Melophorus minimus Heterick, Castalanelli & Shattuck

Heterick, Brian E., Castalanelli, Mark & Shattuck, Steve O., 2017, Revision of the ant genus Melophorus (Hymenoptera, Formicidae), ZooKeys 700, pp. 1-420 : 182-184

Melophorus minimus Heterick, Castalanelli & Shattuck sp. n.

Types.

Holotype minor worker (bottom ant) from St George ('Saint George’ -sic), Queensland, 14 January 1965, B.B. Lowery, sav. Woodland, ANIC Ants Vial 20.215 [ANIC32-900133] (ANIC). Paratypes: 2 minor workers on same pin and with same details as holotype (ANIC); minor worker from 50 km E of Cunnamulla, Queensland, 17 September 1976, P.J. M. Greenslade, (9), 21 [ANIC32-900132] (BMNH); 3 minor workers on separate pin with same details as holotype (MCZ); minor worker from Milang Conservation Park, Koonamore, South Australia, 24 February 1973, P.J. M. Greenslade, (8), M. sp. 34) (SAM); major and 2 minor workers from Tropicana Minesite 29°15'40"S, 124°35'50"E, Western Australia, January 2009, J. Summerhayes, pitfall trap Casuarina , CA 1:5 [JDM32-005036] (WAM).

Other material examined.

New South Wales: Warialda (Oliver, T.). Queensland: ‘Merigol’ (Beutel, T.) (QM), Tindaree via Tara (House, A & Brown, S.) (QM).

Diagnosis.

Melophorus minimus can be placed in the M. biroi species-group on the basis of characters of the clypeus, propodeum, mandible and palps. The species is placed in the M. biroi species-complex on the basis of a further suite of characters (viz, metatibia of major worker with only one preapical spur [except rarely in the mjobergi clade]; clypeal psammophore placed anteriorly at or just above anterior margin of clypeus in the minor worker and often in the major worker; head dorsoventrally compressed to varying degrees in the minor worker of most species with the eyes placed high on the sides; compact legs, and small body size [excluding mjobergi clade]; HW of smallest minor 0.36 mm, average HW of smallest minors 0.46 mm; HW of largest known major 1.29 mm, average HW of largest majors [where known] 1.05 mm). Melophorus minimus is the tiniest Melophorus with a head width of 0.47 mm in one measured major worker. Apart from minute size, the minor worker of this species has a distinctive profile with a strongly truncate, cuboidal propodeum that descends into its declivitous face at an angle approaching ninety degrees. The minor worker eye is large (EI 40-41) and the antennal scape is shorter than in most Melophorus (SI as little as 94). The mesosoma and first gastral tergite are always glabrous. The size of the major worker alone is sufficient to identify it, but its large eye combined with its smooth, glabrous mesosoma also separate it from other small Melophorus major workers in the M. biroi species-complex.

Minor worker description.

Head. Head approximately oval with straight sides; posterior margin of head planar to strongly convex; frons shining with superficial shagreenation or microreticulation only; frons consisting exclusively or almost exclusively of well-spaced, appressed setae only (small, erect setae, if present, usually confined to ocular triangle or posterior margin of head). Eye moderate (eye length 0.20-0.49 length of side of head capsule); in full-face view, eyes set at about midpoint of head capsule; in profile, eye set anteriad of midline of head capsule; eyes elliptical or slightly reniform. In full-face view, frontal carinae concave; frontal lobes straight in front of antennal insertion. Anteromedial clypeal margin broadly and evenly convex; clypeal psammophore set at or just above anterior clypeal margin; palp formula 6,4. Five mandibular teeth in minor worker; mandibles triangular, weakly incurved; third mandibular tooth distinctly shorter than apical tooth and teeth numbers two and four; masticatory margin of mandibles approximately vertical or weakly oblique. Mesosoma. Integument of pronotum, mesonotum and mesopleuron shining and mainly smooth, vestigial shagreenation most noticeable on humeri and mesopleuron; anterior mesosoma in profile weakly elevated anteriad, thereafter gently sinuate, pronotum and mesonotum on same plane; erect pronotal setae absent; in profile, metanotal groove a weak or vestigial furrow; propodeum shining and shagreenate; propodeum angulate, propodeal angle blunt; length ratio of propodeal dorsum to its declivity about1:1; erect propodeal setae always absent; appressed propodeal setulae sparse or absent, if present then not regularly spaced; propodeal spiracle situated on or beside declivitous face of propodeum, and longer (length ≥ 0.50 × height of propodeum). Petiole. In profile, petiolar node squamiform; in full-face view, shape of petiolar node uniformly rounded; node shining and smooth with vestigial sculpture. Gaster. Gaster shining, shagreenate ('LP record’ appearance); pilosity of first gastral tergite consisting of well-spaced short, inconspicuous, appressed setae only, erect setae always absent. General characters. Colour mainly concolorous brown; mesonotum may be orange tan.

Major worker description.

Head. Head square; posterior margin of head planar; cuticle of frons shining and smooth except for piliferous pits and a few striolae around antennal insertions and frontal carinae; frons consisting exclusively or almost exclusively of well-spaced, appressed setae only (small, erect setae, if present, usually confined to ocular triangle or posterior margin of head). Eye moderate (eye length 0.20-0.49 length of head capsule); in full-face view, eyes set at about midpoint of head capsule; in profile, eye set anteriad of midline of head capsule; eyes elliptical, or roughly ovoid, eye narrowed posteriad. In full-face view, frontal carinae straight, divergent posteriad; frontal lobes curved towards antennal insertion. Anterior clypeal margin broadly and evenly convex; clypeal psammophore set below midpoint of clypeus; palp formula 6,4. Five mandibular teeth in major worker; mandibles triangular, weakly incurved; third mandibular tooth distinctly shorter than apical tooth and teeth numbers two and four; masticatory margin of mandibles approximately aligned vertically or weakly oblique. Mesosoma. with weak to moderate sheen and superficial microreticulation; anterior mesosoma in profile broadly convex; erect pronotal setae absent; in profile, metanotal groove a narrow but deep slit; propodeum shining and superficially microreticulate; propodeum smoothly rounded; propodeal dorsum and declivity confluent; erect propodeal setae absent; appressed propodeal setae sparse or absent, if present then not regularly spaced; propodeal spiracle situated on or beside declivitous face of propodeum, and shorter (length less than 0.50 × height of propodeum). Petiole. In profile, petiolar node squamiform; in full-face view, shape of petiolar node tapered with blunt vertex; node shining and smooth with vestigial microreticulation. Gaster. Gaster shining, shagreenate ('LP record’ appearance); pilosity of first gastral tergite consisting of short, well-spaced appressed setae only. General characters. Colour dark brown, lateral mesonotum lighter in colour.

Measurements.

Worker (n = 4): CI 88-96; EI 40-41; EL 0.14-0.17; HL 0.37-0.44; HW 0.33-0.47; ML 0.43-0.55; MTL 0.23-0.28; PpH 0.06-0.06; PpL 0.19-0.24; SI 94-104; SL 0.34-0.39.

Comments.

With a total length of only around one millimetre, the minor workers of this species are probably the smallest Melophorus and are likely ecological competitors with small Monomorium of the same dimensions. At present the species is known from a handful of records from arid and semi-arid NSW (TERC), QLD, SA and WA, but it has probably been overlooked in other mainland states because of its minute size. This species can be identified by a combination of its small size, short antennal scape, glossy appearance, anteriorly located clypeal psammophore, and square propodeum. No specimens were available for sequencing. Recorded habits are remnant brigalow, savanna woodland and mulga. Specimens from Merigol Station, QLD and Tindaree via Tara, QLD were pitfall-trapped but there are no other data.

Etymology.

Latin (dim.) minimus ( ‘least’); adjective in the nominative singular.