Japan, Honshu: Nara Prefecture, Nara City, Nara Park, 34°41'4"N, 135°50'36"E (DMS).

Holotype , ♂ (OSAKA. Type No.: OMNH TI 528): Japan, Honshu, Nara Prefecture, Nara City, Nara Park, 34°41'4"N, 135°50'36"E (DMS), 16.xii.2018, F. Itô leg. (OSAKA).

Triarthron maerkelii Märkel, 1840. 3 ♂♂, 1 ♀ (EUMJ), Japan – Honshu, Gunma Prefecture, Tsumagoi Village, Mt. Asashiki, 24.vii.1979, K. Itô leg.; 1 ♂ (FU), Japan – Hokkaido, Ebetsu City, Nopporo Forest Park, 29.vi.2000, S. Hori leg.; 1 ♂ (FU), Sweden, SM. Hornsö, 27.vi.1998, B. Andersson leg.

Body length about 5 mm. Dorsum is almost concolorous, brown. Both mandibles were sharply curved inwardly at about apical 1/4. Mesofemur bearing five small teeth at dorsal lamina of posterior margin. Metafemur strongly expanded anteriorly at about half of the antero-apical margin and bearing a relatively long tooth and two tiny teeth at the dorsal lamina of the posterior margin. The median lobe of aedeagus weakly curved at lateral margins in dorsal view.

Measurement of holotype . Body 4.8 mm in length; head 1.1 mm in length (from the front margin of the clypeus to base) and 1.3 mm in width; pronotum 1.2 mm in length and 2.0 mm in width; elytra 2.8 mm in length and 2.2 mm in width.

Coloration. Dorsum shining and almost concolorous, brown (Fig. 1); clypeus and labrum light brown; antennae brown and terminal three antennomeres slightly lighter than others; legs brown in general, but all trochanters and about basal 2/5 of metafemora blackish brown; mesoventrite, metaventrite, and abdominal ventrites light brown.

Figure 1. 

Habitat of Triarthron itoi Hoshina, sp. nov. Scale bar: 1 mm.

Body approximately 2.2 times as long as wide.

Head almost smooth, minutely, and densely punctate (Fig. 2), and bearing a few short and very fine setae near each eye and a few long and fine setae near each lateral-basal corner of the clypeus; both mandibles sharply curved inwardly at about apical 1/4 and lacking large teeth at internal margins; antennomeres 1–3 longer than wide; antennomeres 4 and 11 about as long as wide; remaining antennomeres wider than long (Fig. 3).

Pronotum almost smooth and glabrous, widest at about basal 2/5 of lateral margins, minutely and densely punctate as head (Fig. 2), and with a transverse fine groove along the basal margin, which is interrupted at the central part (Fig. 2).

Scutellum almost smooth and distinctly punctate (Fig. 2).

Elytra almost smooth and glabrous except for very sparse and fine setae along lateral margins, widest at about basal 1/3 of lateral margins (Fig. 2); each elytron bearing nine rows of punctures and ninth row present along lateral-downside margins and invisible in dorsal view (Fig. 2); punctures comprising nine rows of punctures distinct and larger than those of head and pronotum (Fig. 2); punctures between rows of punctures dense and minute (Fig. 2).

Hind wings fully developed.

Mesoventrite weakly microreticulate and almost glabrous; metaventrite and abdominal ventrites distinctly microreticulate, and densely and finely pubescent.

Legs with many small spines as other species of the genus Triarthron; mesofemur approximately 2.8 times as wide as long, weakly expanded anteriorly at about half of antero-apical margin, and bearing five small teeth at dorsal lamina of posterior margin (Fig. 4); mesotibia weakly curved inwardly; metafemur approximately 2.8 times as wide as long, strongly expanded anteriorly at about half of antero-apical margin, and bearing a relatively long tooth and two tiny ones at dorsal lamina of posterior margin (Fig. 5); metatibia almost straight.

Aedeagus slender in general (Figs 8, 9); median lobe weakly and simply curved at lateral margins, round at apex in dorsal view (Fig. 8) and almost straight in lateral view (Fig. 9); both parameres almost symmetrical and round at apex (Fig. 8) and almost straight in lateral view (Fig. 9); each paramere bearing three apical setae (Figs 8, 9).

The specific name is dedicated to Mr. Fukuo Itô, the collector of the holotype.

Japan: Honshu (Nara Prefecture).

Collecting the tribe Sogdini is generally not easy in Japan and identified Japanese specimens of the tribe are very small in quantity. Triarthron itoi Hoshina, sp. nov. is described based on only one specimen; therefore, the degree of individual variation cannot be determined in this species. However, I found some morphological features on that specimen that are clearly different from two known species of Triarthron, and recognize it as a new member of the genus. Triarthron itoi sp. nov. can be distinguished from T. maerkelii Märkel, 1840 by the following features: it has a large body whose length is 4.8 mm, mesofemur with relatively large teeth at the dorsal lamina of the posterior margin (Fig. 4), metafemur strongly expanded anteriorly at about half of the antero-apical margin (Fig. 5), and median lobe of aedeagus weakly and simply curved at lateral margins in dorsal view (Fig. 8). In contrast, T. maerkelii differs in the following ways: it has a relatively small body whose length is 2.5–3.8 mm (Daffner 1983), mesofemur with teeth less-visible because of being hidden by ventral side of mesofemur (Fig. 6), metafemur relatively weakly expanded anteriorly at about half of antero-apical margin (Fig. 7), and median lobe sharply narrowed from about apical 2/5 towards apex (Fig. 10).

Figures 2–7. 

Triarthron itoi Hoshina, sp. nov. (2–5). 2 Body 3 right antennomere, dorsal view 4 left mesofemur, ventral view 5 left metafemur, ventral view. T. maerkelii Märkel (6, 7). 6 Left mesofemur, ventral view 7 left metafemur, ventral view. Scale bar: A: 1 mm (2), B: 0.2 mm (3), C, D: 0.5 mm (4–7).

Moreover, T. itoi sp. nov. can be separated from T. lecontei Horn, 1868 by having both mandibles sharply curved inwardly at about apical 1/4 and lacking large teeth at internal margins, and metafemur strongly expanded at the anterior margin (Fig. 5). In contrast, T. lecontei has both mandibles relatively weakly curved inwardly at internal margins, right one with an elongated sub-apical tooth, and metafemur almost straight at the anterior margin (Hatch 1957; Peck and Cook 2009).

Figures 8–10. 

Triarthron itoi Hoshina sp. nov. (8, 9). 8 Aedeagus, dorsal view 9 ditto, lateral view. T. maerkelii Märkel (10) aedeagus, dorsal view. Scale bar: 0.5 mm (8–10).

Life history of Triarthron itoi Hoshina, sp. nov. is not known.