Acropyga oceanica

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Acropyga oceanica
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Subfamily: Formicinae
Tribe: Plagiolepidini
Genus: Acropyga
Species: A. oceanica
Binomial name
Acropyga oceanica
Emery, 1900

Acropyga oceanica casent0905162 p 1 high.jpg

Acropyga oceanica casent0905162 d 1 high.jpg

Specimen Labels

Synonyms

Acropyga oceanica has been recorded from leaf litter, and been collected in bamboo forest from a pitfall trap. An Eumyrmoccus sp. mealybug was recorded with this species by Williams (1998), but the record must be considered questionable given the difficulty in the past in identifying this Acropyga species. (LaPolla 2004)

Identification

LaPolla (2004) - Worker: 7-8 segmented antennae; small species (TL: 1.16-1.43); 4 distinct teeth on mandible; torulae very closely set together; antennal apical segment long, about as long as preceding 4 segments combined. Queen: unknown. Male: unknown. Compare with Acropyga yaeyamensis.

This is the smallest known Acropyga and the antennal segment count is the lowest of any known Old World species. Its torulae are also extremely closely set together with virtually no space between them. It can relatively easily be separated from other small Acropyga species by its low antennal segment count (7-8) and the resence of 4 distinct teeth on the mandible. Morphologically the worker of this species resembles Acropyga smithii of the New World. Both of those species have as few as 7 antennal segments, 4 distinct mandibular teeth and similar mesosomal structures. This similarity however is probably the result of convergence.

Keys including this Species

Distribution

Distribution based on Regional Taxon Lists

Indo-Australian Region: Borneo, Indonesia, Malaysia, New Guinea (type locality), Singapore, Solomon Islands.
Palaearctic Region: China.

This species ranges throughout the Indo-Australian region, reaching as far north as southern China.

Distribution based on specimens

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The above specimen data are provided by AntWeb. Please see Acropyga oceanica for further details

Biology

Little is known about Acropyga oceanica. Until further studies reveal more about this species we can infer that its natural history and biology should be similar to other Acropyga. LaPolla published a worldwide revision of the Acropyga in 2004 and the following synopsis is based on this excellent treatment of the genus.

In overall appearance Acropyga are small, robust, yellowish ants possessing a thin, easily collapsible cuticle. The species generally appear rather similar to each other morphologically. In some species workers and queens display an unusual range of phenotypic variation. Antennal segment number, for example, can vary within and between species. Even a single specimen may posses antennae with a different number of antennal segments and workers in numerous species possess one more antennal segment than conspecific males.

The small eyes, reduced antennae segmentation, lightly pigmented cuticle, and hairs covering the cuticle of Acropyga species are suggestive of a completely subterranean existence. Species also display photophobic behavior (Weber, 1944; LaPolla et al., 2002). Acropyga can survive in a wide range of habitats, from deserts to rainforests, though they do not seem able to survive in regions where temperatures below freezing persist for several months at a time. Some species, such as Acropyga pallida and Acropyga silvestrii for example, are found within a very wide range of habitats. Undoubtedly, the Acropyga lifestyle of existing below the surface buffers them against extremes of the outside environment.

Acropyga nests are found in leaf litter, under stones, in rotten wood (lying on or near the soil surface) and in the soil. Observations of nests of various species show the nests are large, consisting of at least several thousand individuals. The nest structure is diffuse with apparently no central nesting location (LaPolla et al., 2002). Tunnels and indistinct chambers stretch out over large areas through the nesting medium. Polygyny has been suggested for several species. The origins of polygyny remains uncertain, but two routes are suggested based on field observations. Biinzli (1935) found both the occurrence of pleometrosis (founding of a colony by multiple queens) and the acquisition of young queens by established colonies in Acropyga exsanguis.

All Acropyga are thought to be hypogaeic (living entirely underground), surviving primarily by "tending" mealybugs (Hemiptera: Pseudococcidae) on underground roots for their exudate (sometimes referred to as "honeydew") (Weber, 1944; Williams, 1998). This mutually beneficial relationship is called trophobiosis (Holldobler and Wilson, 1990).

Acropyga species are all believed to be obligate coccidophiles (dependent on their tended mealybugs for survival). The strength of this trophophitic relationship is clarified by a number of observations. Queens of eleven species have been observed emerging from their nests prior to their mating flight with a mealybug held in their mandibles (Biinzli, 1935; Wheeler, 1935b; Brown, 1945; Eberhard, 1978; Prins, 1982; Buschinger et al., 1987; Williams, 1998; Johnson et al., 2001). The mealybug that each queen carries presumably serves as a "seed individual" from which a new generation of mealybugs will be started in the newly founded ant colony (Weber, 1944; Williams, 1998). This behavior is called trophophoresy (LaPolla et al. 2002) with queens exhibiting this behavior said to be trophophoretic. The mealybugs utilized by Acropyga belong to the subfamily Rhizoecinae, and it is likely that the mealybugs are not able to survive independently of the ants (Williams, 1998). LaPolla et al. (2002) observed that Acropyga epedana keeps mealybugs with their brood. When a nest in captivity was starved, workers refused a variety of food items presented to them, suggestiving that the ants are completely dependent on the mealybugs as a food source. Fossil evidence suggests that the trophobiotic behavior ofAcropyga ants is an ancient one. Johnson et al. (2001) reported that Acropyga queens were discovered in Dominican amber, either holding a mealybug or with a mealybug nearby in the amber matrix. The amber was dated to the Miocene and is at least 15-20 million years old.

Castes

Nomenclature

The following information is derived from Barry Bolton's New General Catalogue, a catalogue of the world's ants.

  • oceanica. Acropyga (Rhizomyrma) oceanica Emery, 1900c: 333, pl. 8, fig. 44 (w.q.m.) NEW GUINEA. Senior synonym of septemstruma: LaPolla, 2004a: 76.
  • septemstruma. Acropyga (Rhizomyrma) septemstruma Terayama, Fellowes & Zhou, 2002: 24, figs. 7-10 (w.) CHINA. Junior synonym of oceanica: LaPolla, 2004a: 76.

Unless otherwise noted the text for the remainder of this section is reported from the publication that includes the original description.

LaPolla (2004) - The lack of worker-associated males for this species is frustrating since Old World males have not been recorded with antennal segment counts below 12 segments. With the worker of this species only possessing up to 8 segments, males would be expected with as few as 9 antennal segments. This could be phylogenetically very interesting, but will have to await future discovery of the males.

Emery recorded the worker as having up to 9 segments, but that is erroneous, because one of the specimens he determined as this species was in actuality an Acropyga ambigua specimen with 9 segments. Nine segmented antennae have not been recorded in the worker of this species.

Description

Worker

LaPolla (2004) - (n=5): TL: 1.16-1.43; HW: 0.329-0.351; HL: 0.345-0.396; SL: 0.22-0.264; ML: 0.34-0.415; GL: 0.437-0.653; CI: 87.83-95.36; SI: 66.87-79.52.

Head: yellow; head longer than broad; posterior margin entire; 7-8 segmented, incrassate antennae; apical antennal segment about as long as preceding 4 segments combined; scape fails to reach posterior margin by about length of pedicel; torulae very closely set together, so that little space exists between them; clypeus narrow, slightly convex medially, with erect hairs on surface; mandible narrow with 4 distinct, nearly even teeth; dorsal surface of mandibles with dense cluster of hairs near masticatory margin (only easily seen with SEM examination); gap exists between inner mandibular margin and anterior clypeal margin. Mesosoma: yellow; in lateral view pronotum with short anterior shelf; pronotum rises steeply toward mesonotum; pronotum without hairs except for a few erect hairs posteriorly; mesosomal dorsum flat; mesonotum with a dense layer of appressed to erect hairs; propodeum with erect hairs; declivity steep. Gaster: petiole thick and erect; gaster yellow, covered in a thick layer of appressed hairs, with scattered erect hairs throughout.

Type Material

LaPolla (2004):

Acropyga (Rhizomyrma) oceanica Emery, 1900: 333 (w.q.m.). 2 syntype workers, PAPUA NEW GUINEA: N.E.: Friedrich-Wilhelmshaften (L. Birö) (Museo Civico di Storia Naturale, Genoa) [examined]. The designated lectotype is a worker labeled JSL TYPE # 128 and is deposited at MCSN. The lectotype is the worker specimen whose head is off of the card; the worker whose head is glued to the card is Acropyga ambigua. Though the queen and male were briefly described in the original description they were not part of the collection at MCSN, and were not able to be located in other collections known to contain a large number of Emery type material.

Acropyga (Rhizomyrma) septemstruma Terayama, Fellowes, and Zhou, 2002: 24 (w.). Holotype worker, CHINA: Hong Kong, Northeast New Territories, Sha Lo Tung (J.R. Fellowes) (ICKH) [not examined]. NEW SYNONYM.

References