Strumigenys dexis group

Strumigenys dexis group Bolton (2000)

Species

Malagasy

Worker Diagnosis

Mandibles with asymmetrical dentition: left mandible with 5 teeth and denticles in total, right mandible with 6. Left mandible has 3 long teeth (second or third from the base is the longest) followed by either a denticle and a smaller tooth or 2 denticles that share a common base, these located ventral to the third tooth. Right mandible has a long basal tooth followed by a much smaller second tooth; these are followed by two more long teeth (the third from the base longer than the fourth) and the fourth is subtended by a pair of denticles, or a denticle and a short tooth, below the base of the fourth tooth. Therefore in full-face view the left mandible has 3 long or spiniform teeth and the right has 4, the apical denticles not being visible in this aspect. Mandible short (MI range 17-33), with a broadly rounded prominent basal external angle visible; the dorsum proximal of the basal most tooth distinctly concave, the concave area margined distally by a rim or carina that traverses the mandible. Mandible broad basally and tapering strikingly toward the apex in full-face view, leaving a subovate gap between the inner margins distal of the basal lamella.

Leading edge of scape a dorsoventrally flattened convex lamella.

Upper scrobe margins in full-face view widely divergent behind the frontal lobes.

Ventrolateral margin of head without trace of a preocular notch, the margin continuous in front of the eye. Ventral surface of head without a preocular transverse groove but postbuccal groove is distinct.

Promesonotum usually flat and bluntly marginate dorsolaterally (rounded in Strumigenys carolinae).

Spongiform appendages of petiole well developed, with a ventral curtain and lateral lobes. Postpetiole with lateral and ventral lobes. Lamella of propodeal declivity usually well developed into a wide strip below the triangular propodeal spines (narrow in 2 species).

Pilosity. Apicoscrobal hair absent. Pronotal humeral hair absent. Head with broadly spatulate to spoon-shaped hairs present on mandible, lateral clypeal margin (curved anteriorly), upper scrobe margin (curved anteriorly) and leading edge of scape; entire body lacking flagellate hairs. Dorsal surface of mandible with elongate spatulate hairs that are directed towards the midline.

Sculpture. Two forms of basic sculpture are developed within the group, described under the species-complexes, below.

Glands. Gland of scape visible as an elongate patch of pale tissue near the apex of the ventral scape. Femoral gland bulla absent to well developed; decreasing in size from hind femur where it is easily visible to fore femur where it is minute to absent. Absent on all femora of Strumigenys sylvaini and Strumigenys wardi. Tibial gland bulla absent on all tibiae (sylvaini, wardi), or present on hind tibia (Strumigenys peyrierasi, Strumigenys gorgon, Strumigenys dexis, Strumigenys coveri), or present on hind and fore tibia (Strumigenys alperti, Strumigenys carolinae). Gland at base of calcar conspicuous. Tarsal glands not visible. Mesopleural gland visible and set in a narrow circular notch.

Notes

This group shows a morphocline of shortening mandibles. At the longest (sylvaini, MI 28-31; carolinae, MI 29-33) the species are more or less “ordinary” Strumigenys, but at the other end the mandibles are shorter and more powerfully constructed (dexis, MI 17-20), closely resembling short-mandibulate groups of Pyramica. However, even at this end of the morphocline the species retain the apomorphies of Strumigenys. It appears that the members of the group are evolving to fill the “powerfully short-mandibulate Pyramica” niche, which is almost empty in the Malagasy region.

The structure of the mandibles and their dentition is immediately diagnostic of the dexis-group. The presence of a rim or carina that traverses a basal concavity of the mandibles and the asymmetric mandibles are apomorphies for the group.

Members of this group fall into two discrete complexes of related species.

dexis complex

Cephalic dorsum with scattered punctures that are separated by areas of smooth unsculptured cuticle. Promesonotal dorsum with sculpture either the same as the head or even more sparsely punctate and with more extensive smooth areas. Blanketing fine dense reticulate-punctate to reticulate-granulate sculpture never present.

sylvaini complex

Cephalic dorsum blanketed with fine dense reticulate-punctate to reticulate-granulate sculpture and opaque. Promesonotal dorsum blanketed with the same sculpture except in carolinae. Scattered punctures separated by smooth shiny areas never present on head and promesonotum.

References

Bolton, B. 2000. The ant tribe Dacetini. Memoirs of the American Entomological Institute. 65:1-1028.