Proceratium silaceum

More commonly collected than most Proceratium species, silaceum also has a relatively large range.

Identification
A member of the silaceum clade and differing from the other three North American species of this clade, Proceratium croceum, Proceratium mancum and Proceratium crassicorne, by the following characters: from croceum only, in the worker, gyne and male, by the thicker petiole, stronger sculpture and by the larger size (worker TL > 3.7 mm, gyne TL > 5.1 mm, male TL > 4.0 mm, instead of worker TL < 3.5 mm, gyne TL < 4.1 mm, male TL < 3.3 mm); from crassicorne only in the worker, gyne and male, by the sculpture more impressed, in the worker and gyne by the frontal carinae broader and by the hairs of type (1) sparser on the gaster, in the gyne only, by the EL ≥ 0.19 mm instead of ≤ 0.17 mm; from mancum only, in the worker and gyne by the posterior half of the head dorsum more sculptured and by the legs with much shorter hairs. (Baroni Urbani and de Andrade 2003)

This species is similar to Proceratium croceum. It is highly variable in size, color and surface sculpture. In Creighton's manual of North American ants (1950) this variability is discussed at great length, and two subspecies, as well as the species Proceratium crassicorne, are synonymized. Baroni Urbani and de Andrade (2003) show that P. crassicorne is a valid species.

This species is similar to Proceratium crassicorne, differing in the density of hairs on the gaster, especially the second gastral tergite. The tergite of P. silaceum is sparsely punctate, with most punctures separated by several puncture-widths while in Proceratium crassicorne the tergite closely punctate, with most of the punctures only one or two puncture-widths apart when seen under a diffuse light, such as that of a flourescent bulb.

We have seen series of both species from Arkansas that suggest that in that state silaceum tends to be larger than it is farther east, while crassicorne remains small, as in the East.

Distribution
Southern Ontario, south into Florida, west to Illinois and Oklahoma (Smith 1979); it also occurs in eastern Texas, south into Mexico, Guatemala and Honduras (Brown 1974).

Distribution based on Regional Taxon Lists
Nearctic Region: Canada, United States. Neotropical Region: Mexico.

Biology
Wesson & Wesson (1940) from 1934 to 1939 within 45 miles from Jackson (Ohio) collected P. silaceum seven times in open woods in stumps or logs.

Nests are in rotten wood or under stones (Brown 1958b). Colonies are easily collected in north Florida by careful dissection of well-rotted pine logs. This species is also common in litter samples in north Florida. One colony was found under bark flakes at soil level in a living pine tree; this colony contained a queen and alates, as well as workers and brood. Another colony was found between layers of asphalt roofing material that had been illegally dumped in a pine flatwoods site. Colonies of this species have been found in the field feeding on arthropod eggs, and captive colonies have been maintained for weeks almost exclusively on spider eggs (Brown 1958b). Eggs are found stored in chambers in nests in the field, and egg-storing behavior also occurs in the lab (Brown 1958b). When supplied with spider eggs captive colonies ignore other prey, including insect remains and the eggs, larvae or pupae of other ants (Brown 1958b). When P. silaceum is provided with spider eggs in the lab, "the reflexed gastric tip is used to tuck the slippery eggs forward toward the mandibles when the eggs are being carried by the ants" (Brown 1958a).

Nomenclature

 *  silaceum. Proceratium silaceum Roger, 1863a: 172 (w.) U.S.A. Emery, 1895c: 265 (q.); Kennedy & Talbot, 1939: 202 (m.); Crozier, 1970: 116 (k.). Senior synonym of rugulosum, vestitum: Creighton, 1950a: 40. See also: Brown, 1958g: 330; Brown, 1974a: 82; Baroni Urbani & De Andrade, 2003b: 408.
 * vestitum. Proceratium crassicorne var. vestitum Emery, 1895c: 266 (w.) U.S.A. Junior synonym of silaceum: Creighton, 1950a: 40.
 * rugulosum. Proceratium silaceum subsp. rugulosum Wheeler, W.M. 1915b: 390 (w.q.) U.S.A. Junior synonym of silaceum: Creighton, 1950a: 40.

Taxonomic Notes
When Creighton (1950) synonymized Proceratium silaceum and Proceratium crassicorne, he included a detailed commentary on the illusory nature of the supposed differences in the shape of the petiole in the two species. In spite of this, there really does seem to be a small but consistent difference in the petiolar profile, with P. crassicorne less narrowed above than P. silaceum. In addition, the petiole of P. crassicorne is less shining at the base of the petiolar scale, the basal carina of the petiolar scale is less developed, the petiole is more pubescent, especially the anterior face near the base. The frontal carinae of P. crassicorne project more above the surface of the head and are less expanded than in P. silaceum.

Worker
Baroni Urbani and de Andrade (2003) - Head slightly longer than broad or almost as long as broad and with subparallel sides. Vertex in full face view straight or weakly convex. Clypeus reduced, as long as or slightly longer than the antennal sockets. Anterior border of the clypeus truncate. Frontal carinae far from each other, poorly covering the antennal insertions. Lateral expansions of the frontal carinae not very broad, weakly raised, diverging on the two anterior fourths, converging on the third fourth, diverging and carinate only on the last fourth. Frontal area gently concave and with a median, longitudinal carina starting on the posterior fourth and prolonging posteriorly. Head anterolaterally with a variably marked longitudinal carina. Genal carinae marked, each carina corresponding to the external border of a deep sulcus. Eyes visible as a dark dot below the integument, small and placed on the middle of the head sides. First funicular joint almost as long as broad or slightly longer than broad. Funicular joints 2-10 broader than long. Last funicular joint as long as the sum of joints 6-10 or slightly longer than the sum of joints 7-10. Scapes not reaching the vertexal margin posteriorly and gently thickening apically. Masticatory margin of the mandibles with 7- 11 denticles before the pointed apical tooth. Palp formula 2,2.

Mesosoma in side view weakly convex on the two anterior thirds, gently sloping on the posterior third. Pronotal and propodeal sutures superficially impressed. Basal face of the propodeum declivous posteriorly. Area between the basal and declivous faces of the propodeum gently concave, dorsally variably carinate and laterally with a triangular tooth. Declivous face of the propodeum gently sloping posteriorly. Sides of the declivous face of the propodeurn carinate. Propodeal spiracle round and above mid height in lateral view.

Petiole subrectangular, broad. Anterior border of the petiole straight and anterolaterally carinate. Petiole in dorsal view with the posterodorsal border of the node superficially carinate. Ventral process of the petiole large, lamelliform and pointed posteriorly. Postpetiole in dorsal view with the sides diverging on the anterior half and gently convex on the posterior half. Postpetiolar sternite anteromedially with a marked subtriangular projection, gently convex posteriorly in side view. Constriction between postpetiole and gaster impressed. Gastral tergite I about 1/3 longer than the postpetiole and convex on the curvature. Remaining gastral tergites and sternites curved ventrally.

Legs moderately elongate. All tibiae with a pectinate spur. Spurs of fore legs without basal spine. Fore basitarsi longer than the mid ones. Hind basitarsi about 1/5 shorter than hind tibiae. Second tarsomere of hind legs shorter than pretarsus. Pretarsal claws simple. Arolia absent.

Sculpture. Head, mesosoma, petiole and postpetiole minutely punctate, reticulorugose, the reticulation and rugosities larger on the sides of the head, the rugosities variably impressed on the petiole and postpetiole. First gastral tergite smooth and with minute piligerous punctures. Legs punctate.

Body covered by hairs of three types: (1) short, suberect or subdecumbent but less dense than in crassicorne, erect and sparse on the funicular joints; (2) longer than type (1), erect on the whole body, slightly longer on the gaster, absent on the funicui; (3) shorter than hair type (1), dense and decumbent on the funicular joints only. In addition the funicular joints bear whitish, thick, appressed, sparse hairs, and the scapes with sparse hairs similar to type (2) but slightly shorter.

Colour. Light or dark ferrugineous-brown with slightly lighter antennae and legs.

Measurements in mm and Indices: TL 2.56-3.40; HL 0.59-0.74; WW 0.56-0.72; EL 0.03-0.05; SL 0.38-0.51; WL 0.69-0.94; PeL 0.18-0.25; PeW 0.25-0.38; HFeL 0.42-0.61; HTiL 0.35-0.49; HBaL 0.25-0.40; LS4 0.31-0.43; LT4 0.55-0.78; CI 94.9-98.5; SI 64.4-68.9; IGR 0.51 -0.56.

Queen
Baroni Urbani and de Andrade (2003) - Differing from the worker in the following details: eyes large, about 1/4 of the head length, composed by many facets and with ocular pilosity. Ocelli well developed.

Mesosoma robust and gently convex in side view. Parapsidal furrows marked. Scutellum with the sides converging posteriorly and with the posterior border subtruncate. Posterior half of the dorsum of the mesonotum and scutellum with a longitudinal carina more marked posteriorly. Metanotum with a small tooth. Basal face of the propodeum almost flat laterally and gently concave medially.

Fore wings of our type 4, hind wings of our type 3 as defined in the description of the genus.

Colour. As in the worker but some specimens with a dark brown macula on the posterior part of the head, on the mesosoma and on the postpetiole.

Measurements in mm and Indices: TL 3.40-4.05; HL 0.69-0.81; HW 0.64-0.78; EL 0.19-0.22; SL 0.43-0.54; WL 0.96- 1.14; PeL 0.22-0.26; PeW 0.34-0.41; HFeL 0.53-0.68; HTiL 0.43-0.55; HBaL 0.34-0.46; LS4 0.43-0.53; LT4 0.82-0.97; CI 94.1-98.5; SI 63.2-69.2; IGR 0.51-0.56.

Male
Baroni Urbani and de Andrade (2003) - Head slightly broader than long. Vertex in full face view strongly convex. Vertexal margin superficially carinate. Clypeus reduced, truncate and slightly longer than the antennal sockets. Frontal carinae thin, low and parallel. Frontal area with a round or obtuse swelling anteriorly, concave posteriorly. Ocelli large. Compound eyes large and mostly on the anterior part of the head sides. Scapes short of the anterior ocellus posteriorly. First funicular joint slightly shorter than the second. Joints 2-12 longer than broad. Last funicular joint slightly longer than the sum of joints 10-11. Mandibles edentate and only with a pointed apical tooth. Palp formula 5,2.

Mesosoma robust and convex in profile. Pronotum and anterior third of mesonotum almost perpendicular to the posterior two thirds of the mesonotum. Parapsidal furrows marked. Scutellum weakly convex in side view; posterior border of scutellum round; dorsum of the scutellum with a longitudinal carina prolonging up to the mesonotum. Basal face of the propodeum slightly declivous and medially with a variably impressed incision prolonging up to the anterior part of the declivous face. Sides between basal and declivous faces of the propodeum subangulate. Declivous face of propodeum flat. Metanotum with a median, broad, spinirorm tooth. Lower part of the propodeal lobes obtuse and upper part round and partially lamelliform. Propodeal spiracles small.

Petiole in side view with declivous anterior face, the node scale-like. Anterior border of the petiole laterally wealtly carinate. Subpetiolar process small, triangular. Postpetiole anteriorly broader than the petiole; postpetiolar sides convex. Anterior border of the postpetiolar sternite with a superficial triangular "lip". Gastral tergite I convex in side view. Gastral sternite I large. Remaining gastral tergites and sternites slightly curved ventrally.

Legs as in the worker but more elongate. Wind basitarsi slightly shorter than hind tibiae.

Fore wings of our type 4, hind wings of our type 3 as defined in the description of the genus.

Sculpture. Head and mesosoma reticulate-punctate, irregularly and variably rugulose, the rugosities longer and longitudinal on the posterior half of the mesonotum and on the scutellum, the reticulation broader on the basal face of the propodeum. Petiole, postpetiole and gaster smooth and with minute piligerous punctures. Legs smooth and minutely punctate.

Pilosity as in the worker.

Colour. Black with slightly lighter legs, antennae and mandibles.

Measurements in mm and Indices: TL 2.84-3.15; HL 0.52-0.59; HW 0.53-0.60; EL 0.26-0.28; SL 0.26-0.29; WL 0.99-1.04; PeL 0.17-0.21; PeW 0.22-0.30; HFeL 0.56-0.62; HTiL 0.42-0.47; HBaL 0.40-0.42; LS4 0.36-0.43; LT4 0.65-0.75; CI 101.7-103.6; SI 47.4-51.8; IGR 0.55-0.57.