Myrmecia pilosula

Taylor, Robert W., 2015, Ants with Attitude: Australian Jack-jumpers of the Myrmecia pilosula species complex, with descriptions of four new species (Hymenoptera: Formicidae: Myrmeciinae), Zootaxa 3911 (4), pp. 493-520 : 509-513

Myrmecia pilosula Fr. Smith (Eastern Race)

( Figs 10–12)

The Eastern Race of Myrmecia pilosula is abundant at moderate to high elevations in components of the Great Dividing Range in eastern Victoria, eastern NSW and the ACT. Its most northern records are in the high ranges east of the New England Tableland in northern central-eastern NSW, including mile-high Point Lookout and nearby Cathedral Rock National Park. It has not been collected at lower elevations westward on the Tableland proper towards Armidale, where M. croslandi and M. impaternata are sympatrically common. In the southwest of its range M. pilosula (Eastern Race) is found in the vicinity of Melbourne, west to Ballarat across central Victoria and southeast to Mornington Peninsula, where its distributional range encounters that of the putatively conspecific Western Race.

JACP vouchers recognized here as the Eastern Race of M. pilosula were discussed by Imai, Taylor et al. (1994) as possible representatives of an unnamed species which they speculated might have originated as a hybrid between M. pilosula and M. banksi , identified by the codes PB–1 (= pilosula banksi –1) and PB–2 (see following paragraph). Appropriate species-characteristic karyological differences between M. banksi and “ M. pilosula s. str. ” were reviewed, but they were not cited in support of the hybridization hypothesis, which was based entirely on morphotaxonomic characters involving leg coloration discussed above under M. pilosula (Western Race) (see Imai, Taylor et al., 1994, Fig 10).

The Imai, Taylor et al. (1994) proposition posited that the similarities in leg color between the PB variants (PB1, PB2) and M. banksi (all with brown hind tibiae), and the differences between them and the Western Race of M. pilosula (cited as “ M. pilosula s. str. ”) (with reddish-orange hind tibiae), indicated that the PB entities had originated through hybridization between M. banksi and M. pilosula (Western Race). PB entities were shown (as here) to occupy a (very large) territory situated geographically between the current ranges of banksi and pilosula . I have consistently demurred from this interpretation (e.g. see Imai, Taylor et al., 1994, p 152). In my opinion the leg coloration characters shared by banksi and M. pilosula (Eastern Race) are more likely derived synapomorphically from a common ancestor, or the character “brown tibiae” might be a homoplasy and not homologous at all in these species.

The 1994 geographical argument, while based on confirmed contemporary distribution patterns, failed to acknowledge that the purported hybridization event likely occurred at least hundreds of thousands, or more probably more than a million years ago; that very different distribution patterns on a geographically and environmentally different terrestrial substrate would have prevailed and that chromosomal rearrangements could have occurred subsequently. Nor did it acknowledge that the Eastern Race of M. pilosula has an extensive distributional range which widely separates the current ranges of its implied hybridic parent species M. banksi and western M. pilosula . The argument also failed to consider that a species originally derived as a hybrid between M. banksi (2n=9–10) and M. pilosula (Western Race) (2n=22–30) would likely be allodiploid, with unmatched haploid chromosome sets, as exemplified here by M. impaternata , an evidentially well supported putative hybrid between M. banksi and a component of the Eastern Race of M pilosula (see elsewhere in these pages).

The reported karyological differences contrasting the Eastern and Western Races of M. pilosula are discussed above.

Imai, Taylor et al. (1994) also recognized hybrid “back-cross” variants “P/PB–1, Mix” and “P/PB–2 Mix” known only from Tasmania. These were hypothesized to have originated as crosses between M. pilosula s.str with PB-1 and/or PB-2. They were recognised as discordant worker individuals collected from otherwise normal “ M. pilosula s. str. ” colonies, and identified only by features of leg coloration. They are assigned here to M. pilosula (see above).

Deposition of voucher specimens. AMSA, ANIC, MVMA, QMBA, SAMA, WAMA, TMHA, BMNH, CASC, MCZC, MHNG.

Distribution. M. pilosula (Eastern Race) is known to range southwards in NEW SOUTH WALES from the mountains east of the New England Tableland, along the Great Dividing Range and its flanks, including the Snowy Mountains, the Blue Mountains, and the Brindabella Ranges in the AUSTRALIAN CAPITAL TERRITORY, to the Victorian Alps and their slopes and the Gippsland coast in eastern VICTORIA. The known Victorian distribution carries westwards at least as far as Ballarat in the elevated country which extends the Great Dividing Range westwards across central Victoria north of Melbourne. This distribution evidently fails to carry approximately 120 km further west to the Grampians Range, SA, which is generally considered the western terminus of the East Australian Cordillera, because there the western putative race, M. pilosula (W), is exceptionally common at Halls Gap, its surrounds and elsewhere (as also southwards in the Hamilton district), and the Eastern Race of M. pilosula is unreported. Two damaged specimens from Ararat, VIC (MVMA), not far northeast of The Grampians, are identified as M. pilosula (Eastern Race). There are records of the Eastern Race from the SE corner of Victoria near Mallacoota, westwards to the Mornington Peninsula.

Colonies of the two races of M. pilosula appear to intersperse in Victoria SE of Melbourne, on or immediately east of the Mornington Peninsula, which they enter from the east and west respectively. There is no sound current morphological evidence of gene introgression between these taxa in Victoria, or evidence that they hybridize in the area of range overlap, and on these grounds it could be concluded that they represent separate, related, reproductively isolated, apparently parapatrically distributed, but previously allopatrically evolved, sister species. Their separate distinctive karyologies could support this conclusion. However, several known series of specimens from the Melbourne area are believed to evidence colonies containing workers representative of both forms (though the specimen labels do not specify the defining collection circumstances). Such “mixed” colonies are frequent and well confirmed in collections from Tasmania, where their presence was considered by Imai, Taylor et al. (1994) to evidence introgression of genes from the eastern pilosula race to its western counterpart, even though “pure” colonies of eastern habitus are unknown from Tasmania (see above). This evidence of interbreeding with gene introgression is the reason why the two designated forms are recognized here as geographical races of M. pilosula , rather than separate, putatively reproductively-isolated species. The present argument would presumably apply even if mixed colonies are not present in Victoria, in light of the Tasmanian evidence for crossing with introgression—though it is entirely plausible that two parapatrically distributed species/semispecies could experience introgression in some populations but not in others.

Material examined. The extreme northern record is from Cathedral Rock National Park [-30 25, 152.15], near Ebor, NSW, Imai, Kubota & Taylor, 27–11–1999 (HI 99–08) (Chromosomes: N=14). JACP records cited here have HI– accession numbers. Unless indicated other accessions are AAVAS: AUSTRALIAN CAPITAL TERRITORY: Condor Creek [-35 19, 148 50], HI85–376; Gudgenby [-35 46, 148 55], -Naas Creek HI87–134; Brindabella [-35 34, 148 57], 4/xii/2005; Picadilly Circus [-35 22, 148 48], HI85–373, 374, 378, 379, 89–003, 0 33, 91–051; Tidbinbilla [-35 28, 148 55], HI87–146, 147; Uriarra [-35 15, 148 55], 2/xii/2005. NEW SOUTH WALES: Blackheath [-35 38, 150 17], 950m (-33 38, 150 20) PSW; Braidwood [-35 26, 149 48], 5/xi/2006; Bredbo (-35, 57, 149 09), PSW; Captains Flat [-35 35, 149 57], HI87–141–145; Cathedral Range [-30 25, 152 15], HI87–222, 223; Corang River Bridge (35 12, 150 03), 3/xii/2005; Corang River Bridge [-35 12, 150 03], HI87–137–140; Cudgewa Bluff [-30 10, 149 25], HI87–227–229; Geehi River (-36 23, 48 10), 28/xi/2006; Island Bend, 1350m (-36 20, 148 28) PSW; Jindabyne (-36 23, 148 42), 1/x/2006; Kanangra Boyd Nat. Pk. [-33 59, 150 5], 1200m PSW; Kanangra Walls Road [-33 59, 150 8], 30/xi/2006; Lawson [-33 43, 150 26], HI87–111–115; Leather Barrel Creek (-36 31, 148 11) 28/xi/2006; Lochinvar [-32 42, 151 27], 23/vii/2006; Londonderry, 50m (-33 40, 150 46) PSW; Medlow Bath [-33 41, 150 17], HI87–116; Mongarlowe [-35 21, 150 0], HI85–192, 193–195, 85–227–233; Murray 1 Power Station [-36 24, 148 19], HI87–231–232; Murrumbidgee River, 29/xi/2006; Northangera [-35 28, 149 55], 3/xii/ 2005; Shannons Flat [-35 56, 148 57], HI87–131–133, HI91–077–079; Shoalhaven River nr Larbert [-35 18, 149 46], HI91–081; South Bowenfels [-33 31, 150 7], 1/ii/2007; Tumbarumba [-35 47, 148.1], 14/viii/2006; Wadbilliga [-33 43, 15023], HI87–163, 164; Wambrook Creek [-36 11, 148 56], HI87–128–130; Wentworth Falls [-33 43, 150 23], HI87–117–121; Wyndham [-35 57, 147 7], 5/vii/2007. VICTORIA: Ballarat (-37.32.647, 143.54.984); Ballarat [-37 34, 143 52], (MVMA); Baxter [-38 12, 145 19], (MVMA); Buchan [-37 25, 148 10], RWT; Daylesford [-37 20, 144 9], (MVMA); Christmas Hills [-37 39, 145 18], RWT; Dartmouth [-36 45, 147 39], (MVMA); Dry Diggings (-37.16.982, 44.09.598); Eltham [-32 43, 145 8], (MVMA); Genoa [-37 29, 149 32], RWT; Gisborne [-37 29, 144 36], (MVMA); Harrietville (-36.53.562, 147.03.662); Healesville [-37 39, 145 31], (MVMA); Heathmont [-37 50, 145 15], (MVMA); Jericho [-36 55, 147 3], (MVMA); Langwarrin [-38 9, 145 12], (MVMA); Maldon [-36 59, 144 4], (MVMA); Mansfield [-37 3, 146 5], (MVMA); Marysville [-37 31, 145 45], HI87–220, 221; Merrijig, 12km SE [-37 39, 147 27±12k], (MVMA); Mt Buffalo [-36 44, 16 47], HI87–226; Mt Donna Buang [-37 42, 145 41], (MVMA); Mt Donna Buang [-37 42, 145 41], HI87–219; Mount Martha, N of Heywood [-38 16, 145 2], HI89–015, 016; Orbost [-37 42, 148 27], RWT; Powers Lookout [-36 51, 146 22], (MVMA); Riddell [-37 28, 148 27], (MVMA); Sarsfield [-37 45, 147 44], HI89–022–225; Warrandyte South [-37 46, 145 15], HI87–211, 212; Tawonga South (-36.44.925, 147 09.493); Wandiligong [-36 48, 146 58], 16/xii/2006; W of Genoa [-37 29, 149 32], HI89–026–028; Wesburn [-37 466, 145 39], (MVMA).

Worker diagnosis. General features as illustrated and in key couplets 1, 2, 5 & 6 above. Distinguished morphologically from the Western Race of M. pilosula largely by leg coloration (see key). Expression of this character-set in pilosula (Eastern Race) is generally similar to that of M. pilosula (Western Race) (see below), but the tibiae and tarsi are dull medium brown in color, lighter than the proximal leg segments, where those of M. pilosula (Western Race) are brightly reddish-brown, much like the fore-tarsi. The color difference between these forms is clearly distinguishable even in old, dry cabinet specimens. They correlate exactly with the karyological differences distinguishing the two taxa.

Sculpturation of large individuals is more intensely developed than in small specimens, with gradation in intermediate series. Large workers can be similar to those of M. croslandi (including details of leg coloration) and readily misidentified (see couplet 6 in the key to species above). Such specimens can most confidently be identified if associated with smaller nest mates or field associates. Single, unassociated large workers can be problematic.

Dimensions. (Holotype, smallest paratype, largest paratype (mm): TL = 1 27, 10.27, 12.26; HW = 2.28, 2.04, 2.37; HL = 2.07, 1.81, 2.14; CI = 110, 112, 111; EL = 0.86, 0.79, 0.92; OI = 38, 39, 38; SL = 1.91, 1.65, 1.93; SI = 84, 81, 81; PW = 1.42, 1.18, 1.53; WL = 3.28, 3.02, 3.47; PetW = 0.86, 0.68, 0.87; PpetW = 1.38, 1.10, 1.38.

Karyology. Chromosome numbers determined by Imai are given in Imai, Taylor et al., 1994, appendix. The overall range is slightly greater than in M. pilosula (Western Race), with 2N=18 to 2N=32 (with all intermediate values represented except 2N=27 and 2N=31), and with a single recorded haploid count of N=15. Karyologically M. pilosula (Eastern Race) is distinguished from M. pilosula (Western Race) by remarkably elongated C-band polymorphisms and chromosome arrangements involving centric fusion (fusion burst) (Imai, Taylor et al., 1994, fig. 11). Alternatively, “Fission burst’ characterizes the karyology of M. pilosula (Western Race).

Field associations. Sympatric with M. haskinsorum at most mainland sites from which the latter is known, variously with M. croslandi and with M. pilosula (Western Race) east and southeast of Melbourne. Myrmecia pilosula (Eastern Race) is not known from suburban Canberra where M. croslandi and M. impaternata are common, but it is common at higher elevations in the nearby Brindabella Ranges and other local elevated areas, and in the Snowy Mountains.

Research prospects. Prospective research topics involving the Eastern and Western Races of M. pilosula include (1) genetical investigation of both races, to challenge the Tasmanian inrogression hypothesis discussed above, and to clarify whether these taxa are genuinely parapatric in distribution in Victoria; (2) investigation of the several implied Victorian areas of geographical range contact or intersection, with search for possible genetical introgression between the two taxa, notably in West Gippsland and areas between Ballarat and the Grampians, possibly also at other localities on the eastern flanks of Melbourne; (3) investigation of comparative bionomics of both taxa in contact versus non-contact zones; and (4) locating the distributions of these two races and other Jackjumper species in the Melbourne area and greater Victoria, and of the Eastern Race in the ACT and surrounding NSW.