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Overview of Stenamma in Costa Rica

Stenamma is one of few Costa Rican ant genera that has a diversity and abundance peak in montane forests rather than lowland forests (the others being Cryptopone and Simopelta). They have small colonies and cryptic habits, and very little is known of their biology. Morphologically they are very generalized myrmicines and difficult to define or key out. They may be confused with the genus Rogeria, but Rogeria almost all have a uniformly convex mesosoma with no metanotal groove. All Stenamma have a metanotal groove, with the promesonotum forming a convexity distinct from the propodeum. Workers may also be confused with Pheidole, but the terminal four or more segments of the antenna gradually thicken and do not form a discrete 3-segmented club.

At the species level, Stenamma taxonomy can be remarkably difficult. The Costa Rican species seem very plastic in details of surface sculpture, pilosity, and head shape. Even complex structures, like the deep mandibular notch of the schmidti complex (see below) can vary in distinctness and in some cases can disappear all together.

Preliminary notes on the Stenamma schmidti complex

The smallest and most abundant members of the genus are in what I call the schmidti complex, and these are most similar to the holarctic species.

Much like their holoarctic counterparts, these ants are rarely collected by manual search but occur abundantly in Winkler samples of sifted leaf litter. They have small colonies of only a few workers and a queen, they are brown and slow-moving, and they become motionless on disturbance. They are nearly impossible to see in the leaf litter, yet colonies are relatively high density. Most Winkler samples from montane habitats contain Stenamma workers.

Within the complex there is a high degree of character variation. Some morphological clusters are emerging, and these show some patterning with respect to geography and microhabitat. It is not at all clear what a species is in the complex. I have developed a set of operational morphospecies based on the dominant morphological clusters, but I cannot unambiguously place every specimen.


Basal margin of mandible: On most members of the complex there is a strongly developed notch on the basal margin of the mandible. The notch is formed from a combination of a strongly sinuous margin and a blunt proximal tubercle (figure). There is usually a corresponding angular projection on the anterior margin of the clypeus, such that the projection fits into the notch when the mandibles are closed. This appears to be a unique derived trait in the schmidti complex. Other species in the genus have the basal margin of the mandible flat or slightly sinuous. Most individuals can be unambiguously scored for presence or absence of a mandibular notch (if the mandibles are spread), but all degrees of intermediacy can be found. In spite of occasional intermediacy, I use presence or absence of a notch as a species separatory character (with the exception of the notchless Osa population of JTL-006, see below).

Pubescence on fourth abdominal (first gastral) tergite: Some individuals have a dense layer of pubescence on the fourth abdominal tergite; others have this pubescence weak or absent (figure). In spite of fairly frequent intermediacy, I use this as a species separatory character.

Dorsal macrosetae: longer setae on the meso and metasoma may be thin and flexuous or variously thickened and stiff.

Face sculpture: the face may be completely punctatorugose, or with a variable extent of punctatorugose sculpture anteromedially, grading to smooth and shiny posteriorly. Although differences can be dramatic, variation does not correlate with other characters. I am treating this as intraspecific variation, but the biological significance of these differences in surface sculpture beg investigation.

Lateral profile: some forms are relatively compact and with moderately developed propodeal tubercles; others are more gracile and with a smoothly rounded propodeum that lacks tubercles.

Sculpture and elevation: there is a tendency for sculpture to be reduced at higher elevation, with punctatorugose sculpture being replaced by smooth and shiny surfaces. I am interpreting this as intraspecific clinal variation that occurs in parallel among many species, rather than interspecific variation.


This figure (large file, small file) illustrates my current morphospecies concepts for the Barva Transect in Braulio Carrillo National Park.

As currently defined, schmidti has a mandibular notch, dense pubescence on the gaster, completely punctatorugose face, and somewhat compact lateral profile. The sides of the head and the pronotum tend to become smoother and shinier at higher elevation. Stenamma schmidti occurs from about 1000 to 2000m elevation, only in leaf litter on the forest floor (not under epiphytes). It is one of the most abundant species in collections because it dominates cloud forest Winkler samples. In 1990, before I was very aware of character variation in the complex, I examined the types of schmidti. The types were from "Vara Blanca, 2000m, between Barba and Poas" presumably from very near the 2000m site of the Volcan Barva transect. The types definitely had the mandibular notch, but I do not know what the gastral pubescence was like. Although I am using the name schmidti for the common forest floor form, the types should be re-examined.

JTL-006 also has a mandibular notch, and is essentially all remaining notched forms after a narrowly circumscribed "schmidti" is separated out. The main feature that differentiates it from schmidti is the weak or absent gastral pubescence. It is coextensive with schmidti at upper elevations, but also extends into lower elevations. It is the only member of the complex that makes it into lowland rainforest. At La Selva it is small, compact, with punctatorugose face, and with distinctly stiffened dorsal macrosetae. It occurs as a low-density element in Winkler samples from the forest floor. At about 1000m elevation, where it becomes sympatric with schmidti, it becomes more arboreal, occurring both on the forest floor and under epiphytes. At 1000m elevation and above the sculpture becomes variable, the face is often partially smooth and shiny, the size is somewhat larger, and the macrosetae may become relatively thinner and more flexuous.

An exception is a set of specimens from the lowland rainforest of the Osa Peninsula. A Winkler sample yielded several workers that in almost every respect were identical to JTL-006 from lowland rainforest on the Atlantic slope, but they have no trace of a mandibular notch.

JTL-018 is known from one strange specimen from the 2000m site. It has a strong mandibular notch like schmidti and JTL-006, but is dramatically larger and shinier than any other collection. The anterior clypeal margin is truncate with small medial teeth in addition to the rounded lateral teeth. I wonder if this could be a hybrid with one of the larger non-schmidti complex species like expolitum?

JTL-017 has the habitus of the schmidti complex but lacks the mandibular notch. Some specimens have a trace of a mandibular notch, as though grading into JTL-006. Unlike schmidti and JTL-006, the anteromedian clypeal margin is less protruding, and the downturned portion has a small median notch. The compound eyes are larger than the eyes of schmidti and JTL-006. Like JTL-006 it has weak to absent gastral pubescence, and the face may be completely punctatorugose or partially smooth and shiny. The mesosoma in lateral view is generally more gracile than other forms, with no propodeal tubercles. It occurs from 1000 to 2000m elevation, where it seems to be strictly arboreal, nesting under epiphytes.

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

Date of this version: 29 September 2004.
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