Stenamma

Stenamma Westwood

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

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

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

Diagnosis of Stenamma. A new diagnosis of the Stenamma worker caste is presented followed by a short discussion on interspecific variation and how to distinguish Stenamma from closely related genera. The classification of Bolton (2003) placed Stenamma within the tribe Stenammini, which, as currently defined, is not monophyletic (Brady et al. 2006; Moreau et al. 2006). Stenamma instead is more closely related to the genera Aphaenogaster Mayr and Messor 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, Aphaenogaster, and Messor. Standard images of Stenamma representing species from different biogeographic regions are shown in Figures 2-16.

Diagnosis of the Stenamma worker caste. With characters of the Myrmicinae 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 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 group (S. chiricahua, S. punctatoventre, S. 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 (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, S. debile, S. heathi, S. owstoni, S. sequoiarum, and S. 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, S. diversum, and S. expolitum 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, 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 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 (Figure 12) and S. alas and the latter by S. diversum (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, 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. 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.

Synonymic list of species

Assignment of species to species groups follows Snelling (1973) for Nearctic species and DuBois (1998) for Palearctic species.

Nearctic Species

brevicorne group

brevicorne (Mayr, 1886)

= neoarcticum Mayr, 1886

chiricahua Snelling, 1973

punctatoventre Snelling, 1973

smithi Cole, 1966

= knowltoni Gregg, 1972

diecki group

diecki Emery, 1895

= diecki impressum Buren, 1944

snellingi Bolton, 1995

= occidentale Smith, 1957 (homonym)

sequoiarum Wheeler, 1917

californicum Snelling, 1973

dyscheres Snelling, 1973

heathi group

heathi Wheeler, 1915

exasperatum Snelling, 1973

huachucanum group

huachucanum Smith, 1957

wheelerorum group

wheelerorum Snelling, 1973

Currently unassigned

schmittii Wheeler, 1903

impar Forel, 1901

meridionale Smith, 1957

foveolocephalum Smith, 1930

= carolinense Smith, 1951

Neotropical species

Currently unassigned

alas Longino, 2005

expolitum Smith, 1962

diversum Mann, 1922

felixi Mann, 1922

manni Wheeler, 1941

schmidti Menozzi, 1931

Palearctic Species

owstoni group

kurilense Arnol'di, 1975

nipponense Yasumatsu & Murakami, 1960

ussuriense Arnol'di, 1975

gurkhalis DuBois, 1998

koreanense Lyu, DuBois, & Cho, 2002

owstoni Wheeler, 1906

punctiventre group

punctiventre Emery, 1908

= myops Santschi, 1921

westwoodii group

debile (Foerster, 1850)

= minkii (Foerster, 1850)

= westwoodii polonicum Begdon, 1932

= golosojevi Karavaiev, 1926

= ucrainicum Arnol'di, 1928

georgii Arnol'di, 1975

hissarianum Arnol'di, 1975

kashmirense Urbani, 1977

jeriorum DuBois, 1998

lippulum (Nylander, 1849)

= hirtulum Emery, 1898

= caucasicum Arnol'di, 1975

msilanum Forel, 1901

= africanum Santschi, 1939

= africanum submuticum Santschi, 1939

petiolatum Emery, 1897

picetojuglandeti Arnol'di, 1975

sardoum Emery, 1915

sogdianum Arnol'di, 1975

striatulum Emery, 1895

= tscherkessicum Arnol'di, 1928

westwoodii Westwood, 1839

orousseti Casevitz-weulersse, 1990

Species incertae sedis

berendti (Mayr, 1868) [fossil]

westwoodii asiaticum Ruzsky, 1905