Stenamma

Branstetter, M. G., 2009, The ant genus Stenamma Westwood (Hymenoptera: Formicidae) redefined, with a description of a new genus Propodilobus., Zootaxa 2221, pp. 41-57 : 42-46

publication ID	22826
DOI	https://doi.org/10.5281/zenodo.6217434
persistent identifier	https://treatment.plazi.org/id/91D76131-FEC1-B915-B2E6-B0ECC435865B
treatment provided by	Christiana
scientific name	Stenamma
status

Treatment

Stenamma View in CoL HNS Westwood

Stenamma HNS Westwood, 1839: 219. Type-species: Stenamma westwoodii HNS , by monotypy.

Asemorhoptrum Mayr, 1861: 76. Type-species: Myrmica lippula HNS , by monotypy. [Synonymy with Stenamma HNS by Andre, 1883: 310.]

Theryella Santschi HNS , 1921: 68. Type-species: Theryella myops HNS (provisional junior synonym of Stenamma punctiventre HNS ), by monotypy. [Synonymy with Stenamma HNS by Santschi, 1923: 136.]

Diagnosis of Stenamma HNS . A new diagnosis of the Stenamma HNS worker caste is presented followed by a short discussion on interspecific variation and how to distinguish Stenamma HNS from closely related genera. The classification of Bolton (2003) placed Stenamma HNS within the tribe Stenammini HNS , which, as currently defined, is not monophyletic (Brady et al. 2006; Moreau et al. 2006). Stenamma HNS instead is more closely related to the genera Aphaenogaster HNS Mayr and Messor HNS Forel, suggesting that the older tribal classification of Emery (1921), which included these genera together, should be reconsidered. Some of the characters mentioned here may help to diagnose the group formed by Stenamma HNS , Aphaenogaster HNS , and Messor HNS . Standard images of Stenamma HNS representing species from different biogeographic regions are shown in Figures 2-16.

Diagnosis of the Stenamma HNS worker caste. With characters of the Myrmicinae HNS as described by Bolton (2003), and the following more specific features:

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 HNS and has been useful for distinguishing species groups in western North America (Snelling 1973) and will likely be useful for distinguishing Neotropical groups (pers. obs.; Figures 2, 5, 8, 11, 14, 17-20). The concavity is easy to observe in all species except for those belonging to the smithi HNS group ( S. chiricahua, HNS S. punctatoventre HNS , S. smithi HNS ). In this group, a median lobe projects over the clypeal margin, obscuring the concavity. The best way to observe this character in the smithi HNS group is in a ventral view of the head (as in Figure 20).

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 S. brevicorne, HNS S. debile HNS , S. heathi HNS , S. owstoni HNS , S. sequoiarum HNS , and S. smithi HNS . 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 HNS . alas, S. diversum HNS , and S. expolitum HNS completely lack clypeal carinae (Figures 11, 14). Most other Neotropical taxa have only faint carinae and lack a strong median depression.

7. The frontal lobes are expanded laterally in S. diversum HNS , covering the torular lobe in full-face view (Figure 14).

11. This character can be difficult to assess when looking at a single specimen with an indistinct club. However, Stenamma HNS 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 HNS (Figure 12) and S. alas and the latter by S. diversum HNS (Figure 15).

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 (compare Figures 22 and 24).

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 (compare Figures 21 and 23).

Comments on similarities and differences among Stenamma HNS , Aphaenogaster HNS , and Messor HNS Aphaenogaster HNS and Messor HNS show greater morphological variation than Stenamma HNS and differ from Stenamma HNS in characters 5, 6, 10, 11, 17, and 22. The most notable difference is the structure of the clypeus. In both Aphaenogaster HNS and Messor HNS , 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 HNS and Messor HNS tend to have a row of setae, which are much stouter than the setae observed in Stenamma HNS .

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 HNS and Messor HNS have an ACI that is even lower than Stenamma HNS (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 HNS , some species of Aphaenogaster HNS and Messor HNS have meso- and metasternal processes.