Hydrotarsus compunctus Wollaston 1865

Alarie, Yves & Bilton, David T., 2001, Larval Morhology Of Hydrotarsus Falkenström: Generic Characteristics, Description Of H. Compunctus (Wollaston), And Analysis Of Relationships With Other Members Of The Tribe Hydroporini (Coleoptera: Dytiscidae, Hydroporinae), The Coleopterists Bulletin 55 (3), pp. 341-350 : 342-347

Description of larvae of Hydrotarsus compunctus Wollaston 1865

Figs. 1–5

Diagnostic Combination (Instar III). body fusiformate; stemmata present; cephalic capsule not constricted at level of occipital suture ( Fig. 1 ); antennomere 2 lacking secondary setae; antennomere 3 with a ventroapical spinula, with pore ANf present; maxillary cardo present, primary seta MX 1 inserted on maxillary stipes; primary seta MX 5 absent; prementum lacking elongate lateral spinulae; primary seta TR2 of trochanters present (Fig. 15); spinulae weakly developed on ventral margin of tibiae and tarsi ( Fig. 4 ); legs lacking natatory setae ( Figs. 4–5 ), length of metathoracic leg = 2.20 times HW; posterotransverse carina lacking on thoracic terga; spiracular openings present; siphon strongly constricted posterior to insertion of urogomphi, and short, about 0.20 times LLAS ( Fig. 2 ); primary setae UR2, UR3, and UR4 contiguously articulated on urogomphomere 1 ( Fig. 3 ); urogomphomere 1 lacking secondary setae ( Fig. 3 ).

Description, Instar III ( Figs. 1–5 ). Colour. Cephalic capsule yellowish brown with a reticulate piceous marking meso­posteriorly ( Fig. 1 ); head appendages yellow except A4 predominantly piceous; thorax, terga predominantly dark brown to piceous with a few yellowish maculae; legs yellow; abdomen, terga predominantly brown; urogomphus yellow to pale brown.

Head ( Fig. 1 ). HL = 0.84–0.87 mm (mean = 0.86 mm, n = 2); HW = 0.74–0.78 mm (mean = 0.76 mm, n = 2); FCL = 0.64–0.66 mm (mean = 0.65 mm, n = 2); cephalic capsule ( Fig. 1 ) slightly longer than broad (HL/HW = 1.11–1.13), pear­shaped, tapering posteriorly, not constricted at level of occipital suture; HW/OcW = 1.39–1.46; ecdysial suture well­developed, coronal suture short, 0.24 times HL; occipital suture present; frontoclypeus bluntly rounded, broad (FCL/FCW = 2.39–2.46), 0.76–0.77 times as long as HL, with lateral notches slightly visible in dorsal view; ventroapical margin of frontoclypeus with several spatulate setae (lamellae clypeales of Bertrand 1972), lateroventral margin with several lateral bulges; ocularium present, stemmata not visible ventrally and subdivided into 2 vertical series, stemmata of the posterior row more widely spaced; tentorial pits visible ventrally on each side of middle at about midlength; parietals with 18–25 lateral spines; antenna shorter than HW (length of antenna/HW = 0.56–0.61); A2 = A3> A1> A4, A2/A3 = 0.98–1.04, lateral elongation of antennomere 3 (A3′) shorter than antennomere 4 (A3′/A4 = 0.56–0.63); antennomere 3 with a ventroapical spinula; mandible not toothed on inner margin, = 0.50 times as long as HL; maxilla, maxillary stipes short and thick, incompletely sclerotized ventrally; cardo present, galea and lacinia absent; length of antenna/length of maxillary palpus = 1.14–1.24; MX 1> MX 2> MX 3, palpomere 2, 0.81–0.85 times as long as palpomere 1; labium, prementum trapezoidal, indented medially along anterior margin, = 1.40 times broader than long, lacking marginal spinulae laterally; length of maxillary palpus/length of labial palpus = 1.14–1.24, palpomere 2 subfusiform, 0.96–0.97 times length of palpomere 1; chaetotaxy and porotaxy, head capsule with quite a few secondary setae; head appendages lacking secondary setae except for one lateroproximal seta on mandible; all primary setae and pores of generalized hydroporine head appendages present except seta MX 5; primary seta MX 1 inserted on cardo.

Thorax. pronotum without transverse carina; both meso­ and metathoracic terga with an anterotransverse carina; secondary tergal setation well­developed.

Legs ( Figs. 4–5 ). Metathoracic legs longest, = 1.30 times length of prothoracic legs, = 2.20 times HW; coxa = femur> tibia = tarsus = trochanter; posterior metathoracic claw = 0.57 times as long as metatarsus; spinulae on ventral margin of tibiae and tarsi weakly developedand restricted to the distal 1/3–1/2; chaetotaxy and porotaxy, natatory setae lacking; position and number of secondary setae as expressed in Table 1 .

Abdomen ( Fig. 2 ). Maximum width of body at level of metathorax and abdomen segments 1–2; LLAS = 0.37 mm (n = 2); LLAS/HW = 0.47–0.50, with siphon strongly constricted posterior to insertion of urogomphi; siphon short, broad and bluntly rounded apically, 0.17 times LLAS.

Urogomphus ( Fig. 3 ). Total length of urogomphus = 1.15–1.23 mm (mean = 1.19 mm, n = 2) (length of urogomphomere 1 = 0.53–0.55 mm (mean = 0.54 mm, n = 2)); 1.55–1.57 times HW (length of Uro1/HW = 0.71) and 3.10–3.31 times LLAS (length of Uro1/LLAS = 1.42–1.49); urogomphomere 1 slightly shorter than urogomphomere 2 (Uro1/Uro2 = 0.82–0.84; Uro1/Uro2′ = 0.86–0.88); chaetotaxy and porotaxy, secondary setae absent; basal articulations of primary setae UR2, UR3, and UR4 contiguous; verticillated setae UR5, UR6, and UR7 elongate; seta UR8 subterminal in position, distally articulated on urogomphomere 2 (Uro2′/Uro2 = 0.97).

Description, Instar II. As instar I except for the following characters: Colour. Cephalic capsule more broadly pale yellow anteriorly over frontoclypeus and laterally over parietals.

Head. HL = 0.64 mm (n = 1); HW = 0.57 mm (n = 1); FCL = 0.50 mm (n = 2); cephalic capsule HL/HW = 1.12, HW/OcW = 1.45; occipital suture present; FCL/FCW = 2.27, 0.77 times as long as HL, with lateral notches slightly visible in dorsal view; parietals with = 10 lateral spines; antenna, length of antenna/HW = 0.60; A2 = A3> A1> A4, A2/A3 = 0.93, A3′/A4 = 0.70; maxilla, length of antenna/length of maxillary palpus = 1.24; MX 2> MX 1> MX 3, palpomere 2, 1.20 times as long as palpomere 1; labium, length of maxillary palpus/length of labial palpus = 1.09, palpomere 2, 1.27 times length of palpomere 1; chaetotaxy and porotaxy, head capsule with a lesser number of secondary setae.

Thorax. Pronotum without transverse carina; both meso­ and metathoracic terga with an anterotransverse carina; secondary tergal setation less developed.

Legs. Metathoracic legs longest, = 1.20 times length of prothoracic legs, = 2.20 times HW; coxa = femur> tibia = tarsus = trochanter; posterior metathoracic claw = 0.57 times as long as metatarsus; spinulae on ventral margin of tibiae and tarsi weakly developed and restricted to the distal 1/3–1/2; chaetotaxy and porotaxy, position and number of secondary setae as expressed in Table 1 .

Abdomen. LLAS = 0.28 mm (n = 1); LLAS/HW = 0.48; siphon 0.30 times LLAS.

Urogomphus. Total length of urogomphus = 1.16 mm (n = 1) (length of urogomphomere 1 = 0.61 mm (n = 1)); 2.02 times HW (length of Uro1/HW = 1.07) and 4.18 times LLAS (length of Uro1/LLAS = 2.22); Uro1/Uro2 = 1.13; Uro1/Uro2′ = 1.24; Uro2′/Uro2 = 0.91.

Source of Material. The larvae studied were collected in association with adults by the junior author at the following locality: Canary Islands, La Gomera, Garajonay , 1,500 m. 15.i.2000. The identification is firm as no other species of Hydrotarsus was found at that locality.

Biological Notes. One of us (D.T.B.) has collected all three species of Hydrotarsus in recent years, and has found that the species appear to be restricted to permanent seepages located at altitudes of> 800 m in humid areas of the islands. Here the beetles occur in trickles of water 1–3 mm in depth over vertical rock faces. Adult individuals can be observed crawling rapidly over bare rock, and also resting in crevices and fissures. In some cases sites had growths of Marchantia and/or other bryophyte species, but on most occasions these were sparse and peripheral, the areas inhabited by Hydrotarsus being covered only by epilithic algal films. On the Canaries the most abundant insects in such habitats were the hydrophilid Laccobius canariensis d’Orchymont and larvae of the dipteran Thaumalea subafricana (Becker) . On Gomera adults of Hydrotarsus compunctus were observed feeding on oligochaetes within the seepages, up to 12 individuals aggregating around moribund worms of 10–15 mm in length. Larvae of H. compunctus were taken together with adults, cooccurring in the same patch of seepage.

Distribution. Hydrotarsus compunctus was described from Tenerife. Subsequently it was found on La Palma (Machado 1989), and discovered there in numbers at high altitude in the Caldera de Taburiente ( Balke et al. 1990). On this basis, and the finding of only H. pilosus on Tenerife, Balke et al. (1990) considered the species to be a La Palman endemic, the original specimens of Wollaston (1865) being mis­labelled. The discovery of H. compunctus on La Gomera (first reported here) demonstrates that this species is more widespread in the Canaries, and suggests that it may yet turn up on Tenerife. Such comment fits with the fact that very few coleopteran taxa are endemic to La Palma, which is geologically young (ca. 1.5 my old—cf. 10 my for La Gomera and 11.6 my for Tenerife).

Taxonomic Notes. Larvae of Hydrotarsus are more likely to be confounded with those of either Hydroporus or Sanfilippodytes from which they can be distinguished by the following combination of characters: shorter frontoclypeus (ratio HL/HW <1.10) ( Table 2 ); shorter metathoracic legs (<2.20 times HW) ( Table 2 ), ventral marginal spinulae reduced on tibia and tarsus ( Fig. 4 ), siphon bluntly rounded apically ( Fig. 2 ) and short (ratio siphon length/LLAS <0.20) ( Table 2 ), and primary seta UR4 contiguous to setae UR2 and UR3 ( Fig. 3 ).

Figures

Figs. 1–3. Hydrotarsus compunctus (Wollaston), Instar III. 1) Head capsule, dorsal aspect; 2) last abdominal segment, dorsal aspect; 3) urogomphus, dorsal aspect. Scale bar = 0.10 mm.

Figs. 4–5. Hydrotarsus compunctus (Wollaston), metathoracic legs, Instar III. 4) Anterior face; 5) posterior face. Sensillar series: A = anterior; AV = anteroventral; AD = anterodorsal; Pr = proximal; PV = posteroventral; SP = spinulae; TR2 = primary seta TR2. Scale bar = 0.10 mm.