Acropyga

Small hypogaeic ants that are presumed to subsist entirely on their obligately tended mealybugs.

Identification
LaPolla (2004): Workers can be separated from other formicine ants by a combination of possessing 11 or fewer antennal segments; tubular, elongate torulae; small eyes (generally only a few facets) that are laterally placed at lower 1/4 of head; low palp formula (less than 6:4); large, round propodeal spiracles; and heads and gasters covered in a dense layer of short appressed hairs with scattered erect hairs throughout. Queens are very similar in general appearance to workers, though they always possess large, flat mesonota. Males are more difficult to diagnose, though they always have 12 or fewer antennal segments, palp formulae less than 6:4, and large, prominent genitalia.

Identification Keys
The revision of the genus by LaPolla (2004) contains New World and Old World species keys for workers, queens and males.
 * Key to New World Acropyga Workers
 * Key to New World Acropyga Queens
 * Key to New World Acropyga Males
 * Key to Old World Acropyga Workers
 * Key to Old World Acropyga Queens
 * Key to Old World Acropyga Males


 * Australia

Distribution
LaPolla (2004): Acropyga are limited to warm temperate and tropical areas around the world. In the New World, Acropyga occur from southern Arizona into northern parts of Argentina, and from the Lesser Antilles and Hispaniola (though curiously they are absent from elsewhere in the Greater Antilles). Two species are currently known from Africa, one in southern Africa, the other in West and East Africa. A single species is restricted to the Palaearctic having been found only in Greece. In the Indo-Australian/Oriental regions, species are known from warmer parts of temperate Japan and China, south through Australia. Acropyga have been found as far east as the Solomon Islands, west to India and Sri Lanka. Acropyga are present in all regions except the Malagasy.

Species richness
Species richness by country based on regional taxon lists (countries with darker colours are more species-rich). View Data



Biology
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. There are currently ~ 40 known species. The species diversity of the genus could increase significantly as litter sampling techniques are being more widely adopted and employed by ant collectors. In the past Acropyga have been poorly collected, and underrepresented in major ant collections, likely  due to their being overlooked. An important implication of this poor collecting coverage is that our understanding of the range and habitat affinities of most species should, at best, be considered preliminary in nature.

In some species workers and queens display an unusual range of phenotypic variation. Antennal segment number, for example, can vary within and between species and even a single specimen may possess differing antennal segment numbers from one antenna to the other. Workers in numerous speices 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. Species in which nesting habits are known possess large, consisting of at least several thousand individuals, the structure of which 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, it is believed, 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).

Evidence suggests that all Acropyga species are obligate coccidophiles, in other words the ants are dependent on the mealybugs for survival. The strength of this association is shown 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 ai. (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.

Immature Stages
LaPolla (2004): Immature stages of Acropyga species are poorly known and a thorough analysis cannot be completed at this time. There may be an interesting trend however in that in the New World pupae have been found to be naked, while in the Old World pupae have cocoons. Given that pupae are known for only 3 species however, it is too early to draw any general conclusions.

Nomenclature

 *  ACROPYGA [Formicinae: Lasiini]
 * Acropyga Roger, 1862a: 242. Type-species: Acropyga acutiventris, by monotypy.
 * Acropyga senior synonym of Atopodon, Malacomyrma, Rhizomyrma: LaPolla, 2004: 16. [The former subgenera were listed as junior synonyms in Hölldobler & Wilson, 1990: 18, without reason; reverted to subgeneric status in Bolton, 1994: 51 pending revision; formally synonymised by LaPolla, above.]
 * ATOPODON [junior synonym of Acropyga]
 * Atopodon Forel, 1912m: 771 [as subgenus of Acropyga]. Type-species: Acropyga (Atopodon) inezae, by subsequent designation of Wheeler, W.M. 1913a: 79.
 * [Atopodon also described as new by Forel, 1913k: 100.]
 * Atopodon subgenus of Rhizomyrma: Forel, 1917: 249.
 * Atopodon raised to genus: Wheeler, W.M. 1922a: 695.
 * Atopodon subgenus of Acropyga: Emery, 1925b: 30.
 * Atopodon junior synonym of Acropyga: LaPolla, 2004: 16.
 * MALACOMYRMA [junior synonym of Acropyga]
 * Malacomyrma Emery, 1922d: 109 [as subgenus of Acropyga]. Type-species: Acropyga silvestrii, by monotypy.
 * Malacomyrma junior synonym of Acropyga: LaPolla, 2004: 16.
 * RHIZOMYRMA [junior synonym of Acropyga]
 * Rhizomyrma Forel, 1893g: 347 [as subgenus of Acropyga]. Type-species: Acropyga (Rhizomyrma) goeldii, by subsequent designation of Wheeler, W.M. 1911f: 172.
 * Rhizomyrma raised to genus: Emery, 1906c: 182; Kusnezov, 1956: 33.
 * Rhizomyrma subgenus of Acropyga: Emery, 1925b: 29.
 * Rhizomyrma junior synonym of Acropyga: LaPolla, 2004: 16.

Description
From LaPolla (2004):

Characters marked with an (*) are considered synapomorphies for the genus.

Worker
1) Small, robust, yellow to brownish-yellow (to occasionally slightly reddish-yellow) formicine ants (Total length up to 5 mm as found in Acropyga rubescens, typically however most species smaller, around 2 mm in total length).

2) Monomorphic.

3) Cuticle seemingly always thin, shrinkage and collapsing of cuticle commonly observed in specimens.

4) *Antennae incrassate (apical segment always the largest and thickest) with 7- 11 segments.

5) Surface of scapes and flagellae covered in thick layer of appressed hairs.

6) *Torulae tubular and elongated, touching posterior clypeal margin, typically closely set together (only in Acropyga kinomurai are torulae very widely separated from each other); small triangular area between torulae.

7) *Median portion of anterior clypeal margin always with a single hair; length of hair highly variable, sometimes surpassing apex of closed mandibles.

8) Mandibles variable, narrow to broad, with 3-9 teeth possible, though if with more than 6 teeth, some of those teeth are usually quite small (intercalary).

9) *Palpal formula always less than 6:4, typically 2 : 3 or 3 :3, though up to 5 :4 is known in Acropyga arnoldi and Acropyga paleartica.

10) *Eyes small, typically consisting of fewer than 10 facets, though more than 20 facets observed in Acropyga acutiventris and Acropyga rubescens; eyes laterally placed, at the lower 1/4 of the head.

11) Head typically covered in a thick layer of short, appressed hairs.

12) Maximum width of pronotum wider than that of remainder of mesosoma (in dorsal view).

l3) Mesosoma and legs robust; legs always with at least a layer of appressed hairs, occasionally with scattered erect hairs as well.

14) Metanotal area variable with regards to number of sulci present, though the region is never very deeply impressed.

15) *Propodeal spiracle large and round, placed near declivity border.

16) Metapleural gland present.

17) Petiole always thick and erect, occasionally slightly inclined forward.

18) Gaster robust, rounded and larger toward petiole end from where it becomes narrower (to a point) posteriorly; in lateral view tear-drop shaped; gaster sloping toward acidopore dorsally from anterior to posterior; venter more flattened than dorsum.

19) Proventriculus asepalous (see Eisner, 1957; Prins, 1980; this study [Acropyga ayanganna, new species; Acropyga keira, new species examined]).

20) Declivity distinct and steep.

21) Mesosomal-petiole muscle insertion point round.

22) Ventral mesosomal-petiole articulation point surpasses metacoxal insertions.

Queen
23) Yellow to dark brown (almost black) in color.

24) Three ocelli present, large and distinct at apex of head.

25) Pronotum small and collar-like.

26) Mesonotum dorsum flat and wide.

27) Propodeum short, and low; declivity typically indistinct from dorsal face of propodeum.

28) Similar in overall appearance to workers (though always larger) with modifications expected for caste; antennal segment count and mandibular dentition typically same as found in workers.

Male
29) Yellow to dark brown (almost black) in color.

30) * Antennae with 8- 12 segments.

31) Three ocelli present, large and distinct at apex of head.

32) Eyes large, breaking outline of head in full face view

33) Mandibles narrow to broad, with 1 to 6 teeth possible.

34) Mesosoma robust, mesonotum large, with flat dorsum.

35) Pronotum small and collar-like.

36) Propodeum short, and low; declivity typically indistinct from dorsal face of propodeum.

37) Genitalia large (proportional to body) and prominent.

Larvae and Pupae
38) Larval body hairs adundant and of three types: simple, denticulate, and branched (Wheeler and Wheeler, 1953).

39) Pupae both naked and with cocoons; naked pupae reported for Acropyga ayanganna, new species and Acropyga keira, new species; pupae with cocoons reported for Acropyga acutiventris.