Acanthostichini
Creators: name: Brown, W. L.
Publisher: name: Zenodo
Year: 1975
Subjects: Biodiversity
Taxonomy

description: Tribe Acanthostichini
Though 2 genera, Acanthostichus and Ctenopyga, are recognized in this tribe, only the first is known for all castes. In Ctenopyga, we know for sure only the queen; males assigned to the genus are doubtfully associated, and no worker has yet been described for it. The assumption is that the Ctenopyga worker is much like that of Acanthostichus, but how many characters they actually share is a matter for conjecture. In any case, the characterization that follows is based only on worker, queen, and male of Acanthostichus and queen of Ctenopyga.
Worker: Monomorphic, but often varying considerably in size, even in the same nest. Body compact but somewhat elongate, subcylindrical, TL ranging from 3 mm to about 8 mm; integument thick and hard. Head subrectangular as seen in full-face view, longer than broad, with sides parallel or nearly so and straight or gently convex; posterior border transverse, usually concave, at least slightly in the middle; posterior corners broadly to rather sharply rounded. Cranium convex above and below, rather deep posteroventrally and with the posterior profile broadly rounded as seen from the side, without carinae or sutures posteroventrally or ventrally, except for the median suture; only the extreme dorsal cervical margin of the head is carinate, and a pigmented mark extends forward a little way from this carina in the middle, following an apophysis on the internal surface.
Eyes reduced to minute unpigmented discs or pits (in some large workers, traces of ommatidial facets can still be discerned at high magnification) situated dorsolaterally a little behind the cranial midlength. Ocelli absent, side of head anteroventral to eye with a shallow, longitudinal groove or channel running from near midlength of head to mandibular insertion. The groove runs ventrad around the lateral boss at the end of the clypeus and appears to enter the insertion at a point adjacent to a deep, round fossa in the extreme base of the mandible itself, as though it could conduct some liquid material from the side of the head down to the mandibular fossa. It is not clear from the available specimens whether the groove begins at its posterior end with a pore or merely the insertion of a seta situated at that point. Usually 1 - 3 more setae are found in the groove farther forward toward the mandible. (A groove also exists along the outer margin of the mandible in its apical half, and this is set with long, downcurved setae, but the basal connections of this groove, if any, are not clear.)
Antennae 12 - merous, very short and thick, especially the scapes, which are flattened and shield-like, and only about 2 - 3 times longer than broad, but suddenly narrowed basad. Funiculi markedly but gradually broadened toward apex, most segments wider than long, but apical segment longer than broad. Scapes never reaching much beyond eyes when laid straight back, inserted very close to anterior margin of head and close together. Clypeus reduced almost to the status of an anterior rim of the head, extending back as a narrow median sliver between the nearly approximate frontal lobes, and anteriorly forming a low median lobe or apron that is deeply emarginate in the middle and extends laterad as a low, anterior rim or process fronting each antennal socket. Frontal lobes short, broad, horizontal or obliquely raised, with convex lateral margins becoming concave behind, where they are pinched before terminating well in front of the cephalic midlength. Posterior clypeal lobe and frontal area forming little more than a wide groove between the frontal lobes.
Mandibles thick and downcurved, essentially triangular, but the masticatory margins strongly curving and continued onto narrowed apices, so that the blades may appear broadly subfalcate in oblique end-on view. Masticatory borders edentate and cultrate or with a few spaced denticles.
Under mouthparts (described and figured by Gotwald, 1969: 43 - 44) with labrum not much broader than long, shallowly bilobate; galea elongate; palpi segmented 2,3.
Trunk boxlike and solidly fused into a single structure; dorsum flat or only gently convex, joining the vertical or even slightly concave sides at rectangular margins, or narrowly rounding into the sides. Pronotum with rounded or bluntly subangular humeri as seen from above. Dorsal sutures obliterated, except for metanotal groove, which sometimes shows as a slightly impressed line; in this area there is a coarse median pit. Laterally, the promesonotal and mesometanotal sutures are strongly impressed and lead up to their respective spiracles; the metanotal spiracle is lateral, well below the dorsolateral margin. In dorsal view, sides of trunk subparallel, usually a little wider across pronotum, and sometimes feebly constricted near metanotal groove. Propodeum with steep, distinct declivity, margined or submarginate on sides, and passing into dorsum through a right angle or sharp curve. Propodeal spiracle round, situated near mid-height of trunk and well in front of declivity margin. Metapleural gland bulla and meatus welldeveloped.
Legs short and stout, with thick coxae, incrassate to very strongly incrassate femora, and short, apically thickened tibiae. Middle and hind tibiae each with a single, broadly pectinate apical spur; a stout seta usually present on these tibiae may represent a vestigial lateral spur; the middle tibiae also have hairy, roughened, extensor surfaces with short, oblique, setal bristles, as in Centromyrmex, Cryptopone, etc. Metatarsi of middle legs short, each with a stout, conical, submedian setal spine on outer surface and a circlet of such spines at apex; II to VI tarsal segments also with 2 or more such spines, and some also on tarsomeres of hind legs. Tarsal claws slender, simple.
Petiole boxlike, from above, subquadratic or oblong; from side view, longer than high, sessile, with steep anterior face rounding into flat or weakly convex dorsal face; sides vertical, meeting dorsal face at dorsolateral margins or else rounding sharply into dorsal face; sides parallel or nearly so, straight to weakly convex; node posteriorly broadly attached to postpetiole. Subpetiolar process present, laterally compressed, usually irregularly rounded to angular in side view.
Postpetiole wider, and much deeper in side view, than petiole, about as deep as main gastric segments, or nearly so, rounded in front and with broadly rounded ventral surface; distinctly but shallowly constricted from remainder of gaster. Gaster fairly long, tapered toward both ends, and wider than postpetiole; apical segment downturned, its tergum apically flattened and forming a triangular pygidial area bordered with short, stout, setal spines; sting well developed.
In general, body smooth, shining, and without coarse sculpture; but in some of the species, wide areas, especially sides and posterior of trunk and dorsal face of petiole, are finely and superficially reticulate or reticulate-striolate, and these areas are opaque or subopaque.
Pilosity sparse, consisting mainly of longish, fine setae that are bilaterally matching on head, alitrunk, and petiole, and a little more abundant on gaster, especially near apex. The antennae, mandibular apices, and legs have some erect or oblique hairs, and the legs also may have very sparse, short, appressed pubescence.
Color varying from yellowish through reddish to dark brown, sometimes in life appearing black to the naked eye.
Queen: Like the worker of Acanthostichus, but occurs in 2 types, the subdichthadiiform (Acanthostichus) and the " normal " (Ctenopyga). In the subdichthadiiform type, the body is much larger and broader than in the corresponding worker (body L 10.5 mm in A. quadratus, according to Emery, and 12 mm in the A. laticornis queen, teste Bruch), the head is almost circular in full-face view, and the flat compound eyes, while still small (not longer than the width of the median funicular segments), are distinctly multifacetted. Ocelli lacking or represented by small pits. Frontal carinae deflected strongly behind, laterad toward the eyes, forming demiscrobes for the antennal scapes. Mandibles short but subfalcate, edentate, but each with an acute apex, blades narrower than in worker. Antennae and palpi much as in worker, perhaps a bit more slender. Trunk workerlike in form but much wider, and as wide behind as in front or slightly wider. Node of petiole much wider than long, with rounded sides; broader behind, and very broadly attached to post-petiolar segment. Postpetiole nearly as wide as succeeding (IV true abdominal) segment and not separated from it by a distinct constriction — at least not in dorsal view — so that it forms an integral part of the gaster. Pygidium not bordered with spiniform setae. Gaster including postpetiole quite voluminous and taking up more than half the total length of the body.
Sculpture mostly smooth or sparsely punctulate on head, trunk, and petiole; sides of trunk finely reticulate or reticulostriolate; pronotum smooth or punctulopubescent. Gaster, at least true abdominal terga IV-VII, densely punctulopubescent and subsericeous. Head, trunk, legs, and petiole with or without long, fine, erect hairs, and the gastric apex with erect pilosity.
Figures of the dichthadiiform queen are given by Emery 1895 and 1911, and by Bruch 1934.
The " normal " queen (Ctenopyga) is the kind of winged or dealate female that one would expect to go with the workers of Acanthostichus; this queen has fairly large compound eyes, ocelli, and a well-developed but low pterothorax that bears, in the virgin state, apparently functional wings. I have not studied such a queen directly, but the good figure in M. R. Smith (1947: 526, pl. 1, fig. 4, 529) shows an insect very much like the Acanthostichus workers we know from South America. The cheek grooves, a worker character described here for the first time, have not been checked for on the extant Ctenopyga (or Acanthostichus) queens. Wing venation unknown.
Male: (Acanthostichus): Body size similar to, or slightly greater than, the corresponding workers, but appearance quite different. Head broader than long, including the large compound eyes, which occupy nearly half to more than half of the head length. Ocelli well developed.
Antennae 12 - merous, with short scapes, often with incrassate funiculus, tapered distad in apical segment. First postpedicellar (second funicular) segment very small. Frontal carinae short, with lobes raised more than in worker, forming a more or less boat-shaped median frontal area. Clypeus narrow, with a narrow posterior lobe between frontal lobes; anterior part steeply sloping forward, with a more or less distinct, free lamelliform margin.
Mandibles triangular, with tapered apices and edentate, hollow-curved masticatory borders, tips crossing at full closure. Under mouthparts much as in worker; palpi segmented 2,3. Pronotum narrower than pterothorax, the latter not strongly humped. Trunk robust, with well developed wings (fig. 73) having complete or nearly complete venation; in some species, Rsf 2.3 is absent. No anal lobes on hind wings. Notauli absent or at most weakly indicated at the anterior end, incomplete behind.
Legs fairly long and slender, without the stout spiniform setae of the worker; tibial apices each with a single, narrow pectinate spur; claws slender, simple.
Petiolar node somewhat like that of worker but longer, more slender, subpedunculate in front; the node with sloping anterior face, broadest near front or midlength, slightly constricted posteriad, but broadly attached to postpetiole; subpetiolar process absent or poorly developed. Postpetiole narrow in front, with a sharp transverse ventral lip; broadened caudad.
Gaster fairly long, subcylindrical, often with a pair of light, unpigmented spots beneath on the first segment near the anterior sternal corners; distinct constriction between postpetiole and first segment. Pygidium rounded, with rounded cultrate apical border; cerci vestigial, membranous, not normally exposed in dried or alcoholic specimens. Genital capsule with all main primitive formicid parts; parameres not divided into basal and apical (gonostyli) pieces; aedeagal plates coarsely serrate on ventral edges and with a beaklike, differentiated apex (figs. 140, 141). Parameres rather short; laciniae longer, usually with thickened apices bent outward, overlapping ends of parameres when the capsule is partly extended. Subgenital plate ending in a pair of long, acute, subparallel or diverging spines, usually somewhat upcurved apicad.
Sculpture mostly more or less shining, smooth, but the head often minutely roughened and with scattered coarse or fine punctures; coarse punctures (often elongate or even sulciform) on truncal dorsum, or trunk longitudinally striate dorsally and on sides. Mandibles usually rugulose and hairy. Petiole, postpetiole, and gaster densely punctulate, moderately shining. Body covered with abundant short, brownish, oblique pilosity. Color castaneous to dark brown, nearly black.
distribution: Two genera, Acanthostichus and Ctenopyga, confined to the New World: southwestern United States to northern Argentina.
description: Published as part of Brown, W. L., 1975, Contributions toward a reclassification of the Formicidae. V. Ponerinae, tribes Platythyreini, Cerapachyini, Cylindromyrmecini, Acanthostichini, and Aenictogitini., pp. 1-115 in Search: Agriculture; Cornell University Agricultural Experiment Station 15 on pages 39-41