Aegus robustus Yamamoto & Pham, 2025

Aegus robustus Yamamoto & Pham , sp. nov.

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( Figs 1–10 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 View Figure 9 View Figure 10 )

Aegus sp. : Fujita 2010: 345, partim on pl. 215, fig. 1058-1–2, 2 ♂♂ from Sapa, N. Vietnam.

Aegus sp. 22: Shibata 2023: 87, partim on 22, fig. 117-a–b, 2 ♂♂ from Sapa, N. Vietnam.

“ Aegus imitator View in CoL ”: Huang & Chen 2017: 272, partim, misidentification based on figures of Fujita 2010: pl. 215, fig. 1058-1–2, 2 ♂♂ from Sapa, N. Vietnam.

“ Aegus zhouzii ”: Huang & Chen 2023: 16, partim, misidentification based on figures of Fujita 2010: pl. 215, fig. 1058-1–2, 2 ♂♂ from Sapa, N. Vietnam.

Type material ( 16 ♂♂). VIETNAM: Ha Giang: Holotype : ♂ ( Figs 1 View Figure 1 , 4B View Figure 4 , 5A View Figure 5 , 6 View Figure 6 , 8 View Figure 8 , 9 View Figure 9 , 10D View Figure 10 ), “ N. Vietnam: / Ha Giang, / Tay Con Linh / mountain, / Vi Xuyen / March 2024 ” (IB) ; Paratypes: 1 ♂ ( Figs 3A View Figure 3 , 7B View Figure 7 ), same data as the holotype (IB); 1 ♂ ( Fig. 2D View Figure 2 ), “ Lung Cu / Ha Giang Prov. / Northeast Region / VIETNAM / IV, 2019 / Local collector leg.” (IB) . Yen Bai: Paratypes : 1 ♂ ( Figs 2A View Figure 2 , 4A View Figure 4 , 7A View Figure 7 , 10C, E View Figure 10 ), “Mu Cang Chai / Yen Bai Prov. / Northeast Region / VIETNAM / V, 2023 / Local collector leg.” (IB); 1 ♂ ( Fig. 2C View Figure 2 ), “ N. Vietnam: / Yen Bai, / April 2019 ” (IB); 1 ♂ ( Fig. 3B View Figure 3 ), “ N. Vietnam: / Yen Bai, / April 2020 ” (IB); 1 ♂, “ N. Vietnam: / Yen Bai, / May 2020 ” (IB); 1 ♂ ( Figs 2E View Figure 2 , 5B View Figure 5 ), “ N. Vietnam: / Yen Bai, / June 2023 ” (IB); 3 ♂♂ ( Figs 2F View Figure 2 , 10A, B, F View Figure 10 , all taken from the same specimen), “ N. Vietnam: / Yen Bai, / May 2024 ” (IB); 4 ♂♂ ( Figs 2B View Figure 2 , 3C View Figure 3 , 5C View Figure 5 , all taken from the two specimens), “ N. Vietnam: / Yen Bai, / June 2024 ” (IB); 1 ♂, “ N. Vietnam: / Yen Bai, / July 2024 ” (IB) .

Type locality. Northern Vietnam, Ha Giang, Vi Xuyen, Tay Con Linh Mountain.

Differential diagnosis. Aegus robustus sp. nov. is most similar to A. imitator Nagel, 1941 ( sensu Huang & Chen 2023) from China in general appearance, but is distinguished from the males of the latter by the following character states: body somewhat wider; mandibles only weakly arcuate ( Fig. 5A View Figure 5 ); 2 nd inner mandibular teeth short and wide, directed laterally ( Fig. 5A View Figure 5 ); sternite VIII medially with an inverted-triangular colorless area, accompanied by a small elongate sclerite on its posterior apex ( Fig. 10B, C View Figure 10 ); and sternite IX markedly widened posteriorly, with a long distinctly elongate colorless area along midline, starting immediately from near the base of that abdominal segment ( Fig. 10E, F View Figure 10 ), lacking a short incision directed anteriorly, formed by a colorless area of the posterior end ( Fig. 10E, F View Figure 10 ).

Diagnosis. Male. Body 22.87–40.03 mm long (including mandibles), wide, robust, comparatively large as member of Aegus ; mandibles weakly arcuate with basal (1 st) and middle inner teeth (2 nd), latter being short and somewhat wide, directed laterally (inward), located at slightly beyond middle (anteriad) in major form; head with a pair of large and long frontal projections along frontal ridge; pronotum with a feebly-developed, large central depression, with which scattered large coarse punctures; pronotal anterior corners truncate, excised obliquely; elytra each with seven deep dorsal striae (excluding elytral suture), outer margin widely rugose; metatibiae with well-developed seta-tuft at inner apex; abdomen with conspicuous seta-patches, those of ventrite 5 largest; tergite VIII medially with ill-defined elongate colorless area; sternite VIII medially with well-marked inverted-triangular colorless area, plus tiny elongate chitinized area on its posterior tip; sternite IX distinctly enlarged posteriorly, with a colorless longitudinal stripe along midline, starting immediately from near base of that abdominal segment; flagellum of median lobe of aedeagus fragile, thin, more than 3.5 times longer than aedeagus, non-enlarged at apex.

Description of holotype. Male. Size large for the genus, body with mandibles (TBL) 35.17 mm long, widest in anterior 1/3 of pronotum. Measurements of other body parts: BL, 26.81; BW, 12.18; ML, 8.36; HL, 5.47; HW, 11.79; PL, 6.56; PW, 12.18; EL, 14.42; EW, 11.38.

Body ( Fig. 1 View Figure 1 ) black, broad and flat, robust, about 2.89 times (with mandibles) longer than wide.

Head capsule ( Figs 1A–C View Figure 1 , 4B View Figure 4 , 5A View Figure 5 , 6B View Figure 6 ) transverse rectangular, about 2.16 times as wide as long, widest across ocular canthi, maximum width narrower than that of pronotum but wider than elytra; frontal ridge broadly concaved, with pair of large and acute frontal projections, each located at base of mandibles ( Fig. 5A View Figure 5 , fp); postocular margin, or temple, very weakly produced; frons and vertex with large and shallow depression forming ambiguously inverted-triangle; dorsal surface matte, with fine and inconspicuous scattered punctures. Eyes ( Figs 1A–C View Figure 1 , 4B View Figure 4 , 5A View Figure 5 , 6B View Figure 6 ) moderately large, each almost fully margined and bordered with developed ocular canthus of which sides nearly straight and parallel. Mandibles ( Figs 1 View Figure 1 , 4B View Figure 4 , 5A View Figure 5 , 6B View Figure 6 ) moderate in length, apparently longer than head and slightly longer than pronotum, gently arcuate in dorsal view, each with a short and acute basal (1 st) tooth on ventral side and a short, somewhat wide, acute 2 nd inner tooth dorsally located at slightly beyond middle (anteriad) of mandibles; apical part only gently curved; surface uniformly finely micro-reticulated, weakly glossy. Antennae ( Figs 1 View Figure 1 , 4B View Figure 4 ) composed of ten antennomeres; club three-segmented, with two terminal antennomeres uniformly pubescent, remaining one partly pubescent; antennomere 7 slightly modified at inner apical angle; antennomeres 8–10 lamellate. Mentum ( Figs 1B View Figure 1 , 6D View Figure 6 ) distinctly transverse, about 3.61 times wider than long; surface finely granular, glabrous, with large punctures sparsely on near lateral margins.

Pronotum ( Figs 1 View Figure 1 , 4B View Figure 4 , 5A View Figure 5 , 6A View Figure 6 ) short, transverse, about 1.86 times wider than long, widest at apical 1/3; anterior margin excavated on either side of midline; pronotal anterior angles obliquely truncate, pointed; lateral sides almost parallel, linear and smooth, convergent posteriorly; posterior angles obsolete and blunt, obtusely rounded; posterior margin weakly bisinuate; disc granular, margined by large punctures, with feebly-developed, elongate central depression along midline, containing a few series of large and independent punctures sparsely. Prosternal process ( Figs 1B View Figure 1 , 6D View Figure 6 ) well-developed, produced, strongly enlarged beyond procoxae posteriorly, with roundly produced apex. Scutellum ( Figs 1A View Figure 1 , 4B View Figure 4 , 6E View Figure 6 ) linguiform, with sparse large punctures posteriorly, interstices microrugose.

Elytra ( Figs 1A, C View Figure 1 , 4B View Figure 4 , 6E View Figure 6 ) short, elongate-oval, about 1.27 times longer than wide, broadest in anterior 1/3, with width slightly narrower than pronotum; humeri right-angled; lateral sides broadly arcuate, convergent posteriorly; lateral marginal areas narrowly explanate and little flattened, widely rugose, densely punctate; surface each with seven deep dorsal striae (excluding elytral suture) of which intervals flat with very fine punctures.

Legs ( Figs 1 View Figure 1 , 4B View Figure 4 , 6C View Figure 6 ) short, robust, but with thin and long tarsi. Protibiae nearly straight, flattened, gradually widened apically, with a large apical tooth; outer margin entirely serrate, with small to large blunt teeth ( Fig. 6C View Figure 6 ). Mesotibiae straight, relatively slender, narrower at base, with median and terminal spurs. Metatibiae longer and thicker than mesotibiae, each with well-developed seta-tuft of prominent yellowish-brown setae along inner posterolateral edge (cf. Fig. 7A View Figure 7 ).

Abdomen ( Fig. 1B View Figure 1 ) short, broadly oval. Ventrite V ( Fig. 1B View Figure 1 ) relatively long, about 1.76 times as long as ventrite IV, with posterior margin widely simply rounded; each visible ventrite with a trace of large seta-patch of dense setae, those of ventrite V largest, but most of them lost their setae. Tergite VIII ( Fig. 8A View Figure 8 ) semicircular, with ill-defined elongate colorless area, or longitudinal membranous area in anterior 2/3, along midline; colorless area with reduced number of microsetae. Sternite VIII ( Fig. 8B View Figure 8 ) strongly transverse, simply rounded at posterior margin, with well-marked inverted-triangular colorless area having tiny elongate chitinized area on its posterior tip along midline; colorless area with reduced number of microsetae. Pleurite IX ( Fig. 8D View Figure 8 ) not conterminous dorsally, moderately separated by membranous area. Sternite IX ( Fig. 8C–E View Figure 8 ) narrowly elongate, distinctly enlarged posteriorly, particularly from weak constriction in middle in dorsal and ventral views, gently arcuate in lateral view, with colorless longitudinal stripe along midline, starting immediately from near base of that abdominal segment; posterior margin membranous, boundary line generally comparatively narrowly emarginate but medially slightly produced posteriorly, without short incision directed anteriorly by membranous area at middle (cf. Huang & Chen 2017: 261).

Aedeagus ( Figs 9 View Figure 9 , 10D View Figure 10 ) elongate, about 3.11 times longer than wide (without flagellum) in ventral view; basal piece or phallobase ( Fig. 9B–D View Figure 9 ) elongate, about 1.36 times as long as parameres in ventral view, very weakly constricted in basal part, gently arcuate in lateral view; caudal ventral plate of basal piece bi-lobed, long, narrow, rather deeply excavated by colorless area at middle; median lobe (penis) clearly shorter than paramere and as wide as paramere in ventral view, tapered apically, not enlarged at apex; paramere without basal process, slightly upcurved; flagellum ( Figs 9A View Figure 9 , 10D View Figure 10 ) fragile, markedly long and thin, about 4.1 times as long as aedeagus, apical 1/3 not pigmented, without enlarged apex.

Male paratypes ( Figs 2 View Figure 2 , 3 View Figure 3 , 4A, C, D View Figure 4 , 5B, C View Figure 5 , 7 View Figure 7 , 10A–C, E, F; n View Figure 10 =15). Measurements:

Large to medium-sized forms ( n =10): TBL, 28.37–40.03 (32.69±3.30); BL, 21.48–29.85 (24.70±2.27); BW, 9.19–13.58 (10.81±1.21); ML, 6.78–9.79 (8.12±0.95); HL, 4.21–5.70 (4.83±0.42); HW, 8.98–13.23 (10.65±1.19); PL, 5.14–7.42 (6.07±0.57); PW, 9.19–13.58 (10.81±1.21); EL, 11.93– 16.29 (13.59±1.17); EW, 8.79–12.76 (10.35±1.06).

Small-sized form ( n =5): TBL, 22.87–25.19 (24.00±0.97); BL, 18.39–19.80 (19.10±0.60); BW, 7.36–8.00 (7.70±0.26); ML, 4.65–5.57 (5.10±0.31); HL, 3.32–4.05 (3.66±0.24); HW, 6.39–7.55 (7.05±0.47); PL, 4.33–4.89 (4.65±0.19); PW, 7.03–7.98 (7.52±0.38); EL, 10.44–11.18 (10.81±0.30); EW, 7.36–8.00 (7.70±0.26).

Some morphological variations can be seen along with their body sizes. Abdominal terminalia and genitalia are generally stable, except for some minor differences. In general, the smaller the individual, the more likely the following trends are observed: 2 nd inner tooth of mandibles located more basal area; a pair of frontal projections becoming smaller, more inconspicuous; head and pronotum dorsally with larger and denser punctures; pronotum less wide in comparison with elytral width, point of maximum width gradually lowered; anterior angles of pronotum less truncate; elytra with outermost striae (7 th stria) becoming ambiguous.

Body length. Males ( n =16): 22.87–40.03 (30.13±4.95) mm including mandibles; 18.39–29.85 (23.08±3.29) mm excluding mandibles.

Etymology. The specific epithet is from the Latin robustus (meaning “robust”), referring to the large and robust body of the new species.

Distribution. Northern Vietnam ( Ha Giang and Yen Bai provinces, but also in Sapa of Lao Cai Province based on Fujita (2010) and Shibata (2023)).

Bionomics. No ecological information has been known. All the specimens were collected from March to July.

Remarks. This new species was first recognized by Fujita (2010), who illustrated specimens from Sapa, Lao Cai Province, Northern Vietnam. Later, Huang and Chen (2017) treated the specimens shown in Fujita (2010) as Aegus imitator Nagel, 1941 , known from mainland China, rather than as an undescribed species. However, Shibata (2023) supports Fujita (2010) and reinstates the treatment as Aegus sp. , or more precisely Aegus sp. 22. The situation is further complicated by Huang and Chen (2023) because the species they considered “ Aegus imitator ” in their previous work ( Huang & Chen, 2017) was actually an undescribed species, i.e., “ Aegus zhouzii Huang & Chen, 2023 ”, and the Chinese population they also considered as “ Aegus eschscholtzii (Hope, 1845) ” in that work turned out to be the true A. imitator . As a result, they included Northern Vietnam in the distribution of the new species A. zhouzii based on the illustrated Vietnamese specimens of Aegus sp. in Fujita (2010).

Our study revealed that this northern Vietnamese population of “ A. zhouzii ” has male copulatory organs quite different from the ones of Chinese A. zhouzii (e.g., apex of the flagellum of the aedeagus is thin, not enlarged), as well as other details of the abdominal terminalia. Furthermore, the Chinese A. zhouzii has a smaller sparser seta-patch on abdominal ventrite V and a pair of shorter and smaller frontal projections on near the anterior margin of the head. Thus, it cannot be identified the Vietnamese population as A. zhouzii . Given this, we support the views of Fujita (2010) and Shibata (2023) on the taxon, and described it as a new species.

In reality, Aegus robustus sp. nov. is closely related to A. imitator ( sensu Huang & Chen 2023) in general appearance and even the male genitalia (see Huang & Chen 2017: 261 as “ A. eschscholtzii ”; Huang & Chen 2023). However, the large males of A. robustus sp. nov. have a wider body (in A. imitator , the body appears to be narrower), less arcuated mandibles (in A. imitator , they are rather strongly curved), and a short, laterally directed 2 nd inner tooth of the mandibles (in A. imitator , they are long and conspicuous, directed anterolaterally).

All the male specimens of A. robustus sp. nov., regardless of their body sizes, are easily distinguished from A. imitator by the shapes and locations of colorless areas on abdominal sternites VIII and IX. For example, the sternite VIII of A. robustus sp. nov. has an inverted-triangular colorless area in the central part, which is accompanied by a small elongate sclerite on its posterior tip (in A. imitator , it is somewhat transversely oval, lacking such a tiny sclerite on that position). Besides, the sternite IX of the new species has strongly widened posteriorly from the middle (in A. imitator , it is much less weakly broadened posteriad). Furthermore, several distinct features are appeared in the structures of the sternite IX: (i) a conspicuous colorless longitudinal stripe along the midline is present in A. robustus sp. nov., which appears immediately after the basal marginal area of sternite IX (in A. imitator , the colorless longitudinal stripe is much less conspicuous, located on the middle of the sternite IX with some remote distance from the base of it); (ii) a marginal colorless area at the posterior-end of A. robustus sp. nov. does not sharply expand anteriorly (in A. imitator , it has a short, narrow extension anteriorly in the middle). Considering of these characteristics, the males of A. robustus sp. nov. can be easily distinguished from those of A. imitator .