Rhopalomastix

Rhopalomastix workers chew a network of tunnels under the bark of living trees where they nest. Tunnels are inhabited by large numbers of diaspidid scale insects (Yong et al. 2019). Non-specific associations with different genera of trees and diaspidids were recorded in SIngapore, Thailand and Okinawa (Japan) (C. Peeters unpublished). Rhopalomastix is the second ant genus involved in a mutualism with diaspidids, besides Melissotarsus.

Eguchi, Bui and Yamane (2011) - The worker of Rhopalomastix is easily separated from those of other known Vietnamese myrmicine genera by a combination of the following characters: frontal lobes closely approximated; eye relatively large; mesosoma box-shaped; fore- and hindfemora extremely widened and flattened distally; postpetiole broadly attached to gaster.

Identification
Wang, Yong, and Jaitrong (2018) - Rhopalomastix species in Southeast Asia can roughly be separated into two broad species groups based wholly on morphological similarities; actual phylogenetic relationships may be established in the future when additional genetic markers are available.

Rhopalomastix murphyi group—Relatively small size across all castes; workers monomorphic with little intranidal size variation; head, body surfaces, and femora mostly completely smooth and shining, and/or with fine or superficial sculpture. Two novel species from Southeast Asia belong to this group: Rhopalomastix murphyi and Rhopalomastix glabricephala.

Rhopalomastix rothneyi group—Generally larger in size compared to the murphyi group across all castes; workers monomorphic, tend to exhibit broad but isometric size variation, with allometric variation in minor characters; head, body surfaces, and femora mostly strongly sculptured, sometimes superficial, few or no surfaces entirely smooth and shining. Four species from Southeast Asia fit these criteria: Rhopalomastix johorensis, Rhopalomastix javana, Rhopalomastix striata, and Rhopalomastix tenebra.

Distribution
Wang, Yong, and Jaitrong (2018) - It is striking that so many species of this rather obscure genus can be found in Singapore, a small country of severely limited land area. The existence of multiple species in Singapore alone implies a strong possibility of many more species of Rhopalomastix awaiting discovery across the Southeast Asian region.

Species by Region
Number of species within biogeographic regions, along with the total number of species for each region.

Biology
Yong et al. (2019) - The presence of diaspidid scale insects inside all nests examined is a strong suggestion that they are a source of food for the ants. However, there are no direct observations of feeding or other interactions because ants stop normal activities and take cover as soon as the tunnels are opened. The five genera of diaspidids associated with four species of Rhopalomastix in Singapore all share the remarkable characteristic that females can reach sexual maturity without secreting a hard wax shield. As in Melissotarsus, Rhopalomastix adult females can secrete silk that is used to secure the tunnels. In contrast, Rhopalomastix workers have a sting and lack the extreme leg specialisation legs,



Wang, Yong, and Jaitrong (2018) - While we cannot confirm from limited samples in this study that Rhopalomastix species are specialized colonists of particular species or families of trees, we observed that trees where Rhopalomastix can be found are typically of wide girth, with relatively thick layers of bark, inner layers often succulent and fairly moist. Trees with dry, cracked layers of bark are unlikely to harbour Rhopalomastix. It is also notable that many colonies sampled in this study were found living in close association with armored scale insects (diaspidids) (G.W.J. Yong, pers. comm.). It has been surmised that the ants rear these scale insects in their nests, and feed on proteins and/or waxy compounds secreted by the latter to build their protective armour or shields (Peeters et al. 2017; Yong et al. 2018, in press). Mutualistic relationships between ants and diaspidids have been observed and recorded in Melissotarsus, the Afrotropical cousin of Rhopalomastix from the same tribe (Peeters et al. 2017). Much more awaits to be uncovered on the biology, behaviour, and systematics of this unique but poorly-studied bark-dwelling ant genus in Southeast Asia.

Nomenclature

 *  RHOPALOMASTIX [Myrmicinae: Melissotarsini]
 * Rhopalomastix Forel, 1900a: 24. Type-species: Rhopalomastix rothneyi, by monotypy.

Wang, Yong, and Jaitrong (2018)

Worker Total length approximately 1.59–2.62 mm, HL 0.39–0.58, HW 0.36–0.56. Worker monomorphic with variation in size. Head in full-face view subrectangular or subquadrate; frontal carina and antennal scrobe weak; frontal lobes touching or separated only by a narrow longitudinal impression. Median portion of anterior clypeal margin weakly convex anteriorly and projected forward slightly from lateral clypeal margin, lined with minute denticles. Mandible short, subtrapezoidal, its masticatory margin with a few teeth, usually one large apical tooth, a smaller pre-apical tooth, and 2-3 minute, reduced denticles. Antenna with 10 segments, antennal club 2-segmented; antennal scape short, extending a little beyond midlength of head when laid backwards; antennal funiculus flattened in appearance. Eye relatively large, with 5–6 ommatidia in the longest axis, located well in front of midlength of side of head. Mesosoma box-shaped; promesonotal suture absent dorsally; metanotal groove absent; propodeum unarmed; metapleural gland bulla large; propodeal lobe reduced and indistinct. Procoxa subtriangular and anteriorly expanded into smooth and rounded lobe. Fore- and hind femora extremely widened and flattened distally. Petiole nodiform, with relatively high node, anteroventrally with well-developed subpetiolar process, globular in dorsal view. Postpetiole much shorter than high, broadly attached to anteriormost portion of gaster. Gaster in dorsal view elliptical in shape, first gastral tergite much larger compared to the rest.

Queen Differing from associated workers in the following characteristics: body slightly larger; eye very large in size (>100 ommatidia); ocelli present; promesonotal suture deeply impressed; metapleuron large, in profile view rounded subrectangular, with distinct metapleural furrow that is oblique and linear, directed posterodorsally. Sculpturing on head usually weaker than in worker. Clypeus usually with two pairs of erect hairs—one pair of setae on the median anterior clypeal margin, another pair of longer hairs closer to the posterior clypeal margin.

Male Head including eye in full-face view rounded or broadly ovate. Eye extremely large (>100 ommatidia) and bulging; ocelli relatively larger than in queen. Mandible reduced and nub-like; clypeus in profile strongly projected from anterior margin of head. Ventrolateral corner of head roundly convex. Antenna 12-segmented, without obvious antennal club, scape very short. Mesosoma elongate, not box-shaped. Petiole sometimes similar to that in worker but relatively longer than high, anterior margin always longer than posterior, in dorsal view, petiole including anterior face subcylindrical. Subpetiolar process besides distal anteroventral point typically lighter in colour and more translucent than adjacent sclerites. Postpetiole relatively flat, broadly attached to anteriormost portion of gaster; gaster elliptical in dorsal view. Hairs on dorsum of mesosoma usually most abundant on mesoscutum, sparser or absent on mesoscutal disc and propodeum.