Stenamma excisum

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.

Identification
Branstetter (2013) - 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.

Quite variable across its range. Separable from similar species as none has the same unique clypeal structure. Phylogenetic results indicate that S. excisum is sister to Stenamma lagunum and that this clade is sister to the remainder of Middle American clade Stenamma species (Branstetter 2012, unpublished data).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. 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.

Distribution
Atlantic slope of Mexico to Honduras.

Distribution based on Regional Taxon Lists
Neotropical Region: Honduras, Mexico.

Castes
Males have yet to be collected.

Nomenclature

 *  excisum. Stenamma excisum Branstetter, 2013: 97, figs. 77-81 (w.q.) HONDURAS.

Worker
(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
(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.

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, CASC, CASENT0621799, CASENT0621801, 1w, EAPZ, CASENT0621800, 1w, FMNH, CASENT0623304, 1w, INBC, 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, UCDC, CASENT0621793; 15.69134°N, 86.86137°W ±20m, 280m, 19 Jun 2010 (LLAMA, Wa-C-09-2-40) 1dq, 1w, MGBPC, CASENT0623541, CASENT0623540.

Etymology
The specific epithet excisum refers to the distinctive excised clypeus found in the type population of this species.

References based on Global Ant Biodiversity Informatics

 * Branstetter M.G. 2013. Revision of the Middle American clade of the ant genus Stenamma Westwood (Hymenoptera, Formicidae, Myrmicinae). ZooKeys 295: 1277
 * Dattilo W. et al. 2019. MEXICO ANTS: incidence and abundance along the Nearctic-Neotropical interface. Ecology https://doi.org/10.1002/ecy.2944
 * Longino J. T. L., and M. G. Branstetter. 2018. The truncated bell: an enigmatic but pervasive elevational diversity pattern in Middle American ants. Ecography 41: 1-12.