Non-Costa Rican species with 9-segmented antenna
The majority of the available names in the genus Myrmelachista are for species with a 10- segmented antenna, and almost all of these are from South America. Only three, longinoda , zeledoni and mexicana , are known from Central America. There is little question of the identity of zeledoni and mexicana in Costa Rica. Myrmelachista longinoda is known from a single queen from Panama, and it may be a species similar to but not conspecific with cooperi . There is no indication that there is a large undiscovered or complex community of 10-segmented forms in Central America.
In contrast, the species with 9-segmented antenna are more abundant and diverse in Central America and the Caribbean. The results from this study suggest there is a diverse and complex community and there will be many new species discovered with additional collecting in other parts of Central America. However, it is important to establish the identities of the existing available names, both for the correct assignment of names for the Costa Rican fauna and as a basis for further work on the Central American and Caribbean fauna. Below I review and discuss all the previously known forms with 9-segmented antenna.
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.
The above are all the species and subspecies known from Caribbean islands. My knowledge of the Caribbean fauna is limited, but I have examined 20 collections of ramulorum from Puerto Rico, St. Croix, USA (Florida, possibly introduced and then extirpated, see Deyrup 2003), Santo Domingo, St. Thomas, and the Dominican Republic; three collections of rogeri from Cuba; syntypes of rogeri manni from Cuba, and syntypes of rogeri rubriceps from Cuba. All appear to be similar to plebecula . All are bicolored or various shades of red brown. Unlike plebecula , all have long erect setae projecting from the sides of the head (workers and queens). Eight queens of ramulorum are very small with very narrow, rectangular heads. The largest of these have the narrowest heads, with HW around 0.70mm and CI around 74, a combination not found in any Costa Rican species except for the one small longiceps-like queen described under longiceps . Unlike ramulorum , the longiceps-like queen lacks erect setae on the sides of the head. The smallest ramulorum queens and the queens of rogeri are in the same size range as plebecula queens, but with relatively narrow heads. All measured queens of plebecula have CI 85 or greater. The highest CI among the ramulorum and rogeri queens is 82. Myrmelachista kraatzii , ambigua , rogeri , and ramulorum are all older names than plebecula , and if plebecula proves to be an allopatric variant of a widespread polytypic Caribbean species it will no doubt be a synonym of one of these older names.
It is not clear that rogeri and ramulorum are distinct. When two Cuban rogeri queens I have measured are compared to eight ramulorum queens from other islands, they are at the small end of a continuum of measurements.
Myrmelachista ambigua was described from a single worker from St. Vincent. Given the relative uniformity of workers, the published description and even examination of the type will be of little use. Queen and male-associated collections of Myrmelachista from St. Vincent will be needed to compare with material from other parts of the Caribbean. Wheeler (1908) considered the worker of ramulorum close to ambigua .
Myrmelachista gagates , from Haiti, was described as being close to rogeri but solid black.
It will be important to examine multiple collections of Myrmelachista from Cuba, to ascertain whether there are multiple sympatric species there. It is unknown whether kraatzii and the forms of rogeri are distinct or represent one variable species. Myrmelachista kraatzii from Cuba and M. nigella from Venezuela are the two oldest names in the genus, kraatzii being a 9-segmented form and nigella a 10-segmented form. Thus kraatzii would have priority among all the 9-segmented forms.
One collection from El Yunque, Puerto Rico, is indistinguishable firom M. longiceps . It is a collection of workers and alate queens, collected by Juan Torres. I am reluctant to identify it as longiceps until more Puerto Rican material is obtained, but there is a large size gap between the queen of this El Yunque collection and the various queens of ramulorum from elsewhere in Puerto Rico.
From these observations it is clear that more collections are needed from the Caribbean to better understand species boundaries in this group.
Key to workers (costa rica)
1. Antenna with 10 segments; mesosoma strongly hourglass-shaped, with strong constriction at metanotal groove (Fig. 2); color dark brown to black....................2
- Antenna with 9 segments; mesosoma not strongly hourglass-shaped, moderately constricted at metanotal groove; color variable, from yellow orange to red brown to black....................................................................................................................3
2(1). Surface of head and mesosoma shagreened; HW greater than 0.50mm...... zeledoni
- Surface of head and much of mesosoma smooth and shiny; HW less than 0.48mm. ................................................................................................................... mexicana
3(1). Color nearly uniform light yellow orange or with some degree of infuscation on gaster (Fig. 2C); if clearly bicolored with orange head and mesosoma and dark gaster, pubescence on sides of head and on hind tibia usually abundant, suberect, OI usually less than 22............................................................................................4
- Color black to red brown, usually either uniform red brown or with dark head and gaster and mottled light and dark brown mesosoma (Fig. 2B); if clearly bicolored, pubescence on sides of head very sparse and fully appressed, pilosity on hind tibia fully appressed, and OI greater than 21................................................................10
4(3). Maxillary palpus always 6-segmented; HW of larger workers usually greater than 0.65mm...................................................................................................................5
- Maxillary palpus always 5-segmented; HW of larger workers usually less than 0.65mm (the following set of species cannot be reliably distinguished with workers but can be differentiated with queens)..............................................................6
5(4). Infuscation of gastral tergites weak, confined to posterior third or less of tergites; nests in live stems of various plants in southern mountains of Costa Rica.............. ............................................................................................................. meganaranja
- Infuscation of gastral tergites more extensive, dark brown and covering 2/3 or more of tergite; nests in one species of understory Guarea in Cordillera de Tilarán , Costa Rica.. .......................................................................................... .. flavoguarea
6(4). Gastral tergites completely yellow, without posterior band of infuscation; hind tibia with suberect pilosity ................................................................................... ...7
- Gastral tergites with variably developed infuscated band posteriorly; hind tibia with suberect or appressed pilosity.. ..................................................................... ..8
7(6). Head of larger workers relatively broad (Fig. 4), CI 98-103; Atlantic slope ........... .............................................................................................................. ... flavocotea
- Head of larger workers relatively narrow, CI 87-95; southern Pacific slope ........... ............................................................................................................. lauropacifica
8(6). Hind tibia with pilosity fully appressed.... ............................................. .. nigrocotea
- Hind tibia with pilosity subdecumbent to suberect .............................................. ...9
9(8). Side of head with projecting erect setae, the longest of which are similar in length to length of first funicular segment; Atlantic slope.. ....................... .. lauroatlantica
- Side of head with projecting setae subdecumbent, very short, much shorter than length of first funicular segment; montane areas and southern Pacific lowlands ..... .......................................................................................................... haberi and osa
10(3). Color solid black; clypeus bulging; mandible with teeth 4 and 5 (counting from apex) separated by a wide diastema, teeth 1-4 more closely and evenly spaced; HW of larger workers 0.75mm or greater .................................................. ... cooperi
- Color variable; clypeus not strongly bulging; mandible without diastema between teeth 4 and 5, all 5 teeth relatively evenly spaced; HW of larger workers usually less than about 0.70mm ..................................................................................... ...11
11(10). Head relatively elongate and subrectangular (Fig. 5), CI 89-95; mandible punctatorugose; sides of head with pubescence sparse and very short, subdecumbent to fully appressed ........................................................................................ ... longiceps
- Head relatively shorter, often somewhat cordate, CI 92-107; mandible usually smooth and shining; pubescence on side of head variable, from sparse and fully appressed to abundant and suberect (workers of plebecula and joycei cannot always be distinguished).. ................................................................................... ..12
12(11). Often bicolored, with light red brown head and mesosoma, dark gaster, but may have same coloration as joycei ; pubescence on side of head usually sparse, short, fully appressed; pilosity on hind tibia usually appressed; eyes relatively larger, OI 22-25; HW of larger workers usually less than about 0.53mm (Fig. 5)... plebecula
- Never bicolored (except nanitics), head and gaster dark brown, mesosoma variably mottled light and dark red brown; pubescence on side of head and hind tibia subdecumbent; eyes relatively smaller, OI 19-21; HW of larger workers often greater than 0.53mm.. ...................................................................................... joycei
Key to queens (Costa Rica)
1. Antenna with 10 segments; color dark brown to black..........................................2
- Antenna with 9 segments; color variable, from yellow orange to red brown to black.. ................................................................................................................................4
2(1). Mandible strongly falcate; head dorsoventrally flattened........................... cooperi
- Mandible subtriangular with differentiated basal and masticatory margin; head not strongly dorsoventrally flattened............................................................................3
3(2). Face weakly roughened or sericeous, not strongly shining; HW greater than 0.8mm ..................................................................................................................... zeledoni
- Face smooth and shiny; HW less than 0.6mm........................................... mexicana
4(1). Head orange...........................................................................................................5
- Head dark red brown to black.................................................................................9
5(4). HL greater than 1.2mm (Fig. 6)..............................................................................6
- HL less than 1.2mm................................................................................................7
6(5). Gaster solid dark brown; obligate inhabitant of understory Guarea in Cordillera de Tilarán .................................................................................................... flavoguarea
- Gaster yellow orange with narrow infuscated bands medially; non-specialist inhabitant of live stems in Cordillera de Talamanca............................ meganaranja
7(5). Sides of head and hind tibia with pilosity (pubescence or longer setae) completely appressed; HL less than 0.85mm (Fig. 6); eyes relatively large, OI greater than 30; ocelli small, OcI 4-7; generalist inhabitant of live and dead stems......................... ............................. plebecula (also keys elsewhere due to variability in head color)
- Sides of head and hind tibia with subdecumbent to suberect pilosity; HL greater than 0.85mm; eye size variable, OI 27-37; ocelli larger, OcI 6-10; specialist inhabitant of understory Ocotea .............................................................................8
8(7). Eyes relatively small, OI 27-31; HW greater than 0.8mm (Fig. 6); Atlantic slope and central Cordilleras............................................................................ flavocotea
- Eyes larger, OI 33-37; HW less than 0.8mm; southern Pacific lowlands................ ............................................................................................................. lauropacifica
9(4). HW less than 0.7mm (Fig. 7); generalist inhabitant of live and dead stems............ ................................................................................................................... plebecula
- HW greater than 0.7mm; specialist or generalist inhabitant of live stems...........10
10(9). HL usually between 0.8-1.2mm (Fig. 7); head relatively broad, CI 87-98; eyes relatively small, OI less than 30, often around 27 (Fig. 8); ocelli very small, OW often less than 0.05mm; mandible smooth and shining or weakly rugose; maxillary palpus always 6-segmented (workers brown to mottled red brown; common cloud forest species nesting in many species of trees)................................... joycei
- HL between 0.8-1.4mm; CI 79-93; OI 23-33; OW 0.04-0.10mm; mandible punctatorugose; maxillary palpus 5 or 6-segmented....................................................11
11(10). HW greater than 1.1mm (Fig. 7); head relatively broad, CI 88-93; eyes relatively small, OI 23-27 (Fig. 8); much of face slightly roughened, dull, not strongly shining(Fig. 9A).. .......................................................................................... nigrocotea
- HW less than 1.1mm, if approaching 1.1mm then head relatively elongate (CI less than 86); OI greater than 27; most of face smooth and strongly shining.. ......... ..12
12(11). In full face view with long erect setae projecting from sides of head (Fig. 9D); maxillary palpus 5-segmented... lauroatlantica
- Sides of head with short appressed to suberect pubescence, no longer erect setae; maxillary palpus 5 or 6-segmented.. .....................................................................13
13(12). Workers dark red brown ( longiceps queens bridge the morphological gap between haberi and osa and cannot be distinguished from either).. ........................ longiceps
- Workers yellow orange... ...................................................................................... 14
14(13). Maxillary palpus 5-segmented; CI 83-89; HL 1.00-1.12; sides of head rounding more gradually into rear margin (Fig. 9B) ...................................................... haberi
- Maxillary palpus 6-segmented; CI 79-84; 1.18-1.23; head more sharply rectangular, sides flat and more abruptly rounding into flat rear margin (Fig. 9C) .......... osa
Key to males (Costa Rica)
The male of M. osa is unknown.
1. Antenna with 11 segments; apodeme of penial valve meeting dorsal margin of blade at nearly right angle (Fig. 10A) .................................................................. ...2
- Antenna with 10 segments; apodeme of penial valve rounding into dorsal margin at obtuse angle (Fig. 10B)...4
2(1). Pygostyles absent .................................................................................... ... mexicana
- Pygostyles present.. .............................................................................................. ..3
3(2). Paramere broadly triangular ...................................................................... ... cooperi
- Paramere more elongate, with parallel sides before bluntly rounded apex .............. .................................................................................................................. ... zeledoni
4(1). Basiparamere lobe absent or reduced to short triangular tooth, much shorter than paramere; cuspis absent ....................................................................................... ...5
- Basiparamere lobe well developed; cuspis present or absent .............................. ...6
5(4). Ocelli small, width of median ocellus less than distance between median and lateral ocellus (Fig. 11A); petiolar dorsum with few to no erect setae .......... plebecula
- Ocelli large, width of median ocellus greater than distance between median and lateral ocellus (Fig. 11B); petiolar dorsum with conspicuous tuft of erect setae ...... ............................................................................................................. lauropacifica
6(4). Pygostyles present; basiparamere lobes and parameres very long and thin (Fig. 12A, B) ................................................................................................................ ...7
-. Pygostyles absent or reduced to tiny unsclerotized remnants; basiparamere lobes and parameres thin or broad ................................................................................ ...8
7(6). Digitus evenly tapered to apex (Fig. 12B)............................................. flavoguarea
- Digitus broadening apically to paddle-shaped apex (Fig. 12A).......... meganaranja
8(6). Digitus greatly expanded distally and with thickened posterodorsal and posterior margins, forming a bulla at apex (Fig. 12C, D)......................................... longiceps
- Digitus evenly tapered or scimitar-shaped, never forming a bulla at apex.............9
9(8). Cuspis absent or reduced to sharply triangular or spine-like process, closely appressed to inner surface of paramere and distant from digitus.........................10
- Cuspis present and subrectangular, separated from inner surface of paramere, apex sometimes approaching or touching dorsal margin of digitus..............................12
10(9). Digitus scimitar-shaped (Fig. 10B); cuspis present as a spiniform tooth; maxillary palpus always 6-segmented............................................................................ joycei
- Digitus more evenly tapered from base to apex; cuspis as above or absent; maxillary palpus 5 or 6-segmented................................................................................11
11(10). Maxillary palpus 5 -segmented; cuspis present as a small triangular or spiniform tooth ........................................................................................................................ haberi
- Maxillary palpus 6-segmented; cuspis absent..................................... lauropacifica
12(9). Paramere thin and evenly tapered...................................................... lauroatlantica
- Paramere scimitar-shaped, broadening apically...................................................13
13(12). Basiparamere lobe and paramere very long and thin; digitus rounded apically....... ................................................................................................................... longiceps
- Basiparamere lobe and paramere broad at the base, shorter; digitus tapering to a point at apex..........................................................................................................14
14(13). Ocelli relatively large, width of median ocellus usually greater than distance between median and lateral ocellus; distance between lateral ocelli about equal to distance from lateral ocellus to compound eye (Fig. 11C)..................... flavocotea
- Ocelli relatively small, width of median ocellus usually less than distance between median and lateral ocellus; distance between lateral ocelli less than distance from lateral ocellus to compound eye (Fig. 11D)............................................. nigrocotea
The above are a set of species described together by Wheeler from material collected by Elisabeth Skwarra in southern Mexico. They are all very similar to plebecula and joycei . I cannot distinguish workers of amicta , skwarrae (and its two subspecies picea and laeta ), and plebecula . The males of skwarrae and plebecula are distinct. The male of skwarrae differs from plebecula in having a basiparamere lobe and distinct (though minute) setose pygostyles. Myrmelachista skwarrae males are more like joycei . They differ from joycei in the relatively thinner and more delicate basiparamere lobe and relatively larger ocelli.
If the Mexican forms prove to be geographic variants of plebecula , the latter has priority. On the other hand, M. joycei could prove to be a geographic variant of one of the previously named Mexican forms.
From the descriptions of both species and my examination of workers of guyanensis , these two species are similar to and could be synonymous with plebecula . But further collections of South American Myrmelachista will be necessary to further understand their status.
It is tempting to speculate that the 10-segmented forms, widespread and diverse in South America, have spawned one main radiation of 9-segmented forms in Central America and the Caribbean. In this scenario only plebecula , the "weediest" of 9- segmented forms, has spread into South America, and there is no diverse community of 9- segmented forms there. But the late discovery of a trove of 9-segmented Myrmelachista species in Costa Rica shows that this group may remain hidden from the general collector, even in the face of prolonged collecting effort. Thus we remain ignorant of extent of such forms in the South American hylea.
Canindeyú (ALWC).
Dieses kleine Formiciden-Genus ist durch 9 - gliedrige Fuehler, die eine 3 - gliedrige Keule haben, ausgezeichnet.
[[ worker ]] sehr klein, Kopf laenglich viereckig, vorn etwas verengt, hinten gerundet, kaum ausgerandet, viel breiter als der Thorax. Mandibeln dreieckig, Schneiderand ziemlich kurz mit kleinen Zaehnen. Clipeus gewoelbt, hinten halbkreisfoermig und zwischen die Fuehlerwurzeln eingeschoben, vorn gerundet. Schild- und Fuehlergrobe- vereinigt. Stirnlamellen sehr kurz. Die Fuehler entspringen an den Seiten des Clipeus und sind 9 - (vielleicht auch 10 -) gliedrig; ihr Schaft ist kurz, reicht nicht bis zum Hinterrand, die kuglige Radicula sehr sichtbar. Das erste Geisselglied ist lang, cylindrisch, wenigstens so lang als die naechstfolgenden 3 — 4 Glieder. Diese, das zweite bis vierte, nehmen an Breite nach vorn etwas zu und schliessen sehr eng an einander. Die drei letzten Glieder sind dicker als die uebrigen und bilden eine Keule, die mindestens so lang ist als die andern Glieder zusammen; das Endglied ist lang eifoermig und uebertrifft an Laenge die 2 vorhergebenden bedeutend. Die Netzaugen sind ziemlich gross, in der Mitte des Seitenrandes. Ocellen fehlen. Von Stirnfeld und Stirnrinne keine Spur.
Der Thorax ist dem von Lasius sehr aehnlich, doch ist die Basalflaeche des Metanotums laenger als z. B. bei L. flavus. Das Pronotum ist seitlich gerundet, hinten halbkreisfoermig fuer die Aufnahme des Mesonotums ausgerandet; dieses ist schmaeler als jenes, in der Mitte sehr stark eingezogen, vom Metanotum wieder durch einen queren Eindruck abgegrenzt. Das letztere ist hinten schief abfallend. Die Schuppe ist aufrecht, eifoermig, platt gedrueckt. Beine sind kurz, kraeftig, die Vordertarsen scheinen erweitert zu sein.
