Strumigenys decumbens, Tang & Guénard, 2023

Tang, Kit Lam & Guénard, Benoit, 2023, Further additions to the knowledge of Strumigenys (Formicidae: Myrmicinae) within South East Asia, with the descriptions of 20 new species, European Journal of Taxonomy 907, pp. 1-144 : 32-34

Strumigenys decumbens sp. nov.

urn:lsid:zoobank.org:act:6CC64AB7-09F0-42E7-A57E-756496C3BA1F

Fig. 10 ; Tables 2 , 6

Diagnosis

Strumigenys decumbens sp. nov. can be distinguished from other species in the S. leptothrix -group by a combination of the following characters: cephalic dorsum with appressed plank-like to spatulate setae; clypeal dorsum with appressed spatulate to elliptic setae; without any laterally-projecting seta in full-face view; in profile view, cephalic dorsum without any erect seta; pronotum marginated dorsolaterally; in profile view, propodeal spines subtended by broad lamellae; femora, tibiae and basitarsi with decumbent to appressed setae, without erect seta.

Etymology

The species is named after the lack of erect setae both on the cephalic dorsum and the surface of legs. The epithet ‘ decumbens ’ is the nominative feminine singular present participle of the Latin verb ‘ decumbo ’ (meaning ‘to lie down’).

Type material

Holotype worker ( Fig. 10 )

THAILAND • Sakon Nakhon Province, Site 99; 17.12266° N, 104.00371° E; 443 m a.s.l.; 21 Jun. 2018; IBBL members leg.; Winkler, 4 corners; secondary forest; LKCNHM ANTWEB1011974 (collection code IBBL SN-S99-ST-sp01).

Paratype workers

THAILAND • 10 workers; same collection data as for holotype; HKBM ANTWEB1011975 to ANTWEB1011984 (collection code IBBL SN-S99-ST-sp01) .

Measurements

Holotype worker

TL 2.3, HL 0.57, HW 0.41, MandL 0.09, SL 0.29, EL 0.077, PW 0.21, ML 0.58, PL 0.31, PH 0.13, DPW 0.10, PPL 0.16, GL 0.60, CI 71, MI 15, SI 70, OI 19, LPI 42, DPI 33.

Paratype workers

TL 2.2–2.5, HL 0.57–0.64, HW 0.42–0.44, MandL 0.08–0.09, SL 0.29–0.33, EL 0.064 –0.088, PW 0.20– 0.23, ML 0.56–0.67, PL 0.27–0.32, PH 0.12–0.13, DPW 0.09–0.11, PPL 0.15–0.19, GL 0.59–0.70, CI 69–73, MI 13–15, SI 70–74, OI 15–21, LPI 39–44, DPI 30–37 (n=10).

Worker description

HEAD. In full-face view, occipital margin deeply concave; occipital corners well developed and bluntly angular; occipital carina present. Clypeus slightly broader than long, around 1.1–1.2 times as long, roughly resembling inverted diamond. Scapes subcylindrical, marginated but not converging anteriorly to form thin lamella at leading edge. Mandibles in full-face view triangular; masticatory margins engaged across entire length, without diastema or gap between mandibles; basal lamellae low and broadly triangular, not fully visible at full closure. In profile view, eye with four ommatidia in diameter.

DENTITION ( Fig. 10B ). Principal dental row with eight alternating short triangular and long spiniform teeth (i.e., four consecutive pairs of teeth, each pair consists of short tooth followed by long tooth), second and third pairs subequal in size and longer than other pairs (short tooth of one pair compares with short tooth of another pair only, same for long tooth); followed by 3–4 small teeth and, at down curvature in anterior view, series of four minute denticles, terminating in small apical tooth. Total dental count of 16–17.

MESOSOMA. In profile view, promesonotal dorsum slightly convex, propodeum more or less flat transversely; pronotum marginated dorsolaterally. In dorsal view, lateral margins of pronotum mostly straight (at most very weakly convex), only converge anteriorly to meet anterior margin and posteriorly to meet lateral margins of mesonotum. In profile view, propodeal spines elongated-triangular, subtended on each side by broad lamella with concave posterior margin that broadens basally into rounded convex propodeal lobe. Metapleural gland bulla well-developed.

METASOMA. In profile view, petiolar peduncle grading evenly into node, and about as long as (or slightly shorter than) node. In dorsal view, petiolar node longer than broad, around 1.4–1.6 times as long; disc of postpetiole slightly broader than long, around 1.1–1.3 times as long. Areolate processes (spongiform tissues) present on both petiole and postpetiole; ventral lobes of petiole and postpetiole extensive; lateral lobe of petiole forming moderately extensive flap at posterolateral angle of node in profile; in dorsal view, processes present along lateral and posterior margins of petiolar node, and surrounding disc of postpetiole, thicker along lateral and posterior margins than on anterior margin.

PILOSITY. In full-face view, cephalic dorsum with appressed spatulatiform setae, ranging from plank-like setae near occipital margin to spatulate setae near posterior clypeal margin; clypeal dorsum covered with short spatulate to elliptic setae; mandibular dorsum covered with small oblanceolate setae; masticatory margins each with row of fine medially-directed setae. Head without any laterally-projecting seta; decumbent setae present along dorsolateral margin of head, those along lateral margin of occipital lobe overlap with one another. Surface of scape covered with appressed subspatulate setae; funiculus (except for apical antennomere) covered with appressed acicular setae; progressively finer setae densely covering apical antennomere. In dorsal view, promesonotal dorsum sparsely with appressed subspatulate setae; posteriorly-directed decumbent setae present along lateral margins; appressed stout setae present along lateral margins of petiolar node and disc of postpetiole. Pronotal humeral seta straight and stout, longer than other setae on pronotal dorsum. In profile view, cephalic dorsum without any erect seta; erect stout setae present on mesonotal dorsum as three pairs, and all over gastral tergites; ventral surface of head with decumbent setae; gastral sternites sparsely with suberect to decumbent setae. Hairwheel present at mesopleural excavation. Surfaces of femora, tibiae and basitarsi with long stout decumbent to appressed setae.

SCULPTURE. Cephalic dorsum, clypeal dorsum and ventral surface of head sparsely reticulate-rugulose, with spaces between rugulae densely areolate-rugulose; surface of antennal scrobe, antenna and legs densely areolate. Promesonotal dorsum predominantly densely areolate, with weak longitudinal rugulae and long, weak, longitudinal median striation; propodeal dorsum and dorsum of petiolar node densely areolate-rugulose. Side of pronotum mostly smooth and shining, weakly areolate-rugulose around margins; pleurae and side of propodeum densely areolate-rugulose. Disc of postpetiole smooth and shining. Basigastral costulae short, around one fifth in length of first gastral tergite; rest of gaster smooth and shining.

Comments

Strumigenys decumbens sp. nov. is a member of the elegantula -complex in the S. leptothrix -group and shares all its characters ( Bolton 2000).

Aside from Strumigenys decumbens sp. nov., there are five other SEA species in the same species group ( S. doydeei sp. nov., S. japonica , S. megaera , S. rongi sp. nov. and S. scolopax ) that share the following characters: cephalic dorsum covered with appressed spatulatiform (elliptic, plank-like, linear, subspatulate, spatulate, oblanceolate, obovate or ovate) setae only, head without laterally-projecting seta in full-face view (at most limited to 1–2 at occipital corner for S. doydeei ) ( Table 6 ). Unlike the three existing species and S. rongi , for S. decumbens and S. doydeei the propodeal spines are subtended by broad lamellae (instead of narrow lamellae or carinae).

Strumigenys decumbens sp. nov. shares a very similar cephalic pilosity with S. doydeei sp. nov., though unlike S. doydeei , these appressed setae markedly transit from spatulate to plank-like setae towards the occipital margin (instead of generally more consistent in shape across the cephalic dorsum). Strumigenys decumbens also only has decumbent to appressed setae on the femora, tibiae and tarsi (instead of laterally-projecting as in S. doydeei ). See comments under S. intermedia sp. nov. for a detailed comparison between the newly described species and the existing species of the S. leptothrix -group.

Geographic range

Thailand (Sakon Nakhon).

Ecology

Collected in secondary forest, at an elevation of 443 m.

Figures

Fig. 10. Type specimens of Strumigenys decumbens sp. nov. A, C–D. Holotype worker (ANTWEB1011974). B. Paratype worker (ANTWEB1011984). A. Full-face view. B. Mandible closeup. C. Profile view. D. Dorsal view.