Acanthostichus Mayr, 1887

Borowiec, Marek L., 2016, Generic revision of the ant subfamily Dorylinae (Hymenoptera, Formicidae), ZooKeys 608, pp. 1-280 : 64-67

Taxon classification Animalia Hymenoptera Formicidae

Acanthostichus Mayr, 1887

= Ctenopyga Ashmead, 1906

Type-species.

Typhlopone serratula , by monotypy.

Acanthostichus is a New World genus of termite-hunting dorylines most closely related to Cylindromyrmex .

Diagnosis.

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 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.

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 Cylindromyrmex , 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.

Description.

Worker.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 Pronotomesopleural 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 Pronotomesopleural 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.

Male.Head: Antennae with 12 or 13 segments. Antennal scapes dorsoventrally flattened. Clypeus with cuticular apron, not translucent. Parafrontal ridges absent. Torulo-posttorular complex vertical. Maxillary palps 2-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. Notauli absent or present. Transverse groove dividing mesopleuron absent or present. Propodeal declivity reduced, without distinct dorsal edge or margin. Metapleural gland opening present. Propodeal lobes present. Metasoma: Petiole anterodorsally immarginate, dorsolaterally immarginate, and laterally above spiracle immarginate. Helcium in relation to tergosternal Pronotomesopleural suture placed at posttergite and supraaxial. Prora forming a simple U-shaped margin or V-shaped protrusion. Spiracle openings of abdominal segments IV–VI circular. Abdominal segment III more than half size of succeeding segment IV; latter 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. Girdling constriction between pre- and postsclerites of abdominal segments V and VI absent or present. Abdominal segment IV not conspicuously largest segment. Abdominal sternite VII simple. Abdominal sternite IX distally armed with two spines, 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 or separated. Telomere gradually tapering toward apex. Volsella gradually tapering toward apex. Penisvalva laterally flattened, at apex hooked ventrally. 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 absent. Metabasitarsal glands absent. Hind pretarsal claws simple. Wings: Tegula present, broad, demiovate or narrow, demilanceolate in shape. Abscissa R·f3 present and running toward distal wing margin but not enclosing cell with Rs·f5. Abscissae Rs·f2-3 absent or present, connecting with Rs+M&M·f2. Cross-vein 2r-rs present, connected to Rs·f2-3&Rs·f4, differentiated from Rs·f4 by presence of Rs·f2-3. Abscissae Rs·f4-5 present, fused in absence of 2rs-m or differentiated into Rs·f4 and Rs·f5 by 2rs-m. Abscissa M·f2 in fore wing contiguous with Rs+M. Abscissa M·f2 in fore wing present, separated from Rs+M by Rs·f2. Abscissa M·f4 in fore wing present, not reaching wing margin. Cross-vein 1m-cu in fore wing present. Cross-vein cu-a in fore wing absent or present, arising from M+Cu and proximal to M·f1. Vein Cu in fore wing present, with only Cu1 branch prominent or with both branches Cu1 and Cu2. Vein A in fore wing with abscissae A·f1 and A·f2 present. Vein C in hind wing absent. Vein Sc+R+Rs present. Vein R in hind wing absent. Vein Sc+R in hind wing present. Abscissa Rs·f1 in hind wing present, shorter than 1rs-m, sometimes a stub. Abscissa Rs·f1 in hind wing present, longer than 1rs-m. Abscissa Rs·f2 in hind wing present, not reaching wing margin. Cross-vein 1rs-m in hind wing absent or present, about as long as M·f1. Vein M+Cu in hind wing present. Abscissa M·f1 in hind wing absent or present. Abscissa M·f2 in hind wing absent or present. Cross-vein cu-a in hind wing absent or present. Vein Cu in hind wing absent or present. Vein A in hind wing absent or with abscissa A·f1 present.

Gyne. 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 (see above; Brown 1975, MacKay 1996).

Larva. Described in Emery (1899c), Bruch (1925). Cocoons absent.

Distribution.

Acanthostichus is a genus of 24 described species, occurring in 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.

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 Acanthostichus 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 Cerapachys davisi match the morphology of Acanthostichus males. A specimen of Cerapachys 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 Acanthostichus davisi (then recognized as Cerapachys ) at the type locality of Acanthostichus 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).

Biology.

Along with their close relatives in Cylindromyrmex , Acanthostichus species are predators of termites, unlike most other doryline species which prey on ants ( Kusnezov 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 1962 a, 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.

Species of Acanthostichus

Acanthostichus arizonensis Mackay, W.P., 1996: United States

Acanthostichus bentoni Mackay, W.P., 1996: Brazil

Acanthostichus brevicornis Emery, 1894: French Guiana

Acanthostichus brevinodis Mackay, W.P., 1996: Brazil

Acanthostichus concavinodis Mackay, W.P., 1996: Bolivia

Acanthostichus davisi (Smith, M. R., 1942a): United States

Acanthostichus emmae Mackay, W.P., 1996: Mexico

Acanthostichus femoralis Kusnezov, 1962: Argentina

Acanthostichus flexuosus Mackay, W.P., 1996: Brazil

Acanthostichus fuscipennis Emery, 1895b: Brazil

† Acanthostichus hispaniolicus De Andrade, 1998b: Dominican amber

Acanthostichus kirbyi Emery, 1895b: Paraguay

Acanthostichus laevigatus Mackay, W.P., 1996: Venezuela

Acanthostichus laticornis Forel, 1908: Paraguay

Acanthostichus lattkei Mackay, W.P., 1996: Venezuela

Acanthostichus longinodis Mackay, W.P., 2004: Paraguay

Acanthostichus punctiscapus Mackay, W.P., 1996: United States

Acanthostichus quadratus Emery, 1895: Bolivia

Acanthostichus quirozi Mackay, W.P., 1996: Mexico

Acanthostichus sanchezorum Mackay, W.P., 1985: Colombia

Acanthostichus serratulus (Smith, F., 1858): Brazil

Acanthostichus skwarrae Wheeler, W. M., 1934: Mexico

Acanthostichus texanus Forel, 1904: United States

Acanthostichus truncatus Mackay, W.P., 1996: Colombia