Simopelta Mann

Schmidt, C. A. & Shattuck, S. O., 2014, The Higher Classification of the Ant Subfamily Ponerinae (Hymenoptera: Formicidae), with a Review of Ponerine Ecology and Behavior, Zootaxa 3817 (1), pp. 1-242 : 156-158

Simopelta Mann

Fig. 32

Simopelta Mann, 1922: 10 (as subgenus of Belonopelta ). Type-species: Belonopelta (Simopelta) jeckylli Mann, 1916: 415 ; by original designation.

Simopelta is a moderate-sized genus (21 described species) restricted to the Neotropics. These ants are notable for their army ant-like lifestyles, including apparently obligate group predation on other ants, reproduction by dichthadiiform queens, and nomadism.

Diagnosis. Simopelta workers can be readily diagnosed by the following combination of characters: subtriangular mandibles, raised clypeal rostrum, eyes small and often consisting of only a single enlarged ommatidium, metapleural gland orifice with a posterior U-shaped lip, usually an absence of stout traction setae on the middle and hind legs, and presence of prominent arolia. The clypeal rostrum is autapomorphic within Ponerini . Some Simopelta species ( S. pergandei and its relatives) superficially resemble Feroponera and some Cryptopone , but differ from them in lacking traction setae on the legs, among other differences.

Synoptic description. Worker. Small (TL 2.1–4.9 mm) slender ants with the standard characters of Ponerini . Mandibles subtriangular, with a weak basal angle, a pair of apical teeth and variable dentition proximally, and no basal groove. Median portion of clypeus forming a raised triangular rostrum which projects past the anterior margin of the clypeus, sometimes with a short medial tooth. Frontal lobes small and closely approximated. Eyes small, often reduced to a single enlarged ommatidium, located on the sides of the head anterior of the head midline. Metanotal groove usually only shallowly impressed, though the mesosomal profile is often distinctly discontinuous due to a gradual posterior depression of the mesonotum. Mesonotum and propodeum much narrower than the pronotum, but the propodeum itself not narrowing significantly dorsally. Propodeal spiracles small and round. Metapleural gland orifice with a posterior U-shaped cuticular lip and a lateral groove. Metatibial spur formula (1p). Arolia prominent. Petiole nodiform to nearly squamiform. Gaster with a weak to strong girdling constriction between pre- and postsclerites of A4. Head, mesosoma and petiole usually heavily punctate, striate, or rugoreticulate, the gaster shiny or lightly punctate. Head and body with sparse to scattered pilosity and fairly light pubescence. Color variable, orange to black. Note: An undescribed hypogeic Simopelta species from Costa Rica ( Longino, 2013) differs from this general description in its testaceous coloration, clubbed antennae, presence of stout traction setae on the mesotibiae, and complete absence of eyes.

Queen. Wingless and dichthadiiform, generally morphologically simplified relative to conspecific workers but with an enlarged petiole and gaster. See more detailed descriptions in Borgmeier (1950) and Gotwald & Brown (1967).

Male. Unknown.

Larva. Described by Borgmeier (1950), Wheeler & Wheeler (1957, 1986b), and Gotwald & Brown (1967). Wheeler & Wheeler (1957) describe the young larvae of Simopelta as among the most unusual of any ant.

Geographic distribution. Simopelta is restricted to central and northern South America, including Guatemala, Costa Rica, Brazil, Peru, and Ecuador ( Gotwald & Brown, 1967; Longino, 2013). It primarily inhabits midelevation moist forests ( Longino, 2013).

Ecology and behavior. Little to no focused research has been performed on the habits of Simopelta , but based on morphology and on anecdotal observations it appears that Simopelta has converged extensively on a true army ant lifestyle, much more so than have the mass termite raiders Megaponera , the Leptogenys processionalis species group, and the Neoponera laevigata species group. Morphologically, the reduced eyes of the workers and the dichthadiiform queens (wingless and with greatly distended gasters) are both convergent with ecitonine army ants, and the behavior of individual workers is reportedly much like that of army ants ( Gotwald & Brown, 1967).

The additional similarities between Simopelta and army ants are striking. Simopelta colonies are fairly large by ponerine standards, with estimates of 1,000 to 2,000 workers ( Gotwald & Brown, 1967). Foraging occurs during the day, usually on the ground and arboreally ( Longino, 2013), though the morphological structure of a newly discovered Costa Rican species suggests that it is purely hypogeic ( Longino, 2013). Simopelta workers form large raiding columns of up to several hundred individuals, at least, and these raiding workers apparently follow an odor trail ( Gotwald & Brown, 1967). Anecdotal accounts suggest that they are specialist predators of both the adults and brood of other ants, particularly Pheidole ( Gotwald & Brown, 1967; Longino, 2013). Simopelta construct both temporary nests and long-term nests, which together with the dichthadiiform nature of the queens, the uniform age of the brood (indicating pulses of reproduction), and observations of colony emigrations suggest a nomadic lifestyle with an alternation between migratory and stationary colony phases, as in army ants ( Borgmeier, 1950; Gotwald & Brown, 1967; Brady, 2003; Longino, 2013). Colony reproduction apparently occurs via budding, as in army ants ( Gotwald & Brown, 1967). Interestingly, males have never been observed, suggesting the possibility that these ants may be parthenogenetic ( Longino, 2013). Longino (2013) provides numerous interesting field accounts of Simopelta in Costa Rica.

Phylogenetic and taxonomic considerations. Simopelta was erected by Mann (1922) as a subgenus of Belonopelta , and since then it has variously been considered a subgenus or junior synonym of Belonopelta (e.g., Donisthorpe, 1943c; Baroni Urbani, 1975) or a separate genus (e.g., Wheeler, 1935; Gotwald & Brown, 1967; Bolton, 2003). On the one hand the morphological argument for a close relationship between Simopelta and Belonopelta is a weak one, as they are quite distinct and their similarities seem to be of the sort that are frequently evolved by small or cryptobiotic ponerines. On the other hand, though Schmidt (2013) did not sample Belonopelta in his molecular phylogeny of the Ponerinae, P.S. Ward (pers. comm.) found that Belonopelta is closely related to Thaumatomyrmex , and Schmidt found Thaumatomyrmex to be close to Simopelta . These findings support the close relationship of these three genera, although their exact relationships will require further study.

Schmidt's (2013) phylogeny places Simopelta with strong support as a member of the Pachycondyla group, and resolves it as sister to the remainder of that group, though other possible placements receive some support from Bayesian analyses, including sister relationships with Thaumatomyrmex and the Ponera group. Simopelta , Belonopelta and Thaumatomyrmex both differ from other members of the Pachycondyla group in having only a single metatibial spur. This could be a synapomorphy for these genera, could be independently evolved, or could represent the plesiomorphic condition within the Pachycondyla genus group, though the latter hypothesis is unlikely because it implies the re-evolution of a second spur. For now, the molecular data suggest most strongly that Simopelta is sister to the rest of the Pachycondyla group and closely related to Belonopelta and Thaumatomyrmex .

Figures

FIGURE 32. Worker caste of Simopolta pergandei: lateral and dorsal view of body and full-face view of head (CASENT0178701, April Nobile and www.antweb.org); world distribution of Simopolta.