Pheidole cervicornis species group

As defined by Sarnat E. M., Fischer, G. and E.P. Economo. 2016. Inordinate spinescence: taxonomic revision and microtomography of the Pheidole cervicornis species group (Hymenoptera, Formicidae). PLoS ONE. 11:e0156709. doi:10.1371/journal.pone.0156709

Species

Key to the Pheidole cervicornis group workers

Notes

All known occurrences of species belonging to the Pheidole cervicornis-group are recorded from New Guinea. In addition to the morphological characters common to all Pheidole, the following characters diagnose the worker caste of the P. cervicornis group from those of all other congeners.

Major worker: (1) Large (HW > 1.8 mm); (2) Antennal scrobes absent; (3) Hypostomal bridge with a single stout to moderate median tooth flanked by a pair of weaker submedian teeth and a pair of stout outer teeth; (4) Head vertex strongly convex in profile; (5) Pronotal spines and propodeal spines very long, subequal in length; (6) Mesonotum modified with spines or lamellate lobes; (7) Transition from mesonotum to propodeum abrupt.

Minor worker: (1) Large (HW > 0.55 mm); (2) Antennal scape hairs suberect to erect.; (3) Antennal scapes extend distinctly beyond posterior head margin; (4) Pronotal spines present, longer than eye length, subequal in length to propodeal spines, simple or bifurcate; (5) Mesonotum spinose, posterior face concave; (6)Propodeal spines extremely long and bifurcated.

All four species within the Pheidole cervicornis group are immediately distinguished from the vast majority of congeners by the presence of extremely long pronotal spines and propodeal spines. The only other Pheidole clades with pronotal spines all occur in the Old World. These include the quadricuspis clade, the quadrispinosa clade and the bifurca clade. The quadricuspis group is restricted to Southeast Asia and does not occur as far east as New Guinea. The major workers are superficially quite similar to those of the cervicornis group, but are distinguished by the (1) lack of a central median hypostomal tooth, (2) coarsely rugoreticulate head sculpture, and (3) shorter, thicker and more abundant pilosity. The minor workers of the quadricuspis group are easily distinguished by the (1) propodeal spines, which are simple and very short (subequal to eye length), and (2) lack of mesonotal spines or lobes.

The quadrispinosa group is sympatric with the cervicornis group in New Guinea. In general, quadrispinosa group workers are smaller than those of the cervicornis group, with proportionally shorter limbs. The major workers as distinguished by the (1) lack of a central median hypostomal tooth, (2) coarsely rugoreticulate head sculpture, (3) deeply excavated antennal scrobes, (4) distinctly concave head vertex, and (5) relatively shorter and more triangular pronotal spines. The minor workers are easily distinguished by their smaller size and non-bifurcating propodeal spines (although they can be strongly curved).

The bifurca group is reciprocally monophyletic with the cervicornis group, and its constituent species are unsurprisingly most difficult to distinguish from those of the cervicornis group.

The bifurca group is sympatric with the cervicornis group (and the quadrispinosa group) in New Guinea. Moreover, the bifurca group is represented by a dizzying number of undescribed species and morphological variation. Based on our preliminary survey of the bifurca group, the major workers are best distinguished from those of the cervicornis group by the (1) shorter pronotal spines, (2) less spinose or lobate mesonotal processes, and (3) smaller size. Additionally, the hypostomal bridge of most (but not all) bifurca group majors have a stout median tooth and either a pair of very weak or absent submedian teeth. However, there is at least one species in which the hypostomal arrangement is similar to that of the cervicornis group species (stout median tooth flanked by a pair of stout submedian teeth). The minor workers of the bifurca group are extremely variable with respect to length and shape of their mesosomal armaments. Some have bifurcating propodeal spines, some have very long pronotal spines, and some have mesonotal processes. None that we have examined, however, possess the combination of all three. Pheidole purpurascens perhaps comes closest to the cervicornis group morphotype, but this species only has angled (versus bifurcate) propodeal spines.

The only Pheidole species outside of the aforementioned groups that has pronotal spines are Pheidole aristotelis and Pheidole hainanensis et al. However, the pronotal and propodeal spines of both species are only subequal in length to the eye, and the latter are not bifurcated. Moreover, the major worker lacks distinct pronotal spines.

The Pheidole roosevelti group also deserves mention here. While its constituent members lack pronotal spines, many of the species have strongly bifurcating propodeal spines that strongly resemble those seen in the cervicornis group, in addition to mesonotal processes. These morphological similarities are entirely convergent, however, as the roosevelti group is only distantly related to the cervicornis group.