Figs. 27, 72.
Etymology.- Greek "adiastolos" ("confused") [i.e., with two other very similar species]
Material examined.- Holotype: [[worker]], Prov. Antsiranana, r.s. Manongarivo, 17.8 km 218 sw Antanambao, 1580 m 14°01'3"s, 48°25'1"e 27.x.1998 bl. Fisher 1972/beating low vegetation, montane rainforest /1972(17)-3 (cas). Paratypes: Prov. Antsiranana (specimens with same collection data as holotype): One ergatoid (bmnh); 2 [[worker]] (mcz).
Other material examined: Prov. Antsiranana: 11.0 km wsw Befingotra, Res. Anjanaharibe-Sud, 16.xi.1994 b. (2[[worker]]) 18.xi.1994 (4[[worker]], 5 [[queen]]) 20.xi.1994 (3 [[worker]], 2[[queen]]) l. Fisher; r.s. Manongarivo, 12.8 km 228 sw Antanambao 11.x.1998 b.l. Fisher (2[[worker]]); r.s. Manongarivo, 14.5 km 220 sw Antanambao 20.x.1998 b.l. Fisher (1 [[worker]]).
Worker description.- Head: Head rectangular; vertex planar or weakly concave; frons shining and smooth except for piliferous pits; pilosity of frons a mixture of well-spaced, distinctly longer, erect and semi-erect setae interspersed with shorter decumbent setae or setulae. Eye moderate, eye width 1-1.5x greatest width of antennal scape; (in full-face view) eyes set above midpoint of head capsule; (viewed in profile) eyes set posteriad of midline of head capsule; eye elliptical, curvature of inner eye margin may be more pronounced than that of its outer margin. Antennal segments12; antennal club three-segmented. Clypeal carinae weakly to strongly defined, anteromedian clypeal margin straight or emarginate, clypeal carinae terminating in blunt angles; paraclypeal setae moderately long and fine, curved; posteromedian clypeal margin extending slightly beyond level of posterior margin of antennal fossae. Anterior tentorial pits situated nearer antennal fossae than mandibular insertions. Frontal lobes sinuate, divergent posteriad. Psammophore absent. Palp formula 2,2. Mandibular teeth four; mandibles linear-triangular and smooth (except for piliferous pits); masticatory margin of mandibles strongly oblique; basal tooth approximately same size as t3 (four teeth present).
Mesosoma: Promesonotum shining and smooth on dorsum, entire lower mesopleuron distinctly striolate; (viewed in profile) anterior promesonotum smoothly rounded, thereafter more-or-less flattened, promesonotum on same plane as propodeum; promesonotal setae greater than twelve; standing promesonotal setae consisting of a mixture of incurved, semi-erect setae and slightly shorter decumbent setae; appressed promesonotal setulae few, mainly on sides of promesonotum. Metanotal groove vestigial. Propodeum shining, dorsum and sides of propodeum mainly smooth, with weak to strong striolae on declivitous face and on metapleuron; propodeal dorsum slightly elevated anteriad and sloping away posteriad, propodeal angles not raised propodeum smoothly rounded or with indistinct angle; standing propodeal setae numerous, wholly or mainly erect or sub-erect, without conspicuous paired setae evident; appressed propodeal setulae abundant, particularly on dorsum of propodeum; propodeal spiracle equidistant from metanotal groove and declivitous face of propodeum. Vestibule of propodeal spiracle distinct; Propodeal lobes present as blunt-angled flanges.
Petiole and postpetiole: Petiolar spiracle lateral and situated slightly anteriad of petiolar node; node (viewed in profile) tumular, inclined posteriad, with vertex also tapered posteriad, or, subcuboidal, inclined posteriad; appearance of node faintly striolate, striolae becoming costulate on rear face of node; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) between 4:3 and1:1; anteroventral petiolar process absent or vestigial; ventral petiolar lobe absent; height ratio of petiole to postpetiole between 3:2 and 4:3; height -length ratio of postpetiole between 4:3 and 1:1; postpetiole shining, with vestigial sculpture; postpetiolar sternite not, or, only slightly depressed at midpoint, anterior process prominent.
Gaster: Pilosity of first gastral tergite consisting of a mixture of incurved, semi-erect setae and slightly shorter decumbent setae.
General characters: Color yellowish to yellowish-brown, gaster darker brown. Worker caste monomorphic.
Holotype measurements: HML 2.39 HL 0.82 HW O.64 CeI 78 SL 0.64 SI 100 PW 0.48.
Other worker measurements: HML 2.17-2.41 HL 0.75-0.84 HW 0.63-0.70 CeI 81-89 SL 0.58-0.68 SI 88-97 PW 0.45-0.52 (n=14).
Queen description.- Head: Head square; vertex weakly concave or planar; frons shining and smooth except for piliferous pits, or, shining and smooth except for piliferous pits and a few striolae around antennal sockets and frontal carinae; pilosity of frons a mixture of well-spaced, distinctly longer erect and semi-erect setae interspersed with shorter setae or setulae, which are decumbent or appressed, longer setae thickest on vertex. Eye semi-circular; (in full-face view) eyes set at about midpoint of head capsule; (viewed in profile) eyes set posteriad of midline of head capsule.
Mesosoma: Mesoscutum broadly convex anteriad, convexity reduced posteriad; pronotum, mesoscutum and mesopleuron shining and mainly smooth, vestigial striolae, if present, confined to anterior katepisternum length-width ratio of mesoscutum and scutellum combined between 2:1 and 3:2; axillae separated by width of at least one axilla; standing pronotal/mesoscutal setae consisting of a mixture of incurved, semi-erect setae and slightly shorter decumbent setae; appressed pronotal, mescoscutal and mesopleural setulae few, mainly on sides of pronotum and mesopleuron. Propodeum shining, dorsum and sides of propodeum mainly smooth, with weak to strong striolae on declivitous face and on metapleuron; propodeum distinctly angulate, propodeal angle sharp, or, distinctly angulate, propodeal angles produced as short denticles; propodeal dorsum flat throughout most of its length, or, sloping posteriad, and depressed between raised propodeal angles; standing propodeal setae consisting of a few decumbent setae only; appressed propodeal setulae very sparse or absent; propodeal spiracle equidistant from metanotal groove and declivitous face of propodeum; propodeal lobes present as well-developed, rounded flanges, or, present as vestigial flanges only, or absent.
Wing: Wing not seen (queens dealated).
Petiole and postpetiole: Petiolar spiracle laterodorsal and situated slightly anteriad of petiolar node; node (viewed in profile) cuneate, vertex tapered; appearance of node shining, with vestigial sculpture; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) between 4:3 and 1:1; anteroventral petiolar process absent or vestigial; height ratio of petiole to postpetiole between 4:3 and 1:1; height -length ratio of postpetiole between 4:3 and 1:1; postpetiole shining and smooth; postpetiolar sternite depressed at about its center, with anterior process developed as a short, conspicuous spur angled at 45-90.
Gaster: Pilosity of first gastral tergite consisting of a mixture of incurved, erect and semi-erect setae and slightly shorter decumbent setae.
General characters: Color tawny-orange. Brachypterous alates not seen. Ergatoid or worker-female intercastes seen.
Queen measurements: HML 2.70-3.16 HL 0.85-0.96 HW 0.77-0.88 CeI 91-98 SL 0.62-0.74 SI 78-88 PW 0.68-0.85 (n=2).
REMARKS.- The taxonomic position of M. adiastolon is rather unsatisfactory, as it has more of the appearance of a hybrid between two other taxonomically difficult taxa (namely, M. fisheri and M. hildebrandti ) than of a good species in its own right. I have tentatively identified two morphotypes or forms of this species. Workers from Manongarivo, in Antsiranana Province, have relatively large eyes (15 ommatidia>) for M. hildebrandti group workers, and a broad, asymmetrical petiolar node that is longitudinally striolate and similar in appearance to the node of a distinctive coastal population of M. hildebrandti . This form has been collected on the same transect as an apparently monomorphic, finely sculptured, small-eyed form of M. fisheri . The two ants are visibly distinct taxa. The other form has been collected at Befingotra, in a more eastern region of the Province, where it is adjacent to or sympatric with a population of a different morphotype of M. fisheri . In the latter form, the workers are extremely smooth and shiny and the promesonotum is flattened and protrudes laterally at the humeri. Nonetheless, the Befingotra adiastolon and fisheri workers are much more similar than their counterparts at Manongarivo. At Befingotra M. adiastolon also has a striolate node, but this structure is thinner and more like that of the local M. fisheri , and the brown coloration, though not the rounded promesonotum, is also similar. The eye, in this population of M. adiastolon , is smaller and nearer to that of typical M. fisheri belonging to the shiny morphotype. Several samples of the queen of M. adiastolon and one ergatoid are known. Queens from Befingotra are of a somewhat smaller average size than M. fisheri queens, but otherwise there is no physical difference between the two taxa. All known workers and queens of M. adiastolon have a maximum of four mandibular teeth and denticles, whereas at least some members of all the M. fisheri morphotypes have five or more, but as this character is variable and the amount of available M. adiastolon material is very limited, this fact signifies little.
For the moment, M. adiastolon can be separated from M. hildebrandti and M. fisheri by the characters noted in the key. However, this is provisional, and a better knowledge of the Monomorium fauna in the Antsiranana Province, including information from more nest series, could well change the current situation. Both forms have been collected from rotten wood and sifted leaf litter in montane rainforest. The ergatoid and two workers from the Manongarivo population were collected by beating from low vegetation.
