Myrmelachista

Longino, J. T., 2006, A taxonomic review of the genus Myrmelachista (Hymenoptera: Formicidae) in Costa Rica., Zootaxa 1141, pp. 1-54 : 2-4

[[ Genus Myrmelachista ]]

Introduction

Within the ants a number of lineages have developed exquisitely arboreal habits, nesting entirely within plant cavities and with specialized morphology and behavior for doing so. Species within these clades often show a range of specialization, being generalist inhabitants of dead stems, generalist inhabitants of live stems, or specialist inhabitants of live stems. The third group is often involved in obligate associations with particular lineages of plants and is particularly important in the study of mutualism (Davidson & McKey 1993). Species-level taxonomic work on stem-nesting ants can act in a synergistic way with studies of the community and evolutionary ecology of ant-plant associations and is particularly important for this reason.

Among these stem-nesting ants is the formicine genus Myrmelachista . This group of ants is confined to the Neotropics and is exclusively arboreal. They are inconspicuous and have been little studied, but there are hints from the literature that they are far richer and with more complex plant associations than previously suspected. Early literature contained scattered reports of South American Myrmelachista inhabiting ant-plants (reviewed in Bequaert 1922, Wheeler 1942). Myrmelachista nigella Roger and M. schumanni Emery were reported in internodes of Duroia hirsuta (Rubiaceae) (Ule 1907-1908, Schumann 1889). More recent reports have described "devil’s gardens" in South America, in which large polygynous colonies of Myrmelachista occupy monospecific patches of understory Duroia and Tococa (Melastomataceae) (Morawetz et al. 1992, Svoma & Morawetz 1992, Renner & Ricklefs 1998, Frederickson et al. 2005). These patches stand out in the typically dense and diverse understory vegetation as peculiar zones of a single plant species with bare earth beneath them and a ring of bare soil around the perimeter. These patches are created and maintained by the ants, which attack and kill foreign vegetation by spraying it with formic acid, which acts as an herbicide (Morawetz et al. 1992, Renner & Ricklefs 1998, Frederickson et al. 2005).

The first indication of something interesting in Central American Myrmelachista was Stout’s (1979) observation of a tight association between an unidentified Myrmelachista and an understory tree species in the genus Ocotea (Lauraceae). She examined 50 plants of Ocotea "pedalifolia" (probably a mix of O. atirrensis and O. dendrodaphne [Hammel 1986]) at the La Selva Biological Station, and found the stems of 49 of them inhabited by Myrmelachista . Since then little has been written about the association, although floristic works on the Lauraceae recognize that various species are routinely occupied by ants (Hammel 1986, Burger & van der Werff 1990). Ibarra-Manríquez and Dirzo (1990) reported the same phenomenon at the Los Tuxtlas Biological Station in Veracruz, Mexico. My studies of Myrmelachista in Costa Rica reveal that these observations are just the beginning and that there is a largely overlooked community of Central American Myrmelachista and their associated plants.

Throughout Costa Rica the understory of mature wet forests have high densities of plants that are associated with multiple species of Myrmelachista . Cloud forests likewise have high densities of Myrmelachista living in live stems, but they are more abundant in the canopy than in the understory. Many of these species nest entirely inside of live stems and rarely venture out onto the surface, and the plants they inhabit show no external signs of specialization for ant occupation. There are no preformed domatia, no food bodies, and no extrafloral nectaries. The result is a lineage of ants that in spite of being species-rich and very abundant is almost never collected. Many of the species reported here had never been collected prior to my work in Costa Rica, much less receiving any formal taxonomic treatment.

This paper is a taxonomic review of the genus Myrmelachista in Costa Rica. This regional taxonomy will (1) encourage and facilitate study of ant-plant relationships in the genus, (2) contribute to Costa Rica’s national biodiversity inventory, and (3) inform and guide surveys of Myrmelachista in other parts of the Neotropics.

Generic placement and diagnosis

Bolton (2003) placed Myrmelachista in the tribe Plagiolepidini , in the lasiine tribe group. The diagnosis for the tribe group included (1) widely separated metacoxae and (2) the petiolar foramen long, extending to or beyond the anteriormost points of the metacoxal cavities. The diagnosis for the Plagiolepidini included (1) the petiolar node inclined anteriorly, or with a long posterior peduncle, or both; and (2) the base of abdominal segment III with complete tergosternal fusion on each side of the helcium, with the free tergite and sternite commencing some distance up the sclerite, well away from the helcium. Ants of the genus Myrmelachista can be distinguished from other plagiolepidines by having a 9 or 10-segmented antenna with a 3 or 4-segmented club. Other genera have the antenna with more than ten segments and/or the funiculus filiform or gradually thickening toward the apex (Bolton 2003). Other plagiolepidine genera with 9 or 10- segmented worker antenna are Aphomomyrmex , Brachymyrmex , and Petalomyrmex . Although Bolton (2003) listed 6,4 as the palpal formula for Myrmelachista , some of the species reported here have 5-segmented maxillary palpus.

The taxonomic history of the genus Myrmelachista is covered amply by Bolton (2003). Briefly, Roger (1863) described two genera, Myrmelachista and Decamera , that differed in number of antennal segments, nine in the former and ten in the latter. Decamera was a junior homonym and was changed to Hincksidris by Donisthorpe (1944). Hincksidris (earlier as Decamera ) was placed as a subgenus of Myrmelachista by various authors and ultimately synonymized under Myrmelachista (Snelling & Hunt 1976).

There are 69 available names in the genus, of which only six are currently junior synonyms (Bolton 1995). Although it was recognized that the number of antennal segments could not be relied upon to reveal monophyletic taxa and that the division of Myrmelachista into two groups was artificial (Brown 1973, Snelling & Hunt 1976), number of antennal segments is stable within species and there is geographic patterning with respect to the abundance of one form versus the other. Among the 63 valid speciesgroup taxa, 17 have workers with 9-segmented antenna and 46 have workers with 10- segmented antenna. The 9-segmented forms are mostly concentrated in Central America and the Caribbean, with only two known from South America. The 10-segmented forms are mostly from South America, with only 3 from Central America and Mexico.