Monomorium rothsteini, Forel

Sparks, Kathryn S., Andersen, Alan N. & Austin, Andrew D., 2014, Systematics of the Monomorium rothsteini Forel species complex (Hymenoptera: Formicidae), a problematic ant group in Australia, Zootaxa 3893 (4), pp. 489-529 : 492-497

Biology of the M. rothsteini species complex

Species in the M. rothsteini species complex are a common and abundant component of the ant fauna throughout most of the Australian mainland outside mesic forests. They are predominantly seed harvesters and the nests of many of the species described here, particular the larger species that occur in the arid and northern monsoonal zones, are easily recognised by the dense middens of mostly grass seed husks that have been discarded to one side of the nest entrance ( Fig. 2 a). Worker ants carrying a variety of seeds and arthropod material can be observed foraging in long columns that radiate out from these nest entrances. The nests of smaller species that occur both in the northern monsoonal zone and the central and southern arid zones are less distinctive, with smaller nest middens or no obvious midden at all and with fewer workers ( Fig. 2 b, 2c). In many areas, the nest of one species with a large midden occurs within metres of another species with no nest midden. Although seeds appear to make up the bulk of the diet of the complex generally, species are opportunistic feeders on other food sources, and it is possible that seeds are only a minor component of the diets of some species. For example, one of us (KSS) has observed several colonies of M. speculum sp. nov. in a dry lake bed. There was very little vegetation within foraging distance and all workers moving around the nest entrance were carrying minute gastropod shells. Workers of M. subapterum have been observed feeding on the nectar of Melaleuca flowers ( Fig. 2 d).

Queens and males are rarely seen. Both are rare in collections and during two extensive field trips and several shorter excursions queens and males were each observed on only one occasion. There are published accounts of species in the M. rothsteini complex having brachypterous queens. Briese (1983) observed 63 queens with poorly developed wings and orange gasters emerging from a colony that subsequently underwent colony fission. The colony was within 5 m of another colony that later produced queens described as fully winged with dark brown gasters. Wheeler’s (1917) erection of M. subapterum and M. subapterum var. bogischi is based on queens he received from the South Australian Museum that had been collected from Harding River in northwest Western Australia and Port Wakefield in southern South Australia. He described these specimens as being smaller than the queens of M. rothsteini var. humilior and having very small wings. He alluded to other specific differences but did not discuss them. The lead author has examined queens from both these series in the South Australian Museum that were dealate, having just the remains of both pairs of wings and resembling fully developed queens in all other aspects. Queens that were excavated from a nest in Birdsville in southwest Queensland and here attributed to M. bogischi were all dealate with obvious wing remnants ( Fig. 9 d). Wheeler may have had access to other specimens than those currently available at the South Australian Museum, but based on our observations of specimens collected in the field and examined in collections, we cannot confirm brachyptery exists in this group. Larger workers that appeared to bear some aberrant characteristics (overall larger size, well defined promesonotal suture and coarser sculpture) were found foraging with other workers from a colony of M. pilbara sp. nov., and small, black, heavily sculptured workers have been observed foraging alongside workers of M. topend sp. nov. that are typically large and amber orange in colour. Whether these aberrant forms represent an intercaste, a developmental anomaly or an apterous female is unknown.

A number of morphospecies within the M. rothsteini species complex have been the subject of ecological studies. Table 1 provides a list of these publications, the morphospecies codes that were used and the corresponding species name as per this study.

Diagnosis of the M. rothsteini species complex

Members of the M. rothsteini species complex can be distinguished from all other species in the genus by the presence of a 12 segmented antenna, mandible with three teeth, mildly to deeply concave anterodorsal clypeal margin and alveolate sculpture on the meso- and metapleuron. Members of the M. rothsteini species complex bear a close morphological relationship to M. sordidum but can be easily distinguished by the shape of the anterior clypeal margin, which is rounded in the latter ( Monomorium sordidum is not granivorous).

Description of the M. rothsteini species complex

Worker measurements (n=201). HW 0.62–1.05, HL 0.68–1.03, EL 0.14–0.22, PMH 0.23–0.49, PH 0.21–0.41, PNH 0.14–0.35, LHW 0.39–0.62, EW 0.1–0.14, PML 0.66–0.7, ML 0.64–1.17, PL 0.27–0.55, PNWdv 0.16–0.35. Head: Shape appearing square or rectangular but always longer than wide; posterior cephalic margin in full face view straight to mildly concave or broadly v-shaped, antennae 12-segmented with a well differentiated 3- segmented club; scape reaching approximately three quarters of the way to vertexal margin of head; mandible with three teeth; anterior margin of clypeus mildly to deeply concave; coarse strigae present laterally between mandibular articulation and eye; strigae may be present on frons above antennal lobes or restricted to antennal lobes or area in between; eye oval in shape, ventral margin flattened.

Mesosoma: Pronotum rounded in lateral profile, promesonotal suture clearly present, faint, or absent, posterior mesonotum slightly raised or flat; metanotal groove shallow or deeply impressed, porcate; mesonotum coarsely strigate, punctuate, both, or smooth. Propodeum with or without a raised anterodorsal transverse carina, dorsal margin in lateral view rounded, flat or sloping posteriorly, dorsal surface concave or flat to slightly rounded, with or without longitudinal carinae and transverse strigae, posterior surface near vertical or sloping. Petiole node broad to narrow and rounded dorsally, postpetiole rounded, approximately three quarters the height of the petiole. Mesopleuron and propodeum coarsely alveolate with fine to coarse striations; metanotal groove always with numerous transverse ridges; petiole without sculpture or with fine reticulation, postpetiole almost always finely reticulate; very fine reticulation present or absent on first metasomal segment.

Pilosity sparse with long and short erect or appressed white or yellow setae on head; always with one pair of elongate, inwardly curved setae arising from centre of clypeal lateral carinae, and one pair of elongate erect setae arising from antennal lobes; erect setae on vertex curved anteriorly, setae on dorsal surface of pronotum inwardly curved, elongate erect setae on propodeum curved anteriorly, erect setae on petiole and postpetiole curved posteriorly.

Colour variable; head and mesosoma amber orange, dark orange brown, dark brown or almost black; metasoma almost always dark brown to black, rarely light brown or amber, cuticle with a high gloss on nonsculptured areas.

Taxonomic synopsis

Monomorium bogischi Wheeler, 1917 stat. rev., stat. nov.

Monomorium broschorum sp. nov.

Monomorium capeyork sp. nov.

Monomorium eremoides sp. nov.

Monomorium eremum sp. nov.

Monomorium geminum sp. nov.

Monomorium hertogi sp. nov.

Monomorium hoffmanni sp. nov.

Monomorium humilior Forel 1910 stat. rev., stat. nov.

Monomorium kidman sp. nov.

Monomorium leda Forel 1915 stat. rev., stat. nov.

Monomorium maryannae sp. nov.

Monomorium merepah sp. nov.

Monomorium mitchell sp. nov.

Monomorium oodnadatta sp. nov.

Monomorium pilbara sp. nov.

Monomorium rothsteini Forel 1902 .

= Monomorium rothsteini doddi Santschi, 1919 .

Monomorium speculum sp. nov.

Monomorium stagnum sp. nov.

Monomorium subapterum Wheeler, 1917 stat. rev.

Monomorium tenebrosum sp. nov.

Monomorium topend sp. nov.

Monomorium torrens sp. nov.

Key to species of the M. rothsteini species complex.

This key is designed for the identification of the majority of the most commonly encountered species in the M. rothsteini complex. All of these species, with the exception of M. leda , were recognised as separately evolving lineages in the mitochondrial DNA analysis of Sparks et al. (2014). The phylogenetic tree from that study is reproduced in Fig. 3 with the molecular clades labelled with the corresponding species names. Additional morphotypes are known to exist within the complex, but there is insufficient evidence available at this time to formally describe them. For species identifications that are particularly important it is strongly recommended that the CO1 sequence data are obtained and compared with those deposited in Genbank. All species exhibit some level of intraspecific variation that can in some cases lead to incorrect identification when using this key. It is recommended that a range of specimens from a colony are examined to determine the character state that best represents the majority of specimens.

1. Anterodorsal margin of the clypeus deeply concave, anterolateral carinae produced as acute angles, often with a wavy margin or minute preapical teeth medial to the acute angles ( Fig. 4 h, 6e, 6h, 7k).......................................... 2

- Anterodorsal margin of the clypeus not as above, may be concave but without acute anterolateral angles or preapical teeth... 5

2. Petiole node, postpetiole and anterior half of T1 completely covered in a fine reticulate pattern; very large, robust species (HW and ML> 1mm for a majority of specimens).......................................................... merepah

- Petiole and postpetiole with a fine reticulate pattern that does not extend onto dorsal surface; smaller species (HW and ML <1mm for a majority of specimens)........................................................................ 3

3. First metasomal tergite (T1) with fine reticulate sculpture restricted to anterior third......................... maryannae

- T1 with fine reticulate sculpture covering at least anterior half.................................................. 4

4. A majority of specimens with mesonotum sculpture present over entire area posteriad of promesonotal suture, sculpture consisting of longitudinal rows of shallow alveolae, rarely with a small central area without sculpture; known only from the Kimberley region of Western Australia.................................................................. stagnum

- Mesonotum sculpture restricted to promesonotal suture, posterior mesonotum and lateral curvature; central area always smooth; known only from the Cape York area of Queensland............................................. capeyork

5. Anterodorsal margin of the clypeus distinctly concave, with frontolateral carinae forming angular ridges that extend beyond anteroventral clypeal margin, forming obtuse angles ( Fig. 5 h, 5n, 8b)............................................ 6

- Anterodorsal margin of the clypeus sinuous to almost straight, frontolateral carinae forming smooth ridges that do not extend beyond anteroventral clypeal margin ( Fig. 4 a, 4k, 8e)........................................................ 15

6. Mesonotum sculptured over most of area posteriad of promesonotal suture ( Fig. 6 l), propodeum in lateral view with dorsolateral angles almost forming a right angle, such that propodeum appears cuboid ( Fig. 6 j)........................ mitchell

- Mesonotum with sculpture restricted to at most posterior half of area posteriad of promesonotal suture; propodeum in lateral view with dorsolateral angles forming an obtuse angle or rounded ( Fig. 5 g, 7a)..................................... 7

7. T1 with fine reticulate sculpture over at least anterior quarter; coarse lateral cephalic strigae reaching anterior margin of eye. 8

- T1 without sculpture, smooth and shining; coarse lateral cephalic strigae may or may not reach anterior margin of eye.... 10

8. Anteroventral margin of the clypeus without an anterior median projection, petiole node broadly rounded in posterior view........................................................................................... humilior (part)

- A majority of specimens with a median projection on the anteroventral margin of the clypeus; petiole node tapering to a rounded point in posterior view.......................................................................... 9

9. Mesonotum strigulate on lateral curvature, in metanotal groove and extending onto posterior mesonotum ( Fig. 7 c); petiole node narrow, width less than 2 x eye width when viewed from above; longitudinal strigae absent from dorsal surface of propodeum ( Fig. 7 c); known only from the Pilbara region of Western Australia.................................... pilbara

- Mesonotum almost entirely smooth, a few strigulae on posterior and lateral portions of posterior mesonotum ( Fig. 5 i); petiole node of medium breadth, between 2 and 2.5 x eye width when viewed from above; propodeal longitudinal strigae very weakly present as broken lines on posterior half only; known from the Victoria River district of the Northern Territory.... hoffmanni

10. Coarse lateral cephalic strigae not reaching anterior eye margin and transverse strigae absent from dorsal surface of propodeum

( Fig. 4 f, 5o)......................................................................................... 11 - At least some coarse lateral cephalic strigae reaching anterior eye margin; propodeal transverse strigae present on dorsal surface, although may be very weak ( Fig. 7 f, 6o, 8c)........................................................... 13

11. Medium to large species (ML> 1mm) with medium to small eyes (EW <0.23 x LHW).......................... leda

- Relatively small species (ML <1 mm) with large eyes (EW> 0.23 x LHW)...................................... 12

12. Petiole node without sculpture and narrow (<2 x eye width when viewed from above); head and mesosoma light to dark brown with a reddish-orange tinge; anteroventral margin of the clypeus with a small median projection........ broschorum

- Petiole node with fine reticulate sculpture laterally and a long posterobasal margin, of medium breadth (2–2.5 x eye width when viewed from above); head, mesosoma and legs amber orange to orange brown; anteroventral margin of the clypeus rarely with a small median projection................................................................. kidman

13. Longitudinal strigae absent from dorsal surface of propodeum and longitudinal depression very shallow or absent; propodeum appears dorsally rounded ( Fig. 7 f), dorsolateral angles of the propodeum rounded; petiole node of medium breadth (2–2.5 x eye width when viewed from above)............................................................... rothsteini

- Longitudinal strigae present on dorsal surface of propodeum either as raised convergent ridges or irregular convergent lines; longitudinal depression present; petiole node narrow (<2 x eye width when viewed from above)...................... 14

14. Longitudinal strigae on the dorsal surface of the propodeum converging anteriorly, forming a triangle with a carina that extends between the dorsolateral angles ( Fig. 8 c); head, mesosoma and legs dark chestnut brown, except trochanters light brown; known only from the Pilbara region of Western Australia........................................ tenebrosum

- Longitudinal strigae on the dorsal surface of the propodeum not forming a triangle, carina absent between the dorsolateral angles; head and mesosoma dark brown with a reddish-orange tinge; known only from the stony plains region of northern South Australia............................................................................... oodnadatta

15. Dorsolateral angles of the propodeum almost forming a right angle, such that the propodeum appears cuboid ( Fig. 4 j, 5a); propodeum laterally alveolate with irregular strigae extending onto dorsal surface or on posterior half of lateral surface; mesopleuron with at least a few strigae extending anteriorly from metapleural groove.............................. 16

- Dorsolateral angles of the propodeum clearly forming an obtuse angle ( Fig. 4 a, 5a), laterally alveolate without irregular strigae extending onto dorsal surface or on posterior half of lateral surface; mesopleuron with or without strigae............... 17

16. Mesonotum strigate-rugose over most of surface posteriad of promesonotal suture ( Fig. 4 l, 4o); sculpture on frons absent, a few strigae present only on antennal lobes ( Fig. 4 k, 4n); petiole node narrow, <2 x eye width when viewed from above; colour of head and mesosoma predominantly dark chestnut brown...................................... eremum / eremoides

- Mesonotum rugulose-strigate on lateral curvature and in metanotal groove, smooth medially ( Fig. 5 c, 8f); frons with strigae that extend well above antennal lobes ( Fig. 8 e); petiole node broad,> 2.5 x eye width when viewed from above ( Fig. 5 a); colour of head and mesosoma predominantly dark amber orange................................. geminum /topendese

17. T1 with a fine reticulate pattern over at least anterior third; colour of head and mesosoma predominantly dark amber orange18

- T1 without sculpture, smooth and shining; colour of head and mesosoma predominantly light to dark brown, sometimes with a reddish tinge........................................................................................ 19

18. Petiole node of medium breadth, 2–2.5 x eye width when viewed from above; a majority of specimens with fine reticulate sculpture restricted to anterior half or less; known only from the central desert region of the Northern Territory humilior (part)

- Petiole node width broad,> 2.5 x eye width when viewed from above; all specimens with T1 completely covered in a fine reticulate pattern; known only from Cape York Peninsula................................................. hertogi

19. Anterior transverse carina prominent on propodeum.................................................... bogischi

- Anterior transverse carina on propodeum very weak or absent................................................. 20

20. Dorsal surface of propodeum with a shallow longitudinal depression that runs most of its length; a majority of specimens without sculpture on the posterior mesonotum ( Fig. 8 i)...................................................... torrens

- Dorsal surface of the propodeum appears flat or slightly rounded along most of its length, a depression present only between posterodorsal angles; a majority of specimens finely strigulate on posterior mesonotum ( Fig. 7 i, 7o)... speculum / subapterum

Figures

FIGURE 2. Field images of the M. rothsteini (Forel) species complex. Fig. 2 a – c nest entrances showing (a) single entrance with discarded seed husks, (b) multiple entrances without seed husks and (c) single nest entrance without sees husks. Fig. 2 d M. subapterum sp. nov. feeding on nectar from Melaleuca sp. flowers.

FIGURE 3. Bayesian phylogeny based on 934 bp region of cytochrome oxidase I (COI) (Sparks et al. 2014). Nodes with> 0.95 Bayesian posterior probability support and> 75 % maximum likelihood bootstrap values are indicated with a star. Well supported morphologically cohesive terminal clades have been compressed for ease of interpretation. The molecular clades are labelled with the corresponding species names.

FIGURE 4. Workers of the M. rothsteini (Forel) species complex in lateral, head and dorsal view of the mesosoma. Fig. 4 a – c: M. bogischi, (Port Wakefield, SA, KSS 241), d – f: M. broschorum holotype (Dajarra, QLD, KSS 60), g – i: M. capeyork, holotype (69 km N Archer River Roadhouse, QLD, TERC 4), j – l: M. eremoides, holotype (9 km E 80 mile Beach, WA, KSS 89), m – o: M. eremum, holotype (33 km N Barkley Homestead, NT, KSS 77). Scale bars are 0.5 mm for all views.

FIGURE 5. Workers of the M. rothsteini (Forel) species complex in lateral, head and dorsal view of the mesosoma. Fig. 5 a – c: M. geminum, paratype (Litchfield NP, NT, ANA 10 – 22), d – f: M. hertogi, holotype (Lakefield NP, QLD, TERC 2), g – i: M. hoffmani, holotype (Mt Stanford Station, NT, TERC 70), j – l: M. humilior, (Alice Springs, NT, ANA 11 – 13), m – o: M. kidman, holotype (Top Springs, NT, TERC 71). Scale bars are 0.5 mm for all views.

FIGURE 6. Workers of the M. rothsteini (Forel) species complex in lateral, head and dorsal view of the mesosoma. Fig. 6 a – c: M. leda, lectotype (Kimberley District, WA), d – f: M. maryannae holotype (Berry Springs, NT, ANA 09 – 15), g – i: M. merepah, paratype (Lakefield NP, QLD, TERC 1), j – l: M. mitchell, holotype (Mitchell Falls, WA, TERC 17), m – o: M. oodnadatta, holotype (160 km E Mala, SA, KSS 114). Scale bars are 0.5 mm for all views.

FIGURE 7. Workers of the M. rothsteini (Forel) species complex in lateral, head and dorsal view of the mesosoma. Fig. 7 a – c: M. pilbara, holotype (4 km S Newman, WA, KSS 188), d – f: M. rothsteini, (Alice River, QLD, ES 38), g – i: M. speculum paratype (Niagara Dam, WA, KSS 218), j – l: M. stagnum, holotype (Mitchell Falls, WA, TERC 11), m – o: M. subapterum, (160 km E Mala, SA, KSS 116). Scale bars are 0.5 mm for all views.

FIGURE 8. Workers of the M. rothsteini (Forel) species complex in lateral, head and dorsal view of the mesosoma. Fig. 8 a – c: M. tenebrosum, holotype (10 km N Tom Price, WA, KSS 177), d – f: M. topend, holotype (Darwin, NT, TERC 75), g – i: M. torrens, paratype (Flinders Rangers, SA, KSS 4). Scale bars are 0.5 mm for all views.

FIGURE 9. Queens and males of the M. rothsteini (Forel) species complex in lateral, head and dorsal view of the mesosoma. Fig. 9 a – c: M. bogischi queen (light form), paralectotype (Port Wakefield. SA), d – f: M. bogischi queen (dark form) (Birdsville, QLD, KSS 46), g – i: M. humilior queen, paralectotype (Tennant Ck, NT), j – l: M. humilior male (Tennant Creek, NT), m – o: M. subapterum queen, paralectotype (Harding River, WA). Scale bars are 1 mm for all views.