Melophorus gracilis Hetrick, Castalanelli & Shattuck

Heterick, Brian E., Castalanelli, Mark & Shattuck, Steve O., 2017, Revision of the ant genus Melophorus (Hymenoptera, Formicidae), ZooKeys 700, pp. 1-420 : 167-170

Melophorus gracilis Hetrick, Castalanelli & Shattuck sp. n.

Types.

Holotype minor worker (middle ant) from Nita Downs turnoff, 19°04.712"S, 121°39.178"E, Western Australia, 14 February 2013 (9.45 am), B.E. Heterick, sample 74: beside road leading to station gates, ABRS M256, [ANIC 32-900209] (WAM). Paratypes: major and minor worker on same pin and with same data as holotype (WAM); minor worker with carrion (second point) from 5 km N Pardoo Hill, 19°52.408"S, 120°35.168"E, Western Australia, 14 February, 2013 (12.25pm), B.E. Heterick, sample 80: beside road: carrying carrion: aspirate, ABRS M265, [ANIC32-900212] (ANIC); 3 minor workers from 42 mi N of Mundiwindi, Western Australia, 26 April 1963, McInnes & Dowse, 29782, A395, [ANIC32-900213] (MCZ).

Other material examined.

Western Australia: 4 km N Cape Leveque turnoff (Heterick, B.E. [M251]), 42 mi N Mundiwindi (McInnes & Dowse [ANIC32-900213]), 5 km N Pardoo Hill (Heterick, B.E. [ANIC32-900212]), Nita Downs turnoff (Heterick, B.E. [ANIC32-900209]).

Diagnosis.

Melophorus gracilis 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 gracilis resembles a very small, glossy M. biroi (major worker HW ≤ 0.65 mm, minor worker HW ≤ 0.40 mm). The minor worker of the ant can be distinguished from M. biroi by the possession of a pair or several long, erect setae at the midpoint of the pronotum (the M. biroi minor worker nearly always has a glabrous mesosoma with only a tiny pronotal seta or two in very rare individuals) and erect setae on the first gastral tergite including a line of erect marginal setae. The lack of mesosomal setae also distinguishes tiny minor workers of M. ludius from minor workers of this ant. The major worker can be distinguished from the major worker of M. biroi by the non-protuberant or arched anterior clypeal margin, and from the M. ludius major worker by the restriction of erect mesosomal setae to a pair on the pronotum (more setae and these widely dispersed in the M. ludius major worker).

Minor worker description.

Head. Head approximately oval, sides of head convergent towards mandibular insertions; posterior margin of head strongly convex; frons shining and smooth except for piliferous pits; pilosity of frons consisting almost exclusively of well-spaced, appressed setae only (small, erect setae confined to ocular triangle). Eye moderate (eye length 0.20-0.49 length of side of head capsule); in full-face view, eyes set above midpoint of head capsule; in profile, eye set around midline of head capsule; eye elliptical. In full-face view, frontal carinae straight; frontal lobes straight in front of antennal insertion. Anterior clypeal margin broadly and evenly convex; clypeal psammophore set at or just above anterior clypeal margin; palp formula 6,4.Five mandibular teeth; mandible triangular, weakly incurved; third mandibular tooth distinctly shorter than apical tooth and teeth nos 2 and 4; masticatory margin of mandibles approximately vertical or weakly oblique. Mesosoma. Integument of pronotum, mesonotum and mesopleuron shining and mainly smooth throughout; anterior mesosoma smoothly rounded anteriad and flattened posteriad; mesonotum gently convex; a pair to several pronotal setae about as long as eye and unmodified; in profile, metanotal groove shallow, indicated mainly by an angle; integument of propodeum smooth and shining throughout; propodeum rounded or with indistinct angle; propodeal dorsum and declivity confluent; erect propodeal setae sparse (a couple or several); appressed propodeal setae short, separated by more than own length and inconspicuous; propodeal spiracle situated on or beside declivitous face of propodeum, and longer (length ³ ½ × height of propodeum). Petiole. In profile, petiolar node squamiform; in full-face view, petiolar node rounded; node shining and smooth throughout. Gaster. Gaster smooth and glossy; pilosity of first gastral segment consisting of a few well-spaced erect and semi-erect setae and longish, appressed setae that are separated by about their own length. General characters. Honey-coloured, gaster light brown to brown.

Major worker description.

Head. Head square; posterior margin of head planar or weakly convex; frons shining and smooth except for piliferous pits; pilosity of frons consisting almost exclusively of well-spaced, appressed setae only (small, erect setae confined to ocular triangle). 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 around midline of head capsule; eye elliptical, slightly ovoid posteriad. In full-face view, frontal carinae straight, divergent posteriad; frontal lobes curved towards antennal insertion. Anterior clypeal margin straight; clypeal psammophore set at or just above anterior clypeal margin; palp formula 6,4.Five mandibular teeth; mandible triangular, weakly incurved; third mandibular tooth distinctly shorter than apical tooth and teeth nos 2 and 4; masticatory margin of mandibles strongly oblique. Mesosoma. Integument of pronotum, mesonotum and mesopleuron shining and mainly smooth, superficial microreticulation most noticeable on humeri and mesopleuron; anterior mesosoma smoothly rounded anteriad and more flattened posteriad; mesonotum gently convex; pronotum with a pair of setae shorter than eye and unmodified; in profile, metanotal groove shallow, indicated mainly by an angle; integument of propodeum shining with multiple hair-like striolae on metapleuron; propodeum angulate, propodeal angle blunt; length ratio of propodeal dorsum to its declivity about 1:1; erect propodeal setae few in number and minute; appressed propodeal setae short, separated by more than own length and inconspicuous; propodeal spiracle situated on or beside declivitous face of propodeum, and longer (length ³ ½ × height of propodeum). Petiole. In profile, petiolar node squamiform; in full-face view, petiolar node rounded; node shining and smooth throughout. Gaster. Gaster smooth and glossy with faint transverse striolae visible in some lights; pilosity of first gastral segment consisting of a few well-spaced erect and semi-erect setae and appressed or decumbent setae that are separated by more than their own length. General characters. Honey-coloured, gaster light brown to brown.

Measurements.

Worker (n = 7): CI 92-98; EI 25-31; EL 0.12-0.16; HL 0.42-0.64; HW 0.38-0.61; ML 0.56-0.78; MTL 0.32-0.44; PpH 0.07-0.10; PpL 0.23-0.33; SI 87-110; SL 0.42-0.54.

Comments.

This very small, shining ant is confined to the far north of Australia. In WA, specimens have been taken from the Cape Leveque turnoff, Great Northern Hwy, near Broome, N of Pardoo Hill and from the Nita Downs turnoff, Great Northern Hwy. Ants in the TERC Collection believed to be of the same species have been taken from locations in West Rankin Stn, Connell’s Lagoon and Alexandra Stn., all in the NT. Melophorus gracilis resembles a small, light brown M. biroi but differs in having erect setae present on the pronotum and first gastral tergite. The species can be differ entiated from other, similar species by a combination of its small eye, visibly thin cuticle and lack of sculpture on the mesopleuron. In a five-gene tree (Figure 4), M. gracilis comes out as sister to M. biroi . This species nests directly into sandy soils and workers at one nest were seen carrying small carrion (dead workers of Monomorium rothsteini ). In all likelihood they are general scavengers of plant and dead animal material.

Etymology.

Latin gracilis ( ‘slender’); adjective in the nominative singular.