Acanthostichus

Acanthostichus is a Neotropical genus that reaches the northern limit of its distribution in the southwestern USA and its southern limit in Paraguay, Uruguay and northern Argentina. There is also a single fossil species from Dominican amber. Suspected of being deeply subterranean, little is known of their habits. Specimens have been found under stones and wood, and collections have been made in association with nests of termites. These ants are rarely collected but may be much more common than their representative specimens suggest.

Identification
Borowiec (2016) - Worker. The workers of this distinctive lineage can be recognized by a combination of 12-segmented antennae, absence of ridge on pronotal collar, unfused pronotomesopleural suture, highly positioned helcium, a single pectinate spur on mid and hind tibiae, propodeal spiracle usually positioned below the midheight of the sclerite, and large pygidium armed with modified finger-like setae. Restricted to the New World, the species of Acanthostichus are medium-sized ants that are often brown or yellowish in coloration and lack distinctive sulcate or striate sculpturing characteristic of its close relative Cylindromyrmex or very conspicuous constrictions between gastral segments of Sphinctomyrmex. Other New World dorylines (army ants related to Eciton, species of Leptanilloides) all have simple, small pygidium, at most armed with several thick setae. Workers of Syscia and the introduced Ooceraea biroi, also found in the New World, can be distinguished by antennal segment count reduced to 11 or 9, respectively.

Borowiec (2016) - Male. The male of Acanthostichus can be separated from all other dorylines by a combination of propodeal lobes conspicuous, supraaxial helcium, single spur on each mid and hind tibiae, costal vein (C) present in fore wing, and R·f3 present past pterostigma but not enclosing a cell with Rs·f5. Most species appear to have 12-segmented antennae but at least Acanthostichus texanus and Acanthostichus davisi are known to have 13 antennal segments. Among New World dorylines the habitus of males is similar to that of Cylindromyr-mex, Neocerapachys, Syscia, and Sphinctomyrmex. Sphinctomyrmex has conspicuous constrictions between abdominal segments IV, V, and VI in combination with narrow axial helcium, Cylindromyrmex has two tibial spurs, and Syscia lacks the costal vein in the fore wing. Neocerapachys has either a closed marginal cell or lacks cross-vein 2rs-m. Furthermore, Acanthostichus males that lack 2rs-m have a broader helcium and more poorly developed posterior face of the petiole than is characteristic of Neocerapachys. Other dorylines found in the New World include the army ants, and males in these genera always have marginal cell closed by R·f3 and Rs·f5, only one well-differentiated waist segment, and no constriction between abdominal segments III and IV. The remaining neotropical genus, Leptanilloides, has no conspicuous tegula and venation reduced, without R·f3 or discal cell.

See also notes on the Acanthostichus species groups.

Distribution
Acanthostichus occurs in the southern United States, Mexico, and most of South America. The genus has long been thought absent from Central America, but at least one specimen is known from Costa Rica. This is unlikely due to undersampling, as Central American countries have been the subject to some of the most intensive surveys of ant faunas (Longino et al. 2014). Acanthostichus hispaniolicus has been described from Dominican amber (Miocene) of Hispaniola. (Borowiec 2016)

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



Biology
These subterranean ants are seldom collected and are presumed termite predators although some species may be predators on Pogonomyrmex and other ants. Mackay and Mackay (2002) stated, in the context of the North American ant fauna: collections of this genus are extremely rare, due to the subterranean habits of colonies. Males of this genus are commonly collected at lights in the tropics; males of North American species are either very rare or are not attracted to lights. The males and females are known for only one of the species that occurs in the United States: Acanthostichus texanus.

Borowiec (2016) - Along with their close relatives in Cylindromyrmex, Acanthostichus species are predators of termites, unlike most other doryline species which prey on ants (Kus-nezov 1962, Brown 1975, MacKay 1996). Acanthostichus truncatus has been observed to raid an arboreal termite nest (MacKay 1996) and Acanthostichus hispaniolicus is known from multiple specimens in Dominican amber suggesting that this species was also an arboreal forager. Unlike Cylindromyrmex, however, they nest in soil, under stones, and in rotting wood (Kusnezov 1962a, MacKay 1996). These ants are rarely encountered and little is known of Acanthostichus ecology, nest size or other particulars of their biology. It is unclear whether brood production is synchronized.

Castes
Mackay (2004) - The gynes of this genus are known from only five species: Acanthostichus emmae (texanus species complex) and Acanthostichus texanus (texanus species complex), which are “normal”, winged gynes, with ocelli, and Acanthostichus laticornis (serratulus species complex), Acanthostichus quadratus (serratulus species complex) and now Acanthostichus brevicornis (brevicornis species complex), which are subdichthadiiform (lacking ocelli, lacking flight sclerites, having a distended gaster, and lacking the teeth on the pygidium). The gyne of A. brevicornis easily can be separated from that of A. quadratus, as the malar groove is developed at least near the base of the mandible (apparently lacking in A. quadratus), the propodeum is rounded between the faces (with an angular process in A. quadratus and a carina in A. laticornis) and the subpetiolar process is absent (developed into a tooth in A. quadratus and a broad flange in A. laticornis). Separation from A. laticornis is more difficult. The gyne of A. brevicornis is smaller than that of A. laticornis. The total length of the gyne of A. brevicornis is less than 9 mm, the gyne of A. laticornis is about 11 mm. The malar groove in the gyne of A. brevicornis is relatively short, extending slightly more than 0.1 mm from the point of insertion of the mandible (extending about half of the distance to the eye of A. laticornis or 0.4 mm). The lateral clypeal angles are absent in A. brevicornis (convex border is present), but are well developed in A. laticornis. The frontal carinae of A. brevicornis are very closely placed, with the distance between the external margins 0.23 mm (about 0.4 mm in A. quadratus, 0.47 in A. laticornis). This suggests an additional characteristic to separate the two species complexes. The frontal carinae of the gynes of the brevicornis complex may be closely spaced, those of the serratulus species group are apparently more widely spaced. As gynes have not been collected without workers, the worker characteristics would allow separation of the gynes of the two species. Since the sample size of gynes is so limited, we have very little sense of intraspecific variation, which is really needed to establish how the species differ.

Nomenclature

 *  ACANTHOSTICHUS  [Dorylinae]
 * Acanthostichus Mayr, 1887: 549. Type-species: Typhlopone serratula, by monotypy.
 * Acanthostichus senior synonym of Ctenopyga: Mackay, 1996: 132.
 * CTENOPYGA [junior synonym of Acanthostichus]
 * Ctenopyga Ashmead, 1906: 29. Type-species: Ctenopyga townsendi (junior synonym of Acanthostichus texanus), by original designation.
 * [Ctenopyga Ashmead, 1905b: 382. Nomen nudum.]
 * Ctenopyga subgenus of Acanthostichus: Emery, 1911d: 13.
 * Ctenopyga revived status as genus: Brown, 1975: 42; Bolton, 1990a: 67; Bolton, 1990c: 1357.
 * Ctenopyga junior synonym of Acanthostichus: Snelling, R.R. 1981: 389; Bolton, 1994: 19; Mackay, 1996: 132.

Borowiec (2016) - Taxonomy and phylogeny. Acanthostichus was erected by Mayr (1887) for the species Typhlopone serratula Smith 1858, then known only from workers. Ashmead (1906) later introduced the genus Ctenopyga, based on an alate gyne and males. He differentiated it from Acanthostichus based on the gyne morphology, as by then wingless gynes were found in Acanthostichus (Emery 1895l). MacKay (1996) synonymized the otherwise remarkably similar Ctenopyga under Acanthostichus and I follow his decision here. MacKay (1996) also revised the genus and provided keys to all species, later adding one more species and describing a gyne of A. brevicornis (MacKay 2004). De Andrade described the species from Dominican amber (de Andrade 1998b).

Males described by Marion Smith (1942b) and attributed to ‘Cerapachys’ (here Syscia) augustae and C. davisi match the morphology of Acanthostichus males. A specimen of C. davisi was also included in a molecular phylogeny and was shown to be a close relative of Acanthostichus punctiscapus. Therefore davisi was transferred to Acanthostichus (Brady et al. 2014). It is possible that Smith’s putative males of Syscia augustae and Acanthostichus davisi will turn out to be conspecific with Acanthostichus arizonensis and Acanthostichus punctiscapus, respectively. MacKay (1996) collected numerous males of A. davisi (then recognized as Cerapachys) at the type locality of A. punctiscapus.

It is now established that Acanthostichus is most closely related to Cylindromyrmex (Brady and Ward 2005, Brady et al. 2006, Brady et al. 2014, Borowiec, in prep.). There have been no efforts to infer the internal phylogeny of the genus, but MacKay divided Acanthostichus into three species groups based on morphology (MacKay 1996).

Mackay (1996) - The genus has been subjected to a few inadequate revisions. Kusnezov (1962) repeated the mistakes of Wheeler (1934) and added several of his own, some of which were pointed out by Kempf (1964). Kusnezov's revision (1962) is thus completely unreliable and, except for the description of Acanthostichus femoralis should be disregarded. As a consequence of these works most identifications in museums are incorrect. This is a difficult genus which presently contains only twenty two known species. There are few differences among species; the mesosoma and gaster are nearly identical in most species. The petiole is the most important structure for the identification of workers. The form of the subpetiolar process is also very important. Many characters associated with the head are useless. The shape of the scape and the form of the anterior margin of the clypeus are important in some species. Identifications depend on very careful measurements, especially of the petiole and the scape. Without access to a good reference collection identifications may be impossible.

Worker
Borowiec (2016) - Head: Antennae with 12 segments. Apical antennal segment moderately enlarged, broader than and about equal in length to two preceding segments combined. Clypeus without cuticular apron. Lateroclypeal teeth absent. Parafrontal ridges absent. Torulo-posttorular complex horizontal. 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 2-segmented. Labial palps 3-segmented. Mandibles triangular, with median tooth or triangular, edentate. Eyes present, composed of 1–20 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 absent or present. Mesosoma: Pronotal flange not separated from collar by distinct ridge. Promesonotal connection with suture completely fused. Pronotomesopleural suture visible, unfused partway to notal surface. Mesometapleural groove replaced by cuticular ridge. Transverse groove dividing mesopleuron absent. Pleural endophragmal pit concavity present. Mesosoma dorsolaterally weakly to conspicuously marginate. Metanotal depression or groove on mesosoma absent. Propodeal spiracle situated low or high on sclerite. Propodeal declivity with distinct dorsal edge or margin and rectangular in posterior view. Metapleural gland with bulla visible through cuticle. Propodeal lobes present, well developed. Metasoma: Petiole anterodorsally immarginate, dorsolaterally marginate, and laterally above spiracle marginate. Helcium in relation to tergosternal suture placed at posttergite and supraaxial. Prora forming a simple U-shaped margin or reduced to small longitudinal ridge. Spiracle openings of abdominal segments IV–VI circular. Abdominal segment III anterodorsally immarginate and dorsolaterally immarginate. Abdominal segment III more than half size of succeeding segment IV, which is weakly constricted at presegmental portion (uninodal waist). Girdling constriction of segment IV present, i.e. pre- and postsclerites distinct. Cinctus of abdominal segment IV gutter-like, not sculptured or weakly cross-ribbed. Abdominal segment IV not 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 or present. Pygidium large, with impressed medial field, and armed with modified setae. Hypopygium unarmed. 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 absent or oval patch of whitish cuticle. Metabasitarsal gland absent. Hind pretarsal claws simple. Polymorphism: Monomorphic.

Mackay (1996) - Clypeus with broadly concave medial anterior border, lateral teeth present in many species; frontal carinae usually not covering insertions of antennae; scapes short, antenna with 12 segments; maxillary palps 2 segmented, labial palps 3 segmented; smooth, glossy and shining throughout (unless otherwise mentioned), except for the dorsum of petiole and sides of mesosoma and petiole, which are usually at least lightly sculptured; few erect, simple hairs scattered over most of body surface; mandible without teeth (in most species), except for the apical angle or tooth; malar groove present, extending from base of mandible posteriorly to area below eye; eye usually very small (relatively large in A. texanus). but present, conSisting of a single (or few) ommatidium; metatibial glands present; claws simple; postpetiole (third abdominal segment) separated from remainder of gaster; pygidium with numerous large, upwardly directed teeth; small to moderate sized ants, often polymorphic in size. Almost invariably reddish-brown to dark brown.

Queen
Borowiec (2016) - Acanthostichus gynes are known either as alates or subdichthadiiform, i.e. ergatoid without wing sclerites but possessing hypertrophied gasters. The former are currently known for Acanthostichus emmae and Acanthostichus texanus, the latter in Acanthostichus brevicornis, Acanthostichus quadratus, and Acanthostichus laticornis. In the fully alate gynes the eyes are large and three ocelli are present and abdominal segment III is differentiated from succeeding segments. In the subdichthadiigynes the eyes are present but small, three small ocelli are present, the head is more round than in workers, and mandibles are falcate; there are no flight-associated sclerites, abdominal segment II (petiole) is broadly attached posteriorly to segment III, which is also enlarged, not separated from the rest of the gaster by a constriction (Emery 1895, MacKay 1996). The distinction between alate versus wingless gynes was the basis for the separation of the genus Ctenopyga from Acanthostichus (Brown 1975, MacKay 1996).

Mackay (1996) - Only known from five species (Acanthostichus brevicornis, Acanthostichus emmae, Acanthostichus laticornis, Acanthostichus quadratus, and Acanthostichus texanus). That of A. texanus is very similar to worker in size and form; ocelli well developed; winged. The unknown worker of A. emmae is probably similar to the female. The females of the other two species (A. quadratus and A. laticornis) are subdichthadiiform (possess ocelli, have large eyes, have full complement of flight sclerites, see Bolton 1990b), much larger than worker; wingless; petiole much wider than long; covered with long, flexuous hairs. The malar groove is present in three of these species (A. emmae, A. texanus and A. laticornis), but absent in the fourth (A. quadratus). The pygidium toothed in A. emmae and A. texanus but not in A. laticornis and A. quadratus. Mandibles with or without teeth.

Male
The males of most species are very similar and difficult to distinguish. Therefore, descriptions are limited to characteristics which can be used to separate species. I am providing a key which may be of little use for identifying some species. Size variable within a species, with no constant size differences between species. The males of many species are unknown. The following characteristics fit most species. HL 0.73-1.16, HW 0.80-1.18, SL 0.18-0.38, SW 0.10-0.18, EL 0.39-0.58, WL 1.60-2.45, PW 0.33-0.68, PL 0.43-0.48, SI 24-35, CI 99-115, PI 70-131. Mandible lightly punctate, usually without teeth on masticatory border (except apical angle); scapes short. often in shape of “drum stick”, insertion of scapes completely exposed; frontal carinae closely placed; malar groove (between anterior edge of eye and mandible) well developed, although difficult to see in some species due to short distance between eye and base of mandible; eyes very large. covering most of sides of head; three ocelli large, well developed; antenna with 12 segments (13 in A. texanus), those of funiculus difficult to count due to roughened sculpture and numerous small setae; scutum elongate and together with remainder of mesothorax, cause the fore and mid coxae to be widely spaced; shape of petiolar node variable between species, seems to bear no relationship to corresponding shape of node of worker of same species; subpetiolar process poorly developed; with two lateral “felt” lines of fine hair on underside of petiole (poorly developed in A. texanus); femora not incrassate; pygidial teeth poorly developed or absent; aedeagus well developed. usually elongate, toothed, with well developed apical tooth; volsella also well developed. usually with ventrally directed, apical tooth, which is often twisted laterally; stipites absent; paramere relatively small, usually smaller than volsella; hypopygium fork-like with two teeth. Partially or primarily shining, concolorous dark brown. with scattered erect hairs on most or all bodily surfaces, unless otherwise indicated.