Hydaticus tuyuensis Trémouilles (Coleoptera: Dytiscidae): larval morphology and phylogenetic relationships within Dytiscinae

Primary Research Paper

Hydaticus tuyuensis Tremouilles (Coleoptera: Dytiscidae): larval

morphology and phylogenetic relationships within Dytiscinae

Mariano C. Michat* & Patricia L. M. TorresLaboratorio de Entomologıa, Departamento de Biodiversidad y Biologıa Experimental, Facultad de CienciasExactas y Naturales, Universidad de Buenos Aires. Av. Int. Guiraldes s/n, Ciudad Universitaria, C1428EHA, Buenos

Aires, Argentina(*Author for correspondence: Tel.: +54-11-4576-3384; Fax: +54-11-4576-3384; E-mail: [email protected])

Received 27 November 2005; in revised form 12 January 2006; accepted 15 January 2006; published online 27 April 2006

Key words: Hydaticini, Hydaticus, larvae, chaetotaxy, phylogeny

Abstract

The three larval instars ofHydaticus (Guignotites) tuyuensis Tremouilles are described and illustrated for thefirst time, emphasizing the morphometry and chaetotaxy. Second- and third-instar larvae of the knownspecies of the subgenusGuignotites Brinck are characterized by a trilobate median process of prementum. Alllarval instars ofH. tuyuensis, however, have a bilobate process, similar to that present in the known species ofthe subgenusHydaticus Leach. This implies that a trilobate process cannot be used as a diagnostic characterfor Guignotites. Alternatively, Guignotites as presently defined may not represent a natural group. A cladisticanalysis of 57 larval characters suggests that the genus Hydaticus shares a common origin with the cladecomposed of the genera Eretes Laporte and Thermonectus Dejean, based on the following synapomorphies:(i) abdominal segment VII almost completely sclerotized ventrally; the absence of (ii) additional ventroapicalpores on the third antennomere, (iii) setae FE4 and FE6, (iv) additional setae on the femur and (v) additionalsetae on the tibia; and the presence of (vi) setae on the median process of prementum and (vii) spinulae on thesecond labial palpomere. A bilobate or trilobate median process of the prementum and the submedialinsertion of seta AN3 distinguish Hydaticus from the remaining genera of Dytiscinae studied.

Introduction

Hydaticus Leach includes 135 species of medium-sized diving beetles, which occur in all continents(Nilsson, 2001). This genus is placed within thetribe Hydaticini, subfamily Dytiscinae, and ispresently divided into four subgenera: the mono-basic Hydaticinus Guignot, the dibasic PleurodytesRegimbart, Hydaticus with seven species, andGuignotites Brinck including the remaining species(Nilsson, 2001). An overall of eight species ofHydaticus are recognized from South America, allincluded in the subgenus Guignotites with theexception of H. (Hydaticinus) xanthomelas (Bru-lle) (Tremouilles, 1996).

The subfamily Dytiscinae is composed of seventribes (Miller, 2000; Nilsson, 2001). Cybistrini andAciliini have a relatively high number of genera,six and seven, respectively. The remaining tribesare monogeneric except for Hydaticini, whichcontains two genera. Although there is substantialevidence for monophyly of the Dytiscinae comingfrom both adult and larval characters (Burmeister,1976; Ruhnau & Brancucci, 1984; Miller, 2000,2001), the phylogenetic relationships among tribesand genera within the subfamily are less under-stood. In a recent paper on the phylogenetic rela-tionships of the tribes of Dytiscinae based on adultfeatures, Miller (2000) postulated a sister grouprelationship between Hydaticini and the clade

Hydrobiologia (2006) 563:479–492 � Springer 2006DOI 10.1007/s10750-006-0026-2

composed of the tribes Eretini and Aciliini. Also,he provided evidence for monophyly of the cladeHydaticini+Eretini+Aciliini, with Aubehydrinias its sister tribe. On the other hand, monophyly ofeach of the tribes of Dytiscinae as presentlydefined is supported, based mainly on adult fea-tures (Miller, 2000, 2002). Larval morphology isan important source of characters in analyses ofthe phylogenetic relationships among taxa at dif-ferent levels within Dytiscidae (Alarie, 1995, 1998;Alarie et al., 2002; Michat & Torres, 2005a). Thisis important because most of the phylogenetichypotheses are based on adult characters and havenot been revised considering other character sys-tems. Detailed studies of the larvae, and phyloge-netic analyses considering the data sets theyprovide, are needed to test phylogenies based onadult features.

Larval morphology of Hydaticus is poorlyknown. Descriptions or treatments of larvae of thegenus belong to Fukuda et al. (1959), Dettner(1984), Hilsenhoff (1993), and Galewski (1973a, b;1975; 1983; 1990), mainly for European and NorthAmerican species. A good diagnosis and descrip-tion at the generic level was provided by Larsonet al. (2000). References to other short descrip-tions of larvae of Hydaticus can be found in Ber-trand (1972). The exhaustive analysis of the larvalmorphometry and chaetotaxy, as implemented inmodern papers on dytiscid larvae, allows explo-ration of large sets of characters that contribute todiagnose the taxa under study, and enablesestablishment of phylogenetic hypothesis of rela-tionships among them. The system of nomencla-ture for the primary sensilla of larval Dytiscidaedeveloped recently (i.e., Nilsson, 1988; Alarie,1995, 1998; Alarie et al., 2002), is a very useful toolto evaluate homologies and differences amonglarvae of different taxa within the family.

Hydaticus (Guignotites) tuyuensis Tremouillesis the southernmost distributed species of thisgenus in South America. Its distributional rangecomprises Southwestern Uruguay and EasternBuenos Aires Province, Argentina (Tremouilles,1996). The lack of detailed descriptions of larvaeof Hydaticus, the very deficient knowledge of itslarval chaetotaxy, and the absence of phylogenetichypotheses on the relationships of this genuswithin the subfamily Dytiscinae based on larvalcharacters promoted this study, which has the

following goals: (i) description and illustration ofthe three larval instars of H. tuyuensis, (ii) detailedmorphometric and chaetotaxic analyses of selectedlarval structures, (iii) comparison of the speciesdescribed here with other members of the genusknown as larvae and (iv) study of the phylogeneticrelationships of Hydaticus within Dytiscinae basedon larval characters.

Material and methods

Three specimens of instar I, four of instar II andone of instar III were used for the descriptions.Larvae were collected at the following localities.Argentina: La Escondida, Bragado, Buenos AiresProvince, March 2005, large semi-permanent pondwith abundant aquatic vegetation; Buenos AiresCity, November 2000 through February 2004,large permanent pond with littoral vegetation.Hydaticus tuyuensis is the only species of the genuspresent in Buenos Aires Province.

Specimens were cleared in lactic acid, dissectedand mounted on slides with Hoyer’s medium.Observation (at magnifications up to 1000�) anddrawings were made using an Olympus CX31compound microscope equipped with cameralucida. Scanning and editing of the drawings wasmade using a computer. The material is held in thelarval collection of M.C. Michat (Laboratory ofEntomology, Buenos Aires University, Argentina).

For analyses of morphometry and chaetotaxy,we employed the terms used in previous papersdealing with larval morphology of Dytiscidae(Alarie, 1995, 1998; Alarie et al., 2002). Pairedstructures of each individual were consideredindependently. The following measures weretaken. Total length (excluding urogomphi) (TL).Maximum width (MW). Head length (HL): totalhead length including the frontoclypeus, measuredmedially along epicranial stem. Head width (HW):maximum head width. Length of frontoclypeus(FRL): from apex of nasale to posterior margin ofecdysial suture. Occipital foramen width (OCW):maximum width measured along dorsal margin ofoccipital foramen. Coronal line length (COL).Length and width of mandible (MN). Length ofmaxillary palpifer (PPF). Length of galea (GA).Length of antenna (A), maxillary (MP) and labial(LP) palpi were derived by adding the lengths of

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the individual segments; each segment is denotedby the corresponding letter(s) followed by anumber (e.g., A1: first antennomere). A3’ is usedas an abbreviation for the apical lateroventralprocess of third antennomere. Length of leg (L)including the longest claw (CL) was derived byadding the lengths of the segments; each leg isdenoted by an L followed by a number (e.g., L1:prothoracic leg). Length of trochanter includesonly the proximal portion (the length of distalportion is included in the femoral length). The legsof the larvae studied were considered as beingcomposed of six segments following Lawrence(1991). Dorsal length of last abdominal segment(LAS): measured along midline from anterior toposterior margin. Length of urogomphus (U):total length from base to apex. These measure-ments were used to calculate several ratios, whichcharacterize the body shape.

Primary (present in first-instar larva) and sec-ondary (added in later instars) setae and so-calledpores (campaniform sensilla) were distinguishedfor the head capsule, head appendages, legs, lastabdominal segment and urogomphi. Primary sen-silla were divided into ancestral and additionalaccording to Nilsson (1988). Sensilla were codedby two capital letters, in most cases correspondingto the first two letters of the name of the structureon which are located, and a number (setae) or alower case letter (pores). The following abbrevia-tions were used. AB: abdominal segment VIII,AN: antenna, CO: coxa, FE: femur, FR: frontoc-lypeus, LA: labium, MN: mandible, MX: maxilla,PA: parietal, PT: pretarsus, TA: tarsus, TI: tibia,TR: trochanter, UR: urogomphus. Recent papershave suggested that Lancetinae is the sister sub-family of Dytiscinae, and that Colymbetinae is thesister subfamily of Lancetinae+Dytiscinae (Miller,2001). For this reason, our procedure for namingsetae and pores in H. tuyuensis was to comparethem with those in the subfamilies Colymbetinae(sensu Miller, 2001) and Lancetinae (Alarie, 1995,1998; Alarie et al., 2002), and to establish homol-ogies using the criterion of similarity of position(Wiley, 1981). This procedure is supported bythe relative stability of the character system con-sidered within higher taxa, even subfamilies(Nilsson, 1988). Setae located at the apex ofmaxillary and labial palpi were extremely difficultto distinguish due to their position and small size.

Accordingly, they are not well represented.Additional setae were included in the count ofsecondary setae.

For the study of the phylogenetic relationshipsof the genus Hydaticus within Dytiscinae, larvae ofH. tuyuensis and representatives of five of theremaining six tribes were coded for parsimonyanalysis (Aubehydrini was not included becausethe larva of Notaticus Zimmermann is unknown).Rhantus signatus (Fabricius) (Colymbetinae) andLancetes marginatus (Steinheil) (Lancetinae) wereincluded as outgroups. Data presented for thespecies Rhantus signatus, Megadytes glaucus(Brulle) and Dytiscus harrisii Kirby have not beenpublished yet. The data matrix was analyzed usingthe program TNT (Goloboff et al., 2003). Thesmall size of the matrix allowed for the imple-mentation of an exact solution algorithm (implicitenumeration). Branch support was calculated byjackknifing (p=36, 200 replicates).

Results

Description of the larvae of Hydaticus tuyuensisTremouilles

Instar IColor. Larva entirely yellow except for tibia, tarsusand distal portion of urogomphus light brown.

Body (Fig. 1a). Subcylindrical. Measurements andratios aimed to characterize the body shape areshown in Table 1.

Head. Head capsule (Fig. 1b and c). Flattened,subtriangular, somewhat longer than broad;maximum width at stemmata, slightly constrictedat level of occipital region; occipital suture andecdysial line well marked; occipital foramenslightly emarginate dorsally, deeply emarginateventrally; posterior tentorial pits visible ventrally;surface covered with short rounded spinulae. FRsubtriangular, lateral margins curved, anteriormargin rounded medially, with 65–69 well devel-oped spatulate setae arranged in two rows, sub-marginal row composed of 47–51 setae, marginalrow composed of 18–20 setae; 1 spine-like eggburster present on each side; anterolateral lobesrounded, projecting beyond anterior margin. Four

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Figure 1. Hydaticus tuyuensis, first-instar larva. (a) Habitus, dorsal aspect; (b, c) Cephalic capsule, dorsal and ventral aspects,

respectively; (d, e) Antenna, dorsal and ventral aspects, respectively; (f) Mandible, dorsal aspect; (g, h) Maxilla, dorsal and ventral

aspects, respectively; (i, j) Labium, dorsal and ventral aspects, respectively. TP: tentorial pit. Numbers and lower case letters refer to

primary setae and pores, respectively. Solid squares refer to additional setae. Scale bars=1.00 mm for (a), 0.20 mm for (b, c) and

0.10 mm for (d–j).

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stemmata on upper side of head and two onunderside, arranged in two curved vertical series.Antenna (Fig. 1d and e). Elongate, slender, 4-seg-mented, about as long as HW; A1 the longest, A2and A3 subequal in length; A3 with a ventroapicalspinula; apical lateroventral process of A3 elon-gate; A4 the shortest, with a lateroventral processat distal half, similar to that of A3. Mandible(Fig. 1f). Prominent, falciform; moderately wide atbase, sharp apically, with short rounded spinulaeon external surface of basal third; mandibularchannel present. Maxilla (Fig. 1g and h). Cardowell developed; stipes narrow, elongate, subcylin-drical; PPF short, broad, palpomere-like; MPelongate, slender, 3-segmented, palpomeres subequalin length; GA well developed, subconical Labium(Fig. 1i and j). Prementum subtriangular, broaderthan long, anterior margin projecting forward in abilobate (widely bifid) median process; LP short,2-segmented, with short rounded spinulae onexternal margin; LP2 somewhat shorter than LP1.

Thorax. Terga convex, pronotum about as long asmeso- and metanotum combined, meso- andmetanotum subequal; protergite subrectangular,

margins rounded, more developed than meso- andmetatergite; meso- and metatergite transversal,with anterotransverse carina; sagittal line wellmarked; thoracic venter membranous; spiraclesabsent. Legs (Fig. 2a and b). Long, 6-segmented,L1 the shortest, L3 the longest; CO robust, elon-gate, TR subdivided into two parts, FE, TI andTA slender, subcylindrical, PT with two long,slender, almost straight claws, posterior clawshorter than anterior one; FE, TI, TA, dorsalsurface of CO and ventral surface of distal portionof TR covered with short spinulae; on prothoracicleg, spinulae on anteroventrodistal portion of TIand anteroventroproximal portion of TA long,very slender, forming dense patches; tarsi with arow of ventral spinulae, more developed on pro-tarsus; claws with short spinulae on lateral andventral surfaces of basal half.

Abdomen. Eight-segmented, segments I–VI scler-otized dorsally, membranous ventrally; tergitesI–VI similar to each other, narrow, transverse,laterally rounded, with anterotransverse carina;segment VII elongate, narrow, completely sclero-tized except for a narrow sagittal band ventrally,

Table 1. Measurements and ratios for the three larval instars of Hydaticus tuyuensis

Measure Instar I Instar II Instar III Measure Instar I Instar II Instar III

TL (mm) 7.90–8.00 16.80–17.50 24.00 MP3/MP2 1.03–1.06 0.90–1.01 0.83

MW (mm) 1.00–1.10 1.90–2.00 3.50 GA/MP1 0.26–0.29 0.20–0.23 0.18–0.19

HL (mm) 1.39–1.41 2.16–2.33 2.98 MP/LP 2.04–2.21 1.72–1.92 1.72

HW (mm) 1.29–1.31 1.88–2.05 2.50 LP2/LP1 0.87–0.95 0.68–0.80 0.63

FRL (mm) 0.61–0.64 0.89–0.99 1.19 L3 (mm) 3.32–3.39 4.79–5.54 6.58–6.59

OCW (mm) 0.52–0.53 0.82–0.84 1.19 L3/L1 1.17–1.19 1.20–1.23 1.23–1.25

HL/HW 1.07–1.09 1.14–1.18 1.19 L3/L2 1.02–1.06 1.04–1.06 1.06–1.07

HW/OCW 2.44–2.52 2.35–2.50 2.10 L3/HW 2.55–2.58 2.51–2.71 2.63

COL/HL 0.54–0.56 0.57–0.61 0.60 L3 (CO/FE) 0.89–0.93 0.90–0.92 0.97–0.99

FRL/HL 0.44–0.46 0.39–0.43 0.40 L3 (TI/FE) 0.85–0.94 0.81–0.83 0.73–0.74

A/HW 0.97–1.04 0.85–0.90 0.81 L3 (TA/FE) 0.79–0.84 0.72 0.61–0.62

A3/A1 0.81–0.87 0.65–0.68 0.55 L3 (CL/TA) 0.36–0.38 0.25–0.28 0.23–0.24

A4/A3 0.12–0.14 0.09–0.11 0.08–0.09 LAS (mm) 1.36–1.46 1.88–2.12 2.31

A3’/A4 0.61–0.78 0.53–0.70 0.67–0.79 LAS/HW 1.05–1.11 0.97–1.04 0.92

MN(l/w) 3.78–4.13 3.83–4.13 3.73–3.88 U (mm) 1.02–1.04 1.29–1.61 1.61–1.62

MN(l)/HL 0.58–0.60 0.50–0.56 0.51–0.52 U/LAS 0.69–0.76 0.68–0.76 0.70

PPF/MP1 0.25–0.28 0.26–0.27 0.28 U/HW 0.77–0.80 0.66–0.79 0.64–0.65

A/MP 1.41–1.45 1.43–1.46 1.53

l, length; w, width.

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with anterotransverse carina, covered with shortspinulae; spiracles absent on segments I–VII; LAS(Fig. 2c and d) the longest and narrowest, com-pletely sclerotized, ring-like, without anterotrans-verse carina, covered with short spinulae; siphonreduced. Urogomphus (Fig. 2e). Short, 1-seg-mented, covered with short spinulae.

Chaetotaxy (Figs. 1b–j and 2a–e). Similar to thatof generalized Colymbetinae and Lancetinae larvae(Alarie, 1995, 1998; Alarie et al., 2002) except forthe following characteristics. Pore FRe absent; se-tae FR4 and FR5 somewhat displaced laterally;seta PA10 and pore PAd contiguous to setae PA21and PA22; one additional small seta contiguous topore PAo; pore PAl located far from the occipitalsuture. Seta AN3 submedial; pore ANg dorso-proximal. Pore MNa distal to pore MNb; setaMN1 contiguous to pore MNc. Seta MX4 proxi-mal and close to internal margin of stipes; only two

pores present on the stipes; one additional spine-like seta proximal to seta MX6; pore MXj medial;three additional, subdistal, small setae on MP3.Setae LA3, LA4, LA5 and LA8 inserted on themedian process; median process with 0–1 addi-tional spine-like setae on each side; one additionalminute seta on dorsodistal half of LP2. We wereunable to find seta LA7. Thoracic tergites withseveral hair-like setae on posterior half, protergitewith an anterotransverse row of hair-like setae.Seta CO7 anteromedial; seta CO12 hair-like; setaTR2 hair-like, anteroventral; rows of natatory se-tae on anteroventral and posterodorsal margins ofFE and TI, posterodorsal row of FE only on distalthird; setae FE4, FE5, FE6, FE8, FE9, FE10 andTI4 absent from their usual positions, we could notestablish if they are really absent or, at least someof them, became elongate, hair-like and incorpo-rated to the rows of natatory setae; femur withadditional spine-like setae on anterodistal surface

Figure 2. Hydaticus tuyuensis, first-instar larva. (a, b) Metathoracic leg, anterior and posterior aspects, respectively; (c, d) Abdominal

segment VIII, dorsal and ventral aspects, respectively; (e) Urogomphus, dorsal aspect. Numbers and lower case letters refer to primary

setae and pores, respectively. Scale bars=0.20 mm.

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(1–2 on L1, 0–1 on L2 and L3); setae TI5, TI6 andTI7 elongate, hair-like; seta TA1 inserted far fromthe apex. Abdominal tergites I–VII with severalhair-like setae on posterior half; abdominal seg-ment VII with a row of natatory setae on lateralmargin. Setae AB11 and AB15 inserted far fromthe ventroapical margin; LAS with a row of nata-tory setae on lateral margin. Setae UR2, UR3 andUR4 inserted at basal half; setae UR5, UR6, UR7and UR8 inserted apically.

Instar II

As first-instar larva except for thefollowing featuresColor. Larva entirely yellowish to light brown.Measurements and ratios aimed to characterize thebody shape are shown in Table 1. Head capsule.Strongly constricted at level of occipital region;surface spinulae absent except anterolaterally onFR, near posterior tentorial pits and posterior tooccipital suture. FR with 116–146 well-developedspatulate setae on anterior margin, arranged in tworows, submarginal row composed of 96–125 setae,marginal row composed of 20–21 setae. Antenna.Somewhat shorter thanHW;A2 andA3 subdividedbasally.Mandible. Without spinulae.Maxilla. MP2and MP3 subdivided basally. Labium. Prementumwith short spinulae on anteroventral surface. Tho-rax. Ventral sclerite present on prothorax. Legs.Metathoracic claws subequal in length; short spin-ulae absent on surface of legs; spinulae on clawsminute. Abdomen. Segment VII completely sclero-tized, ring-like, with anterotransverse carina; LASslightly longer than segment VII, with narrow lon-gitudinal bands of short spinulae both dorsally andventrally. Urogomphus. With short spinulae onbasoventral portion. Chaetotaxy. Head capsulewith numerous minute secondary setae on dorsaland ventral surfaces (anterior to occipital suture);PAwith 11–14 temporal and 3–8 ventral secondary,spine-like setae on each side; A1 with 13–14 sec-ondary hair-like setae; MN with about 3–4 sec-ondary hair-like setae on basoexternal margin, andnumerous secondary minute setae; stipes with 6–7secondary setae on ventral margin; prementumwith one secondary pore contiguous to seta LA1,and 3–5 secondary spine-like setae on each side ofanterodorsal surface; secondary leg setationdetailed in Table 2; posterior surface of CO and

proximal surface of TR with 1 secondary pore; rowof natatory setae on posterodorsal margin of FE allalong the segment; rows of natatory setae also ondorsal margin of CO, ventral margin of TR andposterodorsal margin of TA; LAS with numeroussecondary minute and hair-like setae.

Instar III

As second-instar larva except for thefollowing featuresColor. Thoracic and abdominal tergites with a fewbrown maculae. Measurements and ratios aimed tocharacterize the body shape are shown in Table 1.Head capsule (Fig. 3a). Surface spinulae absentexcept anterolaterally on FR and posterior tooccipital suture. FR with 206 well-developed spat-ulate setae on anterior margin, arranged in tworows, submarginal row composed of 186 setae,marginal row composed of 20 setae. Maxilla. MP1and MP2 subequal in length, MP3 somewhatshorter. Thorax. Spiracles present on mesothorax.Abdomen (Fig. 3d and e). Spiracles present on seg-ments I–VII. Abdominal segment VII with numer-ous secondary spine-like setae ventrally. LASslightly shorter than segment VII. Chaetotaxy. PAwith 13–14 temporal and six ventral secondary,spine-like setae on each side; A1 with 14–15 sec-ondary hair-like setae; stipes with about seven sec-ondary setae on ventral margin; secondary legsetation detailed in Table 2 and Figure 3b and c;posterior surface of CO with 2–5 secondary pores.

Character analysisFifty-seven informative characters (47 binary and 10multi-state, treated as unordered) were coded forlarvae of eight species belonging to six tribes of Dy-tiscinae and twooutgroups (Table 3). The charactersused and their states are listed in Table 4. The anal-ysis of the data matrix (Table 5) using the programTNT resulted in a single most parsimonious clado-gram of length 101, CI=0.68, RI=0.59 (Fig. 4).Jackknife values indicate that some clades are wellsupported; other clades show lower support.

Discussion

With the exception of the species described here,the larvae of only two species included in the

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subgenus Guignotites are known: H. grammius(Germar) (Fukuda et al., 1959) and H. leander(Rossi) (Dettner, 1984). According to Dettner(1984) and Larson et al. (2000), second- and third-instar larvae of the species of Guignotites arecharacterized by a trilobate median process ofprementum. This feature differentiates them fromthe known species of the subgenus Hydaticus, inwhich all larval instars have a bilobate process(Galewski, 1983, 1990). The presence of a trilobateprocess was used by Dettner (1984) as an argu-ment to consider Guignotites a derived groupwithin Hydaticus, supporting previous conclusionsof Roughley & Pengelly (1981) based on adultcharacters. All larval instars of H. (G.) tuyuensisare characterized by a bilobate process, similar tothat present in species included in the subgenus

Hydaticus. This implies that a trilobate processcannot be used as a diagnostic character forGuignotites. However, further research including abroad taxon sampling within Guignotites is neededto test the hypothesis that this subgenus, as pres-ently delimited, represents a natural group.

The descriptions provided here represent thefirst detailed treatment of the morphometry andchaetotaxy of a species of Hydaticus. However,since Hydaticus is a large and diverse genus withfour subgenera and a worldwide distribution, thephylogenetic relationships herein hypothesizedshould be considered preliminary. The study isalso hampered by the relatively high number ofmissing character state data introduced for Hyde-rodes shuckardi Hope and Eretes australis (Erich-son), and by the fact that the tribes Cybistrini and

Table 2. Number and position of secondary setae on the legs of larvae of Hydaticus tuyuensis

Segment Position Instar II larva Instar III larva

Coxa A 1–2/0–1/1–3 8/12–15/13–21

AD 1–2/0–2/0–3 7–11/3–6/12–14

AV 0/0–4/2–6 0/2–3/4–6

P 0/0/0 0/3–4/2

PD 0–3/0–3/0–4 14/14–17/6–8

Range 3–5/0–8/5–16 29–33/38–40/37–51

Trochanter Pr 0–1/0–1/0–1 10–12/8–9/10–11

Range 0–1/0–1/0–1 10–12/8–9/10–11

Femur A 0–2/2–4/2–4 9–13/9–14/10–11

AD 7–8/10–11/9 9–10/17–18/17–19

ADi 2–3/1–2/1–2 5/5–6/3–4

AV 10–14/14–17/11–16 24–26/27–31/28–29

P 3–4/4–5/4 4–5/6/4–6

PDi 1–3/0–3/0 3–4/3/1–2

PV 10–12/9–11/7–9 22–23/19–20/18–21

Range 37–41/44–50/36–41 79–83/92/84–89

Tibia A 0/0/0 1–4/3/4–6

AD 5–7/7–10/8–12 8–9/13/11–12

ADi 0–1/1/1–2 1/1–2/2

AV 8–10/11–20/13–18 12/24–27/27–31

P 18–24/18–22/19–27 18–20/15–20/17–19

PDi 1/1/1 2/1/1

PV 6–8/9–12/12–18 9–10/15–16/19–22

Range 40–47/50–66/55–75 53–56/75–79/83–91

Tarsus AD 5–6/7–10/6–9 5/10–11/8–10

AV 0/20–22/21–25 0/20–21/21

Range 5–6/29–30/27–34 5/31/29–31

Numbers between slash marks refer to pro-, meso- and metathoracic leg, respectively. A=anterior, D=dorsal, Di=distal,

P=posterior, Pr=proximal, V=ventral, range=total number of secondary setae on the segment (excluding primary and natatory setae).

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Aciliini are hardly represented in relation to theirdiversity. Accordingly, the discussion providedbelow should be considered only as a basis for thestudy of the phylogenetic relationships of Hydati-cus within Dytiscinae based on larval characters.

The monophyly of the subfamily Dytiscinaehas been suggested by Burmeister (1976), Ruhnau& Brancucci (1984) and Miller (2000, 2001). Ourstudy does not represent the proper context withinwhich this hypothesis should be tested, because the

whole diversity of the subfamily is hardly repre-sented. However, members of Dytiscinae form avery well supported clade based on our data(Jackknife value=100). For this reason, we findworth to mention the synapomorphies thatsupport this clade, which may serve as a basis forfuture, more comprehensive studies at the familyand subfamily levels: (i) the anterolateral insertionof seta FR5 (character 01, not determined forH. shuckardi and E. australis); (ii) ratio A4/A3lower than 0.40 (character 13, reversal in E. aus-tralis); (iii) an elongate, subcylindrical stipes(character 15); and the presence of (iv) additionalsetae on the stipes (character 16, not determinedfor H. shuckardi), (v) natatory setae on the legs offirst-instar larva (character 35), (vi) natatory setaeon the lateral margin of abdominal segment VII ofsecond- and third-instar larvae (character 51) and(vii) natatory setae on the lateral margin ofabdominal segment VIII (character 52).

Within Dytiscinae, Hydaticus shares a commonorigin with the genera Eretes Laporte and Ther-monectus Dejean, though the support obtained forthis clade is low (Jackknife value=42). The group issupported by seven synapomorphies: the presenceof (i) setae on the median process of prementum(character 26) and (ii) spinulae on the second labial

Figure 3. Hydaticus tuyuensis, third-instar larva. (a) Head, dorsal aspect; (b, c) Prothoracic leg, anterior and posterior aspects,

respectively; (d, e) Abdominal segments VII–VIII and urogomphi, dorsal and ventral aspects, respectively. Scale bars=0.50 mm.

Table 3. Taxa of Dytiscidae coded for parsimony analysis

Colymbetinae

Colymbetini Rhantus signatus (Fabricius)

Dytiscinae

Aciliini Thermonectus succinctus (Aube)a

Cybistrini Megadytes glaucus (Brulle)

Dytiscini Dytiscus harrisii Kirby

Eretini Eretes australis (Erichson)b

Hydaticini Hydaticus tuyuensis Tremouilles

Hyderodini Hyderodes shuckardi Hopec

Lancetinae

Lancetini Lancetes marginatus (Steinheil)d

aData taken from Michat & Torres (2005b).bData taken from Miller (2002).cData taken from Bertrand (1934) and Watts (1964).dData taken from Michat et al. (2005).

487

Table 4. Characters and states used for the phylogenetic analysis of eight terminal taxa of larvae of Dytiscidae

No. Character States

00 Body 0=not bent medially; 1=bent medially

01 Seta FR5 0=inserted anteriorly; 1=inserted anterolaterally

02 Additional setae on PA 0=absent; 1=one to three (contiguous to

pore PAo); 2=numerous

03 Secondary temporal, spine-like setae on PA 0=present; 1=absent

04 Secondary ventral, spine-like setae on PA 0=present; 1=absent

05 Stemmata 0=subequal in size; 1=anterodorsal

two strongly developed

06 A1 (instar I) 0=not subdivided; 1=subdivided

07 A1 (instars II–III) 0=not subdivided; 1=subdivided

08 A2 and A3 (instar I) 0=not subdivided; 1=subdivided

09 A2 and A3 (instar II) 0=not subdivided; 1=subdivided into

two segments; 2=subdivided into three segments

10 A2 and A3 (instar III) 0=not subdivided; 1=subdivided into

two segments; 2=subdivided into three segments

11 A3¢ 0=protruding; 1=not protruding

12 Additional ventroapical pores on A3 0=present; 1=absent

13 Ratio A4/A3 0=higher than 0,55; 1=lower than 0,40

14 Seta AN3 0=inserted distally; 1=inserted submedially

15 Stipes 0=short, subtrapezoidal; 1=elongate,

subcylindrical; 2=strongly developed,

anterointernal angle projected inwards

16 Additional setae on stipes 0=absent; 1=one small, spine-like seta near

MX6; 2=row of elongate, spine-like

setae; 3=numerous hair-like setae

17 Spinulae on stipes 0=absent; 1=present

18 GA 0=subconical; 1=spiniform; 2=absent

19 MP1 (instars II–III) 0=not subdivided; 1=subdivided

20 MP2 (instar II) 0=not subdivided; 1=subdivided

21 MP2 (instar III) 0=not subdivided; 1=subdivided

22 MP3 (instar II) 0=not subdivided; 1=subdivided into two

segments; 2=subdivided into three segments

23 Prementum (instar III) 0=completely sclerotized; 1=not

sclerotized ventromedially

24 Spinulae on prementum (instar I) 0=present; 1=absent

25 Median process of prementum 0=absent; 1=not bifid; 2=shallowly bifid

apically; 3=deeply bifid, as two separate lobes

26 Setae on median process of prementum 0=absent; 1=present; 2=inapplicable

27 Additional dorsal pore on prementum 0=absent; 1=present

28 LP2 (instar I) 0=not subdivided; 1=subdivided

29 LP1 and LP2 (instar II) 0=not subdivided; 1=subdivided

30 LP1 (instar III) 0=not subdivided; 1=subdivided

31 LP2 (instar III) 0=not subdivided; 1=subdivided

32 Spinulae on LP1 0=absent; 1=present

33 Spinulae on LP2 0=absent; 1=present

34 Ventral sclerites on prothorax (instars II–III) 0=absent; 1=one; 2=two

Continued on next page

488

palpomere (character 33); the absence of (iii) addi-tional ventroapical pores on the third antennomere(character 12), (iv) setae FE4 and FE6 (character37), (v) additional setae on the femur (character 39)and (vi) additional setae on the tibia (character 41);and (vii) the abdominal segment VII sclerotizedventrally except for a narrow, sagittal, central band(character 48). It must be mentioned, however, thatcharacters 12, 33, 37, 39, 41 and 48 were coded asmissing for H. shuckardi and E. australis. Theabsence of natatory setae on the urogomphus(character 56) also relates these three genera,though this character is considered homoplasticsince the same state is present in M. glaucus(Michat, pers. obs.). A similar sister-group rela-

tionship to that proposed here was found by Miller(2000, 2001) based on adult characters.

The sister group of the clade Hydati-cus+Eretes+Thermonectus is composed of thegenera Hyderodes Hope, Dytiscus Linnaeus andMegadytes Sharp. This group presents a low sup-port (Jackknife value=37), but the clade formedby Dytiscus and Megadytes is well supported by anumber of synapomorphies (Jackknife value=92).The relatively high number of missing characterstate data introduced forHyderodes does not allowmaking serious conclusions about the position ofthis genus in the topology obtained.

The genera Eretes and Thermonectus form awell-supported clade based on this study

Table 4. (Continued)

No. Character States

35 Natatory setae on legs (instar I) 0=absent; 1=present

36 Additional posterior pores on CO 0=absent; 1=present

37 Setae FE4 and FE6 0=present; 1=absent

38 Seta FE5 0=present; 1=absent

39 Additional setae on FE (excluding natatory setae) 0=absent; 1=present

40 Seta TI5 0=spiniform; 1=setiform

41 Additional setae on TI (excluding natatory setae) 0=absent; 1=present

42 Additional setae on TA (excluding natatory setae) 0=absent; 1=present

43 Row of natatory setae on anteroventral surface of TA (instars II–III) 0=absent; 1=present

44 Basoventral patch of dense, slender spinulae on proTA 0=absent; 1=present

45 Claws (instar I) 0=evenly curved; 1=almost straight

46 Basoventral spinulae on claw 0=present; 1=absent

47 Abdominal tergites I to VI (instar I) 0=with anterotransverse carina;

1=without anterotransverse carina

48 Abdominal segment VII (instar I) 0=membranous ventrally; 1=sclerotized

ventrally except for a narrow, sagittal, central

band

49 Abdominal segment VII (instar I) 0=without lateral row of natatory setae;

1=with lateral row of natatory setae

50 Abdominal segment VII (instars II–III) 0=membranous ventrally; 1=sclerotized

ventrally

51 Abdominal segment VII (instars II–III) 0=without lateral row of natatory setae;

1=with lateral row of natatory setae

52 LAS 0=without lateral row of natatory setae;

1=with lateral row of natatory setae

53 Setae AB11 and AB15 0=inserted near ventroapical margin;

1=inserted far from ventroapical margin

54 Additional dorsal pores on LAS 0=absent; 1=present

55 Natatory setae on U (instar I) 0=absent; 1=present

56 Natatory setae on U (instars II–III) 0=absent; 1=present

489

Table 5. Data matrix of 57 larval characters for eight terminal taxa of Dytiscidae

Taxa Characters

00–04 05–09 10–14 15–19 20–24 25–29 30–34 35–39 40–44 45–49 50–54 55–56

Rhantus signatus 00000 00000 01000 00000 00010 02000 00000 00001 01100 00000 10000 01

Lancetes marginatus 00000 00000 10001 00100 01000 02100 01000 00001 01000 01100 10000 01

Megadytes glaucus 01211 01112 20010 13021 11211 10011 11002 11001 01101 01101 01111 00

Dytiscus harrisii 01111 01111 10010 11101 11100 02011 11101 11011 11010 11001 01101 11

Hyderodes shuckardi 0??00 00101 1??1? 1??00 111?? 02?00 00??2 1???? ???1? ?0??? 111?? 11

Hydaticus tuyuensis 01100 00001 10111 11000 11101 31000 00111 10110 10001 10011 11110 00

Eretes australis 1??0? 1?1?? 1??0? 12?10 ?1??? 11??? 00??? 1???? ???1? ????? 111?? 00

Thermonectus succinctus 11000 10000 11110 22110 00000 21100 00111 10110 10010 11010 11110 00

Missing data coded ‘?’

Figure 4. Single most parsimonious cladogram of eight terminal taxa of larvae of Dytiscidae, with character changes mapped for each

clade. Jackknife values are indicated inside the boxes. *=homoplastic character state transformation. Character transformations

marked ‘�’ are those for which more than one equally parsimonious optimization exist.

490

(Jackknife value=85). These two genera sharefour synapomorphies: (i) body bent medially(character 00); (ii) two anterodorsal stemmatastrongly developed (character 05); (iii) the presenceof a row of additional, elongate, spine-like setae onthe stipes (character 16, not determined for H.shuckardi); and (iv) the presence of a spiniformgalea (character 18). A similar sister-group rela-tionship between the tribes Eretini and Aciliini waspreviously proposed by Miller (2000, 2001) basedon adult characters.

A single larval autapomorphy is deemed tosupport the genus Hydaticus: the bilobate shape ofthe median process of the prementum (character25). Nevertheless, as it was mentioned above, thisprocess has either a bilobate or a trilobate shapewithin the genus. The submedial position of theseta AN3 (character 14) also distinguishes Hyd-aticus from the remaining genera of Dytiscinaestudied. However, this character could not bedetermined for H. shuckardi and E. australis. Also,it is homoplastic in the cladogram obtained as asimilar state is present in Lancetes Sharp.

This paper presents a description of the larvaeof H. tuyuensis, and a preliminary study of thephylogenetic relationships of Hydaticus within thesubfamily Dytiscinae. However, the hypotheses ofcharacter state evolution postulated here need tobe tested in a more general context, with increasedtaxon sampling along the whole subfamily Dyti-scinae. Future studies should include descriptionsof the larvae (principally first instar) of Notaticus.A recent paper on the phylogeny of Dytiscinaebased on adult characters (Miller, 2000) has sug-gested that this monobasic genus represents thesister group of the clade Hydaticini+Eretini+Aciliini.On the other hand, descriptions of the unknownfirst-instar larvae of the genera Hyderodes andEretes are much needed. Studies of the adult andthe last larval instar of species of Eretes (Miller,2000, 2002) have proposed that this genus mightbe closely related to Aciliini, a result supported bythis study.

Acknowledgments

We thank Dr. A. O. Bachmann and an anonymousreferee for their critical review of the manuscriptand valuable comments. Our field and laboratory

work was supported by postgraduate scholarshipsand grant PIP 02541/00 from the Consejo Nac-ional de Investigaciones Cientıficas y Tecnicas dela Republica Argentina (CONICET).

References

Alarie, Y., 1995. Primary setae and pores on the legs, the last

abdominal segment, and the urogomphi of larvae of Nearctic

Colymbetinae (Coleoptera: Adephaga: Dytiscidae) with an

analysis of their phylogenetic relationships. The Canadian

Entomologist 127: 913–943.

Alarie, Y., 1998. Phylogenetic relationships of Nearctic

Colymbetinae (Coleoptera: Adephaga: Dytiscidae) based on

chaetotaxic and porotaxic analysis of head capsule and

appendages of larvae. The Canadian Entomologist 130: 803–

824.

Alarie, Y., M. Archangelsky, A. N. Nilsson & S. C. H. Watts,

2002. Larval morphology of the genus Lancetes (Coleoptera:

Adephaga: Dytiscidae): the hypothesis of sister-group rela-

tionship with the subfamily Dytiscinae revisited. The Cana-

dian Entomologist 134: 467–501.

Bertrand, H., 1934. Notes sur quelques larves de coleopteres

aquatiques. Annales de la Societe Entomologique de France

103: 363–382.

Bertrand, H., 1972. Larves et nymphes des coleopteres aqua-

tiques du globe. F. Paillart, France.

Burmeister, E. G., 1976. Der Ovipositor der Hydradephaga

(Coleoptera) und seine phylogenetische Bedeutung unter

besonderer Berucksichtigung der Dytiscidae. Zoomorphol-

ogie 85: 165–257.

Dettner, K., 1984. Description of the larvae of Hydaticus

leander Rossi (Coleoptera, Dytiscidae) with a key to larvae

of european species of the genus Hydaticus. Entomologica

Basiliensia 9: 108–115.

Fukuda, A., K. Kurosa & N. Hayashi, 1959. Coleoptera. In

Kawada (ed.), Illustrated Insect Larvae of Japan. Hoku-

ryukan, Tokyo, 391–546.

Galewski, K., 1973a. Diagnostic characters of larvae of Central

European species of Hydaticus Leach (Coleoptera, Dytisci-

dae) with some notes on their biology. Bulletin de L’acade-

mie Polonaise des Sciences 21: 511–518.

Galewski, K., 1973b. Generic characters of the larvae of the

subfamily Dytiscinae (Dytiscidae) with a key to the central

European genera. Polskie Pismo Entomologiczne 43: 491–

498.

Galewski, K., 1975. Descriptions of the unknown larvae of the

genera Hydaticus Leach and Graphoderus Dejean (Coleop-

tera, Dytiscidae) with some data on their biology. Annales

Zoologici 32: 249–268.

Galewski, K., 1983. Preimaginal stages of Central-European

species of Hydaticus Leach (Coleoptera, Dytiscidae). Polskie

Pismo Entomologiczne 53: 229–269.

Galewski, K., 1990. First stage larvae of the Central European

species of Hydaticus Leach (Coleoptera, Dytiscidae) with a

key to all larval stages. Annales Zoologici 43: 433–439.

491

Goloboff, P., J. Farris & K. Nixon, 2003. T.N.T.: Tree analysis

using new technology. Program and documentation, avail-

able from the authors, and at http://www.zmuc.dk/public/

phylogeny.

Hilsenhoff, W. L., 1993. Dytiscidae and Noteridae of Wisconsin

(Coleoptera). II. Distribution, habitat, life cycle, and iden-

tification of species of Dytiscinae. The Great Lakes Ento-

mologist 26: 35–53.

Larson, D. J., Y. Alarie & R. E. Roughley, 2000. Predaceous

Diving Beetles (Coleoptera: Dytiscidae) of the Nearctic

Region, with Emphasis on the Fauna of Canada and Alaska.

NRC Research Press, Ottawa.

Lawrence, J. F., 1991. Order Coleoptera. In Stehr, F. W. (ed.),

Immature Insects. Vol. 2. Kendall/Hunt Publishing Com-

pany, Iowa, 144–658.

Michat, M. C., M. Archangelsky & P. L. M. Torres, 2005.

Descriptions of the preimaginal stages of Lancetes margin-

atus (Steinheil) and L. biremis Rıha (Coleoptera: Dytiscidae),

and comparative notes with other Lancetes larvae. Studies

on Neotropical Fauna and Environment 40: 129–142.

Michat, M. C. & P. L. M. Torres, 2005a. Larval morphology of

Macrovatellus haagi (Wehncke) and phylogeny of Hydro-

porinae (Coleoptera: Dytiscidae). Insect Systematics &

Evolution 36: 199–218.

Michat, M. C. & P. L. M. Torres, 2005b. Larval morphology of

Thermonectus succinctus (Aube (1838)) (Coleoptera: Dyti-

scidae: Dytiscinae), with biological notes and chaetotaxic

analysis. Aquatic Insects 27: 281–292.

Miller, K. B., 2000. Cladistic analysis of the tribes of Dytiscinae

and the phylogenetic position of the genus Notaticus Zim-

mermann (Coleoptera: Dytiscidae). Insect Systematics &

Evolution 31: 165–177.

Miller, K. B., 2001. On the phylogeny of the Dytiscidae

(Insecta: Coleoptera) with emphasis on the morphology of

the female reproductive system. Insect Systematics & Evo-

lution 32: 45–92.

Miller, K. B., 2002. Revision of the genus Eretes Laporte, 1833

(Coleoptera: Dytiscidae). Aquatic Insects 24: 247–272.

Nilsson, A. N., 1988. A review of primary setae and pores on

legs of larval Dytiscidae (Coleoptera). Canadian Journal of

Zoology 66: 2283–2294.

Nilsson, A. N., 2001. World Catalogue of Insects, Vol. 3.

Dytiscidae (Coleoptera). Apollo Books, Steenstrup Den-

mark.

Roughley, R. E. & D. H. Pengelly, 1981. Classification, phy-

logeny, and zoogeography of Hydaticus Leach (Coleoptera:

Dytiscidae) of North America. Quaestiones Entomologicae

17: 249–309.

Ruhnau, S. & M. Brancucci, 1984. Studies on the genus

Lancetes. 2. Analysis of its phylogenetic position using pre-

imaginal characters (Coleoptera, Dytiscidae). Entomologica

Basiliensia 9: 80–107.

Tremouilles, E. R., 1996. Revision del genero Hydaticus

Leach en America del Sur, con descripcion de tres nuevas

especies (Coleoptera, Dytiscidae). Physis, Section B 52:

15–32.

Watts, C. H. S., 1964. The larvae of Australian Cybister spp.

Curt., Homoeodytes spp. Reg. and Hyderodes shuckardi

Hope (Coleoptera: Dytiscidae). Transactions of the Royal

Society of South Australia 88: 145–156.

Wiley, E. O., 1981. Phylogenetics. The Theory and Practice

of Phylogenetic Systematics. John Wiley and Sons, New

York.

492