Ooceraea

Ooceraea is an Old World lineage that contains a species emerging as the only model organism among dorylines, the clonal Ooceraea biroi (Borowiec 2016).

Identification
Borowiec (2016) - Worker The workers of Ooceraea can be distinguished by a combination of propodeal spiracle positioned low on the sclerite and pygidium armed with modified setae, antennae with 11 or fewer segments, pronotomesopleural suture developed, two-segmented waist with abdominal segment III strongly tubulated, and no constrictions between abdominal segments IV, V, and VI. The abdominal segment IV is conspicuously the largest and its tergite does not fold over the sternite anteriorly. The habitus of Ooceraea is distinctive, with conspicuously differentiated abdominal segment III forming a postpetiole, eyes small or absent, and coarse cuticular sculpturing. Among the non-army ant dorylines that exhibit reduction in antennomere count Ooceraea can only be confused with Syscia and Parasyscia. The former exhibits a conspicuous folding of the anterior portion of abdominal tergite IV and possesses a unique mid-tibial gland (see below). The few Parasyscia species that have 11 antennal segments can be distinguished from Ooceraea by fused pronotomesopleural suture and larger abdominal segment III.

Male The males of Ooceraea commonly have only 11 antennal segments, which is unique among male dorylines, but a few have 12-segmented antennae, a state shared with most Acanthostichus and all Eusphinctus, Simopone, and Syscia. In Acanthostichus and Eusphinctus the costal vein of fore wing is always present, while missing from the majority of Ooceraea. Additionally, in Acanthostichus the vein R·f3 is visible beyond pterostigma and in Eusphinctus the submarginal cell is closed by Rs·f2–3. Both of these veins are always absent in Ooceraea. Distinguishing between males of Ooceraea and Syscia is difficult. As mentioned above, the majority of species in Ooceraea have 11-segmented antennae, while in Syscia these seem to be always 12-segmented. In Ooceraea the discal cell is closed by cross-vein 1m-cu in the majority of males examined, except for the smallest of specimens, while in the limited material of Syscia I have examined this vein appears to be universally absent. Additionally, most Ooceraea males have a unique specialization of abdominal sternite VII, ranging from a deep cleft in the middle of the posterior margin and denser pilosity on lateral sides, to conspicuous cuticular projections with a brush of hairs. No Syscia have such modifications but in certain Ooceraea this character is not obvious (e.g. Ooceraea biroi) or absent (a male tentatively associated with Ooceraea coeca).

Distribution
Borowiec (2016) - Ooceraea is a lineage confined to the Indomalayan and Australasian regions, including the Fijian archipelago. O. biroi is a tramp species that has been more widely introduced across tropical regions of the world.

Biology
Borowiec (2016) - The members of this lineage are found primarily in leaf litter and soil core samples. Worker morphology (eyes often very small or absent) is also suggestive of subterranean habits.

Ooceraea biroi is perhaps the best studied doryline species. The army ants Eciton and Dorylus have been extensively researched in the field, but their huge colonies are exceptionally difficult to manipulate in laboratory conditions. In contrast, O. biroi is a species much more amenable to experimental manipulation and has been the focal organism for multiple published laboratory-based studies.

O. biroi is a clonal species where all workers in a colony have reproductive potential and multiple individuals are active egg layers (Tsuji and Yamauchi 1995). Brood is synchronized and alternating cycles of reproductive and foraging phases occur, much like in Eciton (Ravary and Jaisson 2002, 2004, Ravary et al. 2006). Much like most other dorylines, Ooceraea biroi is a specialist predator on other ants’ brood, although it can attack other soft-bodied insects (Wetterer et al. 2012). The workers are blind and, like many dorylines, rely solely on chemical communication. A recent study found that O. biroi has the largest number of odorant receptor genes of any insect sequenced (Oxley et al. 2014). The colonies number between a hundred and several hundred individuals. O. biroi is also a ‘tramp species’ whose native range is likely limited to mainland southeast Asia (Kronauer et al. 2012), but it has been established in numerous tropical islands throughout the world, including Japan, Hawaii, Madagascar and Seychelles, and the West Indies (Wetterer et al. 2012). It is the only member of the subfamily whose genome has been published (Oxley et al. 2014).

In addition to offering a rare opportunity for studying the habits of a non-army ant doryline, O. biroi has also provided some important insights into social insect biology in general. A study by Ravary et al. (2007) showed that division of labor is influenced by learning in this species. Individuals that experienced high success rates in foraging would specialize in this task, whereas ants failing at prey discovery would decrease their foraging activity and spend more time on brood care. Teseo et al. (2013) demonstrated that O. biroi workers will execute their genetically identical sisters if they fail to conform to the reproductive activity cycles necessary for synchronized brood development. This behavior in the absence of genetic conflict highlights the importance of worker policing for the economics of a social insect colony (Oldroyd 2013).

It is unknown whether the clonal reproduction and brood production synchronicity in O. biroi is representative of other Ooceraea and if the species is a part of an older clade of parthenogenetic lineages or an exception. Subdichthadiigyne queens of Ooceraea crypta suggest more traditional reproduction in at least one other species. Many males of Ooceraea have a highly modified abdominal sternite VII, suggesting its involvement in copulation (see discussion of male characters above). An Australian species Ooceraea australis is relatively common throughout the continent and has been reported to form colonies with thousands of individuals (Heterick 2009).

Nomenclature

 *  OOCERAEA  [Dorylinae]
 * Ooceraea Roger, 1862a: 248. Type-species: Ooceraea fragosa, by monotypy.
 * Ooceraea subgenus of Cerapachys: Emery, 1902d: 24; Wheeler, W.M. 1902d: 185; Emery, 1911d: 10.
 * Ooceraea junior synonym of Cerapachys: Brown, 1975: 19.
 * Ooceraea as genus: Borowiec, 2016: 191.
 * CYSIAS [junior synonym of Ooceraea]
 * Cysias Emery, 1902c: 24 [as subgenus of Cerapachys]. Type-species: Ooceraea papuana, by original designation.
 * Cysias raised to genus: Ashmead, 1905b: 382.
 * Cysias subgenus of Cerapachys: Wheeler, W.M. 1910g: 137.
 * Cysias junior synonym of Syscia: Emery, 1911d: 10.
 * Cysias junior synonym of Cerapachys: Kempf, 1972a: 76; see also Brown, 1975: 19.
 * Cysias junior synonym of Ooceraea: Borowiec, 2016: 191.

Taxonomic Notes
Borowiec (2016) - The taxonomic history of Ooceraea is complicated. The genus was originally described by Roger (1862) to include Ooceraea coeca from Sri Lanka. Roger did not place Ooceraea in a particular group but subsequent authors classified the genus in Myrmicinae (Mayr 1865, Emery 1877), most likely due to the relatively small abdominal segment III (postpetiole) present in these ants. Dalla Torre (1893) considered it a member of the Ponerinae. Later Forel (1893a) established the tribe “Cerapachysii” within Ponerinae, where he included Ooceraea along with Cerapachys and others. Starting with Emery (1902), Ooceraea was treated as a subgenus of Cerapachys until Brown’s provisional (1973) and formal (1975) synonymizations of all Cerapachys subgenera.

Cysias is a name introduced by Emery (1902) for Ooceraea papuana and Ooceraea pusilla as a subgenus of Cerapachys. In Genera Insectorum (Emery 1911) he considered it a synonym of Syscia, but explained in his diagnosis of the latter that it encompassed species with two distinct morphologies: “Antennae with 9 segments. Without eyes. Basal segment of gaster not much larger than postpetiole (Syscia), or much larger and longer than the latter and covering almost all of gaster (Cysias)”. This was because of his inclusion of species here placed in Syscia, Syscia typhla, which also has 9-segmented antennae but a relatively large abdominal segment III (postpetiole). Based on morphology, papuana and pusilla are here considered species of Ooceraea. See also the discussion of Emery’s Genera Insectorum classifications in the section on doryline taxonomy above.

Genomic data show that Ooceraea is most closely related to Eusphinctus and Syscia (Borowiec, in prep.). No attempts to investigate the internal phylogeny have been made.

Worker
Borowiec (2016) - Head: Antennae with 9, 10 (rarely) or 11 segments. Apical antennal segment conspicuously enlarged, much broader than and longer than two preceding segments combined. Clypeus with or without cuticular apron. Lateroclypeal teeth present. Parafrontal ridges reduced. Torulo-posttorular complex vertical. Antennal scrobes absent. Labrum with median notch or concavity. Proximal face of stipes projecting beyond inner margin of sclerite, concealing prementum when mouthparts fully closed. Maxillary palps 3-segmented. Labial palps 2-segmented. Mandibles triangular, with teeth. Eyes absent or present but small, composed of 1–5 ommatidia, very rarely composed of 6–20 ommatidia. Ocelli absent. Head capsule with differentiated vertical posterior surface above occipital foramen. Ventrolateral margins of head without lamella or ridge extending towards mandibles and beyond carina surrounding occipital foramen. Posterior head corners dorsolaterally immarginate. Carina surrounding occipital foramen ventrally absent. Mesosoma: Pronotal flange not separated from collar by distinct ridge or not. Promesonotal connection with suture completely fused or suture present, weakly differentiated, immobile. Pronotomesopleural suture visible, unfused up to notal surface. Mesometapleural groove not impressed to weakly impressed. Transverse groove dividing mesopleuron absent. Pleural endophragmal pit concavity present. Mesosoma dorsolaterally immarginate. Metanotal depression or groove on mesosoma absent. Propodeal spiracle situated low on sclerite. Propodeal declivity with or without distinct dorsal edge or margin and rectangular in posterior view. Metapleural gland bulla visible or not through cuticle. Propodeal lobes present, well developed. Metasoma: Petiole anterodorsally immarginate, dorsolaterally immarginate, and laterally above spiracle marginate. Helcium in relation to tergosternal suture placed at posttergite and axial. Prora simple, not delimited by carina or a U-shaped margin with median ridge. Spiracle openings of abdominal segments IV–VI circular. Abdominal segment III anterodorsally immarginate and dorsolaterally immarginate. Abdominal segment III about half size of succeeding segment IV, which is strongly constricted at presegmental portion (binodal waist). Girdling constriction of segment IV present, i.e. pre- and postsclerites distinct. Cinctus of abdominal segment IV gutter-like and cross-ribbed. Abdominal segment IV conspicuously largest segment. Abdominal tergite IV not folding over sternite, and anterior portions of sternite and tergite equally well visible in lateral view. Girdling constriction between pre- and posttergites of abdominal segments V and VI absent. Girdling constriction between pre- and poststernites of abdominal segments V and VI absent. Pygidium medium-sized, with impressed medial field, and armed with modified setae. Hypopygium unarmed or armed with modified setae. Legs: Mid tibia with single pectinate spur. Hind tibia with single pectinate spur. Hind basitarsus not widening distally, circular in cross-section. Posterior flange of hind coxa not produced as raised lamella. Metatibial gland present as oval patch of whitish cuticle. Metabasitarsal gland absent. Hind pretarsal claws simple. Polymorphism: Monomorphic.

Queen
Borowiec (2016) - Ergatoid or replaced by fertile workers (Tsuji and Yamauchi 1995). Mesosomal morphology with wing remnants in one undescribed species suggests that brachypterous or fully winged queens may also occur in this genus. In O. crypta the ergatoid queen possesses multifaceted eyes, three ocelli, no sign of additional sutures on the mesosoma, and an enlarged abdominal segment III (Mann 1921); this morphology could perhaps be called “subdichthadiigyne”, although the presence of three well-developed ocelli is atypical. In Ooceraea besucheti the only differences between ergatoid gynes and workers include presence of ocelli and enlarged gaster (Brown 1975).

Male
Borowiec (2016) - Head: Antennae with 11 segments or more rarely with 12 segments. Clypeus with cuticular apron. Parafrontal ridges absent. Torulo-posttorular complex vertical. Maxillary palps 5-segmented. Labial palps 3-segmented. Mandibles triangular, edentate. Ventrolateral margins of head without lamella or ridge extending towards mandibles and beyond carina surrounding occipital foramen. Carina surrounding occipital foramen ventrally absent. Mesosoma: Pronotal flange not separated from collar by distinct ridge, occasionally ridge marked on sides. Notauli present. Transverse groove dividing mesopleuron present. Propodeal declivity reduced, with or without distinct dorsal edge or margin. Metapleural gland opening absent. Propodeal lobes present. Metasoma: Petiole anterodorsally immarginate, dorsolaterally immarginate, and laterally above spiracle marginate, inconspicuously in small species. Helcium in relation to tergosternal suture placed at posttergite and axial. Prora forming a simple U-shaped margin or a U-shaped margin with median ridge. Spiracle openings of abdominal segments IV–VI circular. Abdominal segment III about half size of succeeding segment IV or less; latter strongly constricted at presegmental portion (binodal waist). Girdling constriction of segment IV present, i.e. pre- and postsclerites distinct. Cinctus of abdominal segment IV gutter-like and cross-ribbed. Girdling constriction between pre- and postsclerites of abdominal segments V and VI absent. Abdominal segment IV conspicuously largest segment. Abdominal sternite VII modified, rarely simple. Abdominal sternite IX cleft to modified into two spines, sometimes with additional medial projection or spine, with lateral apodemes about as long as medial apodeme, directed anteriorly (towards head). Genitalia: Cupula long relative to rest of genital capsule and shorter ventrally than dorsally. Basimere broadly fused to telomere, with no sulcus trace at junction, and ventrally with left and right arms abutting. Telomere gradually tapering toward apex. Volsella gradually tapering toward apex. Penisvalva laterally compressed, rounded at apex. Legs: Mid tibia with single pectinate spur. Hind tibia with single pectinate spur. Posterior flange of hind coxa not produced as raised lamella. Metatibial gland present as oval patch of whitish cuticle. Metabasitarsal glands absent. Hind pretarsal claws simple. Wings: Tegula present, broad, demiovate in shape. Vein C in fore wing present or absent. Pterostigma broad. Abscissa R·f3 absent. Abscissae Rs·f2–3 absent. Cross-vein 2r-rs present, forming base of ‘free stigmal vein’ (2r-rs&Rs·f4–5) in absence of Rs·f3 and 2rs-m, although 2rs-m may be present as stub. Abscissae Rs·f4–5 present, fused in absence of 2rs-m or absent. Abscissa M·f2 in fore wing contiguous with Rs+M. Abscissa M·f4 in fore wing absent. Abscissa M·f4 in fore wing present, not reaching wing margin. Cross-vein 1m-cu in fore wing absent or present. Cross-vein cu-a in fore wing present, arising from M+Cu and proximal to M·f1. Vein Cu in fore wing present, with only Cu1 branch prominent. Vein A in fore wing with abscissa A·f1 or with abscissae A·f1 and A·f2 present. Vein C in hind wing absent. Vein R in hind wing absent or present, extending past Sc+R but not reaching distal wing margin. Vein Sc+R in hind wing absent. Vein Sc+R in hind wing present. Abscissa Rs·f1 in hind wing absent. Abscissa Rs·f1 in hind wing present, shorter than 1rs-m. Abscissa Rs·f2 in hind wing absent or present, not reaching wing margin. Cross-vein 1rs-m in hind wing absent. Vein M+Cu in hind wing absent or present. Abscissa M·f1 in hind wing absent. Abscissa M·f2 in hind wing absent. Cross-vein cu-a in hind wing absent or present. Vein Cu in hind wing absent. Vein A in hind wing absent or with abscissa A·f1 present.

Larva
Borowiec (2016) - Larva have been described for Ooceraea australis (Wheeler and Wheeler 1964a, 1973). Cocoons absent.