Megalomyrmex symmetochus

A social parasite that lives within nests of Sericomyrmex amabilis.

Identification
Boudinot et al. (2013) - Worker uniquely identified by the following two characters: (1) meso- and metabasitarsi extremely anteroposteriorly compressed; (2) postpetiole only slightly wider than tall in posterior view. Otherwise recognizable by the following combination: (1) disc of katepisternum costate; (2) dorsal face of mandible coarsely striate; (3) eye not reduced; (4) malar space largely smooth, not covered with longitudinal carinulae. Queen identifiable similarly to worker, alate. Male recognizable by the following combination: (1) scape long, about as long has head width (SI > 75); (2) crossvein 1m-cu present; (3) postpetiolar sternum distinctly bulging; (4) first gastric tergum setae fine, dense.

The males of M. symmetochus and Megalomyrmex adamsae are quite similar morphologically. Megalomyrmex males differ consistently from M. adamsae by the following: (1) postpetiolar sternum with a strong, often subrectangular swelling (vs. weakly convex); (2) setae on first gastric tergum comparatively fine and more dense; (3) clypeal setae numerous; (4) ventral face of petiole with several (~3–6 on each lateral side), slightly divergent longitudinal, infrequently curved carinulae (vs. a single median longitudinal carinula flanked by two slightly divergent longitudinal carinulae); (5) digitus broader, more linear apically; (6) penisvalvar apex truncate (vs. rounded). Although other characters of the petiole and postpetiole show some potential for separating the two species, these characters are either too subtle to observe without direct comparison, or overlap in variation. Geographic variation was also observed; males of M. symmetochus from Costa Rica and Nicaragua have longer setae on most body surfaces, including the scapes, legs, and wings, and have less robust petioles and postpetioles than males from Panama. Specifically, regarding the petiole and postpetiole, the nodes and anterodorsal faces are less produced and/or convex.

The worker of Megalomyrmex symmetochus differs from those of Megalomyrmex adamsae and fungiraptor by the following: (1) smaller (HW 0.69–0.72); (2) without distinct dorsal and posterior faces of the propodeum in profile; (3) malar area and posterior face of propodeum without arcing carinae; (4) petiole stout; node at 90° angle to peduncle; anterior and posterior faces with approximately the same slope; (5) anterior face of scape smooth and shining; (6) setae long, fine, and suberect to subdecumbent on first gastric tergum. Additional characters separate this species individually from each of the others. Workers and queens were observed to have ocular setae.

The worker of M. symmetochus was observed to vary intrapopulationally in the intensity of scape, mandible, and propodeal sculpturation, as well as in the form of the propodeum in profile view.

Intranidal size dimorphism was observed in our sample of males from Panama, having a large and a small class.

Key to Central American Megalomyrmex:
 * workers
 * males

Distribution
sea-level to 1070 m elevation

Distribution based on Regional Taxon Lists
Neotropical Region: Bolivia, Brazil, Costa Rica, Nicaragua, Panama, Peru.

Biology
Boudinot et al. (2013) - Many colonies of this guest ant social parasite have been collected and observed extensively from 1999 to 2012. This species constructs several cavities throughout the Sericomyrmex amabilis fungus garden where they house the queen and brood. Hundreds of parasite workers can be found within a single host colony and in every garden chamber. Parasites can be seen moving from chamber to chamber because the host constructs tunnels in the leaf litter. Most interactions between the two species are amiable (Wheeler 1925) but aggression is sometimes observed in the field and laboratory during the start of the rainy season when the two species are producing sexuals (Adams et al. 2013). Like the guest ant parasite Megalomyrmex adamsae, M. symmetochus workers chew the wings off some of the host female reproductives (Adams et al. 2012). Sericomyrmex amabilis can effectively kill parasite workers by removing legs and antennae with their strong mandibles, giving this host species the ability to reduce fitness impact by the parasite by “culling” the parasite worker population. Young parasitized host colonies have been collected, although none have been found at the foundress stage.

Kaspari et al. (2001) surveyed for alates on Barro Colorado Island, Panama, and found that M. symmetochus peak in alate production in the month of May. Thus future collections in June to August may yield newly infiltrated young host colonies, as has been found in other Megalomyrmex species during these months in Panama. Reproductives in Nicaragua (Parque Nacional Cerro Saslaya) were collected by M. Prebus over the course of six days during May in lowland wet forest at 280 m and in montane forest at 1070 m. Notably, the only reproductives of M. fungiraptor have been collected during September (at La Selva Biological Station, Costa Rica) suggesting possible temporal reproductive isolation between the two species via differing flight phenologies.

Longino (2010) - This species appears to be a specialized associate of small Attini (Wheeler 1925, Brandão 1990, Adams et al. 2000), a habit shared with other species of Megalomyrmex such as Megalomyrmex wettereri, Megalomyrmex mondabora, and Megalomyrmex adamsae. As defined here, M. symmetochus specializes on Sericomyrmex amabilis Wheeler. Wheeler (1925) found numerous colonies nesting with Sericomyrmex on Barro Colorado Island, Panama. He observed a number of colonies in the lab, and made the following observations (in Wheeler's account M. symmetochus is referred to as Cepobroticus Wheeler, a subgenus erected by Wheeler to contain the single species M. symmetochus, later synonymized with Megalomyrmex by Ettershank [1966]):

“The colonies of the Cepobroticus so frequently found living with Sericomyrmex amabilis were decidedly less populous than those of their host. The largest comprised less than 75 individuals, and often the number did not exceed 40 or 50. In every nest a dealated mother queen was present. She usually took up her station, surrounded by a group of her workers, in one of the crypts of the fungus garden a short distance - half to three quarters of an inch - from the Sericomyrmex queen. The guest ants kept their brood in small clusters scattered through the garden and each cluster was cared for by a few workers. Although the ants and their brood were thus intermingled, the workers of each species lavished their attention exclusively on their own eggs, larvae and pupae and were never seen even to transport the progeny of the other species from one part of the garden to another.

“The workers and queens of Cepobroticus are rather alert and move about more rapidly than their hosts. They devote so much time to licking and fondling one another that the observer is somewhat astonished to find them paying little or no attention to the fungus-growers. As a rule the two species are indifferent to one another. One may watch them for hours without observing anything more than rather distant, mutual antennal salutations. On rare occasions a worker Cepobroticus may be seen licking the gaster of a Sericomyrmex worker or of the sluggish queen. More frequently one of the host workers may be observed in the act of lavishing similar but more elaborate attentions on a Cepobroticus worker. The fungus-grower begins by licking the feet or tarsi, the tibiae and femora, then the thorax or abdomen and finally the head and even the mandibles of the guest. During this operation the latter remains motionless and inclines its body somewhat to one side.

“The Sericomyrmex never feed their guests by regurgitation. This is not surprising because they never feed one another thus, but resort individually to the growing fungus bromatia. When hungry the Cepobroticus workers and queen also crop the fungus mycelium, but they do this rather roughly, using their mandibles and even shaking or disturbing the substratum. The guests very rarely transport or rearrange the particles of the substratum or take the slightest interest in the garden, except as a source of nourishment. Only on one occasion did I see a Cepobroticus carry a particle of the substratum to another spot, insert it and pat it down with her fore feet. When fresh fruit was introduced into the nest, it was much less frequently visited and eaten by the guests than by their hosts.”

Numerous collections from Pipeline Road and vicinity (Panama) by R. Adams and colleagues have all been from nests of Sericomyrmex amabilis. At La Selva Biological Station in Costa Rica, alate queens and males have been collected at blacklights. Males and queens were common at the lab clearing blacklight on 4 Aug 1992. A lone worker was obtained by Ronald Vargas, during general collection of ants from trail surfaces and low vegetation.

Nomenclature

 *  symmetochus. Megalomyrmex (Cepobroticus) symmetochus Wheeler, W.M. 1925d: 168, fig. 5 (w.q.m.) PANAMA. Wheeler, G.C. & Wheeler, J. 1955c: 126 (l.). See also: Brandão, 1990: 445.

Worker
Boudinot et al. (2013) - (holotype from Brandão 1990): HW 0.73, HL 0.80, SL 0.78, ML 1.15. (n=13, plus 35 from Longino 2010): HW 0.60–0.84, HL 0.75–0.89, SL 0.69–0.84, OMD 0.15–0.19, EL 0.19–0.25, ML 1.03–1.31, CI 88–95, SI 90–96, EI 27–28, OMI 76–84.

Queen
Boudinot et al. (2013) - (n=5, dealate): HW 0.89–0.93, HL 0.86–0.91, SL 0.79–0.84, OMD 0.15–0.16, EL 0.32, ML 1.37–1.49, CI 102–103, SI 90–92, EI 34–35, OMI 46–50.

Male
Boudinot et al. (2013) - Measurements (n=20): HW 0.68–0.85, HL 0.69–0.84, SL 0.55–0.75, EL 0.37–0.48, ML 1.15–1.50, CI 96–108, SI 78–90, EI 52–59.

Head Antenna with 13 antennomeres; antennomere 3 kinked apically; no other antennomeres kinked; not forming a club; scape length distinctly greater than eye length. Entire antenna honey yellow. Palpal formula 3,2. Mandible triangular; masticatory margin with 6–7 teeth; apical tooth more than twice length of subbasal tooth, which may be notched apically; basal teeth decreasing in size from apical tooth. Dorsal face of mandible densely and finely striate. Minimum distance between lateral ocellus and compound eye dimorphic: less than or somewhat greater than one lateral ocellus length. Compound eye with obvious medium-long ocular setae. Occipital carina not visible in full-face view. Mesosoma Mesosoma attenuate. Notauli absent. Parapsidal lines weak. Foraminal carina present, may be weak; with about four transverse carinulae dorsad, these may also be weak. Basitarsi weakly flattened anteroposteriorly; metabasitarsus neither kinked nor swollen apically. Forewing crossvein 1m-cu present; submarginal cell 1 about one fourth as wide as long; with terminal abscissa of M branching from Rs distad 2rs-rs. Metasoma Basipetiolar carina arc-shaped. Ventrolateral carina of petiole present. Petiolar spiracle in anterior third. Posterior margin of petiole and postpetiole without girdling carinae; postpetiole but with transverse carinae on posterior face. Postpetiolar helcium subcircular. Postpetiolar spiracle placed between anterior third and midlength of tergal lateral margin, on a lateral bulge. Sternum of postpetiole with a blunt triangular process in the anterior half, which slopes strongly to just anterad posterior margin in profile view. Postpetiolar tergum convex, with apex slightly posterad midlength. Genitalia Abdominal sternum IX about as long as broad at base; triangular: lateral margins emarginate about midlength and tapering to apex. Telomere short, triangular with rounded apex; dorsal margin concave, ventral margin convex; medial dentiform process absent; medial face subrectangular; ventral margin without sclerotized denticles. Apical margin of cuspis slightly convex; without apicodorsal process. Digitus narrowing subbasally, broadening near apicodorsal bend; dorsal margin somewhat evenly arched to having three linear segments separated by abrupt curves; ventral margin strongly arched, flattened subapically; apex rounded; ventral margin not obscuring apicodorsal angle of cuspis. Valviceps about half as tall as long, ovate; dorsal margin bulging medially, projecting apicodorsally over the apex to rounding evenly through the valviceps apex to the ventral margin; ventral margin weakly sinuate; penisvalvar teeth truncate and close-set.

Type Material
Boudinot et al. (2013) - Lectotype worker (designated by Brandão, 1990: 445): PANAMA, Panamá: Barro Colorado Island, July 1924 (Wheeler) (lectotype examined,  workers from type series examined).