Sericomyrmex amabilis

A study in Panamá found that 40 out of 44 colonies had a single foundress queen and that the remaining nests had two to four queens (Fernández-Marín et al. 2004). Sericomyrmex amabilis is the host to the socially parasitic ant Megalomyrmex symmetochus, which specializes on Sericomyrmex species (Wheeler 1925b, Adams et al. 2013), and which provides protection to the host-ant colony by defending it from the specialized ant agro-predator Gnamptogenys hartmani (Adams et al. 2013).

Identification
Jesovnik and Schultz (2017) - Medium-sized species; mandible usually striate; frontal carina complete; frontal lobe triangular; eye almost flat, without white layer; posterior cephalic margin with abrupt to gradual emargination; mesosomal tubercles from low and obtuse to well developed; first gastral tergite with lateral carinae strongly developed, dorsal carinae from weak to well developed. S. amabilis differs from the sympatric Sericomyrmex opacus by its larger size, striate mandibles (in opacus always smooth), triangular frontal lobes (rectangular in opacus), the shape of the posterior cephalic corners (in opacus smoothly rounded), and the presence of dorsal carinae on the gaster. The differences in size and mandibles also help in separating queens of these two species. In addition, the notauli and mesoscutal line are often pronounced in amabilis and faint or absent in opacus, although this can vary in both amabilis and opacus.

S. amabilis can be separated from its sister species Sericomyrmex saussurei by its flatter, uncoated eyes (convex, with a thick white layer in saussurei) and usually by geography. It can be separated from S. mayri by its smaller size, narrower head, complete frontal carinae, and wider frontal lobes.

Within S. amabilis, there is variation in the character of smooth versus striate mandibles. In the majority of specimens examined for this revision (including the type specimens) the mandibles are consistently striate across the entire dorsal surface. In some populations, however, some specimens from a given locality have striate mandibles and some have completely smooth mandibles. In those populations intermediate forms are also found, with faint striations most obvious along the posterior lateral edge of the mandible.

The population of amabilis at Costa Rica, Heredia, La Selva Biological Station, exemplifies the polymorphic state in which both smooth and striate mandibles co-occur, whereas the population from Costa Rica, Puntarenas, Osa Peninsula, contains only the striate state. A few specimens from Nicaragua and Ecuador also have smooth or faintly striate mandibles. Specimens of amabilis from Gorgona National Park (an island 35 km off the Pacific coast in the department of Cauca, Colombia) show the complete range of variation, from fully striate through intermediate to completely smooth mandibles. We did not find both forms, fully striate and completely smooth, co-occurring within a single nest, but we examined nest series from only a few localities. The smooth-mandibled populations do not form distinct clusters in molecular phylogenies of amabilis; rather, smooth-mandibled specimens group with striate-mandibled ones and vice versa. Likewise, statistical analyses of morphological measurements do not identify any distinct clusters correlated with mandibular sculpture.

Members of the Gorgona population of amabilis have longer and sharper lateral mesonotal tubercles and more robust anteromedian dorsal carinae on the gaster compared to Central American populations. Some specimens from Nicaragua have a thin, translucent white layer covering part of the eye, similar to the condition observed in S. opacus. Other variable characters in amabilis are the frontal carinae (usually complete, but less developed in some), head shape (from angular to more rounded posterior head corners), and the size of the frontal lobes.

Distribution
Central America, Colombia, Ecuador, Mexico, Venezuela.

Distribution based on Regional Taxon Lists
Neotropical Region: Colombia, Costa Rica, Ecuador, Guatemala, Honduras, Mexico, Nicaragua, Panama, Venezuela.

Biology
These ants are parasitized by Megalomyrmex symmetochus. The Megalomyrmex ants live within the fungal garden and interactions between the two species are generally not aggressive (Wheeler 1925). These two ant species were also found to share similar bacterial symbionts, presumably as part of their utilizing the same fungal gardens for their nutritional needs (Liberti et al. 2015).

Castes
Jesovnik and Schultz 2017. Figure 17. Worker. Striate-mandibled form (USN-MENT00308236) (a, c, e) and smooth-mandibled form (USNMENT01125194) (b, d, f). Figure 19. S. amabilis queen and male; head, profile, and dorsal view. Queen (USNMENT01125871) (a, c, e) Male (USNMENT01125846) (b, d, f).

Jesovnik and Schultz 2017. Figure 18. S. amabilis worker, queen, and male, SEM images. Worker (USNMENT01125862): a head, full-face view b mandibles c mesosoma, lateral view d metasoma, lateral view. Queen (USNMENT01125864): e head, full-face view f mandibles g mesosoma and metasoma, lateral view. Male (USNMENT01125863): h head, full-face view i mandibles j mesosoma and metasoma, lateral view. Figure 20. S. amabilis larva (USNMENT01126216), SEM images. a Lateral view b ventral view c head, frontodorsal view d head, lateral view e mouthparts; f anal setae.

Nomenclature

 *  amabilis. Sericomyrmex amabilis Wheeler, W.M. 1925d: 166, fig. 8 (not fig. 4) (w.) PANAMA. Wheeler, G.C. 1949: 671 (l.). Senior synonym of bierigi: Weber, 1958d: 263.
 * bierigi. Sericomyrmex bierigi Santschi, 1931c: 279 (w.) PANAMA. Junior synonym of amabilis: Weber, 1958d: 263.

Description
Jesovnik and Schultz 2017:

Worker
(lectotype): HWe 0.88–1.21 (1.15) HW 0.88–1.24 (1.15) HW1 0.82–1.18 (1.12) HW2 0.92–1.36 (1.2) HW3 0.58–0.9 (0.75) IFW1 0.56–0.86 (0.78) IFW2 0.19–0.35 (0.3) HL1 0.82–1.2 (1.05) HL2 0.76–1.04 (0.93) SL 0.65–0.87 (0.76) EL 0.11–0.2 (0.162) Om 7–11 (8) WL 1.13–1.6 (1.44) PL 0.22–0.4 (0.3) PPL 0.16–0.29 (0.24) GL 0.78–1.12 (1.03) HFL 0.93–1.38 (1.21) PW 0.58–0.92 (0.78) CI 97–111 (110) FLI 60–72 (68) SI 66–78 (66) OI 12–19 (14) CEI 5–15 (10) [N=70]

Pilosity. Pubescence dense, often lighter colored than integument, appressed to decumbent. Hairs curved, darker in color at base, yellow to gray, appressed to suberect, mostly decumbent.

Head. In full-face view slightly broader than long (CI=104 ± 3, mean ± SD), posterior corner rounded to angular. Posterior cephalic margin with distinct median emargination (CEI=10 ± 2), gradually or abruptly impressed. Vertexal impression and frontal tumuli usually distinct. Mandible with 7–9 teeth, dorsally glossy and striate, striation sometimes reduced. Eye medium-sized (OI=16 ± 1), flat to slightly convex, 7–11 ommatidia across largest diameter. Frontal lobe triangular, posterior margin shorter than medial, lobe diverging laterally, relatively wide (FLI=68 ± 2). Frontal carina complete, reaching posterior cephalic corner. Antennal scape relatively short, never reaching posterior cephalic corner (SI=71 ± 3).

Mesosoma. Mesosomal tubercles from low and obtuse to well developed. Propodeal carinae low, sometimes serrate, each with low posterodorsal denticle.

Metasoma. Petiole and postpetiole each with pair of low, serrate carinae dorsally; in petiole sometimes reduced to two low denticles, seen in dorsolateral view. Postpetiole with another pair of low carinae laterally, sometimes reduced to low denticles. First gastral tergite with lateral carinae strongly developed, dorsal carinae faint in most specimens, sometimes strongly developed.

Queen
HWe 1.27–1.40 HW 0.34–1.45 HW1 1.30–1.52 HW2 1.40–1.52 HW3 0.87–1.04 IFW1 0.90–1.00 IFW2 0.27–0.37 HL1 1.27–1.45 HL2 1.15–1.25 SL 0.81–0.99 EL 0.23–0.29 Om 15–22 EW 0.06–0.1 WL 1.95–2.2 PL 0.45–0.58 PPL 0.28–0.4 GL 1.68–1.87 HFL 1.16–1.58 PW 1.05–1.52 FWg 5.85–6.93 HWg 3.79–4.41 CI 93–104 FLI 70–77 SI 62–74 OI 18–22 [N=10]

Head. Mandible with 8–9 teeth, dorsally glossy and striate. Preocular carina usually fading posterior to eye. Eye large (OI=20 ± 1), nearly flat, 15–22 ommatidia across largest diameter. Frontal lobe as in worker, antennal scape not reaching posterior cephalic corner.

Mesosoma. Scutum in dorsal view with notauli weak, median mesoscutal sulcus reduced. Groove between axillae in dorsal view sometimes weakly transversely costate. Scutellum slightly convex in profile view, narrowing posteriorly in dorsal view, posterior margin medially with wide shallow V-shaped notch, notch sometimes continuing into median impression that divides scutellum into two lateral parts. Propodeum in dorsal view with two low carinae, each with posterodorsal denticle.

Metasoma. First gastral tergite with lateral carinae well developed, dorsal carinae absent, anteromedian groove distinct.

Male
HWe 0.61–0.98 HW 0.6–0.78 IFW1 0.24–0.48 IFW2 0.17–0.25 HL1 0.61–0.78 SL 0.59–0.76 EL 0.24 0.35 Om 20–26 EW 0.1–0.18 WL 1.43–2.02 PL 0.25–0.43 PPL 0.18–0.25 GL 1.08–1.75 HFL 1.48–1.95 PW 0.58–0.90 IOD 0.54–0.63 FWg 4.28–5.77 HWg 2.68–3.68 CI 100–129 FLI 33–60 SI 74–99 OI 34–40 [N=10]

Head in full-face view longer than broad (CI=120 ± 7). Eye large (OI=38 ± 2), 20–26 ommatidia across largest diameter. Preocular carina slightly curved medially, fading pos-terior to eye. In dorsal view, scutum with notauli well developed, mesoscutal line faint, groove between axillae with up to four transverse costae. Propodeal carinae short and faint. Petiole with lateral and dorsal denticles, postpetiole with very reduced lateral denticle.

Larva
About 15 setae on each side of dorsal and lateral body surfaces (i.e., total ~30). Supra-antennal setae present. Four genal setae on each side. Mandibular apical tooth divided. Labial denticles present anterior to sericteries, sparse. First thoracic segment ventrally with multiple multidentate spinules, arranged in transverse rows. Numbers of ventral setae: six on T1, four on T2, four on T3, and around ten on abdomen (not including anal setae). Single pair of setae anterior to anal opening.

Type Material
Lectotype worker (here designated): Panama, Panamá, Barro Colorado Island, [9.1543, -79.8461], 3 Aug 1924, W. M. Wheeler, WMW838 (: 3w, USNMENT00920034, topmost specimen on the pin). Paralectotypes: Same data as lectotype (USNM: 2w, USNMENT00920034, lower two specimens on the pin) (: 2w, 1q, MCZ40-42 21197).