Strumigenys Smith, 1860

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 : 9-14

Genus Strumigenys Smith, 1860

After inspecting specimens of some of the species from Southeast Asia in the Strumigenys leptothrix - group, which was introduced by Bolton (2000), we found that they have several distinctive dentition patterns. Considering this, we introduce three new species complexes within the group, namely the elegantula -complex, leptothrix -complex and zanderi -complex, to accommodate these species, some of which have previously been described while others are firstly described in this study. We also introduce the Strumigenys nathistorisoc -group to accommodate the species described in Tang et al. (2019), which also shares some morphological similarities with the above species group. Worker diagnoses of the newly introduced species group and species complexes are provided below, adopting the format established by Bolton (2000). We urge other species considered to be a member of the S. leptothrix -group to be examined for their dentitions as we were unable to do so in this study. A full species group review is recommended to properly evaluate if the differences in dentitions between species of the S. leptothrix - group should lead to a splitting of the species group into several smaller species groups. Below, species with their designation confirmed will be listed separately from those that are uncertain.

We also raise caution concerning species having mandibles with the masticatory margins engaging across their entire length (i.e., without diastema or a gap between mandibles), that the determination of the dentition may be challenging without specimens in the appropriate conditions (opened mandibles and/or appressed setae along the inner margins of mandible abraded or absent). Depending solely on specimens with fully-closed mandibles and complete mandibular pilosity is prone to misidentification. For verification of species group designations, it is recommended to use specimens with clearly visible dentition. The technique used by Booher et al. (2021) to soften the specimen and manipulate its mandibles may also be helpful when specimens available are abundant. For species identification, when the dentition is not fully visible and the aforementioned technique is not applicable, it may be necessary to rely upon other morphological characters, and species from different species complexes ( elegantula -complex, leptothrix -complex and zanderi -complex) should be considered in conjunction as candidates.

Strumigenys leptothrix -group

Species

Oriental and Sino-Japanese realms

- Strumigenys ailaoshana ( Xu & Zhou, 2004)

- Strumigenys alecto ( Bolton, 2000)

- Strumigenys assamensis De Andrade, 1994

- Strumigenys benten ( Terayama et al., 1996)

- Strumigenys calvus Dong & Kim, 2020

- Strumigenys decumbens sp. nov.

- Strumigenys delicata sp. nov.

- Strumigenys densissima sp. nov.

- Strumigenys doydeei sp. nov.

- Strumigenys elegantula ( Terayama & Kubota, 1989)

- Strumigenys euryale ( Bolton, 2000)

- Strumigenys formosimonticola ( Terayama et al., 1996)

- Strumigenys intermedia sp. nov.

- Strumigenys jacobsoni Menozzi, 1939

- Strumigenys japonica Ito, 1914

- Strumigenys leptothrix Wheeler, 1929

- Strumigenys medusa ( Bolton, 2000)

- Strumigenys megaera ( Bolton, 2000)

- Strumigenys nankunshana (Zhou, 2011)

- Strumigenys rongi sp. nov.

- Strumigenys rudinodis Stärcke, 1941

- Strumigenys scolopax ( Bolton, 2000)

- Strumigenys scylla ( Bolton, 2000)

- Strumigenys stheno ( Bolton, 2000)

- Strumigenys strummeri sp. nov.

- Strumigenys yamanei sp. nov.

- Strumigenys zanderi sp. nov.

- Strumigenys zhenghuii sp. nov.

Comments

When the Strumigenys leptothrix -group was first introduced by Bolton (2000), only the Malagasy species were grouped into species complexes (exiguaevitae -complex and khakaura -complex). Worker diagnoses were also written primarily based on these species and their dentition patterns were described in detail. However, the phylogenetic analysis by Booher et al. (2021) revealed that the Malagasy species are not closely related to the Asian species of the group. Hence, we urge that a separate species group be created for the Malagasy species and the worker diagnosis of S. leptothrix -group be revised based on the remaining Asian species.

After a careful examination of the dentition of specimens with opened mandibles, it is determined that the existing Asian species in the Strumigenys leptothrix -group show more than one dentition pattern. Strumigenys leptothrix and several other species ( S. alecto , S. benten , S. calvus and S. formosimonticola ) share dentitions similar to that of the distantly-related Malagasy species, while some other species ( S. elegantula and S. jacobsoni ) have alternating short triangular teeth and long spiniform teeth as their principal dental row ( Fig. 2A–D ). In addition, S. zanderi sp. nov. and S. strummeri sp. nov. further show the third dentition pattern within the group, with alternating triangular and rounded teeth. Here, we introduce three new species complexes for the Asian species of the group based on their dentition patterns. Some of the existing species cannot be confidently placed within a particular species complex as their dentitions cannot be unambiguously determined based on available specimens, specimen photos or recent publications. These species should be checked for their dentitions in the future. Ultimately, a full species group revision is recommended.

This group shares a superficial resemblance with the S. nathistorisoc -group and S. kichijo -group, such as short and triangular mandibles, relatively elongated head (CI<85) with well-developed occipital corners, and large eyes (OI> 10). It can be differentiated from the other groups by the combination of mandibles with the masticatory margins engaging throughout its length and its dentition. A gap between mandibles is present in the S. nathistorisoc - and S. kichijo -groups.

Placement uncertain

- Strumigenys euryale ( Bolton, 2000)

- Strumigenys japonica Ito, 1914

- Strumigenys medusa ( Bolton, 2000)

- Strumigenys megaera ( Bolton, 2000)

- Strumigenys rudinodis Stärcke, 1941

- Strumigenys scolopax ( Bolton, 2000)

- Strumigenys scylla ( Bolton, 2000)

- Strumigenys stheno ( Bolton, 2000)

elegantula -complex

- Strumigenys decumbens sp. nov.

- Strumigenys densissima sp. nov.

- Strumigenys doydeei sp. nov.

- Strumigenys elegantula ( Terayama & Kubota, 1989)

- Strumigenys intermedia sp. nov.

- Strumigenys jacobsoni Menozzi, 1939

Provisional members

- Strumigenys ailaoshana ( Xu & Zhou, 2004)

- Strumigenys delicata sp. nov.

- Strumigenys nankunshana (Zhou, 2011)

- Strumigenys zhenghuii sp. nov.

Principal dental row with eight alternating short triangular and long spiniform teeth (i.e., four consecutive pairs of teeth, each pair consisting of a short tooth followed by a long tooth), the second and the third pairs subequal in size and longer than the other pairs (short tooth of one pair compares with short tooth of another pair only, same for the long tooth); followed by 3–4 small teeth and, at the down curvature in anterior view, a series of four minute denticles, terminating in a small apical tooth ( Fig. 2 ). Total dental count of 16–17.

leptothrix -complex

- Strumigenys alecto ( Bolton, 2000)

- Strumigenys benten ( Terayama et al., 1996)

- Strumigenys calvus Dong & Kim, 2020

- Strumigenys formosimonticola ( Terayama et al., 1996)

- Strumigenys leptothrix Wheeler, 1929

Provisional members

- Strumigenys assamensis De Andrade, 1994

- Strumigenys rongi sp. nov.

- Strumigenys yamanei sp. nov.

Principal dental row with seven teeth, the first, second and third teeth gradually increase in size, the third tooth the longest on the margin, the fourth tooth about the same size as the second tooth, the fifth tooth slightly longer than the fourth tooth, the six and seventh teeth smaller and about the same size as the first tooth; followed by two small teeth and, at the down curvature in anterior view, a series of four minute denticles, terminating in a small apical tooth. Total dental count of 14.

zanderi -complex

- Strumigenys strummeri sp. nov.

- Strumigenys zanderi sp. nov.

Principal dental row with an elongated-triangular tooth, followed by six alternating elongated-triangular teeth and rounded teeth; the second, fourth and sixth tooth elongated-triangular, longer than the first tooth and neighboring rounded teeth; the third, fifth and seventh teeth broad and rounded; apex of mandible at the down curvature, in anterior view, with an elongated-triangular small tooth, followed by a crowded series of six minute denticles, terminating in a small conical apical tooth. Total dental count of 15.

Strumigenys nathistorisoc -group

Species

Oriental and Sino-Japanese realms

- Strumigenys nathistorisoc Tang et al., 2019

Comments

The Strumigenys nathistorisoc -group currently contains a single member found in the Oriental and Sino-Japanese realms. One undescribed species from Guangdong province of mainland China (specimens have previously been examined by us) also shares the characteristic dentition and likely belong to the same species group.

This group shares superficial resemblance with the S. leptothrix -group and S. kichijo -group, such as short and triangular mandibles, relatively elongated head (CI <85) with well-developed occipital corners, and large eyes (OI>10). It can be easily differentiated from the other groups by the combination of a prominent gap between the mandibles at the basal half and its unique dentition of seven alternating long tapered teeth and reduced low and rounded teeth. A conspicuous gap between mandibles is absent in S. leptothrix -group, while S. kichijo -group’s principal dental row is consistently triangular to elongated-triangular throughout the mandible, not alternating between two different tooth shapes.

Worker diagnosis

MANDIBLE. In full-face view, and at full closure short and elongated-triangular, MI 30–34; in profile view, robust with apical half enlarged and curving downwards; masticatory margins engaged only at apical half of their lengths, basal half with prominent gap between mandibles. Basal lamellae low and broadly triangular, not fully visible at full closure; trap-jaw mechanism most likely absent.

DENTITION. Principal dental row with seven alternating long tapered teeth and reduced low and rounded teeth, four basalmost teeth separated by basal gap between mandibles and following three teeth fully engaging their counterparts from opposing mandible with mandibles at full closure; diastema present between basal lamella and first tooth; first tooth small and triangular, follows by reduced tooth or small denticle, third tooth tapered and larger, fourth tooth reduced, low and rounded, fifth and seventh teeth both tapered, subequal in size and largest among all teeth, sixth tooth reduced, low and rounded, almost squircle in shape and larger than third tooth; apex of mandible at down curvature, in anterior view, with crowded series of around 10–11 minute denticles, terminating in small conical apical tooth. Total dental count of 18–19.

HEAD. Anterior clypeal margin broadly convex. Preocular carina conspicuous in full-face view; occipital carina present. Ventrolateral margin of head between eye and mandible usually marginate; postbuccal impression small.

ANTENNA. Scape short to moderate, SI 68–74, subcylindrical, marginated but not converging anteriorly to form thin lamella at leading edge.

MESOSOMA. In profile view, propodeal spines usually present; propodeal lamella may be present on declivity.

METASOMA. In profile view, petiole elongate and subclavate; petiolar peduncle may grade evenly into node, or node may have differentiated anterior face.Areolate processes (spongiform tissues) of metasoma well developed.

PILOSITY. Clypeal and mandibular dorsum usually with small appressed setae; upper scrobe margins, lateral and anterior clypeal margins usually with decumbent setae; leading edge of scape usually with appressed to decumbent apically-directed setae. Hairwheel present at mesopleural excavation. Pilosity of head and body variable; in dorsal view, lateral margins of mesosoma usually with posteriorly-directed decumbent setae.

SCULPTURE. Basigastral costulae usually short, no more than one fourth in length of first gastral tergite.

Figures

Fig. 2. Mandible close-ups of workers from the elegantula-complex in the Strumigenys leptothrix-group. A–B. S. intermedia sp. nov. (TT01353). C. S. elegantula (Terayama & Kubota, 1989) from Hong Kong (RHL00290). D. S. jacobsoni Menozzi, 1939 from Vietnam (ANTWEB1011973).