Armostus nigricans Hoshina and Satô, 2006

Hoshina, Hideto & Satô, Masataka, 2006, A Taxonomic Study of the Genus Armostus (Coleoptera: Hydrophilidae) from Japan, The Coleopterists Bulletin 60 (2), pp. 95-104 : 97-100

Armostus nigricans Hoshina and Satô , new species

(Japanese name: Seguro-okatsuya-gamushi) ( Figs. 1, 3, 8, 15, 19, 24, 28, and 29)

Diagnosis. Body length is about 2 mm. Dorsum is black except for yellowish brown apex of elytra. Head and pronotum are strongly and densely punctate. Pronotum is more densely pubescent than other Japanese species of Armostus . Elytra are widest approximately 1/7 at base.

Measurement of the Holotype. Body length 2.1 mm; head 0.45 mm in length (from base to front margin of clypeus) and 0.71 mm in width; pronotum 0.55 mm in length and 1.4 mm in width; elytra 1.5 mm in length and width.

Description. Coloration ( Fig. 8). Dorsum bicolorous; head black with brown clypeus; pronotum black or dark brown, rarely dark reddish brown; main parts of elytra black or dark brown, rarely a little reddish, usually with reddish brown or dark brown small patches near humeri, but rarely those patches disappearing; apex of elytra with bright yellowish brown spot ( Fig. 8), showing a certain variability in its extent; maxillay palpi, labrum, and antennae light brown except for brown antennomere 1; prosternum brown with a dark brown median carina; mesoventrite and metaventrite dark reddish brown; legs brown with light brown tarsi; abdominal ventrite brown. Body length 1.9–2.2 mm, about 1.5 times as long as wide ( Fig. 1). Head about 1.6 times as wide as long, smooth, very finely, sparsely, and shortly pubescent; vertex punctate densely, and more strongly than those of other Japanese species of Armostus ( Figs. 2– 5); clypeus minutely and densely punctate; length and width of head about 0.77 times and 0.55 times as long as those of pronotum, respectively. Pronotum widest at base, about 2.4 times as wide as long ( Fig. 1) and smooth; pubescence very fine and short, and more dense than that of other Japanese species of Armostus ( Figs. 2–5); punctures of pronotum dense, consisting of various sizes of punctures ( Fig. 2); lateral margins simply curved and narrowing from base towards anterior margins; length and width of pronotum about 0.40 and 0.93 times as long as those of elytra, respectively. Scutellum smooth, impunctate or having a few very minute punctures. Elytra slightly longer than wide or almost as long as wide, widest approximately 1/7 at base ( Fig. 1), smooth, and very finely and shortly pubescent, almost impunctate between striae; basal starting point of striae 6, 8, and 9 separated from base of elytra more distant than other striae; striae 9 and 10 invisible in dorsal view; lateral margins of elytra sharply narrowed from their breadth maximal points towards apex. Hind wings fully developed. Legs of normal shape as in other species of the genus Armostus . Prosternum less shining, impunctate, and strongly microreticulate, with a carina almost straight at ventral margin in lateral view ( Fig. 15); mesoventrite less shining, impunctate, and strongly microreticulate except for preepisternal elevation of mesothorax; preepisternal elevation of mesothorax about 2.7 times as long as wide, shining, strongly punctate, very finely and shortly pubescent ( Fig. 19); metaventrite less shining, impunctate, and strongly microreticulate except for pentagonal middle portion, with lateral sutures running distant from episternum, and forming triangular area along each lateral margin ( Fig. 19); pentagonal middle portion of metaventrite shining, punctuate, and pubescent as preepisternal elevation of mesothorax ( Fig. 19); abdominal ventrite less shining, impunctate, strongly microreticulate; abdominal ventrite 1 with a distinct carina.

Male. Aedeagus about 0.65 mm in length ( Figs. 28 and 29); median lobe about 0.54 mm in length, slender in general, apically and simply pointed in ventral view, almost straight and narrowing from basal 1/5 towards apex in lateral view; parameres about 0.43 mm in length; lateral sclerites of genital segment almost symmetric, apically pointed, and about 1.8 times as long as a middle sclerite ( Fig. 24).

Distribution. Japan: Ryukyus (Nakanoshima Is. and Amami-Ôshima Is.).

Type Series. Holotype, male, Nakanoshima Is., Tokara Island Group , 24–25. X. 1992, S. Kaneno leg. ( MNHA) . Paratypes, 4 specimens, same data as holotype ; 7 specimens, Yuwan, Amami-Ôshima Is., 7. IV. 1968, N. Minakawa leg. All type series except for the holotype are preserved in FU .

Examined Specimens of Related Species. Syntype of Armostus optatus Sharp, 1890 , 1 specimen, Dikoya , Ceylon, 8. XII. 1881, G. Lewis, leg. (preserved in the collection of The Natural History Museum, London) .

Remarks. Armostus nigricans , new species, is similar to A. shenklingi (d’Orchymont, 1914) , but can be distinguished from the latter by having pronotal punctations composed of small and large punctures ( Fig. 3), elytra widest approximately 1/7 at base, and a narrow yellowish brown patch on it ( Fig. 8). In contrast, A. schenklingi has pronotal punctures, all of the same size, elytra are widest approximately 1/3 at base ( Fig. 2), and the yellowish brown patch on elytra is wide ( Figs. 6 and 7). A. nigricans new species is also related to A. optatus Sharp, 1890 , but is separated from it by having the elytra widest about 1/7 at base and sharply narrowed towards apex from the maximal points ( Fig. 1), whereas in A. optatus , the elytra is widest about 1/3 at base and gradually narrowed. Moreover, the present new species resembles A. cuneatus Hebauer 2002 , in appearance, but can be distinguished from the latter by having the concolorous dorsum except for the apex of elytra ( Fig. 8), whereas in A. cuneatus , the head and pronotum are black and the elytra are shining reddish brown with the testaceous apex and sides.

Etymology. The specific name, nigricans is derived from ‘‘nigric‾ans’’ in Latin, meaning blackish.