Spencerhydrus Sharp, 1882

Michat, Mariano C., Alarie, Yves & Watts, Chris H. S., 2019, Dealing with a hairy beast-larval morphology and chaetotaxy of the Australian endemic diving beetle genus Spencerhydrus (Coleoptera, Dytiscidae, Cybistrini), ZooKeys 884, pp. 53-67 : 53

Spencerhydrus Sharp, 1882

Diagnosis.

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 1 , 2 , 31 , 32 ); lateral projections of frontoclypeus entire, with lamellae clypeales directed forward ( Figs 1 , 2 , 31 , 32 ); notches separating medial and lateral projections of frontoclypeus wide ( Figs 1 , 2 , 31 , 32 ); antennomere 1 with two or three additional pores proximally ( Figs 5-7 ); mandible lacking crown of multifid setae on distal fourth ( Figs 9 , 10 ); maxillary palpomere 1 subdivided into two articles ( Figs 11-14 ); median process of prementum strongly developed ( Figs 15-18 ); prothorax with a single ventral sclerite; seta TI7 short, spine-like ( Figs 20 , 22 ); ventral row of setae on tibia and tarsus formed by setae of different lengths ( Figs 19 , 21 ); claws with basoventral spinulae ( Figs 19-22 ); urogomphi very small but still longer than broad, included together with anus in the membranous ventrodistal area of abdominal segment VIII ( Figs 25 , 26 ); urogomphus bearing a single additional pore ( Figs 27 , 29 ).

Instar I ( Figs 1 - 30 ). Color. Cephalic capsule pale yellow with small, irregular, light brown maculae on central portion of FR, on posterior half of PA dorsally, and few on central portion of PA ventrally (maculae weakly marked in some specimens); a light brown ring-like band present on neck region contiguous to occipital suture; AN, MX, and LA either completely pale yellow or with A4 and apices of A3 and MP3 light brown; MN with distal region light brown; thoracic tergites pale yellow, protergite with some small light brown maculae, meso- and metatergite each with four small light brown maculae marginally; abdominal tergites I–VI pale yellow, each with 4-6 small light brown maculae marginally; tergite VII pale yellow; sclerite of segment VIII uniformly pale yellow to light brown; membranous parts creamy white; legs pale yellow, distal portion and claws sometimes light brown; urogomphus light brown.

Body. Elongate, subcylindrical. Measurements and ratios that characterize the body shape are shown in Table 1 .

Head. Cephalic capsule ( Figs 1-4 ). Flattened, subtriangular, longer than broad; maximum width at level of stemmata, constricted at level of occipital region; occipital suture present; ecdysial suture well marked, coronal line long; occipital foramen deeply emarginate ventrally; posterior tentorial pits close to well visible ventral midline; FR subtriangular, anterior margin divided into three well developed projections: medial projection broad, well projected forward, slightly indented apically; lateral projections broad, rounded, less projected forward; anterolateral lobes rounded, not projected beyond anterior margin; egg bursters large, rounded to somewhat pointed, located laterally close to ecdysial line; four stemmata on upper side of head and two on underside, arranged in two vertical series. Antenna ( Figs 5-8 ). Elongate, slender, somewhat longer than HW, composed of four antennomeres; A1 longest, subdivided into two articles, distal one somewhat less than twice longer than basal one; A2 shorter than A1, subdivided into three articles, basal one shortest, medial one longest; A3 shorter than A2, subdivided into three articles, basal one shortest, distal one longest, bearing a ventroapical spinula; apical lateroventral process of A3 slender, elongate; A4 shortest, with a small lateroventral process on distal third. Mandible ( Figs 9 , 10 ). Strong, falciform, broad at base, narrowing to acute apex, more abruptly narrowed on distal fourth; mandibular channel present. Maxilla ( Figs 11-14 ). Premaxillary lobes well developed; cardo sub-ovate to subtriangular; stipes elongate, slender, subcylindrical; galea absent; PPF elongate, palpomere-like; MP elongate, slender, shorter than antenna, composed of three palpomeres, MP1 longest, MP2 shortest; MP1 and MP2 subdivided into two articles, distal one longer than basal one; MP3 subdivided into three articles, basal one shortest, distal one longest. Labium ( Figs 15-18 ). Prementum broader than long, anterodorsal margin projected forward into strongly developed, apically rounded median process; LP much shorter than MP, composed of two palpomeres subequal in length; LP1 more or less weakly subdivided into two articles, distal one somewhat longer than basal one; LP2 subdivided into two articles, distal one much longer than basal one.

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 19-22 ). Long, composed of six articles, L1 shortest, L3 longest; CO elongate, subcylindrical, TR divided into two parts by an annulus, FE, TI and TA slender, subcylindrical, PT with two long, slender, slightly curved claws, anterior claw slightly longer than posterior claw on L1 and L2, claws subequal in length on L3; proTA with a row of well-developed, bifid, ventral spinulae, those on basal third shorter, multifid, forming a dense patch (cleaning device); claws with elongate ventral spinulae on basal 1/2 to basal 2/3.

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 23-26 ) longest and narrowest, completely sclerotized except ventrodistally around anus and urogomphi, lacking anterotransverse carina and sagittal line, basal half covered with short spinulae; siphon short. Urogomphus ( Figs 27-30 ). Strongly reduced, approximately two or three times longer than broad, composed of one urogomphomere.

Chaetotaxy ( Figs 1 - 30 ). Similar to that of generalized Cybistrini larva ( Alarie et al. 2011) with the following remarks: FR with numerous lamellae clypeales distributed on the apices of the anterior projections, those on medial projection longer than the others, arranged in a characteristic V-shaped pattern; A1 with some additional multifid setae distally and two or three additional pores proximally; crown of elongate additional (usually multifid) setae on distal fourth of MN absent; seta LA8 present; ventral row of setae on TI and TA formed by setae of different lengths; setae CO7, TR2, TR3, FE8, FE9, FE10, and pore ABa most likely present but obscured by the presence of additional sensilla; seta AB4 bifid or multifid; U bearing all ancestral setae and one pore; several short additional setae present on membranous area of abdominal segment VIII, near urogomphal base.

Instar II ( Fig. 31 ). As instar I except for the following features:

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 1 .

Head ( Fig. 31 ). Egg bursters absent; A subequal in length to HW; LP1 somewhat longer than LP2.

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. 32 ). As instar II except for the following features:

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 1 .

Head ( Fig. 32 ). Somewhat more robust and parallel sided. Antenna shorter than HW.

Thorax. Meso- and metatergite covering most of dorsal surface; well-developed spiracles present on mesothorax.

Abdomen. Well-developed spiracles present on segments I–VII.

Figures

Figures 1 - 4. 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.

Figures 31 - 32. Head of Spencerhydrus species 31 S. latecinctus, instar II, dorsal aspect 32 S. pulchellus, instar III, dorsal aspect. Scale bar: 1.00 mm.

Figures 5 - 14. 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: A 1 - 4: antennomeres 1 - 4; AN: antenna; MN: mandible; MP 1 - 3: maxillary palpomeres 1 - 3; MX: maxilla; PPF: palpifer; SP: spinula. Scale bar: 0.30 mm.

Figures 15 - 18. 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; LP 1 - 2: labial palpomeres 1 - 2. Scale bar: 0.15 mm.

Figures 19 - 22. 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.

Figures 23 - 26. 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.

Figures 27 - 30. 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.