Megalomyrmex silvestrii

Boudinot et al. (2013) - Weber (1940) reported three collections of M. silvestrii (then reported as M. wheeleri; M. wheeleri synonymized with M. silvestrii by Kempf & Brown 1968) in nests of Cyphomyrmex costatus on Barro Colorado Island, Panama. In one nest he found a dealate queen. In a second nest he found a dealate queen and a worker. In a third nest he found three dealate queens and 55 workers. However, Brown (in Kempf & Brown 1968) reported finding four nests, two from Barro Colorado Island, one from Cerro Campana, Panama, and one from Santa Teresa, Brazil, that “were nesting independently of other ants so far as he could tell.” Brown's two Barro Colorado nests were “inside a small clod of soil in the leaf litter,” and “in a small piece of rotten wood, 10 mm deep and 15 mm wide, also contained many termites.” J.L.M. Diniz collected isolated nests of M. silvestrii in Betim, Mato Grasso, Brazil (reported in Brandão 1990), and free-living colonies have been collected on Pipeline Road and Barro Colorado Island, Panama. In 1999 a colony was kept for several months in the laboratory and fed Drosophila and honey (RMMA pers. obs.) suggesting predatory tendencies. Kempf and Brown (1968) suggested that the species is “not so much a parasite as it is a mass-foraging predator that specializes in raiding, and sometimes occupying, the nests of small Attini.” This raiding behavior, similar to Megalomyrmex mondaboroides, was observed in a captive colony with C. costatus (RMMA pers. obs.), firmly classifying this species as a facultative thief ant (Kempf and Brown 1968; Adams et al. 2012).

Identification
Boudinot et al. (2013) - Worker unique among Central American Megalomyrmex for the mandibular dentition consisting of two large apical teeth subtended by greater than twelve miniscule, equal-sized denticles. Identification confirmed by the following combination of characters: (1) katepisternum smooth and shining; (2) occipital carina visible in full-face view; (3) postpetiolar node robustly globose, attenuated posteriorly; (4) postpetiolar sternum strongly and smoothly bulging; (5) anterior margin of clypeus recessed. Queen identifiable similarly to worker, alate or ergatoid. Male differs from all other Central American Megalomyrmex by the specific combination of following characters: (1) crossvein 1m-cu present; (2) scape length shorter than eye length; (3) postpetiole with a ventral bulge; (4) mesosoma gracile.

Males of Megalomyrmex silvestrii from Nicaragua to southern Mexico were observed to be large and dark, whereas a single specimen from La Selva Biological Station was smaller and paler. Further characters which distinguish the large, dark form from the small, pale form are as follows: (1) lateral ocellus distant from compound eye by slightly more than one lateral ocellus length (vs. about three lateral ocellus lengths); (2) parapsidal line distinct (vs. indistinct); (3) basipetiolar carina with distinct lateral and posterior margins (vs. arc-shaped); (4) postpetiolar spiracle about midlength of lateral tergal margin (vs. near anterior third); (5) petiole and postpetiole with distinct girdling carinae (vs. weak). This small, pale male could be a representative of a cryptic species of M. silvestrii, a situation which would not be too surprising given the numerous synonyms attributed to this species. COI barcode data is in accord with a cryptic species hypothesis (BOLD 2013), although we were unable to discern any morphologically distinct clusters via examination of workers from La Selva and the rest of Central America.

Across the Central American range of M. silvestrii workers and queens varied in the erectness of tibial and scape setae, the form of the propodeum, and size and shape of the postpetiole. We could find no pattern of variation indicative of distinct species within M. silvestrii. Otherwise, workers and queens were observed to have sparse, short ocular setae. The lancets of the venom apparatus of a worker from Honduras were broad to the apex, which was slightly curved and broadly rounded. The sting shaft subtly widened at the apex, but was not spatulate. An ergatoid queen, along with fully alate queens, were collected in La Ceiba, Honduras. This queen has a more strongly-slanted mesosomal dorsum, while forewing bases are nub-like and the hindwings are not developed.

Key to Central American Megalomyrmex:
 * workers
 * males

Distribution
Mexico to northern Argentina; widespread in mainland Neotropics. Sea-level to 1100 m elevation.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina, Bolivia, Brazil, Colombia, Costa Rica, Ecuador, French Guiana, Guatemala, Guyana, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru, Suriname, Trinidad and Tobago, Venezuela.

Biology
Longino (2010) - This species occurs in moist to wet forest habitats, from sea level to 1100 m elevation. It nests in small chambers in rotten wood or opportunistically in other small cavities in the soil. Colonies have been found in small attine nests and alone, suggesting it is a facultative predator of small Attini. Mann (1916) stated “A good series of workers were taken at Ceiba and San Juan Pueblo, nesting in the ground and in rotten logs. It is a timid species and very active when disturbed.” Weber (1940) reported three collections of silvestrii (reported as wheeleri; wheeleri synonymized with silvestrii by Kempf & Brown 1968) in nests of Cyphomyrmex costatus on Barro Colorado Island, Panama. In one nest he found a dealate queen. In a second nest he found a dealate queen and a worker. In a third nest he found 3 dealate queens and 55 workers. However, Brown (in Kempf & Brown 1968) reported finding four nests, two from Barro Colorado Island, one from Cerro Campana, Panama, and one from Santa Teresa, Brazil, that “were nesting independently of other ants so far as he could tell.” Brown's two Barro Colorado nests were “inside a small clod of soil in the leaf litter,” and “in a small piece of rotten wood, 10 mm deep and 15 mm wide, also contained many termites.” Diniz collected isolated nests of silvestrii in Betim, MG, Brazil (reported in Brandão 1990). Kempf and Brown (1968) suggested that the species is “not so much a parasite as it is a mass-foraging predator that specializes in raiding, and sometimes occupying, the nests of small Attini.” Workers are moderately abundant in Winkler samples of forest floor litter, and workers may visit baits on the forest floor.

Nomenclature

 *  silvestrii. Megalomyrmex silvestrii Wheeler, W.M. 1909b: 235 (w.) MEXICO. Combination in M. (Wheelerimyrmex): Mann, 1922: 29. Senior synonym of brasiliensis, langi, misionensis, sjostedti, wheeleri: Kempf & Brown, 1968: 97. See also: Brandão, 1990: 443.
 * langi. Megalomyrmex (Wheelerimyrmex) sjostedti var. langi Wheeler, W.M. 1925a: 31 (w.m.) GUYANA. Junior synonym of silvestrii: Kempf & Brown, 1968: 97.
 * sjostedti. Megalomyrmex (Wheelerimyrmex) sjostedti Wheeler, W.M. 1925a: 30 (w.q.) PERU. Junior synonym of silvestrii: Kempf & Brown, 1968: 97.
 * brasiliensis. Megalomyrmex (Wheelerimyrmex) brasiliensis Borgmeier, 1930: 34, pl. 5, figs. 33, 35 (w.) BRAZIL. Junior synonym of silvestrii: Kempf & Brown, 1968: 97.
 * misionensis. Megalomyrmex (Wheelerimyrmex) brasiliensis var. misionensis Santschi, 1936d: 405 (w.) ARGENTINA. Junior synonym of silvestrii: Kempf & Brown, 1968: 97.
 * wheeleri. Megalomyrmex (Cepobroticus) wheeleri Weber, 1940a: 425, figs. 15, 16 (w.q.) PANAMA. Junior synonym of silvestrii: Kempf & Brown, 1968: 97.

Worker
Mandibles smooth with 2 apical teeth followed by a series of 1 0-!2 denticles; palpal formula 3:2; anterior border of clypeus without median denticle, median area depressed but not marked by lateral carinae; scape always surpassing the vertex in frontal head view; 10-12 ocular facets at compound eyes largest diameter; occipital margin raised and visible in head full face; promesonotal suture not impressed dorsally; metanotal groove smooth; propodeum may be raised over the metanotal groove in lateral view of the trunk; dorsal face and declivity of propodeum meeting in a gentle angle; declivity with one or two concentric rugosities over the foramen: propodeum spiracles never laterally produced; epipetiolar carina complete; inferior propodeal plates may be quadrate in side aspect; pedunculate petiole with anterior margin of node concave in side view; ventral face with 1 or rarely up to 3 anterior denticles, in general leading to a translucid longitudinal flange; dorsal margin of petiolar node round in frontal view, without an apex; ventral process of postpetiole globose.

Pilosity: long hairs (longer than .15mm) at head, thorax dorsum, gaster and legs (coxae and femora); smaller hairs (ca. .1 mm) at antennae and rest of legs.

Color: mandibles, clypeus, antennae and legs in general bright yellow. Rest of body bright reddish-brown.

Boudinot et al. (2013) - (holotype from Brandão 1990): HW 0.70, HL 0.83, SL 0.94, ML 1.35, CI 84, SI 113. (n=17, plus 30 in parentheses from Brandão 1990): HW 0.60–0.75 (0.60–0.73), HL 0.75–0.94 (0.71–0.88), SL 0.89–1.07 (0.83–1.08), OMD 0.12–0.17, EL 0.18–0.24, ML (1.08–1.45), CI 77–81, SI 110–121, EI 30–33, OMI 60–79.

Queen
Boudinot et al. (2013) - (n=3): HW 0.80–0.86, HL 0.87–0.98, SL 0.97–1.10, OMD 0.15–0.17, EL 0.28–0.31, ML 0.43–1.60, CI 88–91, SI 110–113, EI 35–37, OMI 48–56. Alate (n=3): HW 0.80–0.86, HL 0.87–0.98, SL 0.97–1.10, OMD 0.15–0.17, EL 0.28–0.31, ML 1.43–1.60, CI 88–91, SI 110–113, EI 35–37, OMI 48–56. Ergatoid (n=1): HW 0.70, HL 0.83, SL 0.94, OMD 0.13, EL 0.24, ML 1.20, CI 84, SI 112, EI 35, OMI 54.

Similar to worker and alate queen. Differing from worker by having a reduced alate-condition mesosoma, ocelli, and larger compound eyes. Differing from alate queen by having less voluminous and more slanted mesosoma, relatively smaller ocelli and a narrower head.

Male
Boudinot et al. (2013) - (n=2): HW 0.64–0.78, HL 0.61–0.74, SL 0.23–0.29, EL 0.33–0.40, ML 1.06–1.37, CI 103–108, SI 39–40, EI 46–51.

Head Antenna with 13 antennomeres; antennomere 3 apically kinked; apical antennomeres not clavate; scape length shorter than eye length. Antennae white, grading into honey yellow apically. Palpal formula 4,2. Mandible triangular; masticatory margin with 8 teeth; apical tooth about twice length of subapical tooth which is about twice length of the remaining basal teeth; 6 basal teeth small, subequal. Dorsal face of mandible smooth and shining, with weak piligerous punctae. Minimum distance between lateral ocellus and compound eye somewhat greater than one lateral ocellus length or about three lateral ocellus lengths in. Occipital carina visible in full-face view. Mesosoma Mesosoma attenuate. Notauli absent. Parapsidal lines distinct or weak. Basitarsi tubular. Pterostigma narrow, elongate. Forewing with crossvein 1m-cu; submarginal cell 1 about one third to one fourth as wide as long; terminal abscissa of M branches from Rs+M basad 2r-rs. Metasoma Basipetiolar carina with distinct lateral and posterior margins or arc-shaped. Ventrolateral longitudinal carina of petiole present. Petiolar spiracle placed in anterior fourth. Petiole and postpetiole with girdling carinae or these carinae weak. Postpetiolar helcium somewhat elliptical in the horizontal plane. Postpetiolar spiracle placed at about midlength of lateral tergal margin or near anterior third. Sternum of postpetiole strongly bulging in anterior half, and sloping posteriorly. Postpetiolar tergum convex, with apex posterad midlength. Genitalia Abdominal sternum IX lateral margins weakly convex, curving to base of apical triangular lobe; triangular lobe about as long as broad at the base. Telomere short, triangular, acute apex angular; dorsal and ventral margins convex; medial dentiform process absent; medial face weakly crescentiform; ventral margin without sclerotized denticles. Cuspis with weak apicodorsal process; apical margin linear, meeting ventral margin at a narrowly rounded angle. Digitus narrowing subbasally, becoming apicodorsally explanate; explanate apicodorsal margin distinct from relatively narrow apical portion; ventral margin weakly arched; apex bluntly rounded; ventral margin not obscuring apicodorsal angle of cuspis. Valviceps about half as tall as long, ovate; dorsal margin linear to slightly convex, curving evenly through the apex to the ventral margin; basal half of ventral margin linear to slightly convex; penisvalvar teeth about as long as wide to longer than wide, close-set.

Type Material
Boudinot et al. (2013) - Holotype worker: MEXICO, Veracruz: Cordoba (not examined).