Strumigenys calvus

Two colonies of this species are known, both under stones in a broad-leaved humid forest in mountainous area (alt. 600~700m). Both colonies were collected during August and consisted of more than 70 workers (72, 86) and about 20 alate gynes (18, 7) with a single queen. A couple of foraging workers were collected through sifting from broad-leaved humid forest in the island (alt. ca. 60m) as well (Dong & Kim, 2020).

Identification
Following Bolton (2000), this species belongs to the Strumigenys leptothrix-group, and can be distinguished from other similar species by the combination of the following characteristics: In full-face view, head entirely areolate-rugose excluding cephalic vertex and epistomal sulcus. Occipital margin straight or slightly concave, without projecting erect simple hairs. Lack of laterally projecting hairs along the dorsolateral margin of head in full-face view. Apicoscrobal hair absent. Antennae entirely with short appressed hairs on antennal scape and funiculus, without suberect or erect hairs. Mandibles densely covered with appressed spatulate hairs in full-face view. Pronotal humeral hair usually absent, rarely short and erect, not flagelliform. Pronotum and mesonotum with short appressed hairs, without erect hairs in profile view. Propodeal spines acute, not elongated. Post-ventral propodeal lamella broad. Mesosoma largely smooth excluding propodeum in profile view. Distinct blackish spot on 3rd abdominal sternite (1st gastral segment).

Distribution based on Regional Taxon Lists
Palaearctic Region: Republic of Korea.

Biology
We were able to collect the colonies of this species twice, and both of them made their nest under the stone of broad-leaved humid forest in mountainous area (alt. 600~700m) (Fig. 3). Both colonies were collected at August and consisted of more than 70 workers (72, 86) and about 20 alate gynes (18, 7) with a single queen. A couple of foraging workers were collected through sifting from broad-leaved humid forest in the island (alt. ca. 60m) as well.

Nomenclature

 * . Strumigenys calvus Dong & Kim, 2020: 4, figs. 1-3 (w.q.) KOREA.

Description
Worker (Fig. 1)

Coloration: Body entirely rust orange ground color.

Head (Fig. 1A, B): Head longer than wide (CI: 71–79). Occipital margin straight or slightly concave, not deeply concave. Cephalic posterolateral margin roundly convex. Cephalic frons posteromedially convex in profile view. Eyes small and circular, completely invisible in full-face view. Antennae 6-segmented, scape cylindrical-shaped, basally thin. Clypeus diamond- shaped, anterior clypeal margin convex, converged at frontal apex. Mandibles triangular shaped in full-face view, fully closed without diastema. Mandibular external margin evenly curved, masticatory margin with well-developed 8~9 triangular teeth, basal tooth acute but distinctly smaller (c.a. 1/2) than other typical tooth. Second and third tooth distinctly longer, almost half the length of the basal margin of mandible.

Mesosoma (Fig. 1B, C): In profile view, pronotum anteriorly curved, not angulated. Dorsum of pronotum flat. Promesonotal suture gently raised, not distinctly concave. Dorsum of mesonotum also flat. Metanotal groove indistinct. Dorsum of propodeum flat or slightly raised. Pronotal humeral angle absent in dorsal view. Propodeal spines short, acute, not elongated in profile view. Propodeal spines parallel, not convergent in dorsal view. Sparse spongiform tissue (propodeal lamella) post-ventrally attached between propodeal spine and propodeal lobe. Propodeal declivity evenly concave.

Metasoma (Fig. 1B): Petiolar peduncle long and elongated, as long as petiolar node. Petiolar node round, evenly convex, not angulated in lateral view. In dorsal view, petiolar node longer than wide. Postpetiole gently convex when seen in lateral view. Petiole lateroventrally covered with stretched semicircular spongiform tissue. Broad spongiform tissue attached on the anteroventral portion of the petiole. Stretched semicircular spongiform tissue attached on postpetiole lateroventrally, but much more extensive than tissue on petiole. Gaster oval shaped, black round spot on 3rd abdominal sternite (1st gastral segment) remarkably visible in lateral and ventral view.

Pilosity: Head covered with short appressed hairs except malar area (gena) in lateral view. Occipital margin without projected erect hairs in full-face view. Frons and clypeus densely covered with short hairs, less densely covered in vertex. Lack of laterally projecting hairs along the dorsolateral margin of head in full-face view. Antennal scape and funiculus entirely covered with short appressed hairs. Hairs on antennal funiculus denser than hairs on the antennal scape. Mandibles densely covered with ovate to subspatulate hairs, long fine hairs on posterior side. Pronotum and mesonotum with fine appressed hairs, lacking erect hairs. Propodeum with fine appressed hairs as well. Pronotal humeral hair usually clearly absent, but rarely seen in dorsal view. Petiole and postpetiole covered with appressed to suberect hairs. Gaster covered with fine appressed hairs and erect hairs.

Sculpture: Head densely areolate-rugose. Cephalic vertex weakly rugose or smooth. Epistomal sulcus smooth. Clypeus smooth without punctation. Gena (malar area) distinctly alveolate in lateral view. Antennal scape densely but weakly verrucose. Dorsum and side surface of pronotum weakly costate anteriorly. Mesonotum including mesopleuron, metathorax smooth and shiny. Propodeum irregularly wrinkled, but some specimens seem smooth. Dorsum of petiolar peduncle densely areolate-rugose. Side surface of petiolar node weakly rugose while dorsum is shiny. Dorsum of postpetiole entirely shiny. Gaster also entirely smooth and shiny excluding relatively short basigastral costulae.

Queen (Fig. 2) Morphologically similar to worker except following characteristics; presence of a pair of erect apicoscrobal hair and distinct pronotal humeral hairs in dorsal view, presence of 3 ocelli, and extremely enlarged compound eyes and thorax. Coloration: Body entirely rust orange ground color, slightly darker than worker. Wing venation: Both forewing and hindwing entirely covered with short fine hairs densely. Posterior margin of forewing with distinctly long hairs. Forewing venation vestigial, not developed, even almost invisible excluding pterostigma, costal (C) vein, radial (R1) veins and thick r-rs cross vein.