combination of characters: HL = 1.24–1.36 mm; HL/HW <1.40; nasale broad, subtriangular, not spatulate apically (Figs. 1, 15–16); parietals constricted at level of occipital suture (Figs. 15–16); primary seta FR7 hair-like (Fig. 1); A4/A3 >0.30; labial palpus composed of two palpomeres (Figs. 5–6); prementum lacking spinulae along lateral margins (Figs. 5–6); L3/HW <3.50; primary seta FE7 present (Fig. 8); natatory setae present on dorsal margin of femora, tibiae, and tarsi (Fig. 18); U1/U2′ <3.50; U1/HW <1.70.
Description. Body: Subcylindrical, narrowing towards abdominal apex. Measurements and ratios that characterize the body shape are shown in Table 1. Head: Head capsule (Figs. 1–2) pearshaped, tapering posteriorly, not constricted at level of occipital suture; ecdysial suture welldeveloped, coronal suture short; frontoclypeus elongate, bluntly rounded, subtriangular with welldeveloped lateral branches, anteroventral margin with 12 ventral lamellae clypeales (Bertrand 1972); dorsal surface with 2 spine-like egg bursters (ruptor ovi of Bertrand 1972) at about mid-length; ocularium present, stemmata not visible ventrally and subdivided into 2 vertical series; epicranial plates meeting ventrally; tentorial pits visible medioventrally at about mid-length. Antenna (Figs. 3–4) elongate, slightly shorter than HW; composed of 4 antennomeres, A2 and A3 longest, A1 shortest; A3′ relatively elongate, shorter than A4; A3 lacking ventroapical spinula. Mandible (Fig. 7) prominent, falciform, distal half projecting inwards and upwards, apex sharp; mandibular channel present. Labium (Figs. 5–6), prementum subrectangular, about as long as broad, lacking lateral marginal spinulae; LP elongate, distinctly shorter than MP, composed of 2 palpomeres; LP2 subfusiform, distinctly longer than LP1. Maxilla (Figs. 13–14) with short, thick stipes, incompletely sclerotized ventrally; cardo fused to stipes; galea and lacinia absent; MP elongate, slightly shorter than antenna, composed of 3 palpomeres; MP1 and MP2 longest, MP2 distinctly longer than MP1. Thorax: Thoracic terga convex, pronotum slightly shorter than meso- and metanota combined, meso- and metanota subequal; protergite subrectangular to subovate, more developed than meso- and metatergites; [We were unable to determine the presence of an anterotransverse carina owing to the bad condition of the only specimen available.]; thoracic sterna membranous; spiracles absent. Legs: Long (Figs. 8–9), composed of 6 articles (sensu Lawrence 1991); L1 shortest, L3 longest; CO robust, elongate, TR divided into 2 parts by an annulus, FE, TI, and TA slender, subcylindrical, PTwith 2 long, slender, slightly curved claws; posterior claw shorter than anterior claw on L1 and L2, posterior claw longer than anterior claw on L3; femora, tibiae, and tarsi lacking spinulae along ventral margin. Abdomen: Eight-segmented (Figs 10–11); segments I–VI sclerotized dorsally, membranous ventrally; segment VII sclerotized both dorsally and ventrally, ventral sclerite independent from dorsal one; tergites I–VII narrow, transverse, rounded laterally, lacking sagittal line; [We were unable to determine the presence of an anterotransverse carina owing to the bad condition of the only specimen available.]; segment VIII (=LAS) longest, completely sclerotized, ring-like, strongly constricted at point of insertion of urogomphus; projecting backwards into a very short, subconical siphon; spiracles absent lateroventrally on segments I–VII. Urogomphus very long, composed of 2 urogomphomeres; U1 long, much longer than segment VIII; U2 narrower, setiform, much shorter than U1. Chaetotaxy: Similar to that of generalized Hydroporinae larva (Alarie and Harper 1990; Alarie et al. 1990; Alarie 1991; Alarie andMichat 2007a) except for the following features (Figs. 1–12): Pores PAd and ANf absent; pore ANh distal; setae MX4 and TR2 absent; pore FEa articulated distally, close to seta FE5; seta TI7 short, spine-like; seta AB10 spine-like; setae UR2 and UR3 contiguous, seta UR4 articulated posteriorly; setae UR5, UR6, and UR7 elongate; seta UR8 inserted subapically. [Pore PAk and seta PA13 could not be located. We are reluctant to consider them as lacking due to the condition of the only instar I specimen available.]
No specimens were available for study.
Description. As instar I except as follows: Body: Measurements and ratios that characterize the body shape in Tables 1 and 2. Head: Head capsule (Figs. 15–16) constricted at level of occipital suture; egg bursters lacking. Antenna elongate, distinctly shorter than HW. LP2 subequal to slightly shorter than LP1. MP subequal in length to antenna, MP1 slightly shorter than MP2 (Figs. 13–14). Thorax: Protergum lacking anterotransverse carina; both meso- andmetathoracic terga with an anterotransverse carinae; sagittal line visible;mesopleural region with a spiracular opening on each side. Legs: Position and number of secondary setae in Table 3; natatory setae present on dorsal margin of femora, tibiae, and tarsi (Figs. 17–18). Abdomen: Segment VII completely sclerotized both dorsally and ventrally, all tergiteswith anterotransverse carina (Fig. 19); mesopleural region of segments I–VII with a spiracular opening on each side. Chaetotaxy: Head capsule with numerous secondary setae; lateroventral margin of PA with several secondary spine-like setae (Figs. 15–16); anteroventral margin of nasale with half circle of about 60 lamellae clypeales of different lengths, directed downwards; AN, MX, and LA lacking secondary setae; MN with 1 hair-like secondary seta on basoexternal margin; thoracic and abdominal sclerites I–VIIIwith numerous secondary setaemainly on posterior half; natatory setae present on dorsal margin of femora, tibiae, and tarsi; secondary leg setation in Tables 3-4 and Figs. 17–18; U with secondary setae (Fig. 19).
Remarks. Larvae of Barrethydrus can readily be distinguished from those of other Australian Hydroporini described in detail (i.e., Paroster and Antiporus) by the presence of natatory setae on the dorsal margins of the femur, tibia, and tarsi and metric characters presented in Table 2, and, superficially, from the less well-studied genera by the strong, dark yellow banding on the body.
Distribution. Endemic to Australia.
Barretthydrus tibialis Lea, 1927
(Figs. 1–14, 16–19)
Source of Material. Larvae were collected in association with adults at the following localities: Australia, New South Wales, 20 km North of Nelligan, 15-VIII.1997, coll. C. H. S.Watts; Sardine Creek, 30 km North of Orbost V, 30-XI-1998, C. H. S. Watts leg.
Diagnostic Combination (Instar III). The third instar of B. tibialis can easily be distinguished from that of the closely similar B. geminatus by the presence of a broad, blackish macula on the anterior portion of the frontoclypeus (Fig. 16) in addition to a relatively shorter head capsule compared to the length of abdominal segment VIII (HL/LAS <4.50 compared to >4.60) and a larger total number of secondary setae on selected leg articles (Table 3).
Description. Color: [Not available owing to the bad state of preservation of the only specimen available.] Body: Measurements and ratios that characterize the body shape in Table 1. Head: Head capsule as in Figs. 1–2, HL=0.60 mm, HL/LAS=4.61. Abdomen: As in Figs. 10–11, U1 = 0.75 mm; U1/HW = 1.55.
No specimen available for study.
(Figs. 13–14, 16–19)
Description. Color: Head capsule predominantly yellow; frontoclypeus with a broad, black, subapical macula (Fig. 16); head appendages dark yellow; thoracic tergites dark brown; abdominal tergites I, IV–VII dark brown, II and III dark yellow to pale brown and VIII pale yellow; urogomphi dark yellow to pale brown; legs dark brown proximally, pale yellow distally. Body: Measurements and ratios that characterize the body shape in Table 1. Head: Head capsule as in Fig. 16, HL=1.24–1.30, HL/LAS<4.50. Abdomen: U1 = 1.51–1.69 mm, U1/HW = 1.56–1.67 (Fig. 19). Chaetotaxy: ProFE with more than 52 secondary setae; mesoFE with more than 56 secondary setae; mesoTI with more than 47 secondary setae; mesoTA with more than 34 secondary setae;metaCO withmore than 26 secondary setae; metaFE with more than 66 secondary setae; metaTI with more than 50 secondary setae; metaTA with more than 43 secondary setae.
Measurements and ratios for the larvae of Barretthydrus tibialis (BATI) and Barretthydrus geminatus (BAGE). n = number of specimens studied; ** = missing values. See text for definitions of variable codes.
Barretthydrus geminatus Lea, 1927
Source of Material. Larvae were collected in association with adults at the following locality: Australia, New SouthWales, 13 km NNW Dungog, Williams River, Tillegra Bridge, 109 m., 19. X.2006, 32.19.078S 151.41.250E, L. Hendrich leg. (NSW 84).
Diagnostic Combination (Instar III). The third instar of B. geminatus can easily be distinguished from that of the closely similar B. tibialis by the absence of maculae on the dorsal surface of the head capsule (Fig. 15), in addition to a relatively longer head capsule compared to the length of abdominal segment VIII (HL/LAS >4.60 compared to <4.50) and a lower total number of secondary setae on selected leg articles (Table 3).
Measurements and ratios for instar III of selected genera of Australian Hydroporini: ANTI = Antiporus; BARR =Barretthydrus; PARO=epigaeic Paroster. n= number of species; **, missing values. See text for definitions of variable codes.
No specimen available for study.
No specimen available for study.
Description. Color: Head capsule testaceous (Fig. 15); head appendages pale yellow; thoracic tergites dark brown; abdominal tergites I–III brown mesally, broadly yellow laterally, IV–VII dark brown, VIII pale yellow; urogomphi dark yellow; legs predominantly dark yellow except coxae brownish. Body: Measurements and ratios that characterize the body shape in Table 1. Head: Head capsule as in Fig. 15, HL = 1.27–1.36, HL/LAS >4.60. Abdomen: U1=1.29–1.45 mm, U1/HW= 1.37–1.49. Chaetotaxy: ProFE with less than 45 secondary setae; mesoFE with less than 51 secondary setae; mesoTI with less than 45 secondary setae; mesoTA with less than 30 secondary setae; metaCO with less than 18 secondary setae; metaFE with less than 53 secondary setae; metaTI with less than 46secondary setae; metaTA with less than 38 secondary setae.
The dytiscid subfamily Hydroporinae is generally recognized as monophyletic (Burmeister 1976; Wolfe 1985; Miller 2001; Ribera et al. 2002; Miller et al. 2006; Michat et al. 2007, 2017; Miller and Bergsten 2014). This subfamily is presently composed of 10 tribes (Miller and Bergsten 2016), including the very speciose Hydroporini. Hydroporini remains a difficult group to diagnose, and, as shown in this study (Fig. 20), no really clear larval morphological synapomorphies yet support the monophyletic origin of this group.
Our results, however, posit Barretthydrus as closely related to Antiporus and members of the tribe Vatellini. Larvae of these three lineages are characterized by the presence of well-developed lateral branches on the frontoclypeus (Figs. 1, 15–16; character 003.2), the cardo fused to the stipes (Fig. 14, character 036.1), the primary setae LA10 and LA12 short (Figs. 5–6; character 063.1), the presence of natatory posterodorsal setae on the femur (character 076.1), tibia (character 082.1), and tarsus (ch. 087.1) (Fig. 18), a very short siphon (Figs. 10, 19; character 097.0), and the seta AB10 spine-like (Fig. 11; character 105.1). All these features, however, are homoplasious within the Hydroporinae, which results in weak support for this clade. In our study, the tribe Vatellini is recovered as monophyletic with strong support (Fig. 20; Bremer support >10, Bootstrap value = 99). Larvae of Vatellini have evolved a large number of synapomorphies (Fig. 21), which reinforces the hypothesis that Vatellini is a natural group (e.g., Miller 2001, 2005; Michat and Torres 2005, 2011; Miller et al. 2006; Ribera et al. 2008; Michat et al. 2017). The sister group relationship of Vatellini with Antiporus, however, is only supported by scarce and homoplasious characters. Larvae of this clade share a narrow, more or less parallel-sided (character 001.1) and spatulate (character 002.1) nasale as well as the absence of secondary spine-like setae on the parietals (Alarie and Watts 2004). The phylogenetic relationship between the Vatellini and Antiporus deserves to be further tested using a larger data set that include more taxa.
Number of secondary setae on the legs of third instars of Barretthydrus geminatus (BAGE) and Barretthydrus tibialis (BATI). Four specimens of each species were studied. CO = coxa; FE = femur; TA = tarsus; TI = tibia; TR = trochanter. Sensillar series: A = anterior; AD = anterodorsal; Adi = anterodistal; AV = anteroventral; D = dorsal; NS = natatory setae; PD = posterodorsal; Pr = proximal; PV = posteroventral; V = ventral. Total is the total number of secondary setae on the segment.
Although weakly supported, the close relationship of Baretthydrus with Antiporus is worth noting knowing that both genera were included in the subtribe Sternopriscina by Miller and Bergsten (2016). Larvae of Barretthydrus differ from those of Antiporus by the presence of a hair-like seta FR7 (Fig. 1; character 008.0) (spine-like in Antiporus), the parietals constricted at the level of the occipital suture in instars II–III (Figs. 15–16; character 013.1), and the absence of spinulae along the lateral margin of the prementum (Figs. 5–6; character 051.0).
This paper is the third of a series of articles aiming to study the larvae of the strictly Australian radiation Sternopriscina. Along with Antiporus (Alarie and Watts 2004) and Barretthydrus (this paper), Paroster is the only other genus that has been studied in much detail (Alarie et al. 2009). One may wonder why Paroster was not included in our data matrix. (Tables 5–6). The first instar of only one Paroster species (the hypogaeic Paroster darlotensisWatts and Humphries) has been described to date. Hypogaeic Paroster are morphologically strongly modified (Alarie et al. 2009) and exhibit some parallelisms (e.g., absence of stemmata) with other subterranean dytiscids that we judged would interfere with the larval phylogenetic signal and, therefore, hamper the phylogenetic resolution. Thus, the genus was excluded from the analysis.
Number of secondary setae on the legs of third instars of selected genera of Australian Hydroporini: Antiporus (ANTI); Barretthydrus (BARR); epigaeic Paroster (PARO). n = number of species studied. CO = coxa; FE = femur; TA=tarsus; TI=tibia; TR, trochanter. Sensillar series:A=anterior;AD=anterodorsal; Adi=anterodistal;AV= anteroventral; D = dorsal; NS = natatory setae; PD = posterodorsal; Pr = proximal; PV = posteroventral; V = ventral. Total is the total number of secondary setae on the segment.
Larvae of Paroster are unique within the Hydroporinae in that their labial palpus is composed of three palpomeres (in comparison to two in every other hydroporine species) (Alarie et al. 2009). They also strongly differ from larvae of Antiporus and Barretthydrus by the absence of natatory setae on their legs (Table 4). On the other hand, Paroster larvae share with these two Sternopriscina genera the presence of well-developed lateral branches on the frontoclypeus, the cardo fused to the stipes, the primary setae LA10 and LA12 short, a very short siphon, and the seta AB10 spine-like, which would support their inclusion within the Sternopriscina. A more thorough study including the larvae of additional Sternopriscina genera would be needed to confirm and refine such hypotheses.
Characters used for the cladistic analysis and the coding of states using selected genera of Hydroporinae as outgroup.
Data matrix used for the cladistic analysis. Missing data coded ‘?'. See Table 5 for characters used for the cladistic analysis and the coding of states.
Financial support was provided by the Natural Sciences and Engineering Research Council of Canada in the form of a discovery research grant to Y. Alarie. The work of M. C. Michat was supported by ANPCyT under Grant PICT–2014–0853, and by University of Buenos Aires under Grant UBACyT–20020150100170BA.