Stenamma

Stenamma is a cryptic “leaf-litter” ant genus that occurs in mesic forest habitats throughout the Holarctic region, Central America, and part of northwestern South America (Colombia and Ecuador). The genus was thought to be restricted primarily to the temperate zone, but recent collecting efforts have uncovered a large radiation of Neotropical forms, which rival the Holarctic species in terms of morphological and behavioral diversity. (Branstetter 2013)

Identification
Branstetter (2009) - Stenamma is closely related to the genera Aphaenogaster and Messor. Aphaenogaster and Messor show greater morphological variation than Stenamma and differ from Stenamma in characters 5, 6, 10, 11, 17, and 22 (see nomeclature section below). The most notable difference is the structure of the clypeus. In both Aphaenogaster and Messor, the posteromedial portion of the clypeus is broadly inserted between the frontal lobes and when looked at in full-face view is generally much wider than either lobe at the broadest point. Additionally, along the apical margin of the clypeus, Aphaenogaster and Messor tend to have a row of setae, which are much stouter than the setae observed in Stenamma.

Notable similarities among the genera also exist. In most species of all three genera, the frontal lobes do not completely cover the antennal insertions, allowing the torular lobes to be visible in full-face view. Aphaenogaster and Messor have an ACI that is even lower than Stenamma (53–57); however, many species have a distinct to indistinct 4-segmented antennal club. In some species the club is not distinctly broader or longer that the previous segments, but is covered with a denser layer of setae, sometimes giving these segments a noticeably lighter color. Lastly, like Stenamma, some species of Aphaenogaster and Messor have meso- and metasternal processes.

Distribution
Stenamma has been collected from North America south through Central America to northern South America (Colombia and Ecuador), Europe, northern Africa (Morocco and Algeria), temperate Asia, and Japan. No species are known to occur in the Indo-Australian region.

Species by Region
Number of species within biogeographic regions, along with the total number of species for each region.

Biology
Branstetter (2012, 2013) - Neotropical Stenamma species are found mainly in mesic forest habitats from sea level to around 3000 m elevation, with the highest record reported at 3700 m at Pico Orizaba in Mexico (S. manni Wheeler). Stenamma species have been collected mostly in lowland rainforest, montane wet forest, cloud forest, and dwarf forest. A few species (Stenamma huachucanum, Stenamma lagunum, Stenamma manni) also have been found in seasonal woodland habitats in drier areas of Mexico and the southwestern United States. Stenamma has never been collected in tropical dry forest or scrub environments. One hypothesis for the nearly complete geographic separation of the Holarctic clade and Middle American Clade is the presence of extensive xeric environments in northern Mexico and the southwestern United States.

An exceptional characteristic of Stenamma is that many species seem to be well adapted to cool, wet environments. Although present in the lowlands, the diversity and abundance of Stenamma species peak at mid-elevations between 800–1600 m (Branstetter unpublished data). Also, it has been found that Stenamma can be the most common ant genus in leaf-litter samples collected from very wet and cool, cloud forest localities. These ecological traits are in contrast to the pattern seen in ants generally, in which diversity and abundance decrease with elevation. Biogeographic results indicate that Stenamma originated in the Nearctic, potentially preadapting it to thrive in cool montane forest environments.

Most Stenamma species have very cryptic habits. Nests are usually small, and workers are slow moving, often becoming immobile upon disturbance. Consequently, Stenamma is rarely found by the casual observer and most collections are made by sifting leaf-litter from the forest floor. This fact has given Stenamma its stereotype as a “leaf-litter ant genus.” Although nests of many species do occur in the leaf litter, and foragers are common there, recent collecting efforts have revealed that Middle American Clade Stenamma species nest in a variety of microhabitats. I have found nests in large logs, in small rotting branches, in and under bark, in steep clay or mud banks, in and under epiphytes, under rocks, in the ground, and under leaves in leaf litter. At least a few Stenamma species nest arboreally. For example, several variants of Stenamma schmidti Menozzi have been found reliably underneath epiphyte clumps in the canopy and by canopy fogging, and Stenamma longinoi is known only from one collection under epiphytes in a tree-fall. Based on circumstantial evidence, I surmise that a few other species have arboreal nests or at least forage arboreally. Stenamma stictosomum and Stenamma felixi Mann have been found in epiphytic orchids inspected at quarantine in the U.S., and Stenamma callipygium has been collected most often by beating vegetation.

One of the most intriguing recent discoveries has been the observation that some Stenamma species nest in clay banks. Longino (2005) documented in detail the complex nesting behaviors of Stenamma alas and Stenamma expolitum. Both of these species nest in clay banks in wet forest habitats often along streams, and they exhibit a unique set of behaviors that are thought to be used in evading predation by other ants (possibly army ants specifically). These species construct multiple nests, but only occupy one with brood and a queen; they build each nest with a vertical (S. expolitum) or horizontal (S. alas) turret sunk into a shallow alcove; and they maintain a small clay “doorpebble,” which is used to block the nest entrance upon encounter with the appropriate disturbance. Reported here for the first time, several other Stenamma species are now known to nest in clay banks. I have found Stenamma diversum, Stenamma megamanni, Stenamma muralla, and Stenamma pelophilum all nesting in clay banks along streams or on steep clay slopes. Stenamma diversum is particularly interesting, because it has convergently evolved the same nest architecture as S. alas (Branstetter pers. obs.). It nests exclusively in steep clay substrate, and builds a nest entrance with an ear-like turret sunk into the clay. It does not, however, appear to build multiple nests per colony or maintain a door-pebble. Based on where I have collected workers and how the workers are sculptured, I believe other Stenamma species nest in clay banks as well (e.g. Stenamma llama, Stenamma lobinodus). Why does Stenamma nest in clay banks? I hypothesize that like cloud forests, the clay bank habitat is less hospitable to the average ant, and thus provides Stenamma species with a more protected and less competitive environment in which to nest and forage.

Nomenclature

 *  STENAMMA [Myrmicinae: Stenammini]
 * Stenamma Westwood, 1839: 219. Type-species: Stenamma westwoodii, by monotypy.
 * Stenamma senior synonym of Asemorhoptrum: André, 1883a: 310.
 * Stenamma senior synonym of Theryella: Santschi, 1923a: 136.
 * ASEMORHOPTRUM [junior synonym of Stenamma]
 * Asemorhoptrum Mayr, 1861: 76. Type-species: Myrmica lippula, by monotypy.
 * Asemorhoptrum junior synonym of Stenamma: André, 1883a: 310.
 * [Asemorhoptrum as junior synonym of Proatta: Snelling, R.R. 1981: 395 (in error).]
 * THERYELLA [junior synonym of Stenamma]
 * Theryella Santschi, 1921d: 68. Type-species: Theryella myops (provisional junior synonym of Stenamma punctiventre), by monotypy.
 * Theryella junior synonym of Stenamma: Santschi, 1923a: 136.
 * [Theryella junior synonym of Proatta: Snelling, R.R. 1981: 395 (error).]

Branstetter (2009):

Workers

1. Mandible triangular to elongate triangular; masticatory margin usually with 6–8 teeth or denticles (rarely 9 or 10) which decrease in size irregularly from apex to base; teeth on basal half frequently reduced and poorly defined.

2. Palp formula 4,3.

3. Apex of anterior clypeal margin with a small to prominent notch or concavity, never smoothly convex or with a projecting tooth.

4. Anterior clypeal margin usually lacking a strong isolated median seta.

5. Median portion of clypeus often longitudinally bicarinate and with area between carinae slightly to strongly depressed.

6. Posteromedial margin of clypeus narrowed and prolonged backward between frontal lobes; width not exceeding that of frontal lobes in full-face view.

7. Frontal lobes small and closely approximated, not entirely covering antennal insertions.

8. Antennal scrobes and frontal carinae absent.

9. Torular lobe present and visible in full-face view projecting over condylar bulb.

10. Compound eyes located slightly to distinctly in front of midlength of side of head (excluding mandibles), small to moderate in size, usually with 2–12 ommatidia across greatest diameter.

11. Antenna 12 segmented and terminating in a distinct to indistinct 4-segmented club (ACI 60-70).

12. Posteroventral corners of head lacking grooves.

13. Promesonotum convex in profile, often low domed-convex and very prominent; faint impression or line marking track of former promesonotal suture sometimes present dorsally.

14. Metanotal groove present.

15. Propodeum usually armed with a pair of teeth or short spines (rarely unarmed or with long spines).

16. Propodeal lobes present and prominent, rounded to quadrate in shape, never long and projecting dorsally.

17. Middle and hind tibiae lacking spurs.

18. Pretarsal claws small, simple.

19. Petiole with a long, anterior peduncle and sometimes with an anteroventral process.

20. Postpetiole with short peduncle and low node often slightly longer than broad, never distinctively broader than long.

21. Postpetiolar node always wider than petiolar node.

22. Basigastral striae often present on anterior margin of abdominal tergite 4.

23. Metasternal process present and often well developed.

Comments on worker characters

3. The structure of the clypeus varies greatly among species of Stenamma and has been useful for distinguishing species groups in western North America (Snelling 1973) and will likely be useful for distinguishing Neotropical groups. The concavity is easy to observe in all species except for those belonging to the smithi group (Stenamma chiricahua, Stenamma punctatoventre, Stenamma smithi). In this group, a median lobe projects over the clypeal margin, obscuring the concavity. The best way to observe this character in the smithi group is in a ventral view of the head.

4. Several species commonly display a short- to medium-sized median seta that is located between two longer setae. It is never stouter or longer than the two surrounding setae. This character has been observed in Stenamma brevicorne, Stenamma debile, Stenamma heathi, Stenamma owstoni, Stenamma sequoiarum, and Stenamma smithi. Additionally, as observed by Bolton (2003), the median seta can be variable among specimens within a nest series.

5. There are many exceptions to this character among Neotropical species. Stenamma alas, Stenamma diversum, and Stenamma expolitum completely lack clypeal carinae. Most other Neotropical taxa have only faint carinae and lack a strong median depression.

7. The frontal lobes are expanded laterally in S. diversum, covering the torular lobe in full-face view.

11. This character can be difficult to assess when looking at a single specimen with an indistinct club. However, Stenamma never has a distinct 2- or 3-segmented antennal club and in all observed specimens, it is possible to see a marked increase in antennal segment length between segments 8 and 9, indicating the beginning of the club. This is captured by the antennal club index (ACI) which is never more than 70, i.e., the last two antennal segments make up no more than 70% of the total length of the last four segments.

15. Only Neotropical species are completely unarmed or have long projecting spines. The former state is exhibited by S. expolitum and S. alas and the latter by S. diversum.

19.Neotropical species lack a strongly projecting anteroventral petiolar process.

22. Most Holarctic species have short basigastral striae. Neotropical species usually have carinae around the girdling constriction separating the pre- and postsclerites of the third abdominal segment, but never have striae extending further onto the tergites or sternites of the gaster.

23. This character appears to be present only in Holarctic species. Neotropical species have a small raised area, but it is never elongated into a distinct process.