Hydroporus polaris Fall 1923

Hydroporus polaris Fall, 1923

Fall (1923: 92) described this species from two males and one female from Bernard Harbour (Nunavut, Canada, ca. 68.788N 114.82W; square 9 in Fig. 13). In the same work Fall (1923: 93) described Hydroporus subvirescens after specimens collected at Cape Collinson (Alaska, USA, ca. 69.986N 144.861W; square 8 in Fig. 13). He provided for this taxon the following features: "with evident viridi-aeneous surface lustre" on the elytra and a "distinctly alutaceous and dull" upper surface. Additionally, he wrote: "Front tarsi of male moderately dilated, its anterior claw not appreciably different from the posterior." We have studied the male holotype and the female paratype of H. polaris as well as the male holotype of H. subvirescens (all in CNC). To our great surprise, the holotype of H. polaris has a "distinctly alutaceous and dull" upper surface with a well visible metallic sheen on the elytra (sic!), while the holotype of H. subvirescens has a shiny (although reticulate) surface and does not show any metallic sheen. In addition, both male holotypes have the anterior protarsal claws clearly strongly curved near the base and in the more distal part straight. While the appearance of the shape of the protarsal claws depends somewhat on their orientation and can be misinterpreted, this is not true of the reticulation and sheen of the dorsal surface. Thus, we must assume that either Fall himself or any person who dealt with these types after Fall has mixed up both holotypes by mistake. But we made yet another interesting observation: the dull appearance and the metallic sheen can be easily removed mechanically, for instance by means of a brush with hard hairs (tested in a very small area of the elytra). We agree with Larson et al. (2000: 348) who have synonymised H. subvirescens with H. polaris.

One of the main differences between H. polaris and H. acutangulus is the reticulation which is present in males and females of the former on the entire upper and ventral surface, while only present in some females of the latter. The colouration of the upper surface is varying between lighter and darker brown in H. polaris, but generally more brownish than in the mostly blackish H. acutangulus (compare Figs. 1–2). Additionally, we want to report an observation that we have not found in the literature: the shape of the metacoxal lines is rather variable in H. polaris; sometimes, they are only slightly impressed far before reaching the posterior margin of the metaventrite and often vanish there. If they reach that margin, they are in some specimens converging anteriorly, and not parallel.

Nilsson & Holmen (1995: 48) suspected that H. acutangulus and H. polaris might be conspecific and reported H. polaris from the "extreme north-east of Siberia (Wrangel Island, coll. G. Lafer)" (square 7 in Fig. 13) which is "identical with Nearctic material". Larson et al. (2000: 350) listed several characters which separate H. acutangulus from H. polaris and conclude: "Unless it is shown that these characters vary clinally across northern Asia, the differences are sufficient to justify the recognition of two species." We would refrain from confirming the thesis that H. polaris and H. acutangulus are not conspecific, we can confirm, however, most statements of Larson et al. (2000: 349, 350), except one: H. acutangulus appears not to be broader than H. polaris, on the contrary, the latter species appears to be broader, and this is due chiefly to the more rounded body shape of H. polaris (see Figs. 1–2). Our measurements yielded for H. polaris the following values: TL: 3.1–3.4 mm, MW: 1.5–1.7 mm, TL/MW: 1.88–2.00 (Pederzani 2001: 238 provided similar values: TL: 3.15–3.35 mm, TL/MW: 1.88–2.00). The H.

acutangulus studied are distinctly smaller: TL: 2.8–3.0 mm, MW: 1.45–1.5 mm, TL/MW: 1.93–2.00 (Pederzani 2001: 238 provided: TL: 2.90–3.23 mm, TL/MW: 1.83–2.07).

We want to add that at least those H. acutangulus from northern Europe and the Nearctic H. polaris which we have studied appear externally absolutely different, the H. acutangulus being smaller and distinctly more elongate because the sides of the body are rather parallel, while the H. polaris have evenly rounded sides and at a first glance look like a Hydroporus transgrediens Gschwendtner, 1923, or a small Hydroporus planus (Fabricius, 1782). If one compares the habitus photos of both species (Figs. 1–2) it is hardly believable that both taxa shall belong to one and the same species and that characters shall vary clinally so strongly across northern Asia.

Like H. acutangulus, this species can be also easily distinguished from H. sejilashan sp. n. by the arcuate anterior protarsal claws (males) and, additionally, by the colouration of the upper side.