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
This genus is most likely to be confused with the closely related Cerapachys, which also has denticles on the pygidium of the worker. It can be distinguished by its 12-segmented antennae (11 segments in Cerapachys), the strongly flattened antennal scape and the lack of a prominent lateral carina bordering the antennal sockets.

Mackay (1996) - Acanthostichus can be defined by two synapomorphies in the workers and females: the presence of a metatibial gland, and the malar groove present below the eye. The functions of these two structures are unknown, and the groove has apparently been secondary lost in the female of Acanthostichus quadratus. Males also possess the malar groove, although it is difficult to see in some species as the distance between the base of the mandible and the eye may be very short.

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.

See also notes on the Acanthostichus species groups.

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.

Castes
Mackay (2004) - The gynes of this genus are known from only five species: A. emmae Mackay (texanus species complex) and A. texanus Forel (texanus species complex), which are “normal”, winged gynes, with ocelli, and A. laticornis Forel (serratulus species complex), A. quadratus Emery (serratulus species complex) and now A. brevicornis Emery (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.

Mackay (1996):

Worker
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
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.