Melophorus pelorocephalus

The very gracile appearance of this ant suggests workers move very swiftly over the ground surface when they are active. No information on the habits of the species is available, but specimens have been collected on red sand covered by spinifex and low shrubs in the Little Sandy Desert, WA. Elsewhere, this ant has been collected in the Pilbara region, WA and at Three Forges Waterhole in SA. (Heterick et al. 2017)

Identification
Heterick et al. (2017) - Melophorus pelorocephalus can be placed in the Melophorus biroi species-group on the basis of characters of the clypeus, propodeum, mandible and palps. The species is also placed in the Melophorus wheeleri species-complex because it agrees with the following apomorphies possessed by the complex: the minor worker often has more than five teeth, the largest major worker has a short, massive, elbowed mandible directed posteriad; in profile, the maxillary palps are short in the major and generally short in minor workers (in the minor worker, usually only attaining the neck sclerite at their maximum extent when the head is moderately inclined) and, in full-face view, the anterior margin of the clypeus in the large major worker is usually planar or weakly concave (variable in other subcastes but planar or narrowly protuberant anterior clypeal margins predominate). In full-face view, the broad head capsule of the minor worker of M. pelorocephalus is strongly expanded towards the mandibular insertions, giving it a strongly accentuated trapezoidal shape, and the basal margin of the major worker mandible is a carinate ledge set at 90° to the rest of the mandible throughout its length, with the (slightly) offset basal tooth horizontal and the sub-basal tooth with a horizontal and a vertical plane. A mandibular carina is present to varying degrees in media and minor workers. All workers have a glabrous mesosoma. These characters will distinguish M. pelorocephalus from all other Melophorus except for its close relation, Melophorus laticeps. The heads of minor, media and small major workers of M. pelorocephalus, however, are even more exaggeratedly trapezoidal than are the heads of M. laticeps workers and the upper head of this species is distinctly darker in colour than the lower head and the mesosoma (all are uniformly coloured in M. laticeps). Melophorus pelorocephalus is believed to have a large major worker, but only minor, media and small major workers have been seen in the collections examined.

The morphology and genetic sequencing data confirm a sister relationship between this species and Melophorus laticeps, both ants being separated from other M. wheeleri complex members by a combination of the nature of the mandible in the major worker and the size of the workers. In full-face view, the anterior angles of the head capsule in M. pelorocephalus are more protruding than in its sister taxon, allowing for a very wide mandibular gape (bizarrely so in the minor worker, hence the name adopted here).

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  pelorocephalus. Melophorus pelorocephalus Heterick, Castalanelli & Shattuck, 2017: 357, fig. 84 (w.) AUSTRALIA.

Worker
(n = 4): CI 121–133; EI 19–26; EL 0.24–0.28; HL 0.76–1.13; HW 0.92–1.49; ML 1.16–1.76; MTL 0.90–1.38; PpH 0.12–0.18; PpL 0.44–0.64; SI 88–101; SL 0.93–1.32.

Minor. Head. Head square or rectangular, tending to trapezoid; posterior margin of head strongly convex; frons matt or with weak sheen, shagreenate; pilosity of frons a mixture of a few well-spaced, erect setae interspersed with appressed setae only. 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 anteriad of midline of head capsule; roughly ovoid, eye narrowed posteriad. In full-face view, frontal carinae straight or weakly convex; 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 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 matt or with weak sheen and finely striolate; propodeum smoothly rounded or with indistinct angle; propodeal dorsum and declivity confluent; erect propodeal setae always absent; appressed propodeal setulae short, separated by more than own length and inconspicuous; propodeal spiracle situated nearer to midpoint of propodeum than to its declivitous face, and shorter (length < than 0.50 × height of propodeum). Petiole. In profile, petiolar node narrowly conical, vertex blunt, directed posteriad; in full-face view, shape of petiolar node tapered with blunt vertex; node shining and smooth with vestigial sculpture. Gaster. Gaster weakly shining with indistinct shagreenation; pilosity of first gastral tergite consisting of wellspaced short, inconspicuous, appressed setae, erect setae (present in at least some workers) confined to margin of sclerite. General characters. Colour of foreparts tan, head darker, legs and gaster dark chocolate.

Major. Head. Head horizontally rectangular, broader than wide; posterior margin of head weakly convex; cuticle of frons shining with superficial shagreenation and microreticulation only; consisting exclusively or almost exclusively of well-spaced, appressed setae only; Eye small (eye length less than 0.2 × length of head capsule); in full-face view, eyes set above midpoint of head capsule; in profile, eye set anteriad of midline of head capsule; eyes elliptical. In full-face view, frontal carinae straight or weakly convex; frontal lobes straight in front of antennal insertion. Anterior clypeal margin straight; clypeal psammophore set at or above midpoint of clypeus; palp formula 6,4. Mandibular teeth in major worker 4; mandibles strongly incurved, apical sector uniformly carinate and forming a bifurcate, horizontal ledge that terminates in the basal and pre-basal teeth; third mandibular tooth distinctly shorter than apical tooth, but equivalent in length to remaining teeth; 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 a weak furrow; integument of propodeum shining and finely striolate and microreticulate; propodeum with indistinct angle; length ratio of propodeal dorsum to its declivity not applicable, propodeal dorsum and declivity confluent; erect propodeal setae absent; appressed propodeal setae short, separated by more than own length and inconspicuous; propodeal spiracle situated at least twice its length from the declivitous face of the propodeum, and shorter (length less than 0.50 × height of propodeum). Petiole. In profile, petiolar node narrowly conical, vertex sharply defined; in full-face view, shape of petiolar node tapered with blunt vertex; node shining and smooth throughout; in full-face view, shape of petiolar node uniformly rounded; node shining and faintly striolate and microreticulate Gaster. Gaster shining, shagreenate (‘LP record’ appearance); pilosity of first gastral tergite consisting of well-spaced, short, inconspicuous, appressed setae only, erect setae absent. General characters. Colour reddish-brown, gaster black.

Type Material
Holotype minor worker (top ant) from 142.1 km SSE of Newman 24°31'53"S, 120°17'24"E, Western Australia, October 1996, S. van Leeuwen & R. N. Bromilow, Temp. invert. Pitfall trap, B3: Sand dune [JDM32-004714]. Paratypes: 2 minor workers on same pin with same details as holotype (WAM); minor and media worker from 142 km SSE of Newman 24°31'53"S, 120°17'24"E, Western Australia, October 1996, S. van Leeuwen & R. N. Bromilow, Temp. invert. Pitfall trap, B3: Sand dune [JDM32-004852] ; 2 major and a media worker from Tropicana Minesite 29°15'42"S, 124°30'47"E, Western Australia, January 2009, J. Summerhayes, pifall trap: marble dune, MD2:1 [JDM32-004898] (WAM).

Etymology
Greek peloros (‘monstrous’) plus Latinized Greek kephalos (‘head’); noun in the nominative singular standing in apposition to the generic name.