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.
Branstetter (2013): Nicaragua to Ecuador.
J. Longino (Ants of Costa Rica): Costa Rica: Cordillera de Tilaran, C. Volcanica Central, and C. de Talamanca, from 1000-2200m elevation.
Stenamma schmidti, as defined here, is a rather variable species. It inhabits tropical wet forest environments from sea level to about 2400 m, becoming most abundant in cloud forest habitats above 800 m. At some cloud forest sites, S. schmidti can be one of the most common ant species occupying the leaf litter. Despite this fact, the species is very cryptic and finding nests is an uncommon event. Most collections of S. schmidti are from Winkler or Berlese samples of sifted leaf litter or epiphyte mats. Nest collections have been made, but these are very rare for the leaf-litter dwelling variants and only slightly more common for the arboreal forms. Nests are very small, with only tens of workers, and a single egg-laying queen. Workers, when encountered, are very slow moving and freeze upon disturbance. Additional natural history notes specific to particular morphological variants are described below.
J. Longino (Ants of Costa Rica):
This species is a cloud forest specialist. Workers and queens occur in nearly every Winkler sample from montane habitats, in the 1000-2000m zone. Although very common in Winkler samples, I have very rarely found nests. I did once find a colony in leaf litter at the top of the upturned disk of roots and soil at the base of a fallen tree. The workers curled up and froze upon disturbance. The white brood pile gave them away, however.
Worker diagnosis. Note that this species is highly variable. See comments section below discussing morphological variants. Integument mostly dark brown, red-brown, or brown; small- to medium-sized species; petiolar node in profile usually broadly rounded and distinctly angled posteriad; postpetiole in profile subspherical; propodeal spines absent to tuberculate (PSL 0.06–0.13, PSI 0.8–1.5); basal margin of mandible usually with a distinct basal notch and small accompanying tooth, but sometimes with only a small notch, or with basal margin sinuous. If basal margin of mandible with notch and tooth then: anterior clypeal margin forming 2–4 sharp to blunt teeth, with outer teeth more projecting; eye of moderate to large size (EL 0.10–0.18, REL 19–29), with 6–10 ommatidia at greatest diameter. If basal margin with small notch, but no tooth then: face completely sculptured, densely rugoreticulate; mesosoma mostly sculptured, punctate-rugulose; pilosity on first gastral tergite sparse, mostly stout and suberect, with only a few decumbent setae. If basal margin of mandible sinuous (without notch and tooth) then: anterior clypeal margin with a simple median emargination; propodeal spines absent, reduced to blunt angles where propodeal dorsum and declivity meet; eye large (EL 0.15–0.18, REL 22–27), with 8–11 ommatidia at greatest diameter; face usually completely sculptured, but sculpture never very dense, mostly carinulate-punctate; carinulae usually longitudinal, but some specimens with transverse carinulae on anterior half of head; pronotum either completely carinulate-punctate, lightly punctate, or completely smooth; carinulae when present usually transverse in orientation; some specimens noticeably long and gracile, with scape, metafemur, and petiole relatively long (SI 107–121, 86–93; PL/HW 0.58–0.63); gastral pilosity mostly sparse and suberect, with only a few decumbent to appressed setae; erect setae often stout.
Worker description. (63 measured) HL 0.55–0.93 (0.66), HW 0.48–0.80 (0.57), FLD 0.11–0.25 (0.16), PCW 0.02–0.06 (0.04), SL 0.46–0.82 (0.55), EL 0.09–0.18 (0.12), ACL 0.47–0.75 (0.55), ML 0.68–1.21 (0.84), PrW 0.34–0.56 (0.40), PSL 0.06–0.13 (0.09), SDL 0.06–0.11 (0.09). PL 0.25–0.45 (0.31), PH 0.16–0.28 (0.19), PW 0.13–0.21 (0.15), PPL 0.13–0.27 (0.17), PPH 0.14–0.26 (0.18), PPW 0.16–0.28 (0.20), MFL 0.50–0.98 (0.59), MTL 0.41–0.76 (0.45), CI 79–94 (86), SI 85–121 (97), REL 18–29 (21), FLI 21–33 (29), PSI 0.8–1.5 (1.1), MFI 71–108 (96), ACI1 62–68 (66), ACI2 86–105 (100).
Small- to medium-sized species; general body color dark brown, to red-brown, to brown, with appendages brown to orange-brown, lighter at joints and toward extremities; setae golden; mandible with 5–7 teeth (usually 6), with basal tooth often appearing bidentate, inner teeth sometimes worn and indistinct; basal margin of mandible usually sinuous, with a distinct basal notch and accompanying small tooth (type population), but sometimes with a basal notch and no tooth, or only sinuous; mandible mostly smooth and shiny, with scattered piligerous punctae and a variable number of basal and lateral striae; anterior clypeal margin usually forming 2–4 sharp to blunt teeth (type population), but sometimes nearly flat, or with a simple median emargination; median clypeal lobe usually rather distinct, and produced slightly over anterior clypeal margin in full-face view (type population), but sometimes obliquely flattened and not produced; dorsal surface of lobe mostly smooth and shiny (type population), or with a variable number or irregular carinulae, apex of lobe with a short to long transverse carina; area in between carina and anterior clypeal margin, usually forming a distinct cavity where mandibles insert (type population); remainder of clypeal surface mostly smooth; posterior extension of clypeus between antennal insertions of narrow to moderate width (PCW 0.02–0.06; type population), with sides subparallel; frontal lobes moderate (type population) to slightly expanded (FLD 0.11–0.25; FLI 21–33), not greatly covering torular lobes in full-face view; head usually roughly oval-shaped (type population) to subcircular, with a distinct median depression in posterior margin, but head sometimes more elongate, with posterior margin flat (CI 79–94); eye of moderate to large size, sometimes relatively very large (EL 0.09–0.18, REL 18–29), oval-shaped, often bulging, with 5–11 ommatidia at greatest diameter; face sculpture highly variable, ranging from mostly smooth and shiny, to densely rugoreticulate (type population with fine longitudinal carinulae on middle of head and on gena, posterior 1/5 of head smooth); scape ranging from relatively short and somewhat thick to long and slender (SI 85– 121), not surpassing to distinctly passing posterior margin when laid back (moderate length in type population, just reaching posterior margin); scape surface with numerous piligerous punctae, but mostly shiny; flagellum with a distinct (type population) to very distinct 4-segmented antennal club; mesosomal sculpture highly variable, ranging from mostly smooth, to densely rugose-rugoreticulate (type population with promesonotum mostly smooth and shiny, at most a few longitudinal carinulae dorsally); mesopleuron and side of propodeum mostly smooth, with some faint punctae and carinulae, propodeal dorsum and declivity with light transverse carinulae; promesonotum in profile usually low-domed and roughly symmetrical; metanotal groove variable, usually well-demarcated and of moderate width and depth (type population), but sometimes deeper and better defined, with metanotum forming a small welt, or sometimes shallow and indistinct, with propodeum connecting almost continuously to promesonotum; propodeal spines absent to tuberculate (PSL 0.06–0.13, PSI 0.8–1.5; forming a sharp angle in type population); petiole in profile appearing of moderate length (type population) to somewhat elongate (PL/ML 0.48–0.63); petiolar node usually broadly rounded and pointing distinctly posteriad (type population), but sometimes appearing subquadrate and asymmetrical, with an apex occurring at anterior margin of dorsum; petiolar node similar in size to petiolar node and subspherical; petiole and postpetiole usually mostly smooth and shiny, with only a few faint punctae (type population), but sometimes mostly punctate, rugulae sometimes present on posterior half of postpetiolar node; gaster mostly smooth, with scattered piligerous punctae; most of body dorsum with standing pilosity; gastral pilosity highly variable, sometimes distinctly bilayered, with a sparse layer of stout suberect setae, and a dense layer of short, decumbent pubescence (type population), sometimes pubescence absent, leaving only stout, suberect setae and a few decumbent setae, or sometimes all setae of moderate thickness and with variable density and layering (suberect layer usually always present); setae on scape usually relatively dense, and uniformly subdecumbent to appressed; setae on legs decumbent to appressed, with some longer suberect setae on femoral venters and coxae.
Queen description. (16 measured) HL 0.59–0.89 (0.81), HW 0.53–0.76 (0.74), FLD 0.13–0.25 (0.22), PCW 0.03–0.07 (0.06), SL 0.48–0.81 (0.65), EL 0.16–0.25 (0.21), ACL 0.49–0.73 (0.65), ML 0.79–1.30 (1.17), PrW 0.45–0.68 (0.60), PSL 0.08–0.15 (0.12), SDL 0.08–0.13 (0.12), PL 0.30–0.47 (0.44), PH 0.18–0.26 (0.24), PW 0.16–0.23 (0.20), PPL 0.15–0.26 (0.24), PPH 0.16–0.24 (0.23), PPW 0.19–0.28 (0.26), MFL 0.53–0.93 (0.77), MTL 0.43–0.71 (0.60), CI 83–94 (91), SI 84–111 (88), REL 26–34 (29), FLI 24–34 (30), PSI 1.0–1.4 (1.0), MFI 78–103 (96), ACI1 63–67 (63), ACI2 90–106 (99).
Same as worker except for standard queen modifications and as follows (only type population queen considered): pronotum with some transverse rugulae laterad; mesoscutum lightly punctate to foveolate, with a central line of smooth cuticle; scutellum with longitudinal carinulae laterad, and a central smooth patch; propodeum with transverse carinulae that wrap around entire surface; mesopleuron mostly smooth; pilosity on mesoscutum bilayered similar to gaster, with a layer of longer erect to suberect setae, and a layer of short, dense pubescence; wing venation as in specimen INBIOCRI002281270.
Male. See specimen INBIOCRI002281270.
The Stenamma schmidti complex is as remarkable as it is maddening, comprising an amazing radiation of forms, which together occupy more morphospace than all of the Holarctic clade Stenamma species combined. Because many putative morphospecies appear to occur in sympatry, it is tempting to separate S. schmidti into multiple species. But after careful review of many specimens and populations, accompanied by molecular phylogenetic data, I find it very difficult to divide up the complex in a satisfactory way. A major reason for this is the existence of specimens with intermediate phenotypes, forming what appears to be a morphological continuum. Consequently, I have decided to delimit a single polytypic species composed of many variants (described below). Several of these variants may constitute good species, but I believe more genetic and distribution data are needed (especially from Panama and South America) to adequately assess the distinctness of all forms in the complex.
The difficulty in resolving species boundaries in the complex probably stems from several factors, including phenotypic plasticity, morphological convergence, hybridization, and sampling bias. It could also be that if the group represents a recent radiation, there has been insufficient time for some species to reach monophyly. Based on my observations so far, I favor the idea that there are several weakly differentiated forms that occasionally come into contact and hybridize.
An interesting observation about diversification within the schmidti complex is that it seems particularly prone to convergence, with specimens from a particular elevation and microhabitat having similar morphological characteristics. For example, S. nanozoi and variants 7 and 8 of S. schmidti are all small, have densely sculptured faces, and have the gastral pilosity forming a single sparse layer of stout, suberect setae. All of these taxa occur at low elevation from sea level to approximately 1000 m. Counter to expectations, phylogenetic data show that they are not closely related to one another, with S. nanozoi sister to S. sandinista, and variants 6 and 7 of S. schmidti both more closely related to other high-elevation variants (Branstetter unpublished data). This suggests that the characteristics one would probably use to unite these taxa as a single species are convergent, and thus misleading.
By combining many divergent forms into a single species, it has made characterizing S. schmidti rather difficult due to its large phenotypic range. Most specimens that closely match one of the variants below should be identifiable as S. schmidti, but specimens with intermediate phenotypes will probably not key out easily.
It will also be hard to adequately separate S. schmidti from the closely related S. nanozoi and S. sandinista, as well as from the similar looking S. saenzae, based only on morphology. The characters in the key and diagnosis will work for most specimens, but not all. Fortunately, geography should clarify matters, as most of the schmidti complex occurs farther south than the other species. Only S. sandinista is found in sympatry with variant 6 of S. schmidti. These two species both occur at Cerro Musún in Nicaragua, where they are separated by elevation, with S. schmidti occurring below about 800 m and S. sandinista occurring above 900 m.
Defining features of the type population's morphology (CASENT0126701) are indicated in the species description or Branstetter (2013), but main characteristics are as follows: basal margin of mandible with distinct notch and small accompanying tooth; anterior clypeal margin forming 2–4 teeth; median lobe of clypeus produced slightly over anterior clypeal margin in full-face view; head roughly oval-shaped to subcircular (CI 85–93); scape of moderate length (SI 85–97); eye large (EL 0.12– 0.16, REL 19–23), with 6–9 ommatidia at greatest diameter; face sculpture mostly lightly carinulate, with posterior half or less of head usually smooth; mesosoma mostly smooth, with some faint carinulae/rugulae variably present; gastral pilosity distinctly bilayered, with a sparse layer of suberect setae, and a dense layer of decumbent pubescence. The lectotype appears to be a small, less robust specimen of S. schmidti, with much of the face and mesosoma smooth. A paralectotype specimen I observed from the same series as the holotype is slightly larger and has more developed sculpture, with the face carinulae extending close to the posterior margin of the head. What I consider to be the holotype form of S. schmidti is a high-elevation morphospecies occurring in leaf litter from 1500 to over 2000 m. I have observed similar specimens from multiple sites in Costa Rica and in Panama. Specimens from the Cordillera de Talamanca in southern Costa Rica (e.g. Las Alturas, Altamira, Pittier) and from northern Panama (e.g. Cerro Punta, Hartman Finca) usually have more sculpture on the face and mesosoma. Molecular phylogenetic data show some clustering of specimens from Central Costa Rica, but specimens from southern Costa Rica and Panama are scattered throughout the tree. Nests of this form are almost unknown. Longino (pers. comm.) has reported finding a nest in the root disc of a fallen tree.
Variant 1 (INB0003659427) is a densely sculptured version of the type form. It varies as follows: face completely sculptured, with dense costulae in middle changing to rugoreticulae toward sides; pronotum with dense longitudinal costulae/rugulae, sometimes with a patch of smooth cuticle on side and middle of dorsum; mesonotum rugoreticulate; katepisternum mostly smooth; side of propodeum rugose; dorsum and declivity of propodeum transversely carinulate; petiole and postpetiole mostly punctate. Variant 1 occurs at multiple sites in Costa Rica and northern Panama and seems to occur only at mid-elevations from 800–1500 m. It is known only from samples of sifted leaf litter.
Variant 2 (CASENT0600256) is a more robust version of variant 1. It differs as follows: body color darker; sculpture deeper, more developed; petiolar node thicker, more robust; propodeal lobes and spines in profile view noticeably broader; frontal lobes somewhat expanded, almost completely covering torular lobes in full-face view; layer of pubescent setae on gaster absent, with only a few short decumbent setae present under stout, suberect setae. This variant is known from sites in southern Costa Rica and northern Panama and occurs at mid-elevations between 1000–1500 m. It is known only from samples of sifted leaf litter.
Variant 3 (INB0003662571) is similar to variant 2, except as follows: sculpture on pronotal dorsum distinctly transverse and arcuate; sculpture on side of propodeum more dense and oriented so that it wraps up and over the dorsal surface; promesonotum in profile view slightly more robust, and more distinctly separated from metanotal groove. Variant 3 occurs at multiple sites in Costa Rica at mid-elevations between 1000–1500 m. Results from molecular phylogenetic data show that specimens of variant 2 and 3 from several sites in Costa Rica form a clade nested inside the schmidti complex. This might be evidence that these two variants represent a single good species. However, it is strange that the very similar looking variant 1 does not cluster with variant 2 and 3. A broader sampling of specimens from Costa Rica and especially Panama is needed to further investigate species boundaries. This variant is known only from samples of sifted leaf litter.
Variant 4 (INBIOCRI001280989) is similar to the holotype form, except as follows: face and mesosomal sculpture usually reduced, with posterior half of face mostly smooth; notch in basal margin of mandible very deep; anterior clypeal margin forming 4 distinct teeth, with outer teeth strongly projecting (presumably to fit in notch in basal margin of mandible); gastral pilosity mainly forming a single sparse layer of stout, suberect setae. This variant seems to be an arboreal version of the holotype form of S. schmidti, with most specimens coming from canopy fogging samples or Winkler samples of epiphytic material. Some specimens are also known from litter samples taken from the forest floor. Variant 4 occurs mainly at mid elevations at Monteverde and Volcán Barva in Costa Rica. Specimens from the canopy at Monteverde are rather uniform in morphology, but specimens from the forest floor at Monteverde and on Volcán Barva are more variable and become difficult to assign to a particular variant with certainty. These difficult specimens have intermediate sculpture and gastral pilosity.
Variant 5 (INB0003210597) differs drastically from the type form and is potentially a good species nested within the schmidti complex. It has the following characters: basal margin of mandible sinuous, without a basal notch or tooth; anterior clypeal margin with a simple median emargination; head more elongate and narrow (CI 79–84), with posterior margin flat; eye large (EL 0.15–0.18, REL 22–27); scape long, surpassing posterior margin of head when laid back (SI 107–121); propodeal spines absent, at most forming blunt angles; mesosoma noticeably elongate and gracile; petiole noticeably elongate; sculpture on head and mesosoma carinulate-punctate; face sometimes with transverse carinulae on anterior half; pronotum with transverse carinulae; legs elongate (MFI 71–80); gastral pilosity forming a single sparse layer of stout, suberect setae. This variant is known only from Monteverde and Volcán Barva in Costa Rica from 1000–1500 m. It is apparently an arboreal form of S. schmidti with specimens collected from fogging samples and from nests found under epiphytes in the canopy. Longino (pers. comm.) has reported that this variant prefers to nest under small epiphyte clumps on small branches. Molecular phylogenetic results show specimens from Monteverde and Volcán Barva clustering together, suggesting some reproductive isolation. However, I find specimens with intermediate morphology between this variant, the holotype form, and variant 4. Some specimens have the anterior clypeal margin forming sharp teeth, or the mesosoma stockier and less gracile. Counter to expectations, the intermediates do not cluster with variant 5 specimens in the phylogeny.
Variant 6 (INB0002659320) is similar to variant 5 in that it has the same mandible and clypeus structure, it lacks propodeal spines, and it has a relatively large eye. However, it has the following differences: smaller overall size; mesosoma less elongate and gracile; head roughly oval-shaped, less elongate (CI 85–87); scape and metafemur relatively smaller (SI 94–101, MFI 86–93); gastral pilosity not stout; sculpture reduced, usually faintly punctate or smooth. Variant 6 like variant 5 is known only from Monteverde and Volcán Barva in Costa Rica, and it is arboreal, with specimens collected from fogging samples and nests in epiphyte clumps. Molecular phylogenetic data show specimens from Monteverde and Volcán Barva clustering together. But, despite the similarities with variant 5, the two variants do not cluster together. I would not be surprised if variant 6 is a hybrid between variant 5 and the holotype form of S. schmidti or variant 7. It may also be a distinct species that has convergently evolved some features of variant 5.
Variant 7 (CASENT0622465) is a smaller, low-elevation version of the type form. It has the following differences: smaller overall size; face completely sculptured, with carinulae in middle changing to rugoreticulae toward lateral margin; dorsum of promesonotum usually with carinulae encircling margins (humeri), remainder of promesonotum smooth; eye sometimes relatively very large (EL 0.13–0.16, REL 23– 29); gastral pilosity mainly a single sparse layer of stout, suberect setae, with a few decumbent setae underneath. This variant occurs from sea level to about 1000 m elevation and has been collected at multiple sites in Nicaragua and Costa Rica. It is very similar to variant 8, S. nanozoi, and specimens from lowland Panama, Colombia, and Ecuador, but molecular phylogenetic data do not cluster all of these similar-looking forms together. Only specimens from lowland Nicaragua and northern Costa Rica form a clade. This variant is distinct and easy to identify at low elevations, but at mid elevations in Costa Rica, especially on Volcán Barva, it becomes very difficult to separate this variant from the holotype form or several of the other variants. Some of this difficulty might be because smaller specimens of the other variants look like variant 7, but I suspect it is also because of hybridization and perhaps adaptation of variant 7 to the higher elevation environment. Nearly all specimens of this variant are known from leaf litter, but one nest was found at the La Selva Biological Station in Costa Rica. It was in a small branch in the leaf litter (Longino, pers. comm.).
Variant 8 (CASENT0605211) is a small version of S. schmidti and is superficially similar to variant 7, but differs as follows: face mostly rugoreticulae; mesosoma more sculp- tured, mostly punctate to rugulose-punctate; eye usually smaller (EL 0.09–0.11, REL 19–20), with 5–6 ommatidia at greatest diameter; basal margin of mandible sometimes with only a small basal notch and no tooth; anterior clypeal margin sometimes appearing flat, without defined teeth. This variant mainly includes specimens from the Osa Peninsula in Costa Rica (13 km SSW Pt. Jimenez), but many lowland specimens from Panama to Ecuador are similar, with some variation in sculpture and mandible and clypeus structure. Despite similarities with variant 7, molecular phylogenetic data do not show these two variants clustering together, suggesting that their similar morphology is convergent. I find some similarities in mesosoma shape and sculpture between variant 8 and high-elevation specimens from southern Costa Rica that I assign to the holotype form of S. schmidti. Perhaps overall size and pilosity are misleading characters in the schmidti complex.
Variant 9 (CASENT0605896) is a mid- to high-elevation version of variant 8. It differs as follows: larger overall size (similar to type form); eye larger (EL 0.13–0.14, REL 20–21); dorsum of promesonotum with transverse arcuate carinulae; pilosity on gastral dorsum distinctly bilayered, with a sparse layer of somewhat stout suberect setae, and a layer of dense decumbent to appressed pubescence (compared to type form, suberect setae longer and less stout, and pubescence less dense). Variant 9 occurs between 1300–2000 m elevation and is known from a few sites in Colombia. I have not been able to include this variant in the phylogeny, but I suspect it is closely related to variant 8 and the lowland specimens from Panama and Ecuador.
Stenamma schmidti Menozzi, 1931a: 198, fig 6. Lectotype worker: COSTA RICA, [Heredia]: Vara Blanca, [ca. 10.167°N, 84.150°W], 2000m (H. Schmidti) (NHMB, specimen CASENT0126701) [examined]. Menozzi, 1931b: 267, distribution, remarks on type locality. Borgmeier, 1937: 232: distribution, remarks on taxonomy of worker. Branstetter, 2012: phylogeny. Branstetter, 2013: worker, queen description and images; male images.
[Note: In Branstetter (2013) the specimen CASENT0126701 is listed as the holotype. However, notes from Longino (pers. comm.) show that multiple specimens from the type series of S. schmidti bear a label that says "type". This indicates that no holotype was originally designated. Thus, the holotype listed in Branstetter (2013) should be treated as the de facto lectotype.]
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: 461 times found in montane wet forest, 143 times found in cloud forest, 149 times found in mature wet forest, 96 times found in tropical wet forest, 32 times found in primary cloud forest, 53 times found in ridgetop cloud forest, 24 times found in tropical rainforest, 10 times found in wet forest, 7 times found in forest, 7 times found in CES (700.350 GIS), ...
Collected most commonly using these methods or in the following microhabitats: 348 times Mini Winkler, 367 times miniWinkler, 182 times Winkler, 87 times Berlese, 42 times fogging, 22 times search, 14 times maxiWinkler, 7 times Search & Berles, 4 times baiting, 3 times Malaise, 4 times flight intercept trap, ...
Elevations: collected from 50 - 2400 meters, 1194 meters average