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.