Melophorus pusillus

Workers collected in Cambrai, SA were taken in dunes and two workers were collected in a malaise trough at Craven Peak in QLD, but no other ecological data are available.

Identification
Heterick et al. (2017) - Melophorus pusillus is placed in the Melophorus ludius species-group on the basis of molecular data. However, in morphological appearance this taxon shares major diagnostic characters with the Melophorus biroi complex (viz, metatibia of major worker with only one preapical spur; clypeal psammophore placed anteriorly at or just above anterior margin of clypeus in the minor worker and often in the major worker; legs compact, and small body size [HW of smallest minor < 0.40 mm, HW of largest major < 1.10 mm]). Like the other three members of its species-group, M. pusillus is characterized by being weakly sculptured overall, with the cuticle of the mesosoma visibly thin, the mesonotum being translucent to varying degrees and the mesopleuron either smooth or with vestigial sculpture only. Melophorus pusillus is distinguished from Melophorus ludius by the larger eye (EI in 31-40 in M. pusillus compared with EI of 19-30 in M. ludius) and from Melophorus translucens by the less elongate propodeum and shorter propodeal spiracle.

Melophorus pusillus resembles nothing so much as a slightly elongate M. ludius, and is easily confused with that ant. The larger eye (EI 31-40 compared with EI 19-30) and the very thin cuticle on the mesosoma are the best way to distinguish this ant from M. ludius.

Distribution
The species is widely distributed in Australia, mainly in arid inland environments, and has been recorded from all mainland states except the ACT.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  pusillus. Melophorus pusillus Heterick, Castalanelli & Shattuck, 2017: 393, fig. 93 (w.) AUSTRALIA.

Worker
(n = 6): CI 90–106; EI 31–40; EL 0.15–0.21; HL 0.41–0.64; HW 0.36–0.68; ML 0.52–0.86; MTL 0.30–0.52; PpH 0.05–0.08; PpL 0.22–0.37; SI 99–127; SL 0.46–0.67.

Minor. Head. Head approximately oval with straight sides; posterior margin of head 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 large (eye length ≥ 0.50 × length of side of head capsule); in profile, eye set at about midpoint of head capsule; in profile, eye set around midline of head capsule; roughly ovoid, eye narrowed posteriad. In full-face view, frontal carinae distinctly concave; frontal lobes curved toward antennal insertion. Anteromedial clypeal margin straight; clypeal psammophore set at or above midpoint of clypeus; 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 short, separated by more than own length and inconspicuous; 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 distinctly shagreenate-microreticulate. Gaster. Gaster shining with superficial microreticulation; pilosity of first gastral tergite consisting of well-spaced short, inconspicuous, appressed setae only, erect setae always absent. General characters. Colour pale yellow-orange to bright brown.

Major. Head. Head square; posterior margin of head planar or weakly convex; cuticle of frons shining and smooth except for piliferous pits; 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 or weakly convex; frontal lobes straight in front of antennal insertion. Anterior clypeal margin broadly and evenly convex; clypeal psammophore set at or above 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. Integument of pronotum, mesonotum and mesopleuron moderately shining and shagreenate throughout; anterior mesosoma in profile broadly convex; erect pronotal setae absent; in profile, metanotal groove shallow, broadly V- or U-shaped; propodeum shining and finely striolate and microreticulate; propodeum smoothly rounded or with indistinct angle; 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 sharp vertex; node shining and smooth with vestigial microreticulation anteriad. Gaster. Gaster shining, shagreenate (‘LP record’ appearance); pilosity of first gastral tergite consisting of well-spaced, erect and semi-erect setae interspersed with regularly spaced appressed setae, or consisting of well-spaced short, inconspicuous, appressed setae, erect setae (present in at least some workers) confined to margin of the sclerite. General characters. Colour yellow ochre with brownish-yellow gaster to pale brown.

Type Material
Holotype minor worker (bottom ant) from Tropicana Minesite 29°10'11"S, 124°33'38"E, Western Australia, January 2009, J. Summerhayes, pitfall trap Casuarina, CA 2:3 [JDM32-004668]. Paratypes: major worker on same pin and with same details as holotype (WAM); 2 minor workers from Kunoth Paddock, near Alice Springs, Northern Territory, 25 October 1980, P.J.M. Greenslade, SA EAD ; 2 minor workers from Gawler Ranges, South Australia, 1 October 1972, P.J.M. Greenslade, (9) ; 2 minor workers from 30 km S of Granite Downs, South Australia, 23 September 1980 (I), P.J.M. Greenslade, 9-3C ; 2 minor workers from 3.2 km NW of homestead on Plum Pudding track, Cravens Peak Station 23°19'00"S, 138°33'47", Queensland, 21-24 April 2007, C. Lemann, Malaise trap, gidgee flat, ANIC ANTS VIAL 76.104 [ANIC32-036867] ; 3 minor workers from Mt Gunson, South Australia, 1985, T. J. Case, 541.

Etymology
Latin pusillus (‘puny’); adjective in the nominative singular.