The following account is from Sarnat et al. (2015) and figure numbers refer to that publication.
Pheidole megacephala is a medium sized species of variable color that is most easily recognized outside of its native range by the heart-shaped head and bulging postpetiole. It belongs to a diverse and taxonomically confusing clade of morphologically similar taxa centered in the Afrotropical and Malagasy regions. Both major and minor workers are distinguished from all other introduced Pheidole by the swollen shape of the postpetiole (Fig. 1). Pheidole noda also has a swollen postpetiole, but whereas the postpetiole of P. megacephala is characterized by a posterodorsal and anteroventral bulge, that of P. noda is formed as a high dorsally bulging dome that is tallest at its midpoint.
Pheidole megacephala has often been confused for P. pallidula Nylander in Europe, especially in the Mediterranean region. The introduced populations of P. megacephala can be distinguished from P. pallidula by the following characters. For both major and minor workers the postpetiole of P. megacephala has a posterodorsal (Fig. 1a) and anteroventral (Fig. 1b) bulge, while that of pallidula is not swollen relative to petiole (Fig. 3). The propodeal spines of both subcastes are distinct in P. megacephala but are strongly reduced in P. pallidula. Additionally, the major worker of megacephala has a heart shaped head that broadens significantly posterior to eye-level (Fig. 6) while the head of pallidula is more rectangular (more approximate to Fig. 7).
Accurate identification within the Afrotropics is more problematic. While for Madagascar previously described subspecies have been synonymized with megacephala (Fischer and Fisher 2013), the taxonomy of the megacephala group in Africa remains rather chaotic with a number of unrevised subspecies, most of which remain insufficiently characterized. In a taxonomic overview of the group, Emery (1915) studied type and non-type material of megacephala-related species, yet for several subspecies he was not able to define clear species limits from the multitude of different, yet highly similar, phenotypes. We suspect that some of those names are probably due to intraspecific variation within P. megacephala and P. punctulata Mayr. Other, morphologically unique taxa like P. megacephala nkomoana Forel are clearly valid biological species. However, without a comprehensive taxonomic treatment supported by a robust phylogeny, the following species characterizations may be subject to future taxonomic changes.
Within the megacephala group, minor workers are difficult to separate morphologically and thus have only limited use for species identification, but the majors tend to be more distinct in their morphologies and can be separated by differences in head and body shape and sculpture, and in size and pilosity, although the limits are often unclear and characters are sometimes distributed along a continuum rather than being separated into distinct, clear-cut states.
Major workers of P. megacephala melancholica Santschi are characterized by presence of weak punctures on the majority of the head, including the sides in lateral view, promesonotum with punctures and irregular transverse rugulae, and moderately abundant short and stout standing hairs on head and body, whereas major workers of P. megacephala entirely lack punctures on the posterior 1/3 of the head, have a mostly smooth and glossy promesonotum, and often possess longer, more flexuous standing hairs, which often branch at the tips. Pheidole megacephala nkomoana majors are characterized by a weakly defined antennal scrobe and relatively long frontal carinae that reach about ¾ towards the posterior head margin, two well-defined submedian hypostomal teeth, a weak prominence on the promesonotal dome, and very long, flexuous standing hairs on the dorsal promesonotum. Also the spines tend to be shorter than in megacephala, in length almost equal to the diameter of the propodeal spiracle. Both subspecies have been described from and collected in western African forests. Another closely related species to megacephala is P. punctulata. It is very widespread in sub-Saharan Africa and usually found in dry forests and grassland habitats. Morphologically close to megacephala, its major workers can be distinguished by their often enlarged and strongly heart-shaped heads, the presence of a softly or superficially punctuated sculpture on parts of the head dorsum, promesonotum, postpetiole and gaster, and relatively uniform, short and stout, erect hairs covering the body. Minor workers tend to be slightly larger and more robust than in megacephala, often with a few oblique carinae present between the eyes and the mandibles and reaching the posterior eye level, the hairs similar as in major workers and usually more abundant than in megacephala.
Morphologically very similar to punctulata are P. megacephala ilgi Forel, megacephala impressifrons Wasmann, and megacephala rotundata Forel. Like P. punctulata, they are usually found in drier forest and grassland habitats and their workers seem to be highly polymorphic, which means that in addition to normal major workers, colonies are capable of producing so-called supermajors. These supermajors possess a very strongly heart-shaped head, which can be disproportionately big compared to the size of the mandibles and the rest of their bodies. As Emery (1915) stated for megacephala rotundata, on first glace they look quite distinct from punctulata, but at closer examination of series with different major worker sizes it seems impossible to define species limits. From our own observations it seems likely that these subspecies are a result of sampling bias and phenotypic variation within P. punctulata, rather than historic speciation events (Fischer et al., in preparation). Incomplete sampling can also be a problem when only smaller major and minor workers are collected, which are often very similar to those of P. megacephala, with very similar head sculpture and general morphology.
In the Malagasy region, P. megacephala can be confused with three other species: Pheidole punctulata spinosa Forel, which, on average, has longer spines, a slightly higher propodeum and a more extensively smooth and glossy posterior portion of the head in the larger major workers. Pheidole megatron, which was described from the Comoros and is possibly present in the Northwest of Madagascar as well, is characterized by major workers with a less heart-shaped, and slightly more rectangular head shape, and sometimes sculpture and rugulae present on the posterior head portion (see Fischer and Fisher 2013). Finally, P. decepticon, described from Mayotte and distributed over several of the smaller Southwest Indian ocean islands, is characterized by possessing a denser, more prominent and longer pilosity as well as slightly smaller, less rounded ventral bulges on the postpetiole in both minor and major workers (see Fischer and Fisher 2013). It is however possible that P. decepticon is a geographic variation of and conspecific with P. punctulata spinosa.
Pheidole megacephala is listed among the top five invasive ants (Lowe et al. 2000). Although this species prefers humid and disturbed habitats and is usually found in very high abundances (Burwell et al. 2012; Hoffmann et al. 1999; Wilson 2003), it can generally be found in a large variety of landscapes, from coastal habitats to human settlements and plantations in lower elevations, degraded dry forest, to mid-elevation rainforest or even montane forest—in Papua New Guinea up to 2150 meters altitude (Fischer and Fisher 2013). The distribution range and activity of P. megacephala appears to be somewhat limited by susceptibility to desiccation and higher temperatures. Thus, colonies are often found in more humid microhabitats, and workers tend to forage inside the leaf-litter and at night, or even build covered trails (Greenslade 1972, personal observations). However, some studies reported that on smaller islands or after successful introduction in a new area, P. megacephala expanded its range and invaded into the forest interiors where it attacked and displaced other introduced and natively occurring ant species (Burwell et al. 2012; Hoffmann 1998). In a citrus orchard in Tanzania for example, P. megacephala was able to partly displace highly territorial and competitive Oecophylla weaver ants (Seguni et al. 2011). Pheidole megacephala is an especially common and abundant nuisance and pest on islands, which are generally more strongly impacted by invasions of alien species.
Part of the success of P. megacephala as a pantropic pest species is its generalist behavior. Like many other Pheidole species its diet is broadly omnivorous with a large proportion of its food probably acquired by scavenging on the ground. Pheidole megacephala is also a good predator with an efficient nest mate recruitment that enables the species to dominate baits and to retrieve prey too large for single workers to carry (Dejean et al. 2008; Dejean et al. 2007). Devastating effects on the abundance and diversity of native invertebrates, in northern Australia for example, are well documented (Hoffmann 1998; Hoffmann et al. 1999; Hoffmann and Parr 2008). Pheidole megacephala has also been documented to negatively impact agricultural systems. Workers tend plant and crop-damaging scale insects for honeydew (Campbell 1994; Gaigher et al. 2011; González Hernández et al. 1999; Greenslade 1972; Petty and Tustin 1993; Reimer et al. 1993), protect plants with extrafloral nectaries from phytophagous insects and possibly collect seeds (Hoffmann 1998). A recent study experimentally evaluating the performance in interference competition found that P. megacephala ranked lowest among seven of the world’s worst most destructive invasive ant species (Bertelsmeier et al. 2015). The authors, citing Dejean et al. (2008) suggested that P. megacephala does not dominate invaded ant communities through direct physical interactions (interference competition) but by raiding their colonies.
Nesting sites are variable and can occur in any crack and crevice that is large enough for them to enter, including soil, inside rotting logs, under rocks, in houses or in tree bark. As in several other invasive ant species, colonies are polygynous, and dependently founded via budding, with nests in large areas often forming supercolonies (Hoffmann 1998) that aggressively fight other ants or outcompete them by depleting their prey and other resources (Dejean et al. 2008; Fournier et al. 2009; Hoffmann et al. 1999; Vanderwoude et al. 2000).
Pheidole megacephala is known as a major agricultural and ecological pest species (Williams 1994) and its widespread pantropic distribution and often very close association with humans make it a high-risk invasive species with a serious potential for ecological, agricultural and economic damage. In Ward et al. (2006) it has been the most intercepted exotic ant species (890 out of 4355 interception records between 1955 and 2005) arriving with trade products in New Zealand. Many aspects of its biology indicate that it is highly adaptable and thus able to survive outside of its preferred habitat, by finding suitable microhabitats for nesting and by killing or outcompeting native species. Although mutualistic relationships with scale insects and other crop pests are dominant in agricultural systems with introduced P. megacephala, positive side-effects on plant fitness have been observed as well (Bach 1991).
Diagnosis of worker among Antkey species. Worker castes bimorphic. Head shape ovoid (minor workers) or weakly heart-shaped with posterolateral lobes (majors), but never triangular. Antenna 12-segmented. Antennal club 3-segmented. Antennal insertions at least partly covered by frontal lobes; not surrounded by a raised sharp-edged ridge. Frontal lobes do not obscure face outline between mandible and eye; relatively close together so that the posteromedian portion of the clypeus, where it projects between the frontal lobes, is at most only slightly broader than one of the lobes. Posterolateral corners of head unarmed, without spines. Mandibles triangular. Mesosoma with erect hairs. Pronotal spines absent. Propodeum armed with spines or teeth. Slope of mesosoma steep. Waist 2-segmented. Petiole pedunculate with a distinct and upright node; lacking large subpetiolar process. Postpetiole attached to lower surface of gaster. Color yellowish brown to brown. Minor worker characters. Head smooth and shining, lacking punctation. Antennal scrobe lacking. Postpetiole swollen relative to petiole. Major worker characters. Antennal scrobe weak to absent; no depression capable of receiving antennal scapes clearly visible. Posterolateral lobes smooth and shining, lacking rugae and punctation.
Diagnosis among introduced Pheidole. Light brown to dark brown. Major | HW 1.10–1.54, HL 1.04–1.59, SL 0.59–0.76, CI 97–106, SI 47–58 (n=19, Fischer & Fisher 2013). Head heart-shaped (Fig. 6); posterior 1/3 of dorsal surface smooth, glossy and entirely lacking rugoreticulate sculpture. Hypostoma lacking distinct median and submedian teeth. Promesonotum in profile forming a single dome (Fig. 4), lacking a distinct mound or prominence on the posterior slope. Postpetiole with a posterodorsal (Fig. 1a) and anteroventral (Fig. 1b) bulge. Minor | HW 0.50–0.61, HL 0.57–0.68, SL 0.61–0.72, CI 86–92, SI 114–122 (n=20, Fischer & Fisher 2013). Head predominantly glossy (Fig. 36), lacking punctation and or rugae above eye level. Antennal scapes surpass posterior head margin by approximately same length as eye (Fig. 40). Promesonotum in profile forming a single dome (Fig. 42), lacking a distinct mound or prominence on the posterior slope. Postpetiole with a posterodorsal (Fig. 1a) and anteroventral (Fig. 1b) bulge.
Pheidole megacephala is a cosmopolitan species that has established across the globe as a household and agricultural pest throughout the tropics. Wetterer (2012) provided a detailed review of the worldwide spread of P. megacephala, and cites Wheeler’s statement (Wheeler 1922a) that it is most likely of Afrotropical or Malagasy origin, the only two regions with a diversity of related species (“subspecies and varieties”). Theoretically it is possible that a common ancestor of P. megacephala and the Malagasy endemics P. punctulata spinosa, P. megatron and P. decepticon arrived on the islands in prehistoric times, diversified there, and that P. megacephala was later transported to all other regions including Africa only after the arrival of humans. But the distribution of P. megacephala on Madagascar strongly resembles the distributions of other invasive species on the island—e.g. those of Monomorium floricola, M. pharaonis, Tapinoma melanocephalum, Technomyrmex albipes, Trichomyrmex destructor. While P. punctulata spinosa has established a broad distribution range across the island’s variable habitats and elevations, P. megacephala, like the other invasives, is found mostly along the coast, in low elevation and disturbed habitats or near human settlements.
Similar to Wheeler’s observation, our argument for the “out-of-Africa” hypothesis is an overall much higher complexity in different morphotypes and species-level diversity in African megacephala group taxa and the presence of both, very closely, but also more distantly related taxa (e.g Pheidole aurivillii Mayr). For these reasons and for the purposes of this study, we consider all records from Africa to represent the native range of Pheidole megacephala. However, a further resolution will require a comprehensive phylogeographic study of the species and its allied taxa, especially from the poorly studied and sampled African region.
We treat populations of P. megacephala recorded from the southwestern extent of the Arabian Peninsula as native as this region is commonly considered as belonging to the Afrotropics. However, recent studies on generic distributions of global ant diversity that find little support for including any portion of the Arabian Peninsula in the Afrotropics (unpublished data). Until robust phylogeographic data is available for P. megacephala, this decision must be considered tentative and open to future revision.
We do agree with Wetterer’s (2012) conclusions that records of P. megacephala from Mediterranean Europe northward are either temporary indoor records or misidentifications of P. pallidula. Outside of Africa, the Malagasy region and the range of P. pallidula (western Palearctic), P. megacephala is easily recognized as it does not co-occur with species of similar morphology. We therefore consider all records reviewed from outside the aforementioned regions as confirmed unless otherwise stated.
Dubious records. The following records are considered dubious mostly because there is reason to believe they represent misidentifications of P. pallidula. However, it is possible that some of the following literature records were based on accurate identifications, but that P. megacephala was since extirpated from the referenced localities. This latter possibility is plausible especially for the Mediterranean region where L. humile has established a stronghold. For example, (Heer 1852) described Oecophthora pusilla (=P. megacephala) as ubiquitous on the island of Madeira, “In the town of Funchal there is probably not a single house that does not harbor millions of the tiny creatures…” Less than a century later Wheeler (Wheeler 1927b) reported, “Now it is an interesting fact that the Argentine ant, soon after its arrival in Madeira, completely replaced the Pheidole as a house ant.” Similar instances of well-established populations of introduced ant species becoming locally extirpated have been documented (Moreau et al. 2014; Wetterer 2006).
Algeria: The material referred to by André (1883) P. megacephala is distinguished by that author from P. pallidula only by the difference in size of the propodeal spine, and was otherwise observed to be identical. Considering the other characters separating these two species discussed earlier, we tentatively consider this record to be a misidentification of P. pallidula. Croatia: The material listed from this country (Petrov and Collingwood 1992; Petrov and Legakis 1996) is considered to refer to P. pallidula according to Bračko (2006). Egypt: Egypt is the type locality of the nomen oblitum Formica edax Forskål. Emery (1892) wrote that edax is undoubtedly a small Pheidole, and possibly refers to P. megacephala. Dalla Torre (1892) was also uncertain as to which species (or even genus) the name edax referred to. Given the uncertainty of these two authors, the occurrence of P. pallidula in Egypt and the unconfirmed single literature record of Bakr et al. (2007), it is difficult to know when P. megacephala was first reported from Egypt, and whether the species occurs there at all. France: Bignell (1901) reported the ant species listed in his study of Corsica were identified by Saunders, who is known to have confused P. pallidula for P. megacephala. As P. pallidula was not listed in the publication, we consider the record to either be a misidentification of that species or from an extirpated population. Greece: The only primary references to an outdoor occurrence we could confirm are Collingwood (1993) and Borowiec and Salata (2012). The former authors reported P. megacephala was found only once during their study of five Greek islands on the threshold of a small hotel in Pigadhia on Karpathos. The second study reported finding the species on a road in Crete. The record from Macedonia in (Karaman 2011) is from material identified by Petrov. We tentatively follow (Bračko 2006) as treating this as a misidentification of P. pallidula. Italy: Piedmont is the type locality for Myrmica trinodis Losana which was synonymized with megacephala by Roger in 1863. Losana also lists a M. megacephala Latrielle in the same publication. Latrielle never described any species by the name megacephala, however. Losana might have instead been referring to Messor megacephala Leech (= Messor barbarous Mayr). Regardless, the original description of M. trinodis states that the species was collected from outdoor gardens. There is some reason to suspect this name might refer instead to P. pallidula, as the only verifiable occurrences of megacephala in Italy since are for specimens collected from plant nurseries, greenhouses and cargo hangars used for holding imported plants, fruits and vegetables (Jucker et al. 2008; Limonta and Colombo 2003). Morocco: Saunders (1888) appears to be the only primary reference for P. megacephala occurring in Morocco, but it is likely that the author was referring to misidentified material of P. pallidula (Wetterer 2012). This view is further evidenced by Cagniant and Espadaler (1993) who were unable to find the species in their survey. Spain: We consider the following records from the Balearic Islands and Gibraltar to refer to P. pallidula (Saunders 1888; Saunders 1904; Walker 1889). USA: The specimens reported in Fischer and Fisher (2013) from Arizona were from a quarantine collection intercepted from Florida, and there is no reason to believe the species has ever established in Arizona. Wetterer (2012) cited a specimen record of P. megacephala from Catalina Island (California). If the identification proves accurate, it is the only known record from that island and the population has since been extirpated (perhaps by L. humile). However, a population (CASENT0248690) has been discovered recently in southern California (Orange Co.). Although P. megacephala is listed in the Missouri Ants web page (2015), we cannot verify the entry with any specimen or literature record.