Royidris robertsoni

AntWiki: The Ants --- Online
Royidris robertsoni
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Subfamily: Myrmicinae
Tribe: Crematogastrini
Genus: Royidris
Species group: robertsoni
Species: R. robertsoni
Binomial name
Royidris robertsoni
(Heterick, 2006)

Monomorium robertsoni casent0035010 profile 1.jpg

Monomorium robertsoni casent0035010 dorsal 1.jpg

Specimen labels

Collections have been made in sifted litter in spiny forest and associated thickets, in a pitfall trap at the base of a baobab, and in a malaise trap. The latter capture suggests this species may forage arboreally.


A member of the robertsoni species group. Bolton and Fisher (2014) - The most densely and universally sculptured member of the robertsoni group, this species can be immediately characterised by the long, very smoothly rounded, even transition from propodeal dorsum to declivity.

Keys including this Species


Endemic to Madagascar.

Latitudinal Distribution Pattern

Latitudinal Range: 23.15° to -23.52722222°.

Tropical South

Distribution based on Regional Taxon Lists

Malagasy Region: Madagascar (type locality).

Distribution based on AntMaps


Distribution based on AntWeb specimens

Check data from AntWeb

Countries Occupied

Number of countries occupied by this species based on AntWiki Regional Taxon Lists. In general, fewer countries occupied indicates a narrower range, while more countries indicates a more widespread species.

Estimated Abundance

Relative abundance based on number of AntMaps records per species (this species within the purple bar). Fewer records (to the left) indicates a less abundant/encountered species while more records (to the right) indicates more abundant/encountered species.



Known only from the worker caste.

Images from AntWeb

Monomorium robertsoni casent0077935 head 1.jpgMonomorium robertsoni casent0077935 profile 1.jpgMonomorium robertsoni casent0077935 dorsal 1.jpgMonomorium robertsoni casent0077935 label 1.jpg
Worker. Specimen code casent0077935. Photographer April Nobile, uploaded by California Academy of Sciences. Owned by CAS, San Francisco, CA, USA.


The following information is derived from Barry Bolton's Online Catalogue of the Ants of the World.

  • robertsoni. Monomorium robertsoni Heterick, 2006: 93, figs. 14, 37 (w.) MADAGASCAR.
    • Type-material: holotype worker, 3 paratype workers.
    • Type-locality: Madagascar: Prov. Toliara, Ranobe, 23°02’03’’S, 43°36’43’’E, 30 m., 13-19.ii.2002, Frontier Project, MGF 056 (5), CASENT0003501 (hlotype), sifted litter, spiny forest thicket (no collector’s name); paratypes: 1 worker with same data but MGF 054, 2 workers Prov. Toliara, Mandarano, 23°31’38’’S, 44°05’15’’E, 70 m., 10.v.2002, Frontier Project, MGF 030, sifted litter, gallery forest (no collector’s name).
    • Type-depositories: CASC (holotype); BMNH, CASC, MCZC (paratypes).
    • Combination in Royidris: Bolton & Fisher, 2014: 64.
    • Status as species: Bolton & Fisher, 2014: 64 (redescription).
    • Distribution: Madagascar.

Unless otherwise noted the text for the remainder of this section is reported from the publication that includes the original description.



Holotypes: HML 1.50 HL 0.53 HW 0.45 CeI 86 SL 0.45 SI 100 PW 0.32. Paratypes: HML 1.50–1.82; HL 0.53–0.61; HW 0.45–0.54; CeI 83–88; SL 0.45–0.52; SI 97–104; PW 0.32–0.40 (n=3). Non types: HML 1.49–1.67; HL 0.52–0.58; HW 0.43–0.49; CeI 81–84; SL 0.44–0.51; SI 99–107; PW 0.32–0.36 (n=4).

HEAD: Head square; vertex planar; frons finely striolate throughout; pilosity of frons a mixture of well-spaced, erect and semi-erect setae interspersed with appressed setulae only. Eye large, eye width 1.5x greater than greatest width of antennal scape; (in full-face view) eyes set at about midpoint of head capsule; (viewed in profile) eyes set around midline of head. Antennal segments 12; club three-segmented. Clypeal carinae indistinct; anteromedian clypeal margin broadly convex; paraclypeal setae moderately long and fine, curved; posteromedian clypeal margin approximately level with antennal fossae. Anterior tentorial pits situated nearer antennal fossae than mandibular insertions. Frontal lobes straight, parallel. Psammophore absent. Palp formula 5,3. Mandibular teeth five; mandibles triangular and striate; masticatory margin of mandibles approximately vertical or weakly oblique; basal tooth approximately same size as t4 (five teeth present).

MESOSOMA: Promesonotum finely striolate throughout; in profile broadly convex anteriad, convexity reduced posteriad; promesonotal setae seven to twelve; standing promesonotal setae consisting of short, erect or semi-erect bristles; appressed promesonotal setulae few, mainly on dorsum of promesonotum. Metanotal groove vestigial. Propodeum uniformly finely striolate; propodeal dorsum flat throughout most of its length; always smoothly rounded, no separation between dorsal and declivitous faces when seen in profile; standing propodeal setae absent; appressed propodeal setae very scarce or absent; propodeal spiracle nearer declivitous face of propodeum than metanotal groove; vestibule of propodeal spiracle absent or not visible; propodeal lobes present as rounded flanges.

PETIOLE AND POSTPETIOLE: Petiolar spiracle laterodorsal and situated slightly anteriad of petiolar node. Node (viewed in profile) conical, vertex tapered; appearance of node shining and smooth throughout; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) about 1:1. Anteroventral petiolar process absent or vestigial; ventral petiolar lobe absent. Height ratio of petiole to postpetiole between 4:3 and 1:1; height–length ratio of postpetiole about 1:1; postpetiole shining and smooth; postpetiolar sternite depressed near its junction with gaster, and sloping anteriad at angle of 45–60 to form large conspicuous lip at its anterior end.

GASTER: Pilosity of first gastral tergite consisting mainly of short, appressed setulae, together with a few erect and semi-erect setae.

GENERAL CHARACTERS: Color of head, mesosoma and nodes orange, legs, mandibles and apical portion of gaster straw-colored, basal portion of gaster brown. Worker caste monomorphic.

Bolton and Fisher (2014) - TL 2.4–2.6, HL 0.56–0.62, HW 0.46–0.54, CI 82–87, SL 0.50–0.54, SI 100–109, PW 0.34–0.38, WL 0.66–0.78 (4 measured).

Antennal club 3-segmented. Mandible longitudinally costulate. Scapes of short to relatively moderate length (SI 100–109); when laid straight back in full-face view the apex of the scape just exceeds the posterior margin of the head. With head in full-face view the eyes very close to the midlength of the side of the head capsule. EL 0.12–0.14 (EL/HW 0.24–0.28). Dorsum of head to the level of the posterior margins of the eyes finely, densely shagreenate, with some weak, superficial striolae, the whole surface appearing silky; behind the level of the eyes, close to the posterior margin, the sculpture is mainly of extremely fine, dense transverse striolae. Promesonotum in profile convex and swollen, the mesonotum posteriorly descends abruptly to the commencement of the propodeum; there is no strongly defined, V-shaped metanotal groove. In absolute profile the propodeal dorsum slopes downward posteriorly and rounds evenly into the declivity through a broad, shallow curve, without an angle between dorsum and declivity; the declivity itself continues the outward slope posteriorly, its outline not vertical or concave. The whole propodeal dorsum is strongly depressed with respect to the highest point of the promesonotum, which is on a considerably higher level. Pronotum with 1–3 pairs of setae (usually 2); mesonotum with 2 pairs; dorsum of propodeum lacks setae. Promesonotal dorsum shagreenate-striolate; propodeal dorsum shagreenate, sometimes also with feeble striolae present. Side of mesosoma finely shagreenate everywhere, the sculpture tends to be weakest on the side of the pronotum, coarser and sometimes somewhat punctulate on the mesopleuron, and extremely fine and dense on the metapleuron, where it is usually distinctly finer than on the mesopleuron. Metafemur with MfL 0.50–0.58 (MfL/HW 1.07–1.09, MfL/MfH 3.86–4.50). Petiole with a single pair of setae posteriorly, near the apex of the node; postpetiole with setae dorsally and posteriorly; first gastral tergite with numerous suberect setae. Postpetiole relatively narrow in dorsal view, maximum width 0.17–0.20 (0.37 × HW). Dorsum of postpetiole and first gastral tergite unsculptured. Colour uniformly yellow, or the gaster may be paler or partially infuscated.

Type Material

Holotype: Prov. Toliara, Ranobe, 23°02′03″S, 43°36′43″E, 30 m., 13–19.ii.2002, Frontier Project, MGF056/sifted litter spiny forest/thicket/ CASENT 0003501 MGF056(5) (California Academy of Sciences). Paratypes: Prov. Toliara: one worker with same collection data as holotype, collection code MGF054 (Museum of Comparative Zoology); two workers, Mandarano [‘Manderano’], 23°31′38″S, 44°05′15″E, 70m, 10.v.2002 Frontier Project, MGF030/ sifted litter gallery forest (1worker - CAS) (1 worker – The Natural History Museum).


In honour of Dr. Hamish Robertson (South African Museum).


References based on Global Ant Biodiversity Informatics

  • Bolton B., and B. L. Fisher. 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): 1-99.