Pheidole megacephala

An aggressive species that can form large colonies. Fischer and Fisher (2013): Pheidole megacephala – commonly also known as the ‘African big-headed ant’ – is a globally distributed invasive ant species and can be found on almost every island in the Malagasy region. This species is considered one of the most problematic invasive ant species and has caused devastating environmental problems all over the world (Hoffmann et al. 1999, Hoffmann & Parr 2008, Wetterer 2012). First described by Fabricius from the former ‘Ile de France’, called Mauritius today, the origins of this species remain unknown. Pheidole megacephala and the megacephala group are probably native to the Afrotropics (Wetterer 2012), although a Malagasy origin is also a possibility, as the whole group seems to be quite diverse in Madagascar.

Identification
Wilson (2003), in his description of the megacephala species group: The single species of this African complex known from the New World, megacephala, is a tropicopolitan tramp globally spread by human commerce. In the New World, it occurs spottily in disturbed habitats from southern Florida throughout the West Indies to Mexico, Central America, and South America as far as southern Brazil. Its affinities will be unknown until the African Pheidole fauna is better analyzed. If I were handed specimens and told they belonged to a species native to the New World, I would be inclined on the basis of superficial appearance to place megacephala in the tristis group--except for its possession of only 2 hypostomal teeth, unique postpetiole shape, and unicolonial, polygynous colony structure. A good formula for recognizing megacephala at a glance is: heart-shaped head, circular mesonotum, oval postpetiole.

Fischer and Fisher (2013) - Moderately small species (WL major 0.99–1.22 mm, WL minor 0.62–0.84 mm), with moderately short scapes (SI minor 114–122, major SI 47–58) and legs (FI minor 114–125, FI major 65–73), both workers subcastes with acute short-spinose propodeal spines, absent promesonotal process, and postpetiole with large convex ventral process, mesosoma with relatively few, usually stiff and often blunt or apically bifurcate standing hairs, scape pilosity subdecumbent to suberect, hind tibia pilosity on inner edge decumbent, along outer edge subdecumbent to suberect. Major head often heart shaped, with moderately deep posterior emargination, short-irregular longitudinal rugae anteriorly and sculpture absent from posterior 2/5 of head, except for two weak to superficial longitudinal rugulae medially, hypostomal margin without median process and very small to inconspicuous submedian teeth, promesonotum almost completely smooth and shiny, remainder of mesosoma weakly punctate, smooth or superficially sculptured spots usually present on both katepisternum and lateropropodeum, postpetiole on average 1.9 times wider than petiole (PpWI 171–210). Minor head shape usually ovoid and posterior head margin slightly compressed, head almost as wide as long (CI 86–92), face completely smooth and shiny, except for sometimes weakly developed concentric rugulae around antennal insertions, postpetiole on average 0.8 times as long as petiole (PpLI 70–91).

Malagasy Region
Morphologically, both the minor and major workers of P. megacephala display relatively few diagnostic characters to set them apart from other members of the group. On the Comoros P. megacephala also occurs together with the two newly described species Pheidole decepticon and Pheidole megatron. The minor workers of the latter two species are very similar in morphometric measurements and external morphology to those of P. megacephala, and single major workers possess few clearly defined features that can be used for definite species identifications (also see the identification section of these other two species). Because P. megacephala is a morphologically variable species, series with several major workers are much more helpful for positive identifications. Especially the majors of P. megacephala and P. decepticon are in many characters almost identical and both vary significantly in their development of the typically heart-shaped head. The most apparent and stable differences, however, seem to be the amount of sculpture on the posterior half of the major worker’s head, the type and overall quantity of standing hairs on the meso- and metasoma, as well as the shape of the postpetiolar ventral processes. Further minor differences in the morphologies of P. decepticon suggest ongoing differentiation between populations on the islands where it occurs.

Distribution
New World - Widespread although spottily distributed, and sometimes locally very abundant, from southern Florida, Bermuda, and the Bahamas south through the West Indies, southern Mexico, and Central America, to as far south in South America as Santa Catarina, Brazil (Wilson 2003). Detected in southern California in 2014 (see AntWeb) and currently subject of active control measures.

It is widespread in south and central Florida north to St. Augustine (St. Johns Co.), but sporadic, often occurring as a huge polydomous population, for example at the Selby Gardens in Sarasota County. In Hawaii this is the dominant ant in many habitats (Huddleston and Fluker 1968), with disastrous effects on the native arthropods, but in Florida it is a minor species. Pest status: where concentrations of this species occur, large groups of scavenging workers may invade buildings. First published Florida record: Smith 1933. (Deyrup, Davis & Cover, 2000.)

Distribution based on Regional Taxon Lists
Afrotropical Region: Cape Verde, Gambia, Guinea, Ivory Coast, Kenya, Mali, Mozambique, Rwanda, Senegal, Sierra Leone, Uganda, United Arab Emirates, Zimbabwe. Australasian Region: Australia, New Caledonia, New Zealand. Indo-Australian Region: Borneo, Cook Islands, Fiji, Guam, Hawaii, Indonesia, Kiribati, Malaysia, Marshall Islands, Micronesia (Federated States of), New Guinea, Niue, Northern Mariana Islands, Palau, Philippines, Samoa, Solomon Islands, Tokelau, Tonga, Vanuatu, Wallis and Futuna Islands. Malagasy Region: Madagascar, Mauritius, Mayotte, Réunion, Seychelles. Nearctic Region: United States. Neotropical Region: Aruba, Bahamas, Barbados, Bermuda, Brazil, Colombia, Costa Rica, Cuba, Dominican Republic, Ecuador, French Guiana, Greater Antilles, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Lesser Antilles, Mexico, Netherlands Antilles, Nicaragua, Panama, Peru, Puerto Rico, Saint Lucia, Saint Martin (French part), Suriname, Trinidad and Tobago, Venezuela. Oriental Region: India, Nicobar Island, Thailand, Vietnam. Palaearctic Region: Canary Islands, China, Egypt, Greece, Iran, Italy, Japan, Montenegro, Oman, Republic of Macedonia, Romania, United Kingdom of Great Britain and Northern Ireland.



Biology
This ant is predaceous and scavenging, and may attend Homoptera for honey dew.

From Wilson (2003): The colonies, which are continuous, with no evident pheromone-based boundaries, and large numbers of fertile queens, are able to reach enormous size. In some areas, especially islands such as Madeira, Culebrita, and the Dry Tortugas, they form a virtually continuous supercolony that excludes most other ant species. They do best in relatively moist, disturbed habitats, thus thrive around human habitations and in cultivated land. Nest sites are highly variable, from within and beneath rotting logs and underneath rocks and sidewalk flagstones to the bark and trunk-based detritus of standing trees. Columns of foragers travel substantial distances from one nest site to another and to food sources. P. megacephala are aggressive toward other species, and war with populations of such locally dominant species as the Indo-Australian weaver ant Oecophylla smaragdina and the cosmopolitan Argentine ant Linepithema humile (= Iridomyrmex humilis). General accounts of this important species are given by Wilson (1971), Holldobler and Wilson (1990), and D. F. Williams et al. (1994). A bibliography of the ant for North America is provided by D. R. Smith (1979). The devastating effect on the native Hawaiian insect fauna was described by the pioneering entomologist R. C. L. Perkins (1913).

While this species nests most commonly outside, when found in buildings they occupy ceilings, wall cavities, power outlets and nearly any suitable space.

Bertelsmeier et al. (2015a, b) examined elements of interspecific aggression, and food resource discovery and dominance, between this species and several other highly invasive ants. In laboratory assays Pheidole megacephala was aggressive when confronted with workers of other invasive ants. Of the group of four species that were found to be aggressive, P. megacephala was the most adept at finding and recruiting to food in a laboratory arena experiment. It was also found to be the least effective aggressor of the group of four invasive ants examined.

Eguchi (2008) - For detailed information on biology and ecological and economic impacts of this species see Reimer et al. (1993), Campbell (1994), Hoffmann (1998), Wetterer (1998), Hoffmann et al. (1999), Vanderwoude et al. (2000), etc.

WA Australia
Though identified from stray specimens (possibly in cargo) much earlier, the pest first made its presence known as a colonist in South Perth in 1942 (May and Heterick 2000). The ant now occupies much of the metropolitan area, and is probably assisting, along with the Argentine Ant (Linepithema humile), with the ongoing demise of Perth’s native ant fauna (Heterick et al. 2002). This pest can also be seen in many cities and towns throughout WA.

Florida
Deyrup, Davis & Cover (2000): Pheidole megacephala, like Linepithema humile, has been in Florida for a long time and is the dominant ant on a few sites. These two species appear to be mutually exclusive, and vie for supremacy in Bermuda (Haskins and Haskins 1965) and Hawaii (Reimer 1994). In Florida, P. megacephala occurs from Tampa through the Florida Keys, with one record from the coast of St. Johns Co. Its rarity in north Florida is probably due to cooler winter temperatures. We have no explanation why it is not a dominant species in tropical and subtropical parts of the state.

Castes
For additional images see Pheidole megacephala image gallery.

Worker
Minor

Nomenclature

 * edax. Formica edax Forskål, 1775: 84 (w.) EGYPT. Junior synonym of megacephala: Emery, 1892b: 160; Dalla Torre, 1892: 90. [If synonymy correct then edax is the senior name; however, under Art. 23.9 of ICZN (1999) edax is a nomen oblitum.]
 *  megacephala. Formica megacephala Fabricius, 1793: 361 (s.) no locality given. Latreille, 1802c: 232 (q.); Mayr, 1861: 70 (s.w.q.m.); Wheeler, G.C. & Wheeler, J. 1953b: 75 (l.). Combination in Pheidole: Roger, 1863b: 30. Senior synonym of trinodis: Roger, 1863b: 30; of edax: Dalla Torre, 1892: 90; Emery, 1892b: 160 (see note under edax); of perniciosa: Emery, 1915j: 235; of pusilla (and its junior synonyms janus, laevigata Smith, laevigata Mayr): Wheeler, W.M. 1922a: 812; of suspiciosa: Donisthorpe, 1932c: 455; of testacea: Brown, 1981: 530; of agilis: Eguchi, 2008: 56; of bernhardae, gietleni, picata, scabrior: Fischer & Fisher, 2013: 333. [P. megalocephala Schulz, 1906: 155; unjustified emendation.] Current subspecies: nominal plus costauriensis, duplex, ilgi, impressifrons, melancholica, nkomoana, rotundata, speculifrons, talpa. See also: Eguchi, 2001b: 77; Wilson, 2003: 549.
 * trinodis. Myrmica trinodis Losana, 1834: 327, pl. 36, fig. 6 (w.) ITALY. Junior synonym of megacephala: Roger, 1863b: 30.
 * pusilla. Oecophthora pusilla Heer, 1852: 15, pl. 1, figs. 1-4 (s.w.q.m.) PORTUGAL (Madeira I.). Combination in Pheidole: Smith, F. 1858b: 173. Subspecies of megacephala: Emery, 1915j: 235. Senior synonym of janus: Mayr, 1886c: 360; of laevigata Smith: Roger, 1859: 259; Emery, 1915j: 235; of laevigata Mayr: Mayr, 1870b: 981 (footnote). Junior synonym of megacephala: Wheeler, W.M. 1922a: 812.
 * agilis. Myrmica agilis Smith, F. 1857a: 71 (w.) WEST MALAYSIA. Combination in Pheidole: Donisthorpe, 1932c: 449. Junior synonym of megacephala; Eguchi, 2008: 56.
 * janus. Pheidole janus Smith, F. 1858b: 175, pl. 9, figs. 13-17 (s.w.) SRI LANKA. Junior synonym of pusilla: Mayr, 1886c: 360.
 * testacea. Atta testacea Smith, F. 1858b: 168 (s.w.) BRAZIL. Combination in Pheidole: Mayr, 1886c: 360. Junior synonym of megacephala: Brown, 1981: 530.
 * perniciosa. Oecophthora perniciosa Gerstäcker, 1859: 263 (w.) MOZAMBIQUE. [Also described as new by Gerstäcker, 1862: 516.] Combination in Pheidole: Roger, 1863b: 31. Junior synonym of megacephala: Emery, 1915j: 235.
 * suspiciosa. Myrmica suspiciosa Smith, F. 1859a: 148 (w.) INDONESIA (Aru I.). Junior synonym of megacephala: Donisthorpe, 1932c: 455.
 * scabrior. Pheidole megacephala var. scabrior Forel, 1891b: 178 (s.w.) MADAGASCAR. Junior synonym of megacephala: Fischer & Fisher, 2013: 333. See also: Forel, 1897c: 188.
 * picata. Pheidole megacephala var. picata Forel, 1891b: 178 (s.w.) MADAGASCAR. Subspecies of megacephala: Forel, 1895a: 49; of punctulata: Forel, 1897c: 186; Forel, 1905b: 163; Santschi, 1910c: 370. Raised to species: Emery, 1915j: 245; Wheeler, W.M. 1922a: 1019. Junior synonym of megacephala: Fischer & Fisher, 2013: 333.
 * gietleni. Pheidole punctulata r. gietleni Forel, 1905b: 164 (s.w.) MADAGASCAR. Subspecies of picata: Emery, 1915j: 245. Junior synonym of megacephala: Fischer & Fisher, 2013: 333.
 * bernhardae. Pheidole picata var. bernhardae Emery, 1915j: 245 (s.w.) MADAGASCAR. [First available use of Pheidole punctulata r. spinosa var. bernhardae Forel, 1905b: 164; unavailable name.] Junior synonym of megacephala: Fischer & Fisher, 2013: 333.

Fischer and Fisher (2013) - Possibly as a result of its wide distribution and local abundances, a number of subspecies and synonyms have accumulated during the nineteenth and early twentieth century. Several of the subspecies are probably synonyms (unpublished data), and a number of described and undescribed related species add to the taxonomic confusion within the megacephala group.

Description
Wilson (2003) - Major and minor: in side view, entire postpetiole oval in shape, with all of the ventral margin bulging in a conspicuous convexity, and the node oval, low, and weakly developed; mesonotal convexity absent, the promesonotal profile forming a nearly smooth semicircle; color brownish yellow.

Major: outline of head plus mandibles in full-face view forms a near-perfect heart shape; rugoreticulum present between eye and antennal fossa.

Minor: occiput broad, lacking an occipital collar.

MEASUREMENTS (mm) Major (Grand Bahama Island): HW 1.32, HL 1.32, SL 0.64, EL 0.18, PW 0.60. Minor (Grand Bahama Island): HW 0.54, HL 0.62, SL 0.66, EL 0.12, PW 0.34.

COLOR Major and minor: brownish yellow.

Fischer and Fisher (2013) - Major Measurements (neotype): HW 1.29, HL 1.28, SL 0.69, MDL 0.67, EL 0.17, WL 1.04, PNH 0.45, PNW 0.60, MNH 0.45, PDH 0.34, PTL 0.38, PPL 0.26, PTH 0.22, PPH 0.26, PTW 0.18, PPW 0.36, PSL 0.15, MFL 0.92, MTL 0.66, CI 101, SI 54, MDI 52, EI 13, FI 71, PSLI 12, LPpI 100, DPpI 137, PpWI 197, PpLI 68, PpHI 118.

(n=19): HW 1.10–1.54 (1.30), HL 1.04–1.59 (1.30), SL 0.59–0.76 (0.67), MDL 0.59–0.82 (0.70), EL 0.15–0.19 (0.17), WL 0.94–1.22 (1.06), PNH 0.36–0.54 (0.45), PNW 0.54–0.74 (0.62), MNH 0.61–0.80 (0.68), PDH 0.33–0.53 (0.37), PTL 0.31–0.44 (0.38), PPL 0.23–0.29 (0.26), PTH 0.20–0.28 (0.23), PPH 0.22–0.31 (0.26), PTW 0.14–0.22 (0.18), PPW 0.27–0.45 (0.34), PSL 0.13–0.20 (0.16), MFL 0.77–1.06 (0.89), MTL 0.59– 0.77 (0.67), CI 97–106 (101), SI 47–58 (52), MDI 51–57 (54), EI 11–15 (13), FI 65–73 (69), PSLI 11–14 (12), LPpI 90–114 (100), DPpI 108–156 (134), PpWI 171–210 (190), PpLI 61–81 (69), PpHI 105–123 (114).

Head as long as wide (CI 97–103), usually heart shaped with strongly convex sides and posterior emargination moderately deeply impressed. Mandibles moderately long (MDI 51–57), smooth and shiny. Clypeus smooth with median carina short or absent and one or two pairs of lateral carinae. Frons and sides of head anteriorly, longitudinally rugose, interspaces superficially punctate at frons, punctate or weakly punctate at sides, with a few longer rugae ventral of eyes, posterior 2/5 of head smooth and shiny, except for two weakly to faintly developed longitudinal rugae medially and, rarely present, short, faint rugulae submedially at posterior emargination. Sides of head dorsal and ventral of eyes in profile with few irregular rugae extending up to posterior 1/4 of head, space in between, posterior of eyes, largely unsculptured. Frontal carinae and antennal scrobe absent. Scapes of medium length (SI 47–58) with decumbent to subdecumbent pilosity and few longer, suberect hairs along outer edge. Submedian hypostomal teeth very small to inconspicuous, median process absent. Promesonotum moderately high-domed, in profile usually subangulate posteriorly, humeri in dorsal view not produced, promesonotal process not conspicuously produced, posterior declivity often weakly marginate anteriorly (at posterior end of pronotum), oblique and angulate at its lower portion. Surface of promesonotum and lateropronotum mostly smooth to superficially punctate, with few faint, irregular, transverse rugulae, remainder of mesosoma weakly punctate, with smooth or superficially punctate spots between. Metanotal groove narrow, barely or not impressed, and with few superficially developed cross-ribs. Spines short-spinose to subtriangular in profile (PSI 11–14). Metatibia relatively short (FI 65–72), metafemur pilosity on inner edge decumbent, on outer edge suberect to subdecumbent. Postpetiole in profile about as high as long (LPpI 90–114), with large convex ventral process, in dorsal view trapezoid and on average 1.9 times wider than petiole (PpWI 171–210). Dorsum of petiole and postpetiole smooth, remainder weakly punctate. Gaster smooth. Amount and length of standing hairs on body dorsum variable, usually with few long hairs and some shorter hairs, with several hairs apically blunt or bifurcate. Fine, short pilosity not abundant. Color variable from light to dark brown, head and gaster often darker than the rest.

Minor Measurements (n=20): HW 0.50–0.61 (0.55), HL 0.57–0.68 (0.62), SL 0.61– 0.72 (0.65), MDL 0.32–0.41 (0.36), EL 0.12–0.15 (0.13), WL 0.62–0.84 (0.73), PNH 0.22–0.29 (0.25), PNW 0.32–0.39 (0.35), MNH 0.42–0.53 (0.46), PDH 0.21–0.27 (0.24), PTL 0.22–0.28 (0.24), PPL 0.17–0.22 (0.19), PTH 0.13–0.16 (0.14), PPH 0.16–0.19 (0.14), PTW 0.09–0.11 (0.10), PPW 0.17–0.21 (0.18), PSL 0.07–0.10 (0.08), MFL 0.57–0.76 (0.67), MTL 0.44–0.57 (0.51), CI 86–92 (90), SI 114–122 (118), MDI 63–70 (66), EI 22– 26 (24), FI 114–125 (121), PSLI 13–16 (14), LPpI 94–120 (109), DPpI 89–112 (98), PpWI 164–200 (180), PpLI 70–91 (79), PpHI 110–146 (125).

Head shape roundly ovoid, slightly longer than wide (CI 86–92), sides convex, posterior head margin rounded laterally, medially sometimes slightly compressed, occipital carina very narrow in full-face view, often weakly impressed medially. Mandibles relatively short (MDI 63–70), smooth. Clypeus and remainder of face smooth, clypeal carinae absent or inconspicuous, around antennal insertion few concentric rugulae present and often weakly to superficially developed. Scapes relatively short (SI 114–122), when laid back surpassing posterior head margin by slightly more than the length of tenth funicular segment, with subdecumbent to suberect pilosity. Promesonotal outline in lateral view roundly convex, usually evenly declining from highest point toward metanotal groove, posterior process absent. Metanotal groove in profile weakly impressed, with dorsally weak to laterally superficial cross-ribs. Propodeum about as long as high, in profile slightly declining toward spines. Propodeal spines much shorter than distance between their bases, short-spinose to subtriangular and acute (PSLI 13–16, mean: 14). Promesonotum largely smooth, except superficial punctures anteriorly near the neck, mesopleuron and propodeum punctate to weakly punctate, often with superficially sculptured to smooth areas laterally. Metafemur short (FI 114–125), metatibia with decumbent pilosity and scattered suberect longer hairs on outer edge. Postpetiole in profile with large convex ventral process, on average 1.1 times longer than high (LPpI 94–120), and 0.8 times as long as petiole (PpLI 70–91). Dorsum of petiole and postpetiole smooth, remainder weakly to superficially punctate. Gaster smooth and shiny. Standing hairs short to moderately long, relatively coarse, erect to suberect, not abundant, and usually with some to several hairs apically split or bifurcate. Color usually light brown, sometimes brown, gaster darker brown.



'''Wilson 2003. Upper: major. Lower: minor. BAHAMAS: Grand Bahama Island. Scale bars = 1 mm.'''

Type Material
Fischer and Fisher (2013) - The types are presumed lost. No type specimens could be located in both of the two possible collections – the Zoological Museum of the University of Copenhagen and the Muséum National d'Histoire Naturelle in Paris. To stabilize the name P. megacephala we designated a neotype in this publication. The neotype, together with many associated specimens, was collected on Mauritius. The material is morphologically concordant with previous redescriptions (Wilson 2003, Eguchi 2008), and identical to P. megacephala material from other collection localities (e.g. Papua New Guinea and Australia).



Myrmica agilis

Three worker minor syntypes in. Labelled “35 Malac.” (= Malacca, West Malaysia).

Myrmica suspiciosa

Two worker minor syntypes in. Labelled “Aru.”

Pheidole megacephala

Holotype worker major in. Labelled “Bac. 53.”

Etymology
Gr L megacephala, large-headed, referring to the major. (Wilson 2003)

Other Resources and Articles
Martindale, S. 2014. Discovery of an aggressive ant variety in Costa Mesa touches off a massive search to see how far they've spread. Orange County Register Identification guide to Invasive Ants of the Pacific Islands