Strumigenys subterranea Brassard, Leong & Guenard, 2020

Brassard, Francois, Leong, Chi-Man, Chan, Hoi-Hou & Guenard, Benoit, 2020, A new subterranean species and an updated checklist of Strumigenys (Hymenoptera, Formicidae) from Macao SAR, China, with a key to species of the Greater Bay Area, ZooKeys 970, pp. 63-116 : 63

Strumigenys subterranea Brassard, Leong & Guenard sp. nov.

Type locality.

Macao SAR, China: Coloane Island, Coloane North East hiking trail, 22.1351°N, 113.5700°E, ca. 80 m, subterranean trap placed at a depth of 12.5 cm, 14 May-4 June 2019, F. Brassard leg.

Repository institution.

Insect Biodiversity and Biogeography Lab (IBBL), School of Biological Sciences, Hong Kong University

Type specimen.

Holotype. Pinned worker. Original label: "China SAR: Macau, Coloane. Coloane North East Hiking Trail. 78 m, 14v-4vi.2019, 22.13510°N, 113.57000°E, Subterranean Trap 12.5 cm depth. F. Brassard" “MAC_S12_12.5_q4_Sp.2” [IBBL: ANTWEB1010847].

Worker measurements.

(n = 1): TL 1.809 mm, HL 0.454 mm, HW 0.348 mm, CI 77, MandL 0.098 mm, MI 22, SL 0.226 mm, SI 65, PrW 0.189 mm, PI 54, EL 0.006 mm, OI 2, WL 0.458 mm, PetH 0.124 mm, PetL 0.183 mm, MtfmL 0.295 mm, MttbL 0.230 mm, LPI 68, DPetW 0.117 mm, DPI 64, PosPetL 0.184 mm, ATL 0.356 mm.

Diagnosis.

Mandibles in full-face view triangular, eyes with a single ommatidium, anterior margin of clypeus shallowly convex, clypeal margin fringed with a continuous row of appressed spatulate hairs incurved towards midline of head, conspicuous preocular carina, dorsoventrally flattened scape, spatulate to spoon-shaped hairs on leading edge of scape, pair humeral hairs present, dorsum of head behind clypeus reticulate-punctate, side of mesosoma and disc of postpetiole smooth, postpetiole with concave anterior margin and a projecting lobe on convex posterior margin, total dental count of eight, lack of propodeal spines, and propodeal declivity angular.

Worker description.

(Figs 2 - 4 ). Head. In full-face view, head noticeably longer than wide (CI: 77) (Fig. 3A ), with its widest portion nearby the anterior end of the posterior third of its length. In lateral view, eye with a single facet, inconspicuous, and located at the widest level of the head (Fig. 3E ). Posterior cephalic margin shallowly concave; corners of posterior margin of head weakly developed and evenly rounded through the lateral margins. Posterolateral margins evenly rounded on half of their length, then converging at a slightly steeper angle towards the center of the head. Anteromedian clypeal margin slightly convex. Scapes with a moderately developed subbasal lobe on their anterior portion. Apex of scape not reaching posterior margin of head, antenna including scape with six articles, with the last two articles distinctly enlarged and forming a club; ratio of antennal segments from 2nd to 6th segment = 3.83: 1.33: 1: 3.05: 11.56 (Fig. 3D ). Mandibles triangular with eight teeth (T) and four denticles (D), arranged from basal to apical as such: T-D-T-T-T-T-T-T-D-D-D-T (Fig. 3C ). Basal angle between the basal margin and masticatory margin rounded triangular. Basal lamella a thin strip, widest at the basal tooth and almost fully disappearing at the midpoint of the masticatory margin. Labrum terminates in a pair of short triangular lobes (Fig. 3F ).

Mesosoma. In lateral view, dorsum of mesosoma broadly convex but slightly concave at the metanotal groove (Fig. 4A ). Anterior portion of promesonotum in dorsal view convex (Fig. 4B ), with its widest point slightly posterior to the humeral hairs. Median anterior margin of promesonotum slightly convex. Lateral margin of premosonotum subparallel and slightly convex. Metanotal groove distinct but weakly incised. In dorsal view, propodeum approximately half of the maximal width of the promesonotum. In lateral view, propodeum with an angular declivity. Propodeal declivity with a spongiform lamella.

Metasoma. Petiole in lateral view elongate (LPI: 68) and subclavate, with long and thin peduncle. Petiolar node well developed; dorsum of node convex, with its widest point at the posterodorsal corner. Petiolar node in dorsal view subcircular (DPI: 64), widest towards the posterior part. Postpetiolar disc in dorsal view suboval and distinctly wider than long; the median portion of the anterior margin distinctly concave whereas the posterior margin convex with a lobe projecting from the median portion (Fig. 4C ). Limbus in dorsal view strongly concave, with a thin spongiform pad along its length. Spongiform tissues present on both petiole and postpetiole. Spongiform tissue on the lateral side of petiole restricted to the posterior portion of the node in profile. Excluding the anteriormost part of the ventral portion of petiole, spongiform tissue covers the ventral portion of both the petiole and postpetiole entirely. Depth of spongiform tissue under petiole nearly as much as petiole height. Spongiform tissue particularly extensive on the ventral lobes of the postpetiole. In dorsal view spongiform tissue present along the posterior margin of the petiolar node, and surrounding disc of postpetiole.

Pilosity. On head, spatulate hairs arising from their base and then abruptly curving towards the mandibles, forming a space between the scale of the hair and the head surface. In full-face view of head, numerous evenly spaced spatulate hairs (ca. 95) along the frons, with around two-thirds as much spatulate hairs (ca. 60) evenly spaced but more densely arranged on the clypeus. A total of 16 smaller spatulate hairs present on anterior margin of clypeus. On each side of the anterior margin of the clypeus, three hairs on lateral portion and five on the anterior portion, all incurved towards the midline of the head. Largest spatulate hairs (n = 6) fully extending and found on subbasal lobe of antennal scape; with the first two basal hairs curved towards the apex of the scape, whereas the four most posterior hairs are curved towards the base of the scape. Two pairs of thin remiform hairs on the vertex; with one pair on the lateral portions of vertex and the other in posteromedial position (Fig. 3B ). In profile view, appressed simple hairs present below antennal scrobe towards ventral portion of head. On the mesosoma and metasoma, decumbent hairs evenly spaced with a pair of long flagellate humeral hair present on petiolar node; several erected simple, appressed and filiform hairs present on first gastral tergite, whereas other tergites and sternites are mostly covered by appressed simple hairs. Appressed simple hairs present on tibia, femur and tarsus. Meso- and meta-basitarsal hairs flagellate. Flagellate hairs absent from femurs and tibias.

Sculpture. In full-face and lateral view, head covered by areolate sculpturing (0.10 - 0.23 mm). In dorsal view, superficial sculpturing on the surrounding of the promesonotum and on its posterior section. Center of the dorsal portion of the promesonotum and propodeum smooth; lateral portions of mesosoma smooth (Fig. 4A ). In dorsal view, discs of petiole and postpetiole smooth. In lateral view, petiole with weak sculpturing. Basigastral costulae present as weakly developed and irregular imprints on the central part of the limbus, extending around half the length of the postpetiole disc. Sculpturing on tibias and femurs areolate. Leg bullae absent.

Color. Body coloration concolor yellowish brown, with slightly lighter coloration on the legs, antennae, mandibles and at the apex of the gaster. First gastral tergite and sternite with darker coloration.

Comments.

Strumigenys subterranea sp. nov. belongs to the Strumigenys rostrata group of the Malesian-Oriental-East Palearctic region ( Bolton 2000), due to a combination of morphological characters: mandibles in full-face view triangular, basal lamella of mandible low and rounded-triangular, anterior margin of clypeus broad and shallowly convex, clypeal margin fringed with a continuous row of curved spatulate to spoon-shaped hairs, conspicuous preocular carina, dorsoventrally flattened scape, spatulate to spoon-shaped hairs on leading edge of scape, cuticle within scrobe reticulate or reticulate-punctate, pronotum dorsum without a median longitudinal carina, spongiform appendages present on petiole and postpetiole, pronotal humeral hair present, dorsum of head behind clypeus reticulate-punctate, side of propodeum and disc of postpetiole smooth.

However, S. subterranea sp. nov. can be distinguished from the other 17 species within this group (Table 2 ) by a combination of the following characters: eyes with a single ommatidia, clypeal margin shallowly convex, evenly spaced appressed spatulate hairs along frons (with appressed spatulate hairs evenly spaced but more densely arranged on clypeus), postpetiole with concave anterior margin and a projecting lobe on convex posterior margin, total dental count of nine, lack of propodeal spines, and propodeal declivity angular, not rounded.

We found that the most peculiar characteristic of S. subterranea (i.e., having very small eyes) is shared with Strumigenys atropos Bolton, 2000. However, the shape of the postpetiolar node (straight anterior margin in S. atropos but concave in S. subterranea sp. nov.) and the shape of the anterior margin of the clypeus differ (slightly convex in S. subterranea sp. nov., but noticeably concave in S. atropos ). Moreover, large spatulate hairs are present up to two-third of the length of the lateral margins of the head of S. atropos , but not in S. subterranea. Lastly, large spatulate hairs are present on the dorsal portion of the pronotum of S. atropos , whereas hairs on the dorsal portion of S. subterranea are fine.

Etymology.

The name of this new species refers to the stratum it was collected in and to its suggested subterranean ecology.

Ecology.

A single worker from this species has been collected so far, found within a subterranean trap; a 15 mL falcon tube placed at a depth of 12.5 cm below the ground surface. It contained ethanol 70% and was baited with tuna mixed with honey (see Suppl. material 1: Fig. S1 for sampling design). The trap was placed in young secondary forest and was operating continuously for a period of 21 days. Little is known about the ecology of this species. However, due to the extremely reduced eyes present on the specimen and its collection through a subterranean trap, it is here suggested that the species has subterranean habits. Further reinforcing this hypothesis is the fact that extensive sampling in Hong Kong and Macao over the past 6 years focusing on ground-dwelling and leaf-litter ants using Winklers and pitfall traps never yielded this species. Nevertheless, only a single worker was found within one out of 256 subterranean traps retrieved during our sampling on Coloane Island, which indicates this species is uncommon. Our data also suggests it cohabits within the same soil layer with other ant species, including other subterranean species. Indeed, we found within the same trap one worker of Pheidole ochracea Eguchi, 2008 and hundreds of workers of Carebara zengchengensis Zhou, Zhao & Jia, 2006. Additionally, within the same quadrat (1 × 1 m) we also found C. zengchengensis at depths of 25, 37.5 and 50 cm, as well as Solenopsis jacoti Wheeler, 1923 and Buniapone amblyops Emery, 1887 at a depth of 50 cm.

Figures

Figure 2. Strumigenys subterranea sp. nov. (ANTWEB 1010847), worker in profile view.

Figure 3. Strumigenys subterranea sp. nov. (ANTWEB 1010847) A-F worker A full-face view B face view tilted posteriorly to showcase hairs on vertex C mandibles D left antenna E side view to showcase the eye F ventral view to showcase labrum.

Figure 4. Strumigenys subterranea sp. nov. (ANTWEB 1010847) A-C worker A lateral view of mesosoma B dorsal view of mesosoma C dorsal view of metasoma.