Royidris species groups

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These groups are based on: Bolton, B. & Fisher, B.L. 2014. The Madagascan endemic myrmicine ants related to Eutetramorium (Hymenoptera: Formicidae): taxonomy of the genera Eutetramorium Emery, Malagidris nom. n., Myrmisaraka gen. n., Royidris gen. n., and Vitsika gen. n. Zootaxa 3791:1–99. doi:10.11646/zootaxa.3791.1.1

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Three groups of species can be isolated on worker morphological grounds within the genus. The groups are currently defined for convenience and ease of identification, and do not necessarily reflect accurate phylogeny.

admixta species group

Antennal club of 3 segments. Promesonotum not swollen or domed: in profile the dorsum flat or nearly so, evenly sloping posteriorly to the metanotal groove. Propodeal dorsum behind the metanotal groove approximately continues the line of the promesonotal outline; the propodeal dorsum is not considerably depressed below the level of the promesonotum. Scapes and metafemora in the group are among the shortest in the genus (SI 90–95, MfL/HW 0.90–0.95), and the postpetiole is the broadest (maximum dorsal width 0.42–0.48 × HW). In all other species combined, these measurements show ranges of SI 90–158 (SI < 100 only in some workers of gravipuncta and clarinodis), MfL/HW 0.95–1.50 (again, MfL/HW < 1.00 only in some workers of gravipuncta and clarinodis), maximum dorsal width of postpetiole 0.27–0.39 × HW (outside the admixta group 0.39 × HW is achieved only in longiseta).

The two species included here form a closely related pair, separated by differences in sculpture and pilosity. Both were inexplicably included as conspecific with clarinodis by Heterick (2006), whose description obviously included elements of several species that are recognised as separate here.

robertsoni species group

Antennal club of 3 segments. Promesonotum swollen or domed: in profile the dorsum convex and appearing swollen, the posterior outline of the mesonotum descends steeply, and usually abruptly, to the metanotal groove (or the junction with the propodeum in those species where the groove is suppressed). Propodeal dorsum behind the metanotal groove horizontal or sloping, decidedly not continuing the line of the promesonotal outline; the propodeal dorsum is strikingly depressed below the level of the promesonotal dorsum. The scape is generally shorter in this group (SI 90–110) than in the notorthotenes group (SI 95–158). In the robertsoni group the only workers with SI < 95 are some individuals of clarinodis, while in the notorthotenes group only some individuals of gravipuncta have SI < 100.

Together with the two species now placed in the admixta group, four of the five names included in this group were regarded as conspecific with clarinodis by Heterick (2006); only robertsoni was treated by him as a species separate from clarinodis. The group falls into two uneven complexes of species depending on whether the metanotal groove is conspicuously retained and the eyes are obviously in front of the midlength of the side of the head capsule (clarinodis), or the metanotal groove is suppressed and the eyes are at, or very close to, the midlength (anxietas, pallida, pulchra, robertsoni).

notorthotenes species group

Antennal club of 4 segments. Promesonotum domed: in profile the dorsum convex and appearing swollen, the posterior outline of the mesonotum descends steeply, and usually abruptly, to the metanotal groove. Propodeal dorsum behind the metanotal groove horizontal or nearly so, decidedly not continuing the line of the promesonotal outline; the propodeal dorsum is strikingly depressed below the level of the promesonotal dorsum.

Both of the species that Heterick (2006) recognised, notorthotenes and shuckardi, are now considered to be compound, with diminuta and etiolata being split from his concept of notorthotenes, and longiseta and gravipuncta split from his concept of shuckardi. The two final species, singularis and peregrina, had not been discovered at the time of his study. Three complexes of species can be identified within the group. The first complex includes longiseta, gravipuncta and peregrina, and is characterised by denser, heavier cephalic sculpture, darker colour, relatively shorter scapes (SI 95–117), head that averages relatively broader (CI 82–89), and shorter metafemora (MfL/HW 0.95–1.13). The second complex, which consists of diminuta, etiolata and notorthotenes, has cephalic sculpture that is weak to vestigial, uniform yellow in colour, relatively longer scapes (SI 115–158), head that averages relatively narrower (CI 75–84), and longer metafemora (MfL/HW 1.25–1.50). The final complex, which contains only singularis, exhibits a mixture of the features of the other two complexes. Its sculpture is largely suppressed (the head and dorsal pronotum are glassy smooth), its scapes are long (SI 121), and its body is darkly coloured; it has MfL/HW 1.13, at the top end of the range of the gravipuncta complex but well below that of notorthotenes and its relatives. R. shuckardi cannot be placed in any complex as it is known only from a single queen, but its 4-segmented antennal club indicates its placement within this species group.