Stenamma impar Forel

Stenamma impar, full face view of worker (click photo to enlarge).
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Stenamma impar, side view of worker (click photo to enlarge).
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Stenamma impar, dorsal view of worker (click photo to enlarge).
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Stenamma impar, full face view of worker (MS specimen) (click photo to enlarge).
Stenamma impar, profile view of a worker (MS specimen) (click photo to enlarge).

Stenamma is a small genus of primitive ants that is mostly Holarctic in distribution (some species in Central America and the Oriental region). They typically live in forested areas. Foraging is usually restricted to the cool times of the year. Colonies are relatively small, cryptic, and often difficult to find, and therefore, nests are not frequently seen. However, foraging workers can be commonly collected by litter extraction, pitfall traps, or other methods.

Generic characteristics are as follows: median area of clypeus abruptly raised and somewhat narrowed, with a pair of fine longitudinal carinae that diverge anteriorly, and the area between these carinae concave to flat; propodeum with a pair of teeth; masticatory margin of mandible has 5 or more teeth or denticles; and antenna terminating in a four segmented club. Six described species and two apparently undescribed species (Ipser et al. 2004) are known to occur in the Southeast. Species level identification is difficult due variation in size, color, and sculpture, and reference material is helpful.

Stenamma impar is a relatively small, slender, light yellowish brown species with somewhat weak sculpture and shining surface texture. Workers range from 2.3 - 3.5 mm in total length (TL) and have small eyes that are approximately 0.1 mm in length with 5-6 coarse ommatidia (in greatest diameter of eye) [measurements from Smith (1957) and from specimens]. This species is most similar to S. diecki, from which it differs by being more slender, light brown (instead of dark brown), having slightly larger eyes, less pronounced rugose sculpture on the pronotum, and having sculpture on the dorsum of the postpetiole.

Although Stenamma impar has been collected in Mississippi and Alabama for several years now, it has been reported as S. meridionale. These records were based on misidentifications, and after re-examination of those specimens, it is now thought that they are actually S. impar. Specimens from this region tend to be larger than typical S. impar, but otherwise appear to agree with the description of that species.

Literature Cited

Ipser, R. M., M. A. Brinkman, W. A. Gardner, and H. B. Peeler. 2004. A survey of the ground-dwelling ants (Hymenoptera: Formicidae) in Georgia. Florida Entomologist 87 (3): 253-260.

Smith, M. R. 1957.  Revision of the genus Stenamma Westwood in America north of Mexico (Hymenoptera: Formicidae). The American Midland Naturalist 57:  133-174.


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