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Nesomyrmex asper inhabits second growth and edges in wet forest habitats. Nests are in small live or dead stems. Several Costa Rican collection observations follow:
Guanacaste Conservation Area, Pitilla station: wet forest second growth, single queen colony with alate queens in dead stick.
Cordillera Volcanica Central, Casa Plastico near Rara Avis: 25yr old second growth and primary forest; nest in live branch of rubiaceous shrub; nest 10mm by 26cm; single queen.
Cordillera Volcanica Central, Casa Plastico near Rara Avis: roadside vegetation in wet forest; nest in dead stem, continuing into live stem of Cassia; 2m tall treelet; entire nest contents collected: 34 adult workers, 17 adult males, 1 alate queen, brood of various sizes.
Moravia de Chirripo: 2nd growth patches surrounded by tall rainforest; nest in small chamber, 1cm long, in live petiole of common large herb/shrub (opposite distichous leaves, thick stems, soft pith [Asteraceae?]; stems regularly hollowed out by stem-boring lepidopteran larva). Second nest in same plant species; dead to live transition zone in 11mm dia stem; entire nest collected: 1 dealate queen, 19 alate females, 51 workers, brood.
Braulio Carrillo National Park, Carrillo station: stray worker.
Hitoy Cerrere Biological Reserve: stray worker.
At La Selva Biological Station N. asper has been collected once by Henry Hespenheide, once by J. Dunn in a study of inhabitants of Cordia alliodora, and in 1 of 52 trees fogged by Project ALAS.
The few available observations above suggest that colony reproduction at any one time is biased with respect to sex ratio, with either mostly males or mostly queens being produced. It is also possible that colonies are polydomous, with different nests within a colony specializing in the production of males or queens. One nest that was producing mostly males lacked a dealate queen. The queen could have been lost during collection, or this was one nest of a polydomous colony, or the queen could have recently died, or this was a queenless colony with worker reproduction.
Emery described the species N. tristani in 1896, based on workers and a queen collected by Alfaro in Jimenez, Costa Rica, near present day Guapiles. In Longino (2006) I synonymized tristani under asper, using the following evidence.
Kempf (1959) expressed doubt about the distinctness of the two species asper Mayr and tristani Emery, but he saw evidence of two sympatric species in the vicinity of Caracas, Venezuela. One of the species matched his concept of asper. The other one differed from asper as follows: "1) Longitudinal rugae on cephalic dorsum regular, more widely spaced, often fading out to a variable degree on front, vertex and occiput. 2) Rugae of thoracic dorsum usually less strikingly vermiculate, obsolescent on basal face of epinotum. 3) Epinotal spines shorter, more elevated and more diverging toward apex. 4) Petiolar node, as seen from the side, more depressed; broader, with sides diverging caudad, when seen from above. Each side postero-laterally with at least two prominent teeth. 5) Postpetiole laterally with two prominent teeth. 6) First gastric tergite generally distinctly transverse, broader than long." The only type material of tristani that Kempf was able to examine was the syntype queen. He concluded that the queen was conspecific with the non-asper workers from Venezuela, and thus tristani was a distinct species occurring from Costa Rica to Venezuela, where it was sympatric with asper. Kempf also synonymized Forel's asper antoniensis, from the Santa Marta region of Colombia, with tristani. He identified additional material from Trinidad and Colombia as tristani, and his figures of tristani (figures 11 and 15) were of a worker from Trinidad.
I examined abundant material from Costa Rica, and it all closely matched Kempf's description and illustration of asper, not Kempf's concept of tristani. I have never seen material from Costa Rica that matches Kempf's concept of tristani. In Emery's description of the tristani worker, he states that the side of the postpetiole has two obtuse tubercles that are more or less distinct. In contrast, material Kempf identified as tristani had "two prominent teeth." Workers from Colombia and Venezuela that I have examined have distinct acuminate teeth on the postpetiole, matching Kempf's description of what he thought was tristani, but not matching new material of what must be true tristani from Costa Rica.
I concluded that tristani is a junior synonym of asper, and that what Kempf thought was tristani is a distinct species. Forel's antoniensis was removed from synonymy and raised to species, for the form described as tristani by Kempf.
Costa Rican asper differs from South American asper in having a more elongate petiole. If further research reveals discontinuous variation, tristani might emerge as a distinct Central American species. But Kempf noted that asper itself is highly variable, with two infraspecific synonyms. If tristani is distinct, then asper will probably resolve into multiple differentiated forms.
Longino, J. T. 2006. New species and nomenclatural changes for the Costa Rican ant fauna (Hymenoptera: Formicidae). Myrmecologische Nachrichten 8:131-143.
Alto Paraná, Canindeyú, Paraguarí (ALWC, IFML, INBP, MZSP).
Found most commonly in these habitats: 6 times found in montane wet forest, 1 times found in Roadside vegetation in wet forest, 2 times found in 2nd growth patches surrounded by tall rainforest, 1 times found in 25yr old second growth and primary forest, 3 times found in tropical wet forest, 2 times found in rainforest, 2 times found in Semideciduous seasonal forest, 1 times found in 2º lowland rainforest, 1 times found in Bosque alto, 1 times found in edge of second-growth rainforest, ...
Found most commonly in these microhabitats: 1 times nest in dead/live stem, 1 times nest in live stem Rubiaceae, 1 times Wet forest. Second growth veg. Single-queen colony with alate queens, in dead st, 1 times nest in stem, 1 times nest in live stem, 2 times ex sifted leaf litter, 1 times beating vegetation, 1 times strays, 1 times Sobre Vegetacion, 1 times recent tree-fall, 1 times on recently felled trees, ...
Collected most commonly using these methods: 5 times Malaise, 3 times direct collection, 2 times miniWinkler, 1 times beating, 1 times Manual sampling, 1 times Search, 1 times sweep net, 1 times Sweeping, 1 times Foggin, 1 times fogging.
Elevations: collected from 30 - 706 meters, 445 meters average
Type specimens: syntype of Leptothorax asper: casent0919738; syntype of Leptothorax tristani: casent0904724; syntype of Leptothorax asper rufa: casent0904722; syntype of Leptothorax asper sufurea: casent0909003; syntype of Nesomyrmex asper: casent0901801, casent0909001; type of Leptothorax asper: focol2044