Royidris

Endemic to Madagascar, many species in the genus Royidris are found in dry habitats (spine forest, dry forest). Specimens have most commonly been collected from litter samples, pitfall traps, under rocks and in various types of dead downed wood. Biology details are largely unknown for all of the species in the genus.

Identification
The 15 endemic Madagascan species included in this genus exhibit a habitus that is convergent on some groups of Monomorium. In his study of the Afrotropical members of that genus Bolton (1987: 290) noted two indeterminate Madagascan species which had a high palp formula (5,3), the highest attributed to Monomorium, but did no further analysis of these odd species because the focus of the survey was the extensive Afrotropical fauna. Heterick (2006), in his revision of the Malagasy species of Monomorium, recognised the peculiarity of the high palp formula and utilised it, together with some other characters, to define his M. shuckardi group, all members of which are now transferred to Royidris.

At present, no unambiguous apomorphy can be stated for Royidris, and in fact its habitus is similar to that commonly seen in Monomorium. However, a note of caution must be introduced, because no apomorphies can be cited for Monomorium in its currently accepted form, and it is becoming increasingly obvious that it is polyphyletic. A full survey of its present components, with more detailed morphological and molecular analyses, will be essential if the unwieldy Monomorium is ever to be divided into meaningful monophyletic units that can be awarded generic status.

Members of Royidris can be separated from Monomorium by the presence in Royidris of all the seven following characters in combination: palp formula 5,3; mandible with 5 teeth; antenna 12-segmented, with a club of 3 or 4 segments; propodeal spiracle in profile close to dorsal surface of propodeum; propodeal dorsum usually without setae (one or more pairs of short setae may be present in two species); petiolar spiracle close to midlength of peduncle (not at the node); first gastral tergite does not strongly overlap the sternite on the ventral surface of the gaster.

In Madagascar, and also in Africa, only one Monomorium species, the tramp M. latinode Mayr, has 5 mandibular teeth. However, M. latinode has a palp formula of 3,3, unsculptured mandibles, the eye situated conspicuously far in front of the midlength of the head capsule, the propodeal dorsum transversely costulate and with numerous setae present, the petiolar spiracle at the node, and a first gastral tergite that strongly overlaps the sternite ventrally.

Species Groups

 * Royidris species groups

This genus is one of a number of closely related Myrmicinae genera - the Eutetramorium group - from Madagascar.

Nomenclature

 *  ROYIDRIS [Myrmicinae: Myrmicini]
 * Royidris Bolton & Fisher, 2014: 39. Type-species: Monomorium robertsoni, by original designation.

Worker
Monomorphic myrmicine ants.

Mandible triangular, masticatory margin with 5 teeth, only slightly longer than basal margin.

Palp formula 5,3.

Stipes of maxilla with a vestigial transverse crest or without a crest.

Clypeus posteriorly moderately broadly inserted between the frontal lobes (width of clypeus between the lobes greater than width of one of the lobes); median portion of clypeus with a fine, weak, longitudinal rugulae on each side, the median longitudinal strip unsculptured.

Clypeus with a stout unpaired seta at the midpoint of the anterior margin.

Clypeus with lateral portions not raised into a shielding wall or sharp ridge in front of the antennal sockets.

Frontal carinae short, restricted to well defined but narrow frontal lobes.

Antennal scrobes absent.

Antenna with 12 segments, with an apical club of 3 or 4 segments.

Torulus with upper lobe visible in full-face view.

Eyes present, relatively large, located at about the midlength of the head capsule, or slightly in front of the midlength.

Head capsule without a median longitudinal carina; occipital carina terminates in a distinct ventral prominence in posterodorsal view.

Pronotal humeri rounded in dorsal view.

Pronotum plus anterior mesonotum often swollen and distinctly convex in profile, so that the dorsalmost point of promesonotum is on a considerably higher level than propodeal dorsum (not in admixta group).

Promesonotal suture absent.

Propodeum unarmed, dorsum and declivity separated by a blunt angle; propodeal lobes small and rounded.

Propodeal spiracle at about the midlength of the sclerite and close to the dorsal margin, far in front of the margin of the declivity and separated from apex of metapleural gland bulla by more than the spiracle’s diameter.

Metasternal process absent.

Femora usually incrassate: strongly swollen medially, distinctly tapered proximally and distally.

Tibial spurs: mesotibia 1; metatibia 1; spurs simple.

Abdominal segment 2 (petiole) with an anterior peduncle; spiracle slightly behind the midlength of the peduncle.

Subpetiolar process a minute crest.

Abdominal segment 3 (postpetiole) not dorsoventrally flattened in profile, about as high as broad.

Stridulitrum present on pretergite of abdominal segment 4.

Abdominal tergite 4 (first gastral) does not overlap the sternite on the ventral surface of the gaster; gastral shoulders absent to weakly present.

Sting present, usually weakly developed.

Main pilosity of dorsal head and body: simple, usually absent from propodeal dorsum.

Queen
Known for admixta, diminuta, notorthotenes, peregrina, and shuckardi, plus two unassociated forms. Alate when virgin, considerably larger than the worker. Characters as worker except for those of the mesosoma; with ocelli developed and a full complement of flight sclerites present. Pronotum in profile is slightly overhung by the convex anterior margin of the mesoscutum; in dorsal view the pronotum is not visible anterior to the mesoscutum, but projects beyond the mesoscutum anterolaterally. Propodeum in profile rounded or with an extremely obtuse angle in diminuta and notorthotenes, in all others with a pair of short, stout, broad-based triangular teeth or prominent angles. Venation: see under male.

Male
Known only for notorthotenes and peregrina. About the same size as the worker or slightly smaller, much smaller than the queen. Mandible reduced, short and narrow, with only 2–3 teeth. Palp formula 5,3. Stipital crest absent. Antenna 13-segmented, filiform. SI 30–35. First funicular segment subglobular, about as broad as long and about 0.50–0.60 × the length of the second funicular segment. Eyes large, at or in front of the midlength of the sides. Ocelli large, on a low turret in notorthotenes; in full-face view the potsterior ocelli at the posterior margin of the head. Sides of head behind eyes converge strongly to the posterior ocelli. Mesoscutum in profile strongly overhangs the pronotum, the latter not visible in dorsal view. Notauli feeble or with well developed anterior arms that form a V-shape. Mesopleuron with transverse sulcus present. Mesotibia and metatibia each with a single small, simple spur. Propodeum unarmed and rounded; propodeal lobes rounded. Subpetiolar process minute to vestigial. Cerci present.

Forewing venation. Rs·f4–5 does not meet R·f3 on anterior margin of wing (= marginal cell open). 2rs-m absent. 1m-cu absent (Rs+M extends without any intersecting cross-vein, from the junction of Rs·f1 and M·f1 proximally, to the division of Rs+M into Rs·f3 and M·f3–4 distally; this division is proximal of the junction with 2r-rs). In one notorthotenes queen a stub of 1m-cu is present on the left wing, but not on the right; in another queen a stub occurs on the right wing but not on the left; entirely absent in all other alate material. A·f2 not merely a stub distal of cu-a, the latter retracted toward the wing base and arises from M+Cu, proximal of the point where M+Cu divides into M·f1 and Cu·f1–2.