Ophthalmopone Forel

Ophthalmopone Forel

Fig. 21

Ophthalmopone Forel, 1890: cxi (as genus). Type-species: Ophthalmopone berthoudi Forel, 1890: cxiii; by monotypy. Gen. rev.

Opthalmopone Arnold, 1915: 49 (incorrect subsequent spelling of Ophthalmopone).

Ophthalmopone is a small genus (five described species) restricted to Sub-Saharan Africa. It is notable for its polydomous colonies, specialized termite predation, and reproduction by gamergate workers.

Diagnosis. Diagnostic morphological apomorphies of Ophthalmopone workers include very large eyes located at or posterior to the head midline and a hypopygium armed with stout spines. This combination of characters is unique to Ophthalmopone. Ophthalmopone is similar to Megaponera but lacks the preocular carinae of that genus. Large eyes also occur in Harpegnathos, but those of Harpegnathos are even larger and located at the extreme anterior end of the head, rather than at or posterior to the head midline. Stout hypopygial spines occur in several other ponerine genera, but these groups lack Ophthalmopone’s combination of slender build, dense pubescence, large eyes, nodiform petiole, and obsolete gastral constriction.

Synoptic description. Worker. Large (TL 8–13.5 mm; Emery, 1886, 1902) slender ants with the standard characters of Ponerini. Mandibles triangular and long. Eyes very large, located at or posterior to the head midline. Frontal lobes small, widely separated anteriorly by a triangular extension of the clypeus. Metanotal groove shallowly impressed. Propodeum moderately narrowed dorsally. Propodeal spiracle slit-shaped. Metatibial spurs formula (1s, 1p). Tarsal claws unarmed or armed with a single preapical tooth. Petiole nodiform. Gaster without a girdling constriction between pre- and postsclerites of A4. Stridulitrum present on pretergite of A4. Hypopygium armed with a row of stout setae on either side of the sting. Head and body finely punctate, largely devoid of pilosity but with a dense pubescence. Color black.

Queen. Unknown and apparently absent.

Male. See descriptions in Emery (1911) and Arnold (1915).

Larva. Larvae of O. berthoudi were described by Wheeler & Wheeler (1971a).

Geographic distribution. Ophthalmopone is restricted to Sub-Saharan Africa. O. berthoudi has the widest range of any member of the genus, occurring from Sudan to South Africa (Weber, 1942; Prins, 1978). Other species are restricted to southern Africa (O. hottentota), south-central Africa (O. depilis and O. mocquerysi), or eastern Africa (O. ilgii).

Ecology and behavior. Due to its unusual suite of characteristics, Ophthalmopone has drawn considerable attention from ecologists and ethologists. O. berthoudi is by far the best studied species in the genus, and most of what is known about Ophthalmopone ecology and behavior derives from observations of that species.

O. berthoudi colonies are polydomous, with from two to seven nests located up to 75 m apart under stones, in open ground or in abandoned termitaria (Arnold, 1915; Peeters, 1985; Peeters & Crewe, 1987). O. hottentota nests are also located under stones or in open ground (Dean, 1989). Workers regularly transport brood, other workers, and even males between the nests (Peeters, 1985; Peeters & Crewe, 1987; Sledge et al., 1996). Nests of O. berthoudi have from 20 to 800 workers (mean = 186 workers; Peeters & Crewe, 1987; mean = 89 workers for four excavated nests of O. hottentota; Peeters & Crewe, 1985b; Dean, 1989). A highly variable proportion of workers in each nest are mated (1.4–66% for O. berthoudi; Peeters & Crewe, 1985a; Sledge et al., 1996 and 2001), and these gamergate workers perform all reproduction for the colony.

Males enter foreign colonies and mate preferentially with the younger workers (Peeters & Crewe, 1986a). There is apparently no social regulation over which or how many workers mate. Sledge et al. (2001) found no evidence of aggressive dominance interactions among gamergates or between gamergates and unmated workers in O. berthoudi, though they found clear evidence that gamergates chemically suppress haploid egg production in virgin workers. The fecundity of gamergates is low (fewer than one egg per gamergate per day; Peeters & Crewe, 1985a), which is offset by the presence of multiple reproductives per colony. Sledge et al. (1999) studied the division of labor in O. berthoudi colonies and found that as the percentage of gamergates in a colony decreases over a season, the fecundity of the gamergates increases and their range of activities becomes more restricted. Gamergates are never found outside the nest except during nest transfers (Peeters & Crewe, 1985a).

Like workers of their sister genus Megaponera, Ophthalmopone workers are specialist termite predators, though they are not polymorphic as in Megaponera. It appears that the workers of some species forage in organized raids, like Megaponera, while others forage singly. Arnold (1915) observed “irregular columns” of the exceptionally fast-running foragers of O. berthoudi, and Forel (1928) reported foraging columns of O. ilgii. On the other hand, more recent studies of foraging behavior in O. berthoudi (Peeters & Crewe, 1987) and O. hottentota (Dean, 1989) failed to observe group foraging in these species. Neither study found any evidence of recruitment or of chemical trails, as the workers of both species hunted termites singly. Dean (1989) observed caches of hundreds of paralyzed termites in nests of O. hottentota; prey caching has not been observed in O. berthoudi. Foragers of both species return repeatedly to harvest a single termite source.

Duncan (2001) discussed the energetic challenges facing an Ophthalmopone colony, which depends on an unpredictable and scattered food source (foraging termites), and the paradoxical observation that only a small percentage of workers in a colony forage (Peeters, 1985). She found that foraging workers of O. berthoudi are exceptionally energy efficient, and hypothesized that this, along with the polydomous nature of the colonies, resolves the apparent paradox.

Phylogenetic and taxonomic considerations. Forel (1890) erected Ophthalmopone as a genus along with his description of the type species, O. berthoudi. He correctly recognized the distinctiveness of the taxon, as did all subsequent authors until W. L. Brown (in Bolton, 1994) synonymized it under Pachycondyla without phylogenetic justification.

We are reviving Ophthalmopone to full genus status, based on both molecular and morphological evidence. Schmidt's (2013) molecular phylogeny of the Ponerinae places O. berthoudi with strong support within the Odontomachus group as sister to Megaponera, and not at all close to Pachycondyla. A sister group relationship with Megaponera is also supported by morphology, the workers of the two genera being remarkably similar. These genera also share the ecological and behavioral synapomorphies of specialist predation on termites and an absence of winged queens. See the discussion under Megaponera for more details on the similarities and differences between these sister genera.

Apparent apomorphies of the genus include very large eyes set at or posterior to the midline of the head, stout hypopygeal spines on either side of the sting, specialized termite predation, and reproduction by gamergates. Species-level relationships within Ophthalmopone are unstudied and would provide an interesting opportunity to explore the evolution of mass foraging, given the variability within the genus.