Strumigenys gyges group

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Strumigenys gyges group Bolton (2000)

Species

Malesian-Oriental-East Palaeartic

Worker Diagnosis

Mandibles in full-face view and at full closure elongate-triangular; teeth occupy only the apical half of the mandible but engage through the lengths of the tooth rows. Basal halves edentate and with a large space between them, through which the labral lobes are visible. Close to the clypeus the dorsum of each mandible i s traversed by a weak oblique rim that meets the masticatory margin considerably proximal of the basal tooth. Inventral view outer margin of mandible with a weak prebasal inflection. MI 32-36.

Dentition. Basally with a dental row of 5 low, acute, triangular teeth, followed by 2 slightly shorter teeth, a row of 5 denticles, a preapical tooth that is almost as large as teeth 1-5, and an apical tooth that is markedly enlarged, broad-based, curved and acute, by far the longest and stoutest tooth on the margin. Total dental count 14.

Basal lamella an elongate, very narrow, laminate strip that follows the inner curvature of the mandible. The basal lamella is almost as long as the principal basal dental row but is very much lower than the basal tooth. Lamella visible in full-face view with the mandibles fully closed, terminating just before the basal tooth.

Labrum terminates in a pair of short triangular narrow lobes.

Clypeus with anterior margin broad, evenly shallowly convex across its width. Sides of clypeus proper very short, merging into the preocular ridge almost immediately behind the anterolateral angles.

Clypeal dorsum with minute appressed hairs, the clypeal margins with minute curved hairs; freely projecting specialised hairs do not occur anywhere on the sclerite.

Preocular carina conspicuous in full-face view.

Ventrolateral margin of head extremely poorly developed between eye and mandible, scarcely angular between side and ventre. Postbuccal impression small.

Cuticle of side of head within scrobe reticulate.

Scape short, SI 57-60, broadest at about the basal third and the leading edge rounded where the scape is broadest; proximal to this the scape suddenly narrowing to the base.

Leading edge of scape with 1-2 approximately straight anteriorly projecting hairs near the greatest curvature, the edge from this point to the apex with a row of small, spatulate hairs that curve toward the apex of the scape.

Pronotal dorsum without a median longitudinal carina.

Propodeum unarmed, without trace of spines or teeth but with a distinct narrow lamella that extends the full height of the declivity. Propodeum in profile with its dorsal outline high anteriorly, sloping steeply downward posteriorly.

Spongiform appendages of petiole and postpetiole reduced, small and inconspicuous. Base of first gastral sternite with a weakly developed narrow crest of spongiform tissue. Pilosity. Pronotal humeral hair absent. Erect hairs sparsely present on vertex and first gastral tergite, absent from dorsal surfaces of alitrunk and waist segments. Ground pilosity of head and pronotum of inconspicuous, minute curved hairs. Dorsal (outer) surfaces of middle and hind tibiae with minute apically directed hairs, without elongate freely projecting pilosity.

Notes

Currently this group contains only a single species, Strumigenys gyges, from West Malaysia. The group diagnostic characters above are augmented by the fact that gyges is the only known member of the genus worldwide to have 5-segmented antennae. This has not been included in the diagnosis as groups are known where member-species have a variable antennomere count of 4 or 6, so taxa referable to the gyges-group but with a different number of antennal segments may well await discovery.

At first glance the odd mandibular development described above, with a marked gap between the basal portions when the mandibles are fully closed, seems to ally gyges with the mutica group and extemena group, where a similar development can be seen. However, close examination of mandibular morphology shows that the feature has evolved independently in each case, although the mechanism is similar in the gyges-group and mutica-group, and quite different in the extemena-group.

In both the gyges-group and mutica-group the entire tooth rows of the opposing mandibles engage at full closure and terminate at the apex of the gap: the gap has evolved proximal to the rows of teeth. In the extemena-group the tooth rows extend almost to the clypeal margin and the basal series of teeth do not meet at full closure: the gap has evolved between the masticatory margins of the two mandibles and the basal sections of the tooth rows border each side of the gap. Members of the extemena-group have increased the tooth row to 22-24, to extend the prey-seizing length of the margin. In the mutica-group the tooth row retains the basic Pyramica count of 12, whilst gyges shows a slight increase to 14. The mechanism producing the type of gap evolved by the extemena-group has also evolved independently several other times within the genus, for instance in canina of the sauteri-group, in the Afrotropical terroni-group and in some New World taxa.

Difference in development of the gap between the mutica-group and the gyges-group is more subtle. In the mutica-group the basal lamella of the mandible remains well developed and high, separated from the basal tooth by a long concave diastema. This implies that the gap developed by elongation of the margin (and consequently of the diastema) distal of the lamella whilst the basal lamella and tooth row remained without gross modification. In gyges the basal lamella is a long, narrow strip of cuticle that terminates very close to the basal tooth. The implication here is that elongation of the margin has taken place at the site of the basal lamella rather than distal of it, which has prevented the development of a long diastema.

References

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