Xenomyrmex floridanus Emery (Hymenoptera: Formicidae: Myrmicinae) is a small arboreal ant known only from peninsular Florida and the West Indies. Xenomyrmex floridanus colonies commonly nest in plant cavities, particularly in hollow twigs and dead branches (Creighton 1957). In the Florida Keys, X. floridanus colonies are common on red mangrove (Rhizophora mangle L.; Rhizophoraceae) tree islands (Simberloff & Wilson 1969; Cole 1983). In the Gainesville, Florida, area, X. floridanus colonies nest in sweetgum (Liquidambar styraciflua L.; Altingiaceae) trees (Van Pelt 1947). Here, I examine the biogeography of X. floridanus.

Forel (1884) established the genus Xenomyrmex, based on specimens of Xenomyrmex stolli Forel that Otto Stoll collected in Guatemala, co-inhabiting an oak gall with a carpenter ant (Camponotus abscicus Roger) colony. Forel (1884) assumed that X. stolli was probably a parasitic species, and named the genus Xenomyrmex, meaning “foreign ant.” Forel (1884: 55) wrote (in French): “This genus is nearest to Monomorium, which it closely resembles in habitus. Its petiole, however, is entirely different and is similar to the genus Pristomyrmex, which it otherwise does not at all resemble.”

Emery (1895) described Xenomyrmex stolli floridanus (= X. floridanus) based on specimens that Theodore Pergande collected from a twig of a yellow mastic tree (Sideroxylon foetidissimum Lam; Sapotaceae) in Lake Worth, Palm Beach Co., Florida. Junior synonyms include Xenomyrmex stolli cubanus Wheeler (from Cuba), Xenomyrmex stolli lucayanus Wheeler (from the Bahamas), and Xenomyrmex stolli rufescens Wheeler (from Florida).

The genus Xenomyrmex currently includes 4 other described species, all known only from Central America and/or Mexico: Xenomyrmex panamanus (Wheeler) (known from Panama), Xenomyrmex picquarti (Forel) (known from Costa Rica), Xenomyrmex skwarrae Wheeler (known from Guatemala and Mexico), and X. stolli (known from Guatemala and Mexico) (Kempf 1972; Bestelmeyer et al. 2000; Campbell et al. 2006; Pacheco & Mackay 2013). In addition, unidentified Xenomyrmex specimens have been recorded from South America: Brazil (Delabie & Nascimento 1998; Vasconcelos 2007), Colombia (SIB 2016), Paraguay (Wild 2007), and Peru (Wilson 1987). Because Xenomyrmex workers resemble small Monomorium and Solenopsis workers, they often may be misidentified (Wheeler 1931). It is therefore possible that unrecognized populations of Xenomyrmex are actually fairly widespread in South America.

Materials and Methods

Using published and unpublished records, I documented the known range of Xenomyrmex floridanus. I obtained unpublished site records from museum specimens in the collections of Archbold Biological Station (ABS), the Museum of Comparative Zoology (MCZ), and the Smithsonian Institution (SI). In addition, I used online databases with collection information on specimens by Antweb ( www.antweb.org). I obtained geo-coordinates for collection sites from published references, specimen labels, maps, or geography web sites (e.g., earth.google. com and  www.tageo.com).

In part to define better the geographic range of X. floridanus, I collected ants nesting inside dead twigs of red mangrove at sites along the east coast of Florida, north of the Florida Keys (in St. Johns, Volusia, Brevard, Indian River, St. Lucie, Martin, Palm Beach, Broward, and Miami-Dade counties) and on islands of the Bahamas (Grand Bahama, North Bimini, New Providence, and Half Moon Cay) and the Turks and Caicos Islands (Providenciales, North Caicos, Middle Caicos, and Grand Turk).

Results

I collected Xenomyrmex floridanus nesting in the dead twigs and branches of red mangrove at 5 sites on 3 Bahamian islands (from north to south, geo-coordinates and collection date in parentheses):

I also collected X. floridanus in red mangrove at 43 sites in 8 counties along the east coast of peninsular Florida (from north to south):

I also collected X. floridanus nesting in black mangrove, Avicennia germinans (L.) (Acanthaceae), at 1 site where red mangrove was absent:

In total, I compiled and mapped >100 X. floridanus site records (Fig. 1), documenting the earliest known record for 4 geographic areas (Table 1): Bahamas (Wheeler 1905, 1931; Deyrup 1994; Deyrup et al. 1998), Cuba (Wheeler 1931; Fontenla Rizo 1993a,b; Fontenla 1999; Portuondo Ferrer & Fernández Triana 2004; Portuondo & Fontenla Rizo 2005; Reyes 2005; antweb.org), Florida (Wheeler 1931; Creighton 1957; Van Pelt 1947, 1958; Simberloff & Wilson 1969; Simberloff 1976; Deyrup et al. 1988; Ferster & Prusak 1994; Valles et al. 2007; Moreau & al 2014; antweb.org; ABS), and Jamaica (antweb.org; Table 1). Known records of X. floridanus ranged from Gainesville, Florida (29.7°N; Valles et al. 2007), in the north to Pedro Cross, Jamaica (18.9°N; Table 1), in the south.

Whereas Deyrup et al. (1989) listed X. floridanus from 10 counties in Florida, I found published and unpublished records of X. floridanus from 18 counties in Florida: Alachua, Brevard, Charlotte, Collier, Desoto, Highlands, Hillsborough, Lake, Lee, Marion, Miami-Dade, Monroe, Orange, Palm Beach, Pinellas, Polk, Sarasota, and Volusia. In addition, I collected the first records of X. floridanus from 4 additional counties: Broward, Indian River, Martin, and St. Lucie (see list above), bringing the total to 22 counties in Florida.

Fig. 1.

Known site records of Xenomyrmex floridanus.

Table 1.

Earliest known records for Xenomyrmex floridanus.

North of Volusia County, red mangroves are rare. I did not find X. floridanus at the only red mangrove site I surveyed in St. Johns County (St. Augustine, Fish Island Marina, 29.8705°N, 81.3025°W; 24-Jul-2016), where I only found M. floricola. I did not find any red mangroves at 2 sites in Flagler County dominated by black mangrove: Betty Steflik Memorial Preserve and Gamble Rogers Memorial State Recreational Area.

Discussion

Xenomyrmex floridanus has widespread records from peninsular Florida, the Bahamas, and Cuba, plus 1 record from Jamaica (Fig. 1). The high concentration of X. floridanus records along the east coast of peninsular Florida (Fig. 1) is primarily due to my collecting ants nesting in red mangrove, a tree species restricted to brackish habitats. Although X. floridanus colonies nest in many tree species, it appears that red mangrove provides an important habitat for this species. In my surveys of red mangrove in southeastern Florida and the Bahamas, X. floridanus was the native ant species that I most commonly collected.

Concerning X. floridanus, Wheeler (1905) wrote that “the habits of the workers resemble those of Monomorium floricola, so far as could be ascertained by hasty observation in the field.” Monomorium floricola (Jerdon) is native to the Old World, but has been transported around the world through human commerce (Wetterer 2010) and is the most common exotic ant I collected in red mangrove in Florida. My own observations confirm Wheeler's (1905) supposition that X. floridanus and M. floricola show evolutionary convergence in morphology and behavior. Both species are tiny, thin, and short-legged, a morphology that allows them to nest in very narrow plant cavities, such as inside the hollow dead twigs of red mangrove. Both X. floridanus and M. floricola are slow moving and have an exceptional ability to cling tenaciously to surfaces. As a result, both species can be difficult to collect off twigs and branches using an aspirator and are hard to dislodge through vegetation beating. The ability to grip onto surfaces probably allows these arboreal ants to avoid being blown out of trees, even in high winds. This trait should be particularly important for ants living in red mangrove growing in brackish water, where falling out of the tree would mean almost certain death.

Whereas X. floridanus is extremely common in red mangroves of southeast Florida, additional research is needed to determine whether the same is true in the Bahamas, Cuba, and Jamaica. It is not known whether X. floridanus possesses any special adaptations for living in red mangroves, such as high salt tolerance, or whether the prevalence of hollow spaces in dead twigs of red mangroves may be indicative of a myrmecophyte that benefits from the presence of ants. The close similarity in the behavior and ecology of X. floridanus and M. floricola suggests that these 2 species may be important competitors in areas where M. floricola invades. Monomorium floricola colonies can grow to an enormous size with thousands of workers and dozens of queens (personal observation), giving them a numerical advantage over X. floridanus. Evaluating whether M. floricola has a significant impact on X. floridanus populations in Florida, the Bahamas, or elsewhere would require further research.