Mystrium

Yoshimura, Masashi & Fisher, Brian L., 2014, A revision of the ant genus Mystrium in the Malagasy region with description of six new species and remarks on Amblyopone and Stigmatomma (Hymenoptera, Formicidae, Amblyoponinae), ZooKeys 394, pp. 1-99 : 6-12

Genus Mystrium

Mystrium Roger, 1862: 245. Type-species: Mystrium mysticum , by monotypy.

Mystrium in Ponerinae : Mayr 1862: 715 [in key.]

Mystrium in Amblyoponinae , Amblyoponini : Forel 1893: 162; Bolton 2003: 42, 155.

Mystrium in Ponerinae , Amblyoponini : Emery 1895: 766.

Diagnosis of female.

The characters uniquely observed in Mystrium within the subfamily Amblyoponinae are given in italics.

Compound eye present.

Posterior margin of head strongly expands posteriorly on each side.

Anterior margin of clypeus with specialized conical setae.

Spatulate setae present on clypeus mesal of mandible insertion (Fig. 4A).

Labrum lacking small dentiform setae arranged horizontally.

Palpal formula 4,3.

Distinct extension present on the distal edge of second labial palpomere (Fig. 4B).

Mandible linear without a distinct basal angle.

Masticatory margin of mandible with two rows of projections: true mandibular teeth on dorsal row and a series of basal denticles on the ventral row; basal ventral denticles larger than true mandibular teeth except for apical teeth.

At midlength of mandible, row of basal denticles arranged on ventral edge of mandibular shaft, distant from mandibular teeth (Fig. 4C).

Mandible twisted inward so that apical tooth is located ventrally (Fig. 4D).

No basal large projection present on basal portion of mandibular inner margin.

Ridge present on apical portion of mandible, which is originally dividing ventral and inner surfaces of mandible, inserting dorsally to the apical tooth (Fig. 5A vs. 5B: Stigmatomma mystriops Brown, 1960, see comments).

Constriction between petiole and abdominal segment III present.

Cinctus 3, constriction between pre- and post-sclerites on abdominal segment IV, distinctly present.

One or two stout spines present on posterior portion of abdominal sternum VII (Fig. 5C).

Body surface with spatulate to squamose setae in some workers of all species.

Diagnosis of male.

The diagnostic characters uniquely observed in Mystrium within the subfamily Amblyoponinae are given in italics.

Frontal carinae present.

Anterior margin (=free margin) of clypeus with specialized conical setae.

Antenna consisting of 13 segments.

Mandible with single, blunt apical tooth.

Palpal formula 4,3.

Notaulus distinct or absent.

Mesepimeron often lacking distinct posterodorsal lobe (epimeral lobe).

Mesotibia with one or two spurs in most cases, rarely indistinct.

Metatibia with two spurs.

Distinct constriction present between petiole and abdominal segment III in dorsal view.

Abdominal segment IV with tergosternal fusion.

Pretergite of abdominal segment IV distinctly differentiated from posttergite, with the cinctus between them.

Pygostyles absent.

Distal margin of abdominal sternum IX convex.

Separation between basimere and telomere distinct.

Basal projection on cuspis well developed.

Basoventral portion of the aedeagus in lateral view extended basally, distal margin of extension rounded (Fig. 5D).

Serrate denticles present on basal portion of ventral margin of aedeagus in lateral view.

Pterostigma well developed on forewing.

Radial sector on forewing fully present.

Radial sector on forewing reaches costal margin.

2r-rs on forewing connected with radial sector posterior to pterostigma.

2rs-m present on forewing.

Position of cu-a on forewing variable, close to or far from junction between media and cubitus.

Radius present on hindwing.

1rs-m present on hindwing.

Media on hindwing present apical to 1rs-m.

Comments on generic diagnosis.

The workers of the genus Mystrium can be distinguished easily from those of the other amblyoponine genera by their characteristic head shape and mandibles (Fig. 1). Mystrium has a wide head with posterior margin strongly concave, mandible linear and longer than head with rounded apex in full-face view, and two separated rows of “teeth” on the masticatory margin. In this study, we propose further detailed characters as the result of our comparative study. All Mystrium females share six characters unique to this genus which distinguish it from the other amblyoponine genera: spatulate setae on clypeus mesal of the mandible insertion (character 4: Fig. 4A), a distinct extension on the distal edge of the second segment on the labial palp (character 6: Fig. 4B), mandibular teeth distant from basal denticles (character 10: Fig. 4C), twisted mandible (character 11: Fig. 4D), a ridge on the apical portion of the mandible inserting dorsally to the apical tooth (character 13: 5A), and one or two lateral spines on abdominal sternum VII (character 16: 5C). Character 10, mandible teeth and basal denticles arranged in two rows (Fig. 4C), was used in Bolton (1994) to separate Mystrium from Amblyopone and Stigmatomma , both of which were in Amblyopone at that time. For character 11, Keller (2011: character 29) described the mandibular type as “torqued.” Although all amblyoponine genera share this character state, the mandible in Mystrium is unique: in females the mandible is further twisted so that the apical tooth of queens and minors is directed ventrally (Fig. 4D). For character 13, the ridge (= hump in Gronenberg et al. 1998) on the mandible functions as a pivot when the mandibles snap ( Gronenberg et al. 1998). We confirmed that all females have this ridge, although it is uncertain if all females can snap their mandibles regardless of the shape of the apical tooth (see description for each species group). For character 16, we have confirmed that some species of Stigmatomma have three to nine lateral spines on abdominal sternum VII, while neither Adetomyrma nor Xymmer have this spine.

Brown (1960) described Stigmatomma mystriops (Brown, 1960) as a species that has specialized mandibular characters similar to Mystrium , e.g. teeth arranged in two separated rows, the presence of a ventral ridge on the subapical teeth, and a small apical tooth; however, these characters differ distinctly between Stigmatomma mystriops and Mystrium species. The mandibular teeth and basal denticles are consistently separated and these two rows are almost parallel, except for the apical portion of the mandible in Mystrium , while the separation exists only on the basal portion of the mandible in Stigmatomma mystriops . The ridge in Mystrium is the extension of a carina, which usually divides the lateral and ventral surfaces of the mandible, and the extension of the carina in Mystrium is inserted dorsally to the apical tooth (Fig. 5A). However, the ridge in Stigmatomma mystriops is developed independently from another carina along the ventral one mentioned above, which in Stigmatomma mystriops continues to the ventral margin of the apical tooth the same as a usual mandible of ants (Fig. 5B). The apical tooth in Stigmatomma mystriops is directed mesally, not ventrally as in Mystrium . In addition to the Mystrium -like characters ( Brown 1960), we found two pairs of long setae on the anterior clypeal margin in Stigmatomma mystriops . These strange characters observed in Stigmatomma mystriops may be part of a specialized snapping mandible system.

In addition to characters unique to Mystrium , we discuss two generic characters (characters 5 and 9) that are useful for distinguishing amblyoponine genera. Neither character 5 nor 9 distinguish Mystrium from the other XMAS genera; however, both of these characters do distinguish Mystrium from Amblyopone .

Character 5 is provisionally proposed here to separate Amblyopone from genera in the XMAS clade. In the XMAS clade, the basal part of the labrum is lacking small dentiform setae (Fig. 6A). However, in Amblyopone , small dentiform setae are present on the labrum (Fig. 6B). Mandible character 9 proposed in Yoshimura and Fisher (2012) is unique to the XMAS clade. In our previous work, we described the unique evolution of the mandible in the XMAS clade. We presented as evidence the two-layered “teeth” on the masticatory surface of the mandible, which consists of true mandibular teeth on the dorsal row and apically extended basal denticles on the ventral row (Fig. 6E). We now update the mandibular character description because we have since observed in a few species of Amblyopone two-layered mandible dentition, such as in Amblyopone au02 (QMT152688). Even though two-layered mandible dentition was found in Amblyopone , as we described in character 9, the principal true mandibular teeth on the dorsal row are developed (Fig. 6F) larger than the basal denticles on the ventral row in Amblyopone , while the basal denticles are usually larger than true mandibular teeth on the masticatory margin with the exception of apical teeth in the XMAS clade (Fig. 6E).

The differences in mandibular characters between species in the XMAS clade and those in Amblyopone are associated with the parts of the mandible used for catching their prey. We assume that the masticatory margin is the main functional part of the mandible in XMAS clade species (Figs 6C, 6E), while the whole mandibular shaft is used for this purpose in Amblyopone (Figs 6D, 6F). The presence or absence of dentiform setae on the labrum (character 5: Figs 6B vs. 6A) supports our assumption. Amblyopone species probably “hold” prey using teeth along the whole shaft of the mandible (Fig. 6F), conical setae on the flat clypeus, and dentiform setae on the labrum (Fig. 6B); while species of Stigmatomma (in XMAS clade) “pinch” prey using their masticatory margin (distal part) of the mandible (Fig. 6E), and support it using anterior clypeal conical setae strongly extended anteriorly (Fig. 6A). When the mandibles of both groups elongated under these two use conditions, their morphology adapted differently. The mandibular shaft in Amblyopone became wider and stouter (Fig. 6F), while the basal denticles in XMAS clade became larger than true teeth and distinctly directed basally (Fig. 6E).

Brown (1960) was the first to propose that the dentiform setae on the labrum functioned as a structure to grip active prey. He attributed this character to Amblyoponini but our review suggest that dentiform setae on the basal part of the labrum (as in Fig. 6B) is restricted to three genera: Amblyopone , Onychomyrmex and Apomyrma . Ward (1994) reported that conical setae are present on the labrum in Apomyrma stygia Brown, Gotwald & Lévieux, 1971. Saux et al. (2004) described the presence of these setae in Amblyopone australis Erichson, 1842, Amblyopone hackeri Wheeler, 1927, Amblyopone longidens Forel, 1910, and Stigmatomma gingivale (Brown, 1960). Keller (2011) reported the presence of these setae in Amblyopone australis , Amblyopone mercovichi Brown, 1960, Onychomyrmex doddi Wheeler, 1916, and Apomyrma stygia , but their absence in Concoctio concenta Brown, 1974, and Myopopone castanea (Smith, 1860). Ward (1994) showed in his figure 9 labral setae in Onychomyrmex which is currently identified as Onychomyrmex au01 (PSW10030, CASENT0172779) though in Saux et al. (2004), this taxon is referred to as an undescribed species of Amblyopone . In addition to the above species, we found the presence of dentiform setae on the labrum in Amblyopone michaelseni Forel (CASENT0100441), Amblyopone au01 (CASENT0434465), and Amblyopone au02 (QMT152688). In Apomyrma stygia (CASENT0000077), the dentiform setae are present and distributed not only along the basal portion but also along the whole surface of the labrum; in addition, the conical setae on the anterior clypeal margin are absent. We also confirm the absence of dentiform setae on the labrum in Prionopelta , Concoctio and Bannapone .

Because of the presence of the dentiform labral setae, Stigmatomma gingivale is retransferred to Amblyopone as Amblyopone gingivalis Brown, 1960 comb. rev., although Yoshimura and Fisher (2012) transferred this species from Amblyopone to Stigmatomma . The dentiform labral setae in Amblyopone gingivalis are more abundant than those of Amblyopone australis , and covering on the distinct basal convexity on the labrum. In addition to the presence of the dentiform labral setae, true mandibular teeth larger than the basal ventral denticles are also present in Amblyopone gingivalis and provide an additional supportive character for this generic transfer.

On the other hand, on the basis of the mandible characters, Amblyopone awa , Amblyopone kangba , Amblyopone meiliana , and Amblyopone zomae are transferred to the genus Stigmatomma as Stigmatomma awa (Xu & Chu, 2012), comb. n., Stigmatomma kangba (Xu & Chu, 2012), comb. n., Stigmatomma meilianum (Xu & Chu, 2012), comb. n., and Stigmatomma zomae (Xu & Chu, 2012), comb. n., respectively. According to the original description (Xu and Chu 2012), the mandibular characters in figures 29, 34, and 52 show that Stigmatomma zomae , Stigmatomma meilianum , and Stigmatomma awa have typical forms observed in Stigmatomma , although we did not examine actual specimens of any of these four Chinese species.

The male diagnostic characters used in this revision follow those in Yoshimura and Fisher (2012). All worker diagnostic characters are applicable to both alate and ergatoid queens.