Melophorus sulla

A highly variable, widely ranging, arid habitat Melphorus.

Identification
Heterick et al. (2017) - Melophorus sulla can be placed in the Melophoprus biroi species-group on the basis of characters of the clypeus, propodeum, mandible and palps. The species is also placed in the Melophoprus fieldi species-complex because of the appearance of the anteriorly placed clypeal psammophore, the compact propodeum, the presence of more than one preapical spine on the metatibia, at least in the major worker, and the long, even spindly legs. Most major workers of Melophorus sulla have an unmodified mandible but a few majors from WA have a crushing mandible such as is seen in Melophoprus wheeleri species-complex major workers.. Melophorus sulla (sensu stricto) usually has glabrous workers in all subcastes, although one or two flexuous, erect setae may be seen on the mesosoma. The appressed setae on the gaster of all workers are very small and inconspicuous when the gaster is moderately distended, such setae being separated from one another by at least their own length. These appressed setae are also inconspicuous on the mesosoma and never long and silvery. The node of the minor worker is often squamiform, and most commonly the cuticle is shining or even glossy with vestigial or weak shagreenation. The species is distinguished from similar species like Melophorus turneri or Melophorus longipes by its light yellow or ochre colour, some populations having a darker, brownish gaster along with the pale foreparts. One variant morph has a major worker with a modified, crushing mandible but the taxonomic significance of this modification is not known, especially since the minor workers of this morph are indistinguishable from other M. sulla minors. The “pillipes’’ condition (whorls of fine, erect setae on appendages) has not been seen in this species.

The characteristically pallid Melophorus sulla, which occurs in all mainland Australian states, poses more taxonomic problems than any other  Melophorus except, perhaps, Melophorus biroi. Not the least of these problems is the fact that while the facies of most major workers are typical of the M. fieldi complex, a few workers from midwestern WA have the broad heads and crushing mandible of the M. wheeleri complex. More frequently, the clypeus, especially in major workers, exhibits considerable plasticity; in some cases (mainly in far northern and arid area populations) this sclerite is flattened with a posteriorly placed clypeal psammophore, in other cases it is more-or-less protuberant with the psammophore in the middle of the sclerite. Those workers with a very protuberant clypeus tend to be of deeper colour than the very pale, depigmented individuals with the flattened clypeus. The most common colour form is a concolorous light yellow-ochre with a two-toned head capsule (darker above and conspicuously paler around the lower genae and the clypeus). This is the form that was given the unavailable name Melophorus ludius sulla breviscapa by Forel. Other workers may be uniformly canary yellow or yellow with orange tinges on the head and a light brown gaster. The minor workers of all morphotypes are more uniform in appearance than their major workers and have long spindly appendages, similar to those of Melophorus hirsutipes and Melophorus lanuginosus. However, although closely related, M. sulla does not appear to be a sister group to these taxa.

Distribution
Occurs in all states within drier habitats.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Biology
Heterick et al. (2017) - This arid or semi-arid species is active in a variety of habitats but seems to prefer very sandy areas, hence the frequent mention of ‘dunes’ on label data. However, the ant also occurs in dry watercourses and clay soils. A variety of ecosystems support this ant, and named vegetation in label data includes gidgee, mallee, Casuarina and Eucalyptus dumosa. The manner of leaving the nest is quite intriguing. The ants are extremely timid: the principal author has watched a nest entrance of this species for many minutes. The ants seemed to be most active around the middle of the day and left the nest in pulses of several to many workers, which streamed in and out to forage or deposit sand grains at very high speed. In between these pulses of activity, there was no movement around the nest. Possibly this behavior may reduce predation, with the irregular activity and speed across the hot soil surface lessening the likelihood of a predator being able to pick off workers at regular intervals. Alternately, the behavior may be related to subtle changes in solar radiation, which affects thermoregulation in the ants.

Nomenclature

 *  sulla. Melophorus ludius subsp. sulla Forel, 1910b: 66 (s.q.m.) AUSTRALIA.
 * Heterick, et al. 2017: 312 (w.).
 * Subspecies of ludius: Forel, 1915b: 89; Emery, 1925b: 12; Taylor & Brown, 1985: 124; Bolton, 1995b: 250; Heterick, 2009: 96.
 * Status as species: Heterick, et al. 2017: 308 (redescription).
 * Material of the unavailable name breviscapa referred here by Heterick, et al. 2017: 308.

Heterick et al. (2017) - As previously discussed, the genetics of this species are confusing. The mitochondrial gene suggests several populations; however the morphology and the nuclear genes don’t support this result. Future work is needed to examine the population structure of M. sulla in order to determine if multiple species exist. No samples of the specimens with a wheeleri-type major worker were sequenced (this morphotype appears to be rare) and, apart from the major worker mandible and broad head, these ants are indistinguishable from other M. sulla, and the minor workers cannot be distinguished in any way from those of other populations. There is the possibility that such major workers represent no more than a rare genetic trait. Hybridization with a member of the M. wheeleri complex is unlikely because of the genetic distance between the M. fieldi and the M. wheeleri complexes. The two forms of M. sulla are separated in the taxonomic key, but the unique status of the aberrant member of the nominal species is not established, and this aspect is flagged for further investigation.

Worker
Heterick et al. (2017) - (n = 8): CI 87–116; EI 17–33; EL 0.14–0.28; HL 0.50–1.46; HW 0.43–1.69; ML 0.69–1.79; MTL 0.49–1.24; PpH 0.06–0.17; PpL 0.30–0.72; SI 71–151; SL 0.65–1.20.

Minor. Head. Head square; 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 full-face view, eyes set at about midpoint of head capsule; in profile, eye set around midline of head capsule; eyes elliptical or slightly reniform, or elongate. 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 above midpoint of clypeus; palp formula 6,4. Five to six mandibular teeth in minor worker; mandibles triangular, weakly incurved, or narrow, strap-like, internal and external margins parallel or nearly so; 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 broadly convex; erect pronotal setae absent; in profile, metanotal groove shallow, broadly V or U-shaped; propodeum shining and smooth or with superficial and almost invisible microsculpture; propodeum smoothly rounded or with indistinct angle; propodeal dorsum and declivity confluent; 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 narrowly conical, vertex blunt, directed posteriad; in full-face view, shape of petiolar node uniformly rounded; node shining and smooth throughout. Gaster. Gaster shining, shagreenate (‘LP record’ appearance); pilosity of first gastral tergite consisting of well-spaced short, inconspicuous, appressed setae, erect setae (present in at least some workers) confined to margin of sclerite. General characters. Colour pale yellow-orange (in full-face view, lower frons often visibly depigmented).

Major. Head. Head horizontally rectangular, broader than wide; posterior margin of head weakly concave; cuticle of 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 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, divergent posteriad; frontal lobes straight in front of antennal insertion. Anterior clypeal margin broadly concave; clypeal psammophore set at or above midpoint of clypeus; palp formula 6,4. Mandibular teeth in major worker always 4; mandibles strongly incurved, apical sector weakly carinate or incompletely carinate; 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 shagreenate; propodeum smoothly rounded or with indistinct angle; 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 width from the 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 weakly shining with indistinct shagreenation; pilosity of first gastral tergite consisting of thick, often distally flattened, erect setae over well-spaced, short, appressed setae. General characters. Colour yellow-orange (in full-face view, lower frons often visibly depigmented).

Type Material
Heterick et al. (2017) - Syntype major workers, Tennants [sic] Creek, Northern Territory, the ANIC major worker here designated lectotype. The MHNG major worker (CASENT0909817) is here designated a paralectotype. (See also ‘Comments’ below for other possible paralectotypes.)

The typology of the taxon is also illustrative of outdated taxonomic practice. Two specimens that appear to be part of the original type series and were examined by Forel; both individuals are major workers, and possess a red ‘typus’ label. The ANIC specimen is here designated lectotype to fix the name for this problematic species; the other specimen, held by MHNG, becomes a paralectotype. Forel also examined an unspecified number of queens and males in the original type series. The latter, if they bear a red ‘typus’ label, also become paralectotypes in this revision. In addition to these specimens a pin of two major workers held by SAMA and a major worker held by MSNG are labeled ‘co-type.’ Since the collection data are identical, these specimens may have been regarded by Forel as the equivalent of paratypes, but they are not mentioned as such in his original description and it is not known if they were part of the original group of specimens that he saw. Since the information is equivocal, no paralectotype label has been attached to these specimens. However, should the designated lectotype and paralectotypes become lost or destroyed they could be reconsidered for paralectotype and thus for neotype status (ICZN, section 74F). Definitely removed from the type series are two sets of minor workers labeled as ‘co-types’, one of which is held by MCZ and the other (two workers) by SAM. Forel explicitly states he saw no minor workers of this species in the consignment. In fact, the SAMA workers are M. wheeleri!