Research Article |
Corresponding author: Mariano C. Michat ( marianoide@gmail.com ) Academic editor: Borislav Guéorguiev
© 2019 Mariano C. Michat, Yves Alarie, Chris H. S. Watts.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Michat MC, Alarie Y, Watts CHS (2019) Dealing with a hairy beast–larval morphology and chaetotaxy of the Australian endemic diving beetle genus Spencerhydrus (Coleoptera, Dytiscidae, Cybistrini). ZooKeys 884: 53-67. https://doi.org/10.3897/zookeys.884.38391
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In this contribution, the larval morphology of Spencerhydrus Sharp, 1882 was studied, an Australian endemic genus in the diving beetle tribe Cybistrini. All instars of the only two species included in the genus (S. latecinctus Sharp, 1882 and S. pulchellus Sharp, 1882) are described and illustrated with the exception of the third instar of S. latecinctus. Detailed morphometric and primary chaetotaxic analyses were performed to discover useful characters for generic diagnosis and species distinction. Spencerhydrus can be distinguished from other Cybistrini genera by the medial projection of frontoclypeus slightly indented apically, with lamellae clypeales directed forward in a characteristic V-shaped pattern, the median process of prementum strongly developed, the presence of a single ventral sclerite on prothorax, the presence of basoventral spinulae on claws, and the reduced sclerotization of the abdominal segment VII which covers only the anterior half. Larvae of the two species of Spencerhydrus can readily be distinguished by the shape of the median process of prementum, which is visibly broader in S. pulchellus than in S. latecinctus.
Water beetle, sensilla, larva, morphometry
Spencerhydrus Sharp, 1882 is a small, Australian endemic genus of large diving beetles (adult length 17–18 mm) included in the tribe Cybistrini. It is made up of two species restricted to southern Australia and with clearly separated distributions, S. latecinctus Sharp, 1882 in the south-east, and S. pulchellus Sharp, 1882 in the south-west (
The only treatment of larvae of Spencerhydrus found in the literature is
Larvae were first cleared by submerging them for several days in lactic acid, then dissected in the standard way and mounted on slides with polyvinyl-lacto-glycerol as the medium. Examination (at magnifications up to 1,000×) and drawings were made using an Olympus CX31 (Olympus Corporation, Japan) compound microscope equipped with a camera lucida. Drawings were scanned and digitally inked using a Genius PenSketch tablet (KYE Corporation, Taiwan). After study, the material will be held in the collection of the South Australian Museum.
The methods and terms used herein follow those employed in
Larvae of Spencerhydrus can be distinguished from those of other Cybistrini genera by the following combination of characters: medial projection of frontoclypeus slightly indented apically, with lamellae clypeales directed forward in a characteristic V-shaped pattern (Figs
Cephalic capsule of Spencerhydrus species, instar I 1 S. latecinctus, dorsal aspect 2 S. pulchellus, dorsal aspect 3 S. latecinctus, ventral aspect 4 S. pulchellus, ventral aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Additional setae not labelled. Color patterns not represented. Abbreviations: EB: egg burster; FR: frontoclypeus; PA: parietal; LC: lamellae clypeales; TP: tentorial pit. Scale bar: 0.70 mm.
Head appendages of Spencerhydrus species, instar I 5 S. latecinctus, right antenna, dorsal aspect 6 S. latecinctus, left antenna, ventral aspect 7 S. pulchellus, right antenna, dorsal aspect 8 S. pulchellus, left antenna, ventral aspect 9 S. latecinctus, right mandible, dorsal aspect 10 S. pulchellus, right mandible, dorsal aspect 11 S. latecinctus, right maxilla, dorsal aspect 12 S. latecinctus, left maxilla, ventral aspect 13 S. pulchellus, right maxilla, dorsal aspect 14 S. pulchellus, left maxilla, ventral aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Additional setae and pores not labelled. Abbreviations: A1–4: antennomeres 1–4; AN: antenna; MN: mandible; MP1–3: maxillary palpomeres 1–3; MX: maxilla; PPF: palpifer; SP: spinula. Scale bar: 0.30 mm.
Instar I (Figs
Body. Elongate, subcylindrical. Measurements and ratios that characterize the body shape are shown in Table
Measure | S. latecinctus | S. pulchellus | |||
Instar I (N = 4) | Instar II (N = 1) | Instar I (N = 3) | Instar II (N = 4) | Instar III (N = 4) | |
HL (mm) | 2.55–2.63 | 3.73 | 2.14–2.28 | 2.93–3.14 | 3.94–4.40 |
HW (mm) | 1.86–1.98 | 2.93 | 1.75–1.89 | 2.38–2.63 | 3.20–3.51 |
FRL (mm) | 1.08–1.10 | 1.44 | 0.94–0.98 | 1.11–1.19 | 1.51–1.58 |
OCW (mm) | 0.71–0.83 | 1.19 | 0.60–0.64 | 0.86–0.99 | 1.33–1.55 |
HL/HW | 1.32–1.38 | 1.27 | 1.21–1.22 | 1.19–1.26 | 1.20–1.27 |
HW/OCW | 2.33–2.70 | 2.46 | 2.92–2.96 | 2.47–2.84 | 2.27–2.42 |
COL/HL | 0.57–0.59 | 0.61 | 0.56–0.57 | 0.62–0.63 | 0.62–0.65 |
FRL/HL | 0.41–0.43 | 0.39 | 0.43–0.44 | 0.37–0.38 | 0.35–0.38 |
A/HW | 1.07–1.13 | 0.97 | 1.08–1.16 | 0.93–1.06 | 0.84–0.92 |
A2/A1 | 0.63–0.69 | 0.58 | 0.58–0.60 | 0.56–0.60 | 0.52–0.59 |
A3/A1 | 0.34–0.37 | 0.28 | 0.32–0.34 | 0.31–0.32 | 0.28–0.31 |
A4/A3 | 0.18–0.22 | 0.15 | 0.23–0.24 | 0.15–0.17 | 0.13–0.14 |
A3’/A4 | 0.75–1.00 | 0.77 | 0.76–0.88 | 0.67–0.86 | 0.69–0.85 |
MNL/MNW | 3.40–3.59 | 3.73 | 3.28–3.41 | 3.23–3.50 | 3.31–3.95 |
MNL/HL | 0.48–0.50 | 0.51 | 0.54–0.55 | 0.51–0.54 | 0.50–0.52 |
A/MP | 1.44–1.47 | 1.56 | 1.40–1.48 | 1.52–1.56 | 1.51–1.61 |
PPF/MP1 | 0.49–0.54 | 0.58 | 0.56–0.60 | 0.53–0.60 | 0.57–0.63 |
MP1/MP2 | 2.09–2.17 | 2.22 | 2.17–2.19 | 2.08–2.16 | 2.13–2.30 |
MP3/MP2 | 1.50–1.60 | 1.27 | 1.48–1.53 | 1.27–1.32 | 1.14–1.18 |
MP/LP | 2.62–2.92 | 2.63 | 2.71–2.81 | 2.72–2.89 | 2.51–2.71 |
LP2/LP1 | 1.00–1.04 | 0.75 | 1.00–1.04 | 0.81–0.97 | 0.74–0.80 |
L3 (mm) | 5.50–6.06 | 7.84 | 5.01–5.28 | 6.46–6.72 | 8.14–8.46 |
L3/L1 | 1.21–1.24 | 1.21 | 1.20–1.23 | 1.20–1.23 | 1.20–1.23 |
L3/L2 | 1.10–1.13 | 1.12 | 1.09–1.12 | 1.10–1.12 | 1.10–1.12 |
L3/HW | 2.95–3.07 | 2.68 | 2.80–2.87 | 2.56–2.73 | 2.40–2.54 |
L3 (CO/FE) | 1.04–1.15 | 1.11 | 1.00–1.05 | 1.04–1.07 | 1.04–1.10 |
L3 (TI/FE) | 0.64–0.68 | 0.65 | 0.65–0.67 | 0.62–0.63 | 0.60–0.62 |
L3 (TA/FE) | 0.75–0.78 | 0.65 | 0.77–0.80 | 0.67–0.72 | 0.60–0.64 |
L3 (CL/TA) | 0.44–0.53 | 0.45 | 0.44–0.47 | 0.40–0.48 | 0.37–0.45 |
LAS (mm) | 3.45–3.58 | 5.09 | 3.00–3.23 | 3.99–4.04 | 5.38–5.70 |
LAS/HW | 1.79–1.88 | 1.74 | 1.69–1.73 | 1.54–1.68 | 1.54–1.68 |
U (mm) | 0.09–0.10 | 0.12 | 0.11–0.12 | 0.08–0.10 | 0.10–0.11 |
Head. Cephalic capsule (Figs
Labium of Spencerhydrus species, instar I 15 S. latecinctus, dorsal aspect 16 S. latecinctus, ventral aspect 17 S. pulchellus, dorsal aspect 18 S. pulchellus, ventral aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Additional setae and pores not labelled. Abbreviations: LA: labium; LP1–2: labial palpomeres 1–2. Scale bar: 0.15 mm.
Thorax. Terga convex, pronotum shorter than subequal meso- and metanotum combined; protergite subrectangular, margins somewhat truncate, much more developed than meso- and metatergite; meso- and metatergite small, subrectangular to subtrapezoidal with posterior margin indented; all three tergites with sagittal line, lacking anterotransverse carina; sterna membranous except for a single sclerite on anterior portion of prothorax; spiracles absent. Legs (Figs
Left metathoracic leg of Spencerhydrus species, instar I 19 S. latecinctus, anterior aspect 20 S. latecinctus, posterior aspect 21 S. pulchellus, anterior aspect 22 S. pulchellus, posterior aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Additional setae and pores not labelled. Abbreviations: CO: coxa; FE: femur; PT: pretarsus; TA: tarsus; TI: tibia; TR: trochanter. Scale bar: 0.20 mm.
Abdomen. Eight-segmented; segments I–VI subequal, membranous except for a small anterodorsal sclerite and a minute sclerite on each lateral; tergites I–VI subrectangular, lacking anterotransverse carina, sagittal line visible, posterior half covered with short spinulae; segment VII narrower, main sclerite covering about anterior half of dorsal surface, lacking anterotransverse carina, covered with short spinulae, sagittal line not found; segments I–VII with a minute lateral sclerite, lacking spiracles; segment VIII (= LAS, Figs
Abdominal segment VIII of Spencerhydrus species, instar I 23 S. latecinctus, dorsal aspect 24 S. pulchellus, dorsal aspect 25 S. latecinctus, ventral aspect 26 S. pulchellus, ventral aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. Additional setae and pores not labelled. Abbreviation: AB: abdominal segment VIII. Scale bar: 0.50 mm.
Urogomphus of Spencerhydrus species, instar I 27 S. latecinctus, right urogomphus, dorsal aspect 28 S. latecinctus, left urogomphus, ventral aspect 29 S. pulchellus, right urogomphus, dorsal aspect 30 S. pulchellus, left urogomphus, ventral aspect. Numbers and lowercase letters indicate primary setae and pores, respectively. UR: urogomphus. Scale bar: 0.05 mm.
Chaetotaxy
(Figs
Instar II (Fig.
Color. Light brown maculae on cephalic capsule more numerous; thoracic tergites with some light brown maculae on disc; maculae on abdominal tergites I–VI more extended; sclerite of segment VII with some small light brown maculae.
Body. Measurements and ratios that characterize the body shape are shown in Table
Head
(Fig.
Thorax. Anterotransverse carina present on metatergite, present or absent on mesotergite; spinulae of claws restricted to middle region.
Abdomen. Tergites I–VI with anterotransverse carina, sometimes weakly marked; sclerites I–VIII lacking spinulae.
Chaetotaxy. Secondary setae most likely present on different body parts, although difficult to evaluate due to large number of additional setae.
Instar III (Fig.
Color. Somewhat darker in general; abdominal tergites I–VII predominantly light brown with longitudinal creamy white area centrally.
Body. Measurements and ratios that characterize the body shape are shown in Table
Head
(Fig.
Thorax. Meso- and metatergite covering most of dorsal surface; well-developed spiracles present on mesothorax.
Abdomen. Well-developed spiracles present on segments I–VII.
Source of material. The descriptions of this species are based on nine specimens of instar I and one of instar II (no instar III was available). Larvae were collected in Australia at the following localities: 1) SW Victoria, Kinhil, 14.x.1983 and 9.xi.1983; 2) SA, Watervalley SE 8 km N Mt. Rough, 15.x.2000; 3) SA, 6 km W Penola, 30.x.2001. The association is firm as S. latecinctus is the only species of Spencerhydrus in south-east Australia, and the larvae of the other Cybistrini genera potentially present in the area (Cybister Curtis, 1827, Onychohydrus and Sternhydrus) clearly differ morphologically from the studied material (
Differs from S. pulchellus as follows. Larger size (Table
Source of material. The descriptions of this species are based on nine specimens of instar I, four of instar II, and four of instar III. Larvae were collected in Australia at the following localities: 1) WA, Ellenbrook Nat. Res., 14.ix.2000; 2) WA, 6 km S Pinjarra, 23.x.1996, 23.ix.2000, and 3.x.2003. The association is firm as S. pulchellus is the only species of Spencerhydrus in south-west Australia, and the larvae of the other Cybistrini genera potentially present in the area (Cybister and Onychohydrus) clearly differ morphologically from the studied material (
Differs from S. latecinctus as follows. Smaller size (Table
Similarly to all other members of the tribe Cybistrini known with sufficient chaetotaxic detail (
As mentioned above (see Introduction) the genus Spencerhydrus was included in recent phylogenetic analyses of the Cybistrini (
Although very similar morphologically, we were able to find some characters to confidently separate larvae of the two species of Spencerhydrus (see earlier). The most conspicuous of these characters is the shape of the median process of prementum, which is visibly broader in S. pulchellus than in S. latecinctus (compare Figs
We thank Helena Shaverdo for useful comments on the manuscript. MCM was supported by Agencia Nacional de Promoción Científica y Tecnológica under Grant PICT–2014–0853 and by Universidad de Buenos Aires under Grant UBACyT–20020150100170BA.