Strumigenys leptothrix group

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Strumigenys leptothrix group Bolton (2000)

Species

Malagasy

Malesian-Oriental-East Palaeartic

"Oriental and Sino-Japanese realms" Tang and Guénard (2023)

Worker Diagnosis

Mandibles in full-face view and at full closure short triangular to sub triangular, broad, powerfully developed and with their dorsal surfaces convex from apical to lateral margin and their lateral outlines distinctly convex. Masticatory margins serially dentate and the tooth rows engaging through the lengths of the margins. In ventral view outer margin of mandible without an inflected prebasal angle. MI 14-18.

Dentition. Described under the species-complexes, below.

Basal lamella of mandible an elongate lamina along basal margin proximal of the basal tooth, terminating in a low triangular peak (exiguaevitae-complex) or a short rounded lobe (Strumigenys khakaura) immediately behind basal tooth and lower than the basal tooth; not visible or partially visible in full-face view with the mandibles fully closed.

Labral lobes elongate triangular, conical or digitate.

Clypeus broad, anterior margin convex. Dorsum of clypeus almost flat. Fully closed mandibles in full-face view meeting anterior clypeal margin at or close to the anterolateral angles.

Clypeal dorsum with longitudinal rows of small to minute appressed hairs that may be simple, spatulate or squamate and may be difficult to see; without any other form of pilosity. Lateral margins of clypeus with hairs as dorsum, without a fringe of freely projecting hairs.

Preocular carina conspicuous in full-face view.

Ventrolateral margin of head between eye and mandible angulate to sharply marginate.

Postbuccal impression small but distinct.

Cuticle of side of head within scrobe partially to entirely smooth.

Scape short to moderate, SI 59-83, cylindrical to subcylindrical, the dorsum and ventre not converging anteriorly to form a flange or thinly lamellate leading edge. Ventral surface of scape convex to shallowly concave.

Leading edge of scape with closely appressed minute hairs that are directed toward the apex of the scape.

Petiole in profile elongate and subclavate (except in khakaura). Dorsal outline of peduncle may grade evenly into the node through a shallow curve, or the node may have a differentiated anterior face.

Propodeum may be unarmed, bidentate or bispinose; a lamella may be present on declivity.

Spongiform appendages of waist well developed, the lateral lobes laminate rather than spongiform in some taxa. Base of first gastral sternite with or without spongiform tissue.

Pilosity. Cephalic dorsum behind clypeus with minute appressed ground-pilosity that is simple and difficult to see. Posterior half of vertex usually with simple or flagellate standing hairs. Dorsolateral margins of head hairless or with simple or flagellate hairs projecting beyond the outline, but without isolated bizarre hairs. Pilosity of dorsal body surfaces simple or flagellate when present.

Sculpture. Extremely variable; basigastral costulae short to vestigial.

Notes

Bolton (2000) - This is predominantly a species-group of the Oriental and Malesian regions, but a few species are found in the East Palaearctic and five distinctive species, the isolated khakaura and a complex of four species around exiguaevitae, are found in Madagascar. The group remains unknown in the Afrotropical region. The diagnosis above refers particularly to the Malagasy fauna. For further discussion and wider definition of the group see under the Malesian-Oriental-East Palaearctic fauna.

Madagascar

Strumigenys exiguaevitae and its relatives are easily distinguished from all other Malagasy (and Afrotropical) species of Pyramica by the combination of characters listed above. The closest relative of the exiguaevitae-complex appears to be Strumigenys assamensis of the Oriental region, which has similar dentition and habitus but which has the pronotum almost flat and strongly dorsolaterally marginate.

The two species-complexes in Madagascar are as follows.

exiguaevitae complex

With head in profile the vertex high-domed posteriorly. Anterior clypeal margin broadly evenly convex. Pronotum transversely convex and not marginate dorsolaterally though a narrow oblique carina follows the anterolateral margin of the pronotum. Petiole node in profile long and low, its shallowly convex dorsum much longer than the short anterior face. Anterior margin of postpetiole disc in dorsal view more or less transverse (varying from extremely feebly convex to extremely feebly concave), posterior margin not broadly convex , the disc not transversely kidney-shaped. Base of first gastral sternite without trace of spongiform or fibrous tissue. Simple hairs present but flagellate hairs absent. Eyes with more than 10 ommatidia in total.

Dentition in the complex is consistently of 14 teeth and denticles in total, of which 7 constitute the principal basal row. Counting from the base, teeth 1-3 gradually increase in size, tooth 3 the longest on the margin. Tooth 4 is about the same size as 2; tooth 5 is slightly longer than 4; teeth 6 and 7 are smaller and about the same size as 1. Teeth 8 and 9 are smaller than any preceding and are followed by 4 denticles and a small apical tooth.

khakaura complex

With head in profile the vertex not high-domed posteriorly. Anterior clypeal margin narrowly convex, broadly V-shaped. Pronotum more or less flat and sharply marginate dorsolaterally; remainder of alitrunk with an extensive system of raised cuticular carinae and marginations (described below). Petiole node in profile short and high, its short posteriorly sloping dorsum subequal to its anterior face. Anterior margin of postpetiole disc in dorsal view strongly concave, posterior margin broadly convex so that the disc is transversely kidney-shaped. Base of first gastral sternite with diffuse fibrous tissue present. Flagellate hairs present. Eyes very small, with only 2-3 ommatidia in total.

Dentition is only partially visible as the only known specimen has its mandibles fully closed. The 5 basal teeth conical and approximately the same size, teeth 6 and 7 slightly shorter; beyond this details cannot be ascertained but there are some smaller teeth and denticles and the apical tooth is larger than those immediately preceding it.

Oriental and Sino-Japanese realms

Three species described between 2004 and 2020 were added here: Strumigenys ailaoshana, Strumigenys calvus and Strumigenys nankunshana.

Tang and Guénard (2023) assigned numerous newly described species to this group and created several new species complexes within the S. leptothrix-group. They also concluded a future revision of this group is needed. Tang and Guénard (2023) explain:

"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 and S. strummeri 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.

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."

elegantula complex

Tang and Guénard (2023) - 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.

Pending further analysis of this species group, and the examination of more specimens of these species to allow for more thorough morphological analyses, these species are provisionally placed in this complex:

lepthothrix complex

Tang and Guénard (2023) - 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.

Pending further analysis of this species group, and the examination of more specimens of these species to allow for more thorough morphological analyses, these species are provisionally placed in this complex:

zanderi complex

Tang and Guénard (2023) - 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.

unassigned to a complex

Tang and Guénard (2023) - 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.

References

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