Stenamma diversum

Stenamma diversum occurs in relatively pristine wet forest habitats from sea level to about 1,100 m elevation. It is a particularly interesting species because it has convergently evolved many of the same nesting behaviors as Stenamma alas and Stenamma expolitum, even though it is not closely related (Branstetter 2012, unpublished data).

Identification
Key to Middle America Stenamma

Branstetter (2013) - Integument mostly black and shining; medium-sized species (see HL, ML, PrW below); head mostly smooth and shiny; mesosoma reticulately costate to coarsely rugoreticulate; propodeal spines long and robust (PSL 0.28–0.34, PSI 3.0-3.7); frontal lobes dorsolaterally expanded, obscuring the torular lobes in full-face view (FLD 0.25-0.29, FLI 35-38); eye of moderate to large size (EL 0.11–0.15, REL 16–20), oval-shaped, with 7–9 ommatidia at greatest diameter; anterior margin of clypeus with shallow median emargination; basal margin of mandible straight, without a notch or substantial depression; pilosity on gastral dorsum long, flexuous, and relatively sparse. Similar species: Stenamma lobinodus, Stenamma tico.

Stenamma diversum and S. tico are sister species and together form the diversum species group. This group is defined by the following characters: head mostly smooth and shiny; mesosoma mostly reticulately costate; promesonotum in profile low-domed, roughly symmetrical; postpetiole in profile without a distinct dorsal lobe that projects posteriad over postpetiole (as in S. lobinodus and other lobinodus group species); basal margin of mandible straight; anterior clypeal margin with a median emargination. Based on DNA sequence data Stenamma vexator is likely sister to the diversum species group, but morphological characters could not be found to diagnose the entire clade.

Stenamma diversum is probably the most distinctive Stenamma species, but it does bear some similarity to its sister taxon S. tico and the more distantly related S. lobinodus, both of which share roughly the same color and sculpture pattern. It is easy to separate S. diversum from these other species by comparing the propodeal spines and the frontal lobes. In S. tico, the spines are absent or at most form small dorsally projecting tubercles (PSL 0.14–0.18, PSI 1.4–1.9). In S. lobinodus, the propodeal spines are usually well developed, but they are shorter than those of S. diversum (PSL < 0.23 vs. > 0.27, PSI < 2.6 vs. > 3.0). Stenamma diversum is the only species among these three to have greatly expanded frontal lobes, causing the underlying torular lobes to be covered in full-face view. In the other species, the torular lobes are always visible. The frontal lobes of S. lobinodus are especially reduced. Geography also can help separate these species. Stenamma diversum occurs in sympatry with S. lobinodus only in Oaxaca, Mexico (although never collected together at the same site) and with S. tico only in northern Nicaragua (found in sympatry at Cerro Saslaya and Cerro Musún).

Distribution
Southern Mexico to Nicaragua.

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

Biology
S. diversum is a specialist inhabitant of areas dominated by red clay substrate (Branstetter pers. obs.). Nests are commonly found in banks bordering streams or trails, in steep slopes, and sometimes in small patches of vertical clay that form under roots at tree bases. The architecture of a nest of S. diversum is very similar to that of Stenamma alas. The external portion of the nest consists of a small, ear-like turret that is usually sunk into a shallow alcove. At the center of the ear is the nest entrance, which leads to a single, small chamber. Unlike S. alas or Stenamma expolitum (also a clay nester), there is only a single nest per colony and colonies are quite small, with a single queen and maybe a dozen workers. Also, S. diversum does not have a “door pebble” to be used to block the nest entrance from predators, as reported by Longino (2005) for S. alas and S. expolitum. Workers of S. diversum are slow moving and appear to be most active during the day. The convergent nature of S. alas and S. diversum nests is striking and begs further investigation into the adaptive significance of these structures. It is possible that the structure is an adaptation to living in very wet, exposed environments. Inside mature S. diversum nests I have often noticed a dark material covering the nest walls. What this material is and what function it serves also needs investigation. Given their nesting habits, it is somewhat odd that the collection of the type series was made “beneath a stone.” Perhaps this was on clay bank. (Branstetter 2013)

Castes
Males have been collected but have not been described.

Nomenclature

 *  diversum. Stenamma diversum Mann, 1922: 20 (w.) HONDURAS. Branstetter, 2013: 94 (w.q.m.).

Worker
Branstetter (2013) - (11 measured) HL 0.74–0.81 (0.76), HW 0.69–0.78 (0.72), FLD 0.25–0.29 (0.27), PCW 0.06–0.09 (0.08), SL 0.58–0.66 (0.62), EL 0.11–0.15 (0.13), ACL 0.52–0.56 (0.54), ML 0.95–1.08 (0.99), PrW 0.55–0.61 (0.56), PSL 0.28–0.34 (0.29), SDL 0.08–0.11 (0.08), PL 0.44–0.50 (0.44), PH 0.22–0.25 (0.23), PW 0.18–0.24 (0.20), PPL 0.18–0.21 (0.19), PPH 0.19–0.22 (0.20), PPW 0.18– 0.23 (0.21), MFL 0.71–0.78 (0.72), MTL 0.56–0.63 (0.59), CI 93–97 (95), SI 84–88 (85), REL 16–20 (18), FLI 35–38 (37), PSI 3.0–3.7 (3.5), MFI 96–100 (100), ACI1 65–67 (65), ACI2 85–90 (89).

Medium-sized species; general body color black, with mandibles, clypeus and appendages dark brown to yellow-brown; setae golden; mandible with 6 teeth, consisting of 3 distinct apical teeth, a basal tooth, and 2–3 inner teeth, which are often worn and indistinct; basal margin of mandible straight, without a distinct notch or depression; dorsal surface of mandible mostly smooth and shining, with scattered piligerous punctae, and several weak striae extending from base; anterior clypeal margin with shallow median emargination; median lobe of clypeus with a pair of faint carinulae that diverge toward anterior margin, apex of lobe with a faint transverse carinula, remainder of clypeus mostly smooth and shining; posterior extension of clypeus between frontal lobes broad, with subparallel sides (PCW 0.06–0.09); frontal lobes expanded dorsolaterally (FLD 0.25-0.29, FLI 35-38), with torular lobes obscured in full-face view; head roughly oval-shaped, slightly longer than broad (CI 93-97), with posterior margin flat or gently curving, never depressed medially; eyes of moderate size (EL 0.11–0.15, REL 16-20), oval-shaped, with 7–9 ommatidia at greatest diameter; face largely smooth and shining, with faint carinulae and punctae on gena, scattered piligerous punctae elsewhere; scape short, not surpassing posterior margin of head when laid back (SI 84–88); dorsal surface of scape striate; funiculus with distinct 4-segmented antennal club; mesosoma compact, shiny, and almost entirely reticulately costate (specimens vary considerably in sharpness, coarseness, and orientation of costae); propodeal decliv- ity smooth; promesonotum in profile dome-shaped, roughly symmetrical; propodeal spines long and robust (PSL 0.28–0.34, PSI 3.0-3.7), usually projecting dorsoposteriorly; petiole relatively long and wedge-shaped (PL/HW 0.60-0.65), node variable, appearing rather robust to slightly more gracile, always angled so that the apex points posteriad; anterior slope of petiole usually long and rising gradually from peduncle, but sometimes shorter and rising more abruptly; posterior slope of petiole short and nearly vertical; dorsum of petiolar node viewed from posterior side flat, to depressed medially, to slightly convex; dorsal portion of petiolar node distinctly wider than ventral portion; postpetiolar node in profile smaller than petiolar node (PPH/PH 0.85–0.94), dome-shaped, slightly asymmetrical, with anterior slope longer and more sloping than posterior slope; petiole and postpetiole generally smooth and shiny, nodes with several deep furrows, ventral surfaces faintly punctate; gaster mostly smooth and shiny, with scattered piligerous punctae, and furrows on anterior constriction where gaster inserts into postpetiole; most of body with a relatively sparse layer of long, flexuous setae; setae on scapes and legs varying from mostly suberect to mostly decumbent; setae on femoral venters and coxae always longer and suberect to subdecumbent.

Queen
Branstetter (2013) - (5 measured) HL 0.73–0.80 (0.80), HW 0.69–0.78 (0.78), FLD 0.25–0.30 (0.30), PCW 0.07–0.10 (0.10), SL 0.58–0.65 (0.65), EL 0.17–019 (0.18), ACL 0.51–0.59 (0.59), ML 1.05–1.17 (1.17), PrW 0.63–0.71 (0.69), PSL 0.30–0.33 (0.32), SDL 0.09–0.11 (0.09), PL 0.46–0.53 (0.51), PH 0.24–0.26 (0.26), PW 0.21–0.23 (0.23), PPL 0.21–0.25 (0.22), PPH 0.23–0.25 (0.25), PPW 0.23– 0.25 (0.25), MFL 0.70–0.79 (0.79), MTL 0.56–0.64 (0.64), CI 94–98 (97), SI 82–86 (84), REL 23–27 (24), FLI 26–38 (28), PSI 2.9–3.4 (3.4), MFI 98–99 (99), ACI1 65–66 (65), ACI2 86–90 (90).

Same as worker except for standard queen modifications and the following: costae on mesoscutum with a decidedly longitudinal orientation, but often wavy, and usually with some reticulation anteriorly; costae on side of propodeum longitudinal in orienta- tion; mesopleuron mostly smooth and shiny; wing venation as in specimen CASENT0606782.

Type Material
Branstetter (2013) - Lectotype worker: HONDURAS, [Atlántida]: Lombardia, [ca. 15.567°N, 87.283°W], Feb-Mar 1920, collected beneath a stone (W. M. Mann) (USNM, Cotype No. 24447, specimen CASENT0194018).