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This species belongs to the Middle American clade of Stenamma (see Branstetter 2012). All conent on this page modified from Branstetter (2013) unless noted otherwise.// Distribution
Mexico (Atlantic slope) to Honduras.
A rather uncommon species known almost exclusively from Winkler and Berlese samples of sifted leaf litter, except for one collection from under a stone. Stenamma excisum has been collected from 60–2280 m elevation and occurs in a variety of wet forest environments (e.g. tropical rainforest, cloud forest, disturbed mesophyll forest, pine-oak forest, riparian wet forest). The excision in the anterior clypeal margin of most specimens is unique and suggests some sort of diet specialization different from other Stenamma species.
Worker diagnosis. Integument mostly orange-brown to brown; small-sized species (see HL, ML, PrW below); anterior clypeal margin often with a deep median excision, but sometimes reduced to a shallow median emargination; basal margin of mandible straight; face densely sculptured and mostly rugoreticulate; mesosoma densely sculptured with punctae, rugae, and/or rugoreticulae; eye small (EL 0.04–0.10, REL 8–14), subcircular, with 2–4 ommatidia at greatest diameter; median portion of clypeus angled dorsoventrally, causing it to be mostly hidden in full-face view; posterior extension of clypeus between antennal insertions very narrow (PCW 0.01–0.03), with border of antennal insertions sometimes touching anteriorly; propodeal spines present, short to long (PSL 0.09–0.20, PSI 1.8–5.4); pilosity on gastral tergites distinctly bilayered, with a layer of sparse suberect setae (varying in thickness and length), and a layer of decumbent to appressed setae.
Similar species: Stenamma saenzae.
Worker description. (25 measured) HL 0.49–0.73 (0.71), HW 0.43–0.67 (0.63), FLD 0.09–0.14 (0.14), PCW 0.01–0.03 (0.02), SL 0.38–0.61 (0.59), EL 0.04–0.10 (0.06), ACL 0.41–0.62 (0.62), ML 0.57–0.88 (0.88), PrW 0.29–0.45 (0.43), PSL 0.09–0.20 (0.20), SDL 0.04–0.08 (0.08), PL 0.22–0.34 (0.34), PH 0.13–0.19 (0.18), PW 0.10–0.16 (0.14), PPL 0.11–0.20 (0.19), PPH 0.11–0.16 (0.15), PPW 0.14– 0.22 (0.19), MFL 0.37–0.68 (0.66), MTL 0.31 –0.56 (0.55), CI 86–94, SI 79–97 (94), REL 8–14 (10), FLI 19–24 (22), PSI 1.8–5.4 (2.5), MFI 95 –119 (101), ACI1 64–72 (66), ACI2 98–111 (105).
Small-sized species; general body color mostly orange-brown to brown, with appendages orange-brown to yellow-brown, becoming lighter toward extremities; setae golden; mandible with 5–7 teeth, consisting of 3 distinct apical teeth, a basal tooth, and 1–3 smaller inner teeth/denticles, which are often worn and indistinct; basal margin of mandible straight, without a basal notch or depression; mandible mostly smooth and shiny, with scattered piligerous punctae and some basal striae; anterior clypeal margin often with a deep median excision (type population), but sometimes excision reduced to a shallow emargination; median lobe of clypeus flattened, and angled dorsoventrally, causing it to be mostly invisible in full-face view; surface of clypeus mostly smooth and shiny; posterior extension of clypeus between antennal insertions very narrow (PCW 0.01–0.03), sides subparallel, with border of antennal insertions sometimes touching anteriorly; frontal lobes of moderate width (FLD 0.09–0.14, FLI 19–24), not greatly obscuring torular lobes in full-face view; head appearing subrectangular to oval-shaped (CI 86–94), posterior margin flat to slightly depressed medially; eye small (EL 0.04–0.10, REL 8–14), subcircular, with 2–4 ommatidia at greatest diameter; face densely sculptured, usually mostly rugoreticulate and punctate, with some longitudinal rugae along midline (type population), but sometimes mostly rugose; scape of short to moderate length (SI 79–97), usually not reaching posterior margin of head; scape surface mostly smooth, with scattered piligerous punctae; flagellum with a distinct to very distinct 4-segmented antennal club; mesosoma densely sculptured, dorsum of promesonotum ranging from rugoreticulate (type population), to rugose-punctate, to rugose, to mostly punctate, with rugae longitudinal in orientation; side of pronotum usually punctate, with a few rugulae or rugoreticulae (type population), but sometimes mostly rugose, and with a small patch of smooth cuticle; mesopleuron and side of propodeum mostly punctate, with a variable number of rugulae; dorsum and declivity of propodeum with a few transverse carinulae; promesonotum in profile low-domed and roughly symmetrical; metanotal groove usually well-demarcated and somewhat deep; anterodorsal margin of propodeum often raised into a small to large welt, but sometimes average; propodeal spines present, short to long (PSL 0.09–0.20, PSI 1.8–5.4); petiole appearing moderate to slightly elongate (PL/HW 0.45–0.56), usually with a distinct node, but sometimes node less distinct, making petiole look more wedge-shaped; when distinct, petiolar node in profile, average to slightly enlarged (PH/PL 0.48–0.64), and roughly symmetrical, dorsum of node usually gently rounded and pointing vertically (type population), but sometimes broadly rounded, or nearly angulate and pointing slightly posteriad; postpetiole in profile usually subspherical and appearing similar in size to petiolar node (PPH/PH 0.79–0.96), postpetiole in dorsal view often distinctly wider than petiole, sometimes much wider (PPW/PW 0.55–0.83); petiole and postpetiole usually mostly punctate, with only the anterior faces of nodes smooth, but sometimes nodes completely smooth, or completely punctate; gaster mostly smooth and shiny, with scattered piligerous punctae; most of body dorsum with standing pilosity; pilosity on gastral tergites distinctly bilayered, with a layer of sparse suberect setae (varying in thickness and length), and a layer of decumbent to appressed setae; setae on scapes and legs mostly decumbent to appressed, with some longer suberect setae on femoral venters and coxae.
Queen description. (5 measured) HL 0.55–0.73 (0.73), HW 0.50–0.69 (0.69), FLD 0.11–0.16 (0.16), PCW 0.01–0.02 (0.02), SL 0.42–0.63 (0.63), EL 0.13–0.16 (0.16), ACL 0.43–0.62 (0.62), ML 0.73–1.01 (1.01), PrW 0.40–0.57 (0.57), PSL 0.12–0.22 (0.22), SDL 0.07–0.10 (0.10), PL 0.27–0.38 (0.38), PH 0.15–0.20 (0.20), PW 0.13–0.17 (0.17), PPL 0.14–0.20 (0.20), PPH 0.15–0.19 (0.19), PPW 0.17– 0.23 (0.23), MFL 0.43–0.69 (0.69), MTL 0.37–0.58 (0.58), CI 89–93 (93), SI 81–92 (92), REL 22–27 (23), FLI 22–24 (24), PSI 1.6–2.2 (2.2), MFI 99–118 (99), ACI1 63–74 (63), ACI2 97–107 (98).
Same as worker except for standard queen modifications and as follows (comparison with worker and queen from type population only): pronotum rugoreticulate laterad, and punctate mesad; mesoscutum and scutellum rugoreticulate to foveate; propodeum with transverse carinulae that wrap around surface; katepisternum mostly smooth; petiole more elongate.
This taxon was formerly identified with the morphospecies codes mgb16, mgb37, and mgb65.
Although quite variable across its range (see below), S. excisum is separable from similar species using the characters in the diagnosis above. Several species are superficially similar to S. excisum, but no other species has the same unique clypeal structure. Phylogenetic results indicate that S. excisum is sister to S. lagunum and that this clade is sister to the remainder of Middle American clade Stenamma species (Branstetter 2012, unpublished data).
Stenamma excisum forms a difficult species complex composed of several allopatric variants, which probably constitute distinct species. The type form (CASENT0621834) is known only from two sites in Honduras, where it is unusually abundant in leaf litter samples. The most important features of this form are the rugoreticulate sculpture on the pronotal dorsum and the nearly triangular excision in the anterior clypeal margin. Other characteristics specific to this form are indicated in the species description given in Branstetter (2013). Specimens from the populations in Honduras and the next closest population are quite divergent from one another. Furthermore, the type form seems to be confined to lower elevations, whereas the other variants usually occur at mid elevations. For these reasons, it is very likely that there are multiple species within S. excisum as it is defined here. However, too few collections have been made in intervening areas to adequately assess variation and species boundaries.
Variant 1 (CASENT0605563; previously assigned the code mgb16) is known mainly from mid-elevation sites along the wet eastern slopes of the mountains in Oaxaca, Mexico, but a single specimen is also known from Nahá in Chiapas, Mexico. This variant is very similar to the type population, but differs as follows: smaller overall body size; body color darker, mostly dark orange-brown; sculpture on pronotal dorsum longitudinally rugose, without reticulae.
Variant 2 (CASENT0605441; previously assigned the code mgb37) occurs in the same area as variant 1, but the two forms have not been collected together. It is similar to variant 1 except for the following: anterior clypeal margin with a shallow median emargination, lacking a deep excision; propodeal spines in profile longer and somewhat sinuous; anterior margin of propodeal dorsum with a more distinct welt; postpetiole in dorsal view much wider than petiolar node (PW/PPW 0.55–0.61).
Variant 3 (CASENT0605500) is known from a single aberrant specimen collected near variant 2, but from a slightly higher elevation. It differs from variant 2 as follows: anterior clypeal margin intermediate between variant 1 and 2, with a shallow excision; propodeal spines short, almost tuberculate; antennal segments 11 and 12 noticeably bulging; postpetiolar node mostly smooth; postpetiole in dorsal view not greatly ex- panded; upper layer of setae on gastral dorsum shorter, sparser, and subdecumbent.
Variant 4 (CASENT0605734) is found mainly in northern Mexico in Tamaulipas (El Cielo) and Nuevo León (Monterrey de Chipinque) states. It is a very small version of S. excisum and is quite divergent from the type population. Branstetter (2013) originally intended to describe it as a separate species, but later found intermediate-looking specimens from allopatric populations in Querétaro and Veracruz, Mexico. It differs from the type population as follows: overall body size much smaller; sculpture reduced; face with only light rugoreticulae, punctae, and carinulae; dorsum of pronotum with sparse longitudinal carinulae, interstices shiny; propodeal spines short; decumbent layer of pilosity on gastral dorsum sparse, suberect layer slightly thickened.
Variant 5 (CASENT0126237) occurs in Veracruz, Mexico. It is intermediate between the type population and variant 4. It differs from variant 4 as follows: head and body densely punctate; dorsum of pronotum with longitudinal rugulae/carinulae, but interstices punctate; petiolar node reaching a sharper apex, which points distinctly posteriad; suberect layer of pilosity on gastral dorsum (and most of body dorsum) longer.
Variant 6 (CASENT0126243) is known from two localities in Veracruz, Mexico (Paraje Nuevo, Tetzonapa). It is similar to variant 5, but differs as follows: anterior clypeal margin with a median emargination, lacking an excision; body larger and with dis- tinctly elongate appendages; decumbent layer of gastral pilosity very sparse.
Variant 7 (CASENT0622434) is known from a single locality in Nuevo León, Mexico (38 km SSW Monterrey). It is similar to variant 4, but differs as follows: anterior clypeal margin with a median emargination, lacking a deep excision; larger overall size; head larger, more robust, with a distinct median depression in posterior margin; petiolar node in profile broadly domed and more asymmetrical, with a longer more sloping anterior face. This is likely a new species, but since it is known from only one collection, and the excisum complex shows so much variability, the most prudent course of action is to lump it in S. excisum until more data are available.
The specific epithet excisum refers to the distinctive excised clypeus found in the type population of this species.
Type material. Holotype worker. HONDURAS, Atlántida: 12km SW La Ceiba, 15.69150°N, 86.86151°W ±20m, 280m, 19 Jun 2010, tropical rainforest, ex sifted leaf litter (LLAMA, collection Wa-C-09-2-44) [USNM, specimen CASENT0621834]. Paratypes: same data as holotype but 15.69449°N, 86.86330°W ±20m, 200m (Wa-C-09-1-27) [1dq, 1w, CAS, CASENT0621799, CASENT0621801], [1w, EAPZ, CASENT0621800], [1w, FMNH, CASENT0623304], [1w, IN- Bio, CASENT0623305], [1w, LACM, CASENT0623306]; 15.69175°N, 86.86091°W ±20m (LLAMA, Wa-C-09-2-04) [1dq, 1w, MCZ, CASENT0623307, CASENT0623309], [1dq, 1w, USNM, CASENT0623308, CASENT0623310]; 15.69449°N,86.86344°W ±20m (LLAMA, Wa-C-09-1-24) [1w, UCD, CASENT0621793]; 15.69134°N, 86.86137°W ±20m, 280m, 19 Jun 2010 (LLAMA, Wa-C-09-2-40) [1dq, 1w, MGBPC, CASENT0623541, CASENT0623540].
Branstetter, M. G. 2012. Origin and diversification of the cryptic ant genus Stenamma Westwood (Hymenoptera: Formicidae), inferred from multilocus molecular data, biogeography and natural history. Systematic Entomology 37:478-496. 10.1111/j.1365-3113.2012.00624.x.
Branstetter, M. G. 2013. Revision of the Middle American clade of the ant genus Stenamma Westwood (Hymenoptera, Formicidae, Myrmicinae). ZooKeys 295:1-277. doi: 10.3897/zookeys.295.4905.
Found most commonly in these habitats: 13 times found in tropical rainforest, 12 times found in cloud forest, 1 times found in forest, 2 times found in montane tropical rainforest, 2 times found in tropical evergreen forest, 1 times found in disturbed mesophyll forest, 1 times found in mesophyl forest, 1 times found in pine-oak forest, 2 times found in riparian wet forest, 2 times found in lowland rainforest, ...
Found most commonly in these microhabitats: 37 times ex sifted leaf litter, 1 times ex sifted soil and leaf litter, 1 times leaf litter, 1 times ex sifted litter, 1 times under stone.
Collected most commonly using these methods: 19 times MiniWinkler, 6 times Berlese, 11 times maxiWinkler, 3 times Winkler, 1 times search.
Elevations: collected from 60 - 2280 meters, 903 meters average
Collect Date Range: collected between 1953-08-01 and 2019-06-12
Type specimens: Holotype Stenamma excisum: casent0621834