Female

Body length (head + mesosoma + first two terga) 19.5–21.5 mm (holotype: 19.5 mm); forewing length 14.0–15.2 mm (holotype: 15.2 mm). Head in frontal view (fig. 1) broad, about 1.1 times wider than high. Eyes enlarged, in lateral view strongly swollen at least in ventral half (fig. 2), maximum width about 4.5 times as wide as that of gena. Clypeus convex, with apex sharply pointed triangular; supraclypeal area irregularly punctuate, with small impunctate, flat area below anterior ocellus. Anterior ocellus slightly elliptical in shape (0.32–0.36 mm wide, 0.28–0.30 mm long), about 1.4 times larger in diameter than more-or-less circular posterior ocelli (diameter 0.22–0.26 mm); distance between inner eye margin and posterior ocelli as long as width of anterior ocellus; ocelli close to each other (fig. 3); distance between anterior and posterior ocelli shorter than diameter of posterior ocellus; posterior ocelli separated from each other by about distance equal to their diameter. Mandible with three teeth; proximal tooth small, sharply pointed; middle and distal teeth extended, bluntly projected.

Figure 1–12

Eustenogaster nigra holotype and paratypes. Scale bars are 1 mm. 1–3. Eustenogaster nigra Saito and Nguyen, n.sp., female holotype. Head. 1. Frontal view. 2. Lateral view. 3. Dorsal view. 4–6. Second metasomal tergum in lateral view. 4. Eustenogaster nigra Saito and Nguyen, n.sp., female. 5. E. nigra, male. 6. E. scitula, female. 7. Eustenogaster nigra Saito and Nguyen, n.sp., female paratype. Sixth metasomal tergum in lateral view. 8. Eustenogaster nigra Saito and Nguyen, n.sp., male mandible. 9–12. Eustenogaster nigra Saito and Nguyen, n.sp., male paratype, genitalia. 9. Paramere and volsella in interior lateral view. 10. Volsella in interior lateral view. 11. Aedeagus in lateral view. 12. Aedeagus in ventral view.

Scutum finely, densely punctate (interspaces smaller than punctures), with median carina developed anteriorly. Scutellum hairy, densely punctate, strongly convex medially, separated from metanotum by deep but narrow furrow. Metanotum rather strongly convex, hairy, rugosely punctate in posterior margin. Mesepisternum densely punctate at least in dorsal half. Propodeum hairy, with shallow, small punctures.

First metasomal tergum 8.0–8.6 mm long, about 6.1 times as long as maximum width, and 7.2 times longer than height; second tergum strongly convex dorsally (fig. 4); sixth tergum with a small tubercle (fig. 7).

Color

Body black; head with no spots or markings; paired short lines along posterodorsal margin of pronotum, and scrobal spot and spot below it (both often reduced) on mesepisternum yellow; posterior two-thirds of first tergum dark reddish brown; third tergum anterolaterally with paired, short, yellow lines. Legs dark brown to black; femora pale brown apically. Wings semihyaline, pale brown, darker along anterior margin of forewing; veins brown.

Male

Body length (head + mesosoma + first two terga) 18–20 mm; forewing length 14.5–15.5 mm. Structure and coloration similar to female, but clypeus less convex and with apex bluntly angled, sometimes with a median yellow mark of variable size; mandibular teeth rounded apically (fig. 8); second tergum less strongly convex dorsally (fig. 5); terga with no tubercle.

Genitalia

Parameral spines not dilated, with distinct depression at the base (fig. 9). Volsella bent at anterior margin, with digitus beak-shaped (fig. 10). Aedeagus slender, in lateral view slightly dilated apically (fig. 11), with a pair of projections located laterobasally (fig. 12).

Etymology

The specific name is the feminine of the Latin word niger, referring to nearly entirely black body of this species.

Remarks

This species is most similar to E. scitula, but the second metasomal tergum is more strongly convex (figs. 4–6) and the female mesoscutum is less convex than in E. scitula; furthermore, E. nigra is also easily distinguished from E. scitula by the lack of bright spots on the clypeus, upper part of the mesopleuron, scutellum, metanotum, and propodeum.

The specific name nigra is van der Vecht's manuscript name. van der Vecht undoubtedly distributed to various institutions/museums the specimens with type labels on which the name appears. We have not yet located the specimen with his holotype label.

Description of Nests

Two nests were collected in Tam Dao, northern Viet Nam, on 9 March 2005. One of them (nest A, fig. 13) was about 2.5 m from the ground, at the junction of two thin (0.5–0.8 mm thick) wires hanging from the eaves of a house; the main substrate wire penetrates the basal part of the nest, with a free end hanging beyond the nest; the other wire is tied to the main wire, without a free hanging part. As in all known Eustenogaster species, the nest has an inverted-flask-shaped envelope. As in all species except E. calyptodoma, the envelope is made by extending the outer walls of peripheral cells, and thus its basal portion is comprised of the outer walls of peripheral cells. The apical, tubular part of the envelope is about 40 mm long and about 10 mm in diameter and is mesh-walled, with a broad horizontal flange near the apex. The upper part of the envelope is ornamented with three “wings” that project outward, two of which are closer to each other than to the last. The outer surface of the envelope is furnished with several keels, which run down continuously, slightly spirally, to the apical flange. The color of the nest is basically brown, with transverse stripes (0.5–1.3 mm wide) of lighter and darker brown between the keels, indicating different sources of nest material. The comb consists of 19 cells, which are hexagonal in cross section. The cells in the central part of the comb are 6–7 mm in diameter ( =  side-to-side distance) at opening and 13–17 mm long; peripheral cells are somewhat larger (7–8 mm in diameter) and shorter (9–14 mm in length). The thickness of the cell walls is 0.24 mm on average (0.12–0.38 mm, N  = 10). The nest carton is brittle, made from small chips mixed with a small amount of (possibly salivary) secretion. The part of the main substrate wire hung from the eaves is thinly covered with a single layer of carton sheet for about 70 mm from the base of the nest. The other substrate wire tied to the main wire is also covered by a carton sheet; the covering starts about 10 mm from the nest base and extends for about 100 mm toward the eaves; the part near the nest is tubelike (about 50 mm long and about 4 mm in diameter), and the both ends taper and are closed but it also has an irregularly shaped hole made by incomplete enclosure near the midlength.

Figure 13, 14

Nests of Eustenogaster nigra Saito and Nguyen, n.sp., collected in Tam Dao, northern Viet Nam, on 9 March 2005. 13. Nest A. 14. Nest B. Scale bars are 10 mm.

The other nest (nest B, fig. 14) was attached to a thin (1.5–1.8 mm thick) twig protruding from a concrete basement, at about 2 m from the ground. The nest structure is basically the same as nest A, but the envelope might not have been completed and almost lacks in an apical tubular extension; the cells are very shallow (3–7 mm long), possibly due to the cell wall having been shaved off at adult emergence. The nest color is lighter than in nest A, with the stripes in the envelope varying from pale brown to dark grayish brown. There are 15 cells. Mean thickness of cell walls is 0.43 mm (0.13–0.72 mm, N  = 10).

Wasps in the Nests

Nest A had four dead pupae in the central cells and no adults and no live brood. Two of the four dead pupae were so badly molded that we were unable to identify their sexes; the other two were a male and a female.

In nest B, 11 adult wasps (6 females and 5 males) and no immatures (pupae, larvae, or eggs) were present. When we collected the nest into a plastic bag, we treated it rather roughly, but the wasps came out from the nest only after the nest was kept in the plastic bag for more than 10 minutes. All six females were dissected for ovaries and spermathecae under a stereoscopic dissecting microscope. The spermathecae were removed and mounted on a glass slide in water and examined for presence or absence of sperm under a compound light microscope. All six females had nearly no fat in their metasoma, undeveloped ovaries, and empty spermathecae ( =  uninseminated).