Hydrobius pauper Sharp, 1884

Hydrobius pauper Sharp, 1884 View in CoL

( Figs 1–9, 11, 19, 20)

Hydrobius pauper Sharp, 1884: 452 View in CoL .

Hydrobius pauper View in CoL : Hansen, 1999: 205, 2004: 53;

Fikáček et al., 2015: 49.

Material examined (all in NHML). Lectotype (present designation). Male , Japan, “ Hydrobius pauper , Japan, Lewis” [Sharp’s writing on the face of the card], “ Japan, G. Lewis ” [printed label], “Sharp Coll. 1905-313” [printed label], “ Lectotype ” [purple bordered round printed label], “ Hydrobius pauper Sharp , lectotype, S.K. Ryndevich & R. B. Angus des., 2020” [white printed label] . Paralectotypes. Japan: 1 specimen [originally mounted on one card with lectotype], same data as for lectotype, but “ Paralectotype ” [blue bordered round printed label]; 1 female, “ Hydrobius pauper , Type D.S., Japan, Lewis” [ Sharp’s writing on the face of the card], “Type, H. T.” [red bordered round printed label], “ Japan, G. Lewis ”, “Sharp Coll. 1905-313” [printed labels], “ Paralectotype ” [blue bordered round printed label]; 1 male [specimen lacking the head, mounted upside down on a card and with its wings and elytra spread, and with the aedeagus protruding from the abdomen], “ Hydrobius pauper Japan Lewis” [Sharp’s handwriting on the face of the card] and with “ Japan / G. Lewis ” and “Sharp Coll./ 1905- 313” [printed labels], “ Paralectotype ” [blue bordered round printed label]; 1 specimen, Tochigi Prefecture, “Oyama” [written in pencil on card back], “Oyama/ 24.v – 26.v.80” [handwritten label], “ Japan, G. Lewis / 1910-320” [printed label], “ pauper ” [Sharp’s handwritten label], “ Paralectotype ” [blue round printed label]. All paralectotypes were labelled “ Hydrobius pauper Sharp , paralectotype, S.K. Ryndevich & R. B. Angus des., 2020” [white printed label] .

Note. Although Sharp (1884) labelled one specimen as holotype, he did not publish any type designation, so all these specimens are in fact syntypes. One male ( lectotype) and one unsexed specimen ( paralectotype), originally mounted on one card labelled in Sharp’s handwriting, are now on separate cards and the aedeagus of the male is mounted on the card with the beetle, in a drop of dimethyl hydantoin formaldehyde resin.

Redescription. Form and colour. Body oval, moderately convex ( Figs 1, 2), length 6.5–7.1 mm, width 3.5–3.8 mm. Maximum body width in middle of elytra. Dorsal side black; lateral parts of pronotum and elytra paler, brown or reddish-brown. Maxillary palpomeres and antennae yellowish-brown except for darker club. Last segment sometimes darkened apically. Ventral surface black, metaventral pentagon brown to dark brown, posterior margins of abdominal ventrites paler, brown or reddish-brown. Legs dark brown to yellowish-brown, femora slightly darker, tarsi brownish-yellow ( Figs 1–3).

Head. Dorsal surface densely and evenly punctate with moderately coarse punctures of equal size, without microsculpture. Clypeus with dense, moderately coarse punctures. Anterior margin of clypeus with narrow bead. Eyes big, hardly protruding, not emarginate anteriorly, separated by a distance four times as long as width of one eye ( Fig. 1). Mentum almost flat, glabrous, about 1.8–2.0 times as wide as long. Antennae with nine antennomeres, scapus about 1.2 times as long as antennomeres 2–5 combined, club not compact. Maxillary palpomere 4 asymmetrical, with more straight inner face, about 1.5–1.6 times as long as palpomere 3.

Thorax. Pronotum and elytra with very rare large punctures (with trichobothria), without microsculpture. Pronotum about 0.3 times as long as elytra, about 2.5 times as wide as long, strongly narrowed anteriorly. Pronotal punctation similar to that on head. In lateral parts of pronotum, large punctures forming ovals; these ovals interrupted by centre of pronotum. Lateral margins of pronotum narrowly rimmed; lateral rims overlapping anterior and posterior corners. Anterior and posterior margins without rim. Prosternum slightly convex medially, without longitudinal carina. Elytra almost parallel-sided medially, with their lateral margins more abruptly rounded to apex. Humeral bulge not distinct. Each elytron with ten punctate striae and a short scutellary stria in base of second elytral interval. Intervals flat; ground punctures on intervals fine ( Figs 4, 19, 20); first four to five intervals of female very slightly convex near apex. Large punctures (with trichobothria) in anterior half of elytra situated close to third and fifth elytral striae (Fig. 5). Second interval wider than others, containing 6–8 matted rows of punctation. Remaining intervals with no more than three matted puncture rows. Elytral striae of male weaker apically, reduced to rows of punctures with scarcely any furrows connecting them ( Figs 4, 19, 20). Epipleura rather weakly oblique anteriorly, more so posteriorly. Mesoventral elevation low, acute and dentiform (Figs 6, 7). Metaventrite rather strongly but not abruptly raised, with dense punctate pubescence; median pentagonal area about 0.9–1.0 times as long as wide. Femora with sparse and shallow punctures ventrally, pubescent part of metafemora occupying about half (55–60%) of femoral length. Pubescence sparse near posterior edge of metafemora ( Figs 8, 9). Tarsi with pale setae ventrally. Protarsomere 5 about as long as protarsomeres 2–4 combined, metatarsomere 5 about as long as metatarsomeres 2 and longer than metatarsomeres 3–4 combined.

Abdomen. With five exposed ventrites, each with dense uniform pubescence; first ventrite not carinate, about as long as second ventrite; fifth ventrite arcuate, not emarginate apically ( Fig. 3).

Male genitalia ( Fig. 11). Phallobase symmetrical, about 1.3 times as long as paramere. Parameres narrow in apical half; their outer margin almost straight, slightly concave inward, widely rounded

S.K. Ryndevich & R.B. Angus. Redescription of Hydrobius pauper at apex. Median lobe wide, with widely spaced apodemes; gonopore large, situated subapically.

Comparison. Hydrobius pauper is most similar in habitus to H. pui but has a less convex body. Hydrobius pui also differs from H. pauper in having the parameres shorter, widest and more robust in apical half, distinctly curved inwards and rounded at apex ( Fig. 18).

Figs 5–7. Hydrobius pauper . 5, right elytron in lateral view (yellow arrows indicate large punctures with trichobothria); 6, 7, mesoventral elevation in lateral view (indicated by blue arrow). Female (5, 7) and male (6) paralectotypes. Scale bar: 1 mm. Hydrobius pauper differs from many congeners (except H. pui and H. arcticus ) in its elytra almost parallel-sided medially, with their lateral margins more abruptly rounded to apex ( Figs 19, 20). Hydrobius arcticus also has almost parallel lateral margins of the elytra in middle portion, but they are more smoothly rounded to apex ( Figs 25, 26) and the mesoventral elevation is small and blunt, not dentiform. Hydrobius punctistriatus , H. fuscipes and H. subrotundus have slightly rounded lateral margins of the elytra, which are smoothly rounded to apex ( Figs 21–24). Lateral margins of the elytra of H. rottenbergii are slightly round- ed medially and more abruptly rounded to apex ( Fig. 27). Hydrobius fuscipes , H. subrotundus , H. punctistriatus and H. rottenbergii have deep- er apical section of the elytral striae ( Figs 21–24, 27) and metafemora with coarser punctation and with pubescence unlike H. pauper ( Fig. 10). In addition, H. fuscipes , H. punctistriatus and H. subrotundus have the large punctures (with trichobothria) in the anterior half of the elytra situated in the intervals between the second and third, and between the fourth and fifth punctured striae unlike H. pauper , which has these large punctures in the anterior half of the elytra situated close to the third and fifth elytral striae. Hydrobius rottenbergii , H. punctistriatus , H. fuscipes and H. subrotundus also have the metafemora with the hairline reaching the posterior edge and the pubescent part occupying about 66–71% of femoral length ( Fig. 10) unlike H. pauper , which has the pubescent part of metafemora occupying 55–60% of femoral length ( Figs 8, 9). Hydrobius punctistriatus also differs from H. pauper in its larger size ( 7.8–8.5 mm).

The parameres and median lobe of H. pauper and H. punctistriatus appear very similar. Hydrobius pauper has the parameres almost straight along the interior edge ( Fig. 11), and H. punctistriatus has the parameres with slightly concave interior edge at gonopore level ( Fig. 15). The genitalia of H. fuscipes and H. subrotundus ( Figs 12–14) and H. pauper ( Fig. 11) are also very similar, although the median lobe of H. pauper is slightly wider at the base. It is possible that the slight differences in paramere shape of H. pauper , H. fuscipes , H. punctistriatus and H. subrotundus are a result of clearing the soft tissue. As opposed to this, the differences between H. pauper , H. arcticus , H. rottenbergii and H. pui in male genitalia are more distinct ( Figs 11, 16–18). The last three species have median lobe narrower and parameres wider and more robust in apical half, distinctly curved and rounded at apex.

Distribution. Japan (Honshu,? Hokkaido). The record of this species from Hokkaido ( Minoshima & Hayashi, 2011, 2012) based on the preimaginal stages requires confirmation.

Bionomics. Most likely, like other species of the genus, H. pauper is a water beetle.

Remarks. Shatrovskiy (1989), Hebauer (1995), Hansen (1999, 2004), Ryndevich (2016) and Fikáček et al. (2015) recorded only H. fuscipes for the Russian Far East fauna. According to Fikáček et al. (2015) and Nakajima et al. (2020), the genus Hydrobius in Japanese fauna includes only H. pauper . Sharp (1884) recorded H. fuscipes from Japan based on one specimen from Horubetsu, Yezo ( Hokkaido), which served as the basis for its inclusion in the earlier catalogues ( Hansen, 1999, 2004). This single specimen labelled “Horobetsu, 19.VIII.–20.VIII.80” [printed label], “ Japan G. Lewis 1910 – 380” [printed label] is in Sharp’s collection ( NHML) and is shown to be H. fuscipes by the coarse apical section of the elytral striae and the punctation of the metafemora ( Fig. 10); the presence of H. fuscipes on Hokkaido is confirmed by recent material taken there by Fenglong Jia ( Guangdong, China). Currently, H. pauper is known only from Honshu, the main island of Japan ( Hansen, 1999), where it is the sole recorded species. The record of H. pauper by Uéno et al. (1985) from Japan judging by the photo most likely refers to H. fuscipes . All instars of the larva of Hydrobius were described (as H. pauper ) from South Hokkaido ( Minoshima & Hayashi, 2011, 2012) but their species affiliation requires confirmation. At present there is no information published of any Hydrobius from Honshu since Sharp’s description of H. pauper . Hydrobius fuscipes occurs in the Kuril Islands (Russian Far East) so its presence in northern Japan is not unexpected.