Megalomyrmex species groups

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Brandão first organized Megalomyrmex into species groups in 1990: "My results indicate that the species of Megalomyrmex can be divided in four groups, characterized morphologically and behaviorally. I present below a key for the identification of these species groups, followed by the characterization of each group and a systematic treatment of all species."

More than ten new species have been described since these groups were established. They were not evaluated as to the their potential group affinities hence the total number of species listed here does not include all of the known species.

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Key to Species Groups

1)

  • Dental formula 1 + 4; palpal formula 4:3 . . . . . 2
  • Mandibles with 2 or 3 apical teeth, palpal formula 4:3, 3:2, 3:1 (except M. pusillus, SE Brasil, Pusillus group) . . . . . 3

2)

  • Wl less than 2.00 mm; first funicular segment at least twice as long as the second; eyes with 10-13 facets at the largest diameter (except M. wallacei with 20, sole species with superficial sculpture on at least the interocular region and vertex) . . . . . Modestus group
  • WL more than 2.10 mm; first funicular segment not twice as big as the second; eye with at least 15 ocular facets at the largest diameter . . . . . Leoninus group

3)

  • Club segments broader, but not much,than the other funicular segments; apex of venom apparatus not spatulate but marked with a circular area of very small spines; palpal formula 4:3 or 3:2 . . . . . Silvestrii group
  • Club segments much broader and longer that the other funicular segments: apex of venom apparatus spatulate, not spiny; palpal formula 3:2 or 3:1 . . . . . Pusillus group

Species groups

Leoninus group

Members of the Leoninus group can be characterized as follows: relatively large monomorphic workers (WL between 2.13 and 3.7 mm) with triangular mandibles, dental formula always 1 + 4; palpal formula 4:3; clypeus always smooth; 3- to 5-segmented antennal club (more often 3-). the segments larger but not much than the ordinary ones; first funicular segment never twice as long as the second; compound eyes with 15-25 ocular facets at largest diameter; frontal suture generally impressed; pronotum may have lateral swollings; promesonotal suture distinctly impressed dorsaly in all species, but in Megalomyrmex cyendyra anepisternum and katepisternum never divided by sutures; dorsal faces of propodeum meeting either in an obtuse angle or in lateral tubercles; epipetiolar carina in general complete over the foramen; postpetiole spiracles never laterally produced; genual plate round to acuminate.

Workers of this group never present rugosities on the body surface, except for concentric rugosities on the foramen. The pilosity pattern is also constant, with erect hairs (.2-.3mm) on the clypeus, pronotal disc, dorsum of propodeum, apex of petiolar and postpetiolar nodes and gaster. The rest of the body surface may have small suberect hairs (smaller than .15 mm) specially on the scapes, head dorsum, lateral portions of thorax and legs. Funiculus always with appressed pubescence.

The Leoninus group corresponds roughly to Megalomyrmex s.s.. It includes the largest workers in the genus, comparable in size only to those of a few other myrmicine genera, thus justyfying the name choosen by Forel.

The group comprises free-living species that may tend membracid nymphs, and inhabit preformed cavities among or under stones and rotten logs, with no indications of definite architecture.

Workers forage individually, not showing any kind of recruitment of nestmates, even when faced with large food items pinned on the ground. From the few notes on museum catalogs and labels it can be said that they forage on the ground but visit bushes no higher than 1 m. I found in the stomach contents of the arboricolous lizard Plica umbra L. from Sto. Antonio do Ica, AM, Brasil a worn mandible of a Megalomyrmex from this group (either Megalomyrmex balzani or Megalomyrmex staudingeri).

There are no true queens in the Leoninus group. I found, however, in most species, gamergates taking over the reproductive function. A similar situation was described by Ward (1984) for the New Caledonia fauna of Rhytidoponera, with 19 related queenless species in the impressa group.

The sole participation of gamergates in reproduction affects speciation, dispersion rates and intraspecific variation. Ward (1983) found out that the correlation among workers from species with gamergate is significantly smaller (r = .3) than among workers from colonies with truequeens. (r = .75).

The Leoninus group has a pan-Amazonian distribution, specially on the west, reaching the eastern Andean slopes. In Colombia and Venezuela they show an Andean distribution. In Ecuador and Central America they are Transandean (Nieves, Ecuador). There is only one species in the Brazilian cerrado (Megalomyrmex acauna).

Workers of the Leoninus group share with species of the Modestus group some characters I consider as generalized: mandibular dental formula, palpal formula and a slender dorsal arm on the venom apparatus. In relation to the Modestus group, the Leoninus group is derived as to the loss of true queens and to the tendency toward a progressive modification of the genual plate, from a round plate to a long and sharp spine.

Brandão (2003) - Jones et al. (1999) described the pyrrolidine alkaloids found in extracts of two species in the Leoninus group: Megalomyrmex cyendyra and Megalomyrmex latreillei. Pyrrolidines are well known venom components of other myrmicine ants in general and have been detected also in Megalomyrmex leoninus.

Colonies of Leoninus group Megalomyrmex do not construct or excavate nests, but rather occupy pre-formed spaces among rocks, under bark or within the leaf litter, readily moving away immatures when disturbed. This behavior also prevents their establishment in laboratory nests; in my experience there are no other myrmicine ants as difficult to keep in laboratory conditions as the species belonging to this group. As soon as they are transferred to artificial nests, each worker grabs an immature (or a group of them [if they are small enough]), and runs away to the nearest hiding place without any apparent coordinated behavior, to join nestmates only by chance afterwards.

Modestus group

WL between 1.15 and 2.00 mm; monomorphic; mandibles triangular with dental formula 1 + 4; pal pal formula 4:3; clypeus sometimes depressed medially, but never carinate; first funicular segment twice as long as the second [Brandão (2003) - M. cupecuara workers, that agree in all other characters with other members of the Modestus group except the first segment is larger than the second but not twice as large.]; club with 3 or 4 segments, segments bigger but not much broader than the ordinary segments; compound eyes with 10-13 ocular facets (except for Megalomyrmex wallacei with 20) that, in general, do not interrupt the lateral margin of head in full frontal view; pronotum never impressed medially; epipetiolar carina always complete over the foramen; apex of femora round, apex of venom apparatus as in Fig.

Morphologically, workers of Megalomyrmex iheringi and M. wallacei form a subgroup within the Modestus group, distinguished by head shape, primary allometry and size. M. wallacei shows some derived characters in relation to the other species included, such as superficial sculpture recovering at least part of the tegument and eyes with relatively numerous ocular facets.

Members of the Modestus group live in colonies relatively numerous and sometimes polygynous, always with true queens (in two species only workers are known), that occupy or dig cavities under stones or logs or inhabit hollow twigs. In general nests are not rigidly constructed, but may show definite entrances and chambers. All species inhabit tropical and subtropical forests of South and Central America, occurring in the occidental portion of the Amazon and in southeast Brasil.

Megalomyrmex iheringi is known only from the top of the Serra do Mar, Sao Paulo state, and occurs sympatrically with Megalomyrmex goeldii in at least one locality (Estaciao Biologica de Boraceia, Mun. Salesopolis). The station is one of the best known localities from the viewpoint of the mirmecofauna. M. iheringi is rather common there while M. goeldii has been collected only once. Coming down to sea level, M. goeldii replaces M. iheringi and can be very abundant. In Petropolis, state of Rio de Janeiro, M. goeldii is rather common. On the Atlantic coast M. goeldii is known from Barra de Marica, Rio de Janeiro, to Barra do Una, Sao Paulo; it has been collected as Serra da Mantiqueira, Serra de Itatiaia (2300m) to the southeast of Minas Gerais (between Varginha and Vicosa).

Megalomyrmex modestus occurs in Central America and along the Cauca River, a distribution similar to that of Megalomyrmex foreli of the Leoninus group.

Megalomyrmex wallacei is known from various localities in west central Amazon, being relatively common around Manaus. Unfortunately nothing is known of its biology. Megalomyrmex weyrauchi has the same distribution as Megalomyrmex glaesarius, of the Leoninus group.

Megalomyrmex ayri occurs on the transitional belt between the cerrados and the Amazon forest at the north of Mato Grosso, Brasil.

Megalomyrmex caete. and Megalomyrmex cupecuara show a transandean distribution in forests.

The species assembled in the Modestus group show the largest number of characters I consider generalized. specially with reference to the palpal formula and the venom apparatus. Also they consistently have true queens and recruitment among workers, conditions commonly found among the mymicine genera previously included in the Solenopsidini.

Pusillus group

Workers of this group may be monomorphic or present primary allometry (WL = .75-1.25mm) with smooth slender mandibles with 1-3 apical teeth followed by a variable series of subequal denticles (3-7); median area of clypeus always depressed and sometimes marked laterally by carinae; anterior clypeal border with up to 4 denticles, the inner ones originating the carinae; palpal formula 3:2 (except M. gnomus with 3:1), first funicular segment length always at least twice the second; 3- segmented antennal club, the segments involved distinctly broader and longer than the preceeding ones; compound eyes relatively large occupying generally 1/3 of the head capsule in full face view, with 5-15 ocular facets at their largest diameter; mesosternum and metasternum without acrotergites; genual plates round; apex of venom apparatus spatulate.

In general workers and true queens dorsum of head hairs are longer than ordinary body hairs and inclined toward the head longitudinal axis.

Species of Pusillus group live in colonies with true queens or else with gamergates, as in Megalomyrmex drifti and Megalomyrmex incisus. These two species show the higher degree of variation in the morphometric characters studied for the whole genus. This situation may also be related to the presence of gamergates as in Ward (1983) obsevations in Rhytidoponera.

The species assembled in this group are the smallest in Megalomyrmex. They nest within the litter of forested areas in the tropical Americas and are mostly known from Berlesate samples, a particularly poorly known segment of the Neotropical ant fauna.

As most samples are represented by just a few individuals, collected at sparse localities, an interpretation of their distributional pattern at this moment would be premature.

The Pusillus group includes species with a number of characters I considered derived, specially the falcate mandibles in some species, the very distinct antennal club, loss of palpal segments and the peculiar shape of the venom apparatus apex.

Brandão (2003) - Workers of the species in this group are relatively small inhabitants of the leaf litter, and hence rare in collections, but the most prone to be collected using recently adopted techniques, such as the Winkler extraction apparatus, soil samples or Berlese funnels.

Silvestrii group

Morphologically can be characterized by monomorphic workers with mandible blades with 2-3 apical teeth followed by a series of 3-12 minute denticles; palpal formula 4:3 or 3:2; anterior border of clypeus always round: clypeal median area always depressed (except in Megalomyrmex cuatiara): frontal suture never impressed; first funicular segment length at least twice as long as the second; 3-segmented antennal club with segments distinctly longer but not much broader than ordinary ones: frontal carina always bearing two parallel rows of long hairs, that may reach the vertex; propodeum dorsum never depressed longitudinally: metanotal groove, in general, deeply excavated; mesosternum and metastenum always smooth: dorsal border of petiolar node, in frontal view, always round; venom apparatus with a circular area of diminute spines at the apex; genual plates flat and round.

Queens of Megalomyrmex symmetochus have dental formula 1 + 4. Workers of Megalomyrmex mondabora have palpal formula 4:3. Both conditions. Shared with species in the preceeding groups, may mean retention of generalized characters from the genus basic stock.

Workers of M. symmetochus show characters that isolate this species from all other grouped here, as dental formula and antennal structure. Other species of the Silvestrii group are rather uniform morphologically, albeit easily distinguished by the features appointed in the diagnosis and key. Most new species here described are known by few specimens. indicating how poor collected they are.

Female reproductives in this group, when known, are true winged queens. Most specimens studied came from collections within several Attini genera nests, with whom they maintain apparent lestobiotic relationships. Summing up all informations from museum catalogs, labels and literature. records, however, I verified that workers also forage either individually or in groups above ground, even in species where informations on the association with Attini are copious and reliable. This suggests that either the relation is non-obligatory or that certain food items have to be obtained out of the “host”-nests. Also some free-living colonies of Megalomyrmex silvestrii have been recorded.

The geographical distribution of members of the Silvestrii group roughly follows the Attini distribution. restrained however to the limits of Megalomyrmex.

References

  • Brandão, C. R. F. 1990b. Systematic revision of the Neotropical ant genus Megalomyrmex Forel (Hymenoptera: Formicidae: Myrmicinae), with the description of thirteen new species. Arq. Zool. (São Paulo) 31:411-481.
  • Brandão, C. R. F. 2003. Further revisionary studies on the ant gentus Megalomyrmex Forel (Hymenoptera: Formicidae). Pap. Avulsos Zool. (São Paulo). 43:145-159.