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Xenomyrmex of Costa Rica

These are very small myrmicines with a distinctive petiole structure (Creighton 1957, Ettershank 1966). They are very secretive and there are relatively few collections. They are strictly arboreal, nesting in thin dead branches, small cavities in bark, and other plant cavities. They will nest in preformed cavities or passages they excavate themselves. In observations of captive colonies of X. floridanus skwarrae Creighton observed that workers could produce copious "sawdust" while excavating nest chambers. They fed on sugar solution (juice from canned peaches) and dead termites. His captive colonies were very resistant to dessication.

There are very few known species: floridanus floridanus Emery in Florida and parts of the Caribbean, floridanus skwarrae in eastern Mexico, stollii Forel in Guatemala and southern Mexico, and panamanus (Wheeler) in Panama. I have found what appear to be three distinct species in Costa Rica: JTL-001, JTL-002, and panamanus (see below and key). Wilson (1987) reported the occurrence of a Xenomyrmex species in canopy fogging samples from Peru.

Taxonomic Comments

I have available for examination two collections of Xenomyrmex from Florida, one collection from Jamaica, and four collections from Costa Rica. I see a major difference in petiole shape that divides the Florida/Jamaica material from the Costa Rica material. In the former the petiole is relatively longer and the anterolateral tubercles are not as sharply produced. In the latter the petiole is shorter and the anterolateral tubercles acutely produced (see figure above).

Wheeler described panamanus from two workers from Colon, Panama, and later (1931) reported additional collections from Mt. Hope and Barro Colorado Island, Panama. He described panamanus as having the petiole from above nearly square, and the figures show dorsal and lateral views of the petiole that match the Costa Rican material closely. Additional characters were a pair of strong carinae on the clypeus and a heavily sculptured thorax. These additional characters and the figures closely match two collections from Costa Rica, one from Corcovado and one from La Selva, and I have identified these collections as panamanus. Wheeler also stated that the clypeal lobe of panamanus had "marginate sides and rather sharp corners." Creighton stated that in panamanus the median lobe of the clypeus rounded into the flanking lateral portions through a gradual curve, while in other species of Xenomyrmex there were two prominent angles that flanked the median lobe. However, on the Costa Rican collections of panamanus the clypeus does have two prominent angles. However, the head capsule and mandibles are dark and when the mandibles are closed the clypeal teeth are very hard to see. I think Creighton overlooked them.

Wheeler (1931) placed all other Xenomyrmex as subspecies of stollii, but for subspecies skwarrae stated that it differed from all other stollii forms in, among other characters, having the petiole as broad as long (in contrast to others having petiole longer than broad). He stated "Perhaps this ant, which is more distinct morphologically than any of the other forms of X. stollii should be regarded as an independent species." I suspect my JTL-001 and JTL-002 may be versions of Wheeler's skwarrae. The short petiole may characterize a southern Central American lineage, while the elongate petiole characterizes floridanus forms from northern Central America, the Caribbean, and Florida.

Creighton (1957) ignored petiolar characters and keyed the species of Xenomyrmex as follows:

1. Clypeus with two prominent carinae; lateral cephalic striae extending to the occiput; entire thorax heavily sculptured and opaque ... panamanus.

Clypeus without carinae; cephalic striae largely restricted to the area between the eye and the insertion of the mandible; thoracic sculpture mainly confined to the mesopleurae and the epinotum ... 2

2. The majority of workers in a colony with a head length more than 0.5mm.; clypeal rugules usually prominent; anteroventral tooth on the petiole ordinarily well developed, color piceous brown ... stollii

The majority of workers in a colony with a head length less than 0.5mm.; clypeal rugules usually obscure or absent; anteroventral tooth on the petiole poorly developed or absent; color not as above ... 3

3. Color clear golden or lemon yellow throughout ... floridanus skwarrae

Color dirty brownish yellow to golden yellow, the gaster always extensively infuscated ... floridanus floridanus

In Creighton's key JTL-001 would key to floridanus floridanus and JTL-002 would key to stollii, but if my hypothesis regarding petiole shape is true, neither identification would be correct.

Literature Cited

Creighton, W. S. 1957. A study of the genus Xenomyrmex (Hymenoptera, Formicidae). American Museum Novitates 1843:1-14.

Ettershank, G. 1966. A generic revision of the world Myrmicinae related to Solenopsis and Pheidologeton (Hymenoptera: Formicidae). Aust. J. Zool. 14:73-171.

Wheeler, W. M. 1922. A new genus and subgenus of Myrmicinae from tropical America. American Museum Novitates 46:1-6.

Wheeler, W. M. 1931. Neotropical ants of the genus Xenomyrmex Forel. Rev. Entomol. (Rio de Janeiro) 1:129-139.

Wilson, E. O. 1987. The arboreal ant fauna of Peruvian Amazon forests: a first assessment. Biotropica 19:245-251.

Page author: John T. Longino longinoj@evergreen.edu