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The genus Adelomyrmex in Costa Rica

Introduction

Adelomyrmex is a genus of small, cryptic ants that inhabit wet forest floor leaf litter. Old World representatives of the genus have been described from New Guinea, Fiji, and Samoa. Twenty New World species have been described (Fernández and MacKay 2003, Fernández 2003), ranging from southern Mexico to southern Brazil and Paraguay.

Taxonomic History

Wheeler (1910) described a new genus and species, Apsychomyrmex myops, from Guatemala. Mann (1922) reported a second collection of myops from Honduras, observing that specimens occurred in small colonies beneath stones, the colonies resembling those of Rogeria. Menozzi (1931) described two additional species, silvestrii and tristani, from the central highlands of Costa Rica. Borgmeier (1937) reported additional collections of silvestrii and tristani from Hamburg Farm, Costa Rica, collected by F. Nevermann. Hamburg Farm is a site in the Atlantic lowlands, north of Limon. Smith (1947) reviewed the genus, providing redescriptions of the three known species of what he called "some of the rarest American ants," and providing a few new locality records. Prior to the description of Apsychomyrmex, Emery (1897) described a new genus and species, Adelomyrmex biroi, from New Guinea. Mann (1921) described Adelomyrmex hirsutus from Fiji, and Wilson and Taylor (1967) described Adelomyrmex samoanus from Samoa. Kempf (1972) synonymized Apsychomyrmex under Adelomyrmex. This synonymy was simply stated in the catalog of Neotropical ants, with no discussion or justification. Kugler (1978) examined the sting apparatus, and concluded that Adelomyrmex was related to the Neotropical genus Lachnomyrmex and the African genus Cyphoidris. Bolton (1981) described the African genus Baracidris, and considered it very close to Adelomyrmex. Fernandez and MacKay (2003) described new species. Fernandez (2003) provided a thorough species-level revision of Adelomyrmex and Baracidris and provided evidence that the two genera form a monophyletic group.

Costa Rican Abundance and Distribution

In Costa Rica, the genus is most abundant in wet forest sites above 500m, and is increasingly rare at lower elevations. Workers occur in nearly every litter sample from the Monteverde area, but are relatively infrequently encountered at La Selva. The caveat should be added that this abundance pattern is that revealed by litter sifting. The variation in abundance could be due to real differences in density, or to differences in nesting or foraging behavior that affect catchability.

The Costa Rican species are of relatively uniform habitus, and vary in details of surface sculpture, size, color, and pilosity. They occur in communities of sympatric species, and show patterns of parapatric distributions along elevational gradients. silvestrii is a broad habitat generalist, occurring in wet forest habitats from sea level to cloud forest. myops and longinoi are moderately common in lowland sites, below 500m. foveolatus and microps are very rare species (or at least difficult to collect). They are known from only 3 and 1 specimen, respectively, from La Selva, where ant sampling intensity has been particularly high. tristani occurs at La Selva but is extremely rare. It increases in abundance at higher elevations, and becomes a relatively common element of the leaf litter in cloud forest. At these higher elevations laevigatus appears, and is a relatively widespread mid-elevation montane species. brevispinosus occurs in a slightly higher elevational zone than laevigatus. It is known from the dripping, fog-bathed cloudforest on the narrow ridge crest above Monteverde, and on the slopes of Volcan Barva. In Monteverde, it appears parapatric with laevigatus. brevispinosus inhabits the wet cloud forest of the ridge crest; laevigatus inhabits the surrounding wet to moist forest in the Monteverde community and in the Penas Blancas Valley.

Nesting Habits and Foraging

In my brief examination of this genus, I can say nothing of nesting habits or foraging behavior, because I have never seen them other than as loose workers and queens in the ethanol of Winkler or Berlese samples. Mann (1922) found nests under stones in Honduras.

Distinctive Clypeo-Mandibular Morphology

All the Adelomyrmex specimens I have examined have exhibited a characteristic morphology of clypeus and mandibles, the adaptive significance of which begs investigation. Smith (1947) also observed this morphology. The clypeus has two portions, a dorsal surface (assuming a prognathous head) with an anteromedian bidentate projection, and an anterior surface perpendicular to the dorsal surface. The anterior surface has a broad, concave median portion and concave lateral portions, divided by projecting longitudinal ridges. The ridge forms a distinct tooth, and the lateral concavity a distinct notch. The basal margin of the mandible has a tooth that fits in the notch (Figure 1). It appears to be either a locking mechanism, to keep the mandibles from sliding laterally, or a gripping mechanism, for tightly clamping soft-bodied prey. In some species there is also a pronounced median tooth on the hypostomal margin (Figure 2). Perhaps the dentate anteromedian projection, the lateral interlocking tooth and notch complex, and the pronounced hypostomal tooth function together to hold earthworms or other soft-bodied, slippery or tapered prey, much like the teeth on a pair of pliers.

In the same Costa Rican cloud forests where Adelomymrmex are common, members of the Stenamma schmidti complex also have mandibles with a tooth and notch on the basal margin of the mandibles, and a corresponding tooth and notch on the anterolateral clypeal margin. The structures are remarkably similar between the two genera, but are presumably a result of convergence and not shared ancestry. It will be exciting to discover whether or not Adelomyrmex and the Stenamma schmidti complex share foraging methods or prey preferences.


Literature Cited

Bolton, B. 1981. A revision of six minor genera of Myrmicinae (Hymenoptera: Formicidae) in the Ethiopian zoogeographical region. Bull. Br. Mus. (Nat. Hist.) Entomol. 43:245-307.

Borgmeier, T. 1937. Formigas novas ou pouco conhecidas da America do Sul e Central, principalmente do Brasil (Hym. Formicidae). Arch. Inst. Biol. Veg. (Rio J.) 3:217-255.

Emery, C. 1897. Formicidarum species novae vel minus cognitae in collectione Musaei Nationalis Hungarici quas in Nova-Guinea, colonia germanica, collegit L. Biro. Termeszetr. Fuz. 20:571-599.

Fernández C., F. 2003. Revision of the myrmicine ants of the Adelomyrmex genus-group (Hymenoptera: Formicidae). Zootaxa 361: 1–52.

Fernández C., F., MacKay, W.P. 2003. The myrmicine ants of the Adelomyrmex laevigatus species complex (Hymenoptera: Formicidae). Sociobiology 41:593–604.

Kempf, W. W. 1972. Catalogo abreviado das formigas da regiao Neotropical. Stud. Entomol. 15:3-344.

Kugler, C. 1978. A comparative study of the myrmicine sting apparatus (Hymenoptera, Formicidae). Stud. Entomol. 20:413-548.

Mann, W. M. 1921. The ants of the Fiji Islands. Bull. Mus. Comp. Zool. 64:401-499.

Mann, W. M. 1922. Ants from Honduras and Guatemala. Proc. U. S. Natl. Mus. 61:1-54.

Menozzi, C. 1931. Contribuzione alla conoscenza del "microgenton" di Costa Rica. III. Hymenoptera - Formicidae. Boll. Lab. Zool. Gen. Agrar. R. Sc. Super. Agric. 25:259-274.

Smith, M. R. 1947 ("1946"). Ants of the genus Apsychomyrmex Wheeler (Hymenoptera: Formicidae). Rev. Entomol. (Rio J.) 17:468-473.

Wheeler, W. M. 1910. Three new genera of myrmicine ants from tropical America. Bull. Am. Mus. Nat. Hist. 28:259-265.

Wilson, E. O., Taylor, R. W. 1967. The ants of Polynesia (Hymenoptera: Formicidae). Pac. Insects Monogr. 14:1-109.


Page author:

John T. Longino, The Evergreen State College, Olympia WA 98505 USA.longinoj@evergreen.edu


Date of this version: 2 December 2003.
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