Sericomyrmex opacus

A fungus growing ant with a wide distribution. Most collections have been from forested areas. The biology of Sericomyrmex opacus has not been studied, general details about the biology of the genus are given here.

Identification
Ješovnik & Schultz (2017) - Small species; mandible smooth, glossy; posterior cephalic corner smoothly rounded; frontal lobe rectangular; eye small, often with at least partial white layer; mesosomal tubercles low, reduced; first gastral tergite with lateral carinae weakly developed, dorsal carinae absent.

The species most similar to S. opacus are Sericomyrmex parvulus, Sericomyrmex saramama, and, in Central America, smooth-mandibled populations of Sericomyrmex amabilis. Typical amabilis can be distinguished from opacus by their completely striate mandibles, triangular frontal lobes, and larger size. The distinction between opacus and smooth-mandibled amabilis is less obvious, but the frontal lobes, head shape, and size are still good indicators. An ameliorating factor for this difficulty is that, when sympatric, amabilis and opacus are very distinct; we have not encountered the smooth-mandibled amabilis variant sympatric with opacus, which might indicate character displacement. The queen of S. opacus can easily be separated from that of the sympatric S. amabilis by its smaller size, smooth mandibles (striate in amabilis), and usually less conspicuous notauli on the scutum. The main characters separating S. opacus from saramama are the shape of the frontal lobes (triangular in saramama) and the white layer over the eyes (absent in saramama). The S. saramama queen can easily be separated from the opacus queen by its striate mandibles (smooth in opacus).

S. parvulus can be distinguished from the typical opacus by having smaller, narrower, triangular frontal lobes; smaller overall size; and shorter frontal carinae, often fading well before reaching the posterior cephalic corners. Separating non-typical representatives of opacus, which may also have reduced frontal lobes, from parvulus is difficult. Also, it is very difficult to separate queens of opacus and parvulus. The parvulus queen is slightly smaller and lacks the faint supraocular carinae; however, these carinae are absent in some opacus queens as well. The region of overlap of the known distributions of these two species is limited, so geographic origin can aid in species identification.

S. opacus is morphologically variable across its geographic range. This variation is correlated with patterns in the molecular phylogeny (Suppl. material 1) as well as with geography. The most pronounced variation within opacus occurs in the shape of the frontal lobes and in the eyes. The typical S. opacus has rectangular frontal lobes and the eyes covered with a thin white layer. There are occasional, rare individuals with smaller, almost triangular lobes and eyes lacking the white layer. These rare, odd specimens are also smaller in size, so these atypical character states could be correlated with size (e.g., in nanitic workers). The specimens from the opacus 3 population all have eyes lacking the white layer and the frontal lobes are often (but not always) more triangular than rectangular.

Distribution
Brazil, Central America, Colombia, Mexico.

Distribution based on Regional Taxon Lists
Neotropical Region: Brazil, Colombia , Guatemala , Honduras, Mexico , Panama.

Castes
Jesovnik and Schultz 2017. Figure 43bdf. S. opacus worker without a white layer over the eyes (USNMENT01125118) (b, d, f). Figure 44. S. opacus worker (USNMENT01125331), SEM images. a Head, full-face view b mandibles c mesosoma and metasoma, lateral view d eye.

Jesovnik and Schultz 2017. Figure 45. S. opacus queen and male; head, lateral profile, and dorsal view. Queen (USNMENT00305214) (a, c, e) Male (USNMENT01125314) (b, d, f). Figure 46. S. opacus larva (USNMENT01125317), SEM images. a Lateral view b ventral view c head, frontolateral view d head and thorax, frontolateral view e mouthparts f anal setae.

Nomenclature

 *  opacus. Sericomyrmex opacus Mayr, 1865: 84, pl. 3, fig. 22 (q.) BRAZIL.
 * [Note: type-locality Mexico (Veracruz), after Ješovnik & Schultz, 2017a: 78.]
 * Mayr, 1866a: 506 (m.); Emery, 1894c: 223 (w.); Wheeler, W.M. 1916c: 11 (w.); Ješovnik & Schultz, 2017a: 75 (l.).
 * Status as species: Mayr, 1866a: 506; Dalla Torre, 1893: 150; Emery, 1894c: 223; Forel, 1895b: 138; Wheeler, W.M. 1905b: 130; Forel, 1912e: 191 (in key); Wheeler, W.M. 1916c: 11 (in key); Emery, 1924d: 339; Borgmeier, 1927c: 128; Kempf, 1972a: 229; Bolton, 1995b: 382; Ješovnik & Schultz, 2017a: 71 (redescription).
 * Senior synonym of aztecus, diego, zacapanus: Ješovnik & Schultz, 2017a: 72.
 * aztecus. Sericomyrmex aztecus Forel, 1885a: 363 (w.) MEXICO.
 * Status as species: Dalla Torre, 1893: 150; Forel, 1899c: 37; Forel, 1912e: 192 (in key); Wheeler, W.M. 1916c: 11 (in key); Emery, 1924d: 339; Wheeler, W.M. 1934g: 178; Kempf, 1972a: 229; Bolton, 1995b: 382.
 * Junior synonym of opacus: Ješovnik & Schultz, 2017a: 72.
 * diego. Sericomyrmex diego Forel, 1912e: 192 (w.m.) COLOMBIA.
 * Status as species: Wheeler, W.M. 1916c: 11 (in key); Emery, 1924d: 339; Kempf, 1972a: 229; Bolton, 1995b: 382.
 * Junior synonym of opacus: Ješovnik & Schultz, 2017a: 72.
 * zacapanus. Sericomyrmex zacapanus Wheeler, W.M. 1925a: 54 (w.) GUATEMALA.
 * Status as species: Kempf, 1972a: 230; Bolton, 1995b: 382.
 * Junior synonym of opacus: Ješovnik & Schultz, 2017a: 72.

Description
Ješovnik & Schultz (2017):

Worker
(lectotype): HWe 0.8–1 (0.93) HW 0.8–1 (0.95) HW1 0.78–1 (0.84) HW2 0.88–1.13 (0.96) HW3 0.54–0.74 (0.62) IFW1 0.54–0.73 (0.64) IFW2 0.16–0.28 (0.24) HL1 0.82–1 (0.9) HL2 0.6–0.9 (0.82) SL 0.58–1.08 (0.62) EL 0.12–0.18 (0.15) Om 6–10 (8) WL 0.99–1.3 (1.23) PL 0.2–0.33 (0.21) PPL 0.15–0.25 (0.18) GL 0.78–1 (0.87) HFL 0.68–1.02 (0.93) PW 0.50–0.72 (0.63) CI 95–106 (103) FLI 62–78 (69) SI 65–78 (67) OI 13–19 (16) CEI 5–19 (9) [N=68]

Pilosity. Pubescence dense, often lighter 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 almost equally broad and long (CI=100 ± 3), posterior corner smoothly rounded, posterior cephalic emargination relatively shallow (CEI=9 ± 2), gradually impressed. Vertexal impression and frontal tumuli faint. Mandibles with 7–8 teeth, dorsally smooth and glossy, finely transversely striate only along masticatory margin. Eye medium-sized (OI =16 ± 1), flat to slightly convex, 6–10 ommatidia across largest diameter, with (Figure 43a, 44d) or without (Figure 43b) thin white layer. Frontal lobe wide (FLI=70 ± 3), rarely triangular, mostly rectangular to trapeziform, posterior margin as long, or almost as long, as medial. Frontal carina usually incomplete, fading before reaching posterior cephalic corner. Antennal scape relatively short, never reaching posterior cephalic corner (SI=71 ± 2).

Mesosoma. Mesosomal tubercles small, low and obtuse. Propodeal carinae low and feeble, sometimes serrate, sometimes reduced to just posterodorsal denticles.

Metasoma. Petiole with two low, reduced denticles; postpetiole with two faint, short carina dorsally. First gastral tergite with lateral carinae weakly developed, dorsal carinae absent.

Queen
HWe 1.02–1.16 HW 1.05–1.20 HW1 1.10–1.25 HW2 1.20–1.30 HW3 0.72–0.85 IFW1 0.76–0.85 IFW2 0.24–0.33 HL1 1.09–1.12 HL2 0.98–1.00 SL 0.74–0.78 EL 0.20–0.24 Om 15–17 EW 0.08–0.10 WL 1.58–1.80 PL 0.34–0.48 PPL 0.24–0.28 GL 1.50–1.64 HFL 0.99–1.28 PW 0.90–0.96 FWg 4.85–5.17 HWg 3.60–3.64 CI 93–106 FLI 71–76 SI 64–73 OI 18–22 [N=6]

Head. Mandible with 7–8 teeth, dorsally glossy and smooth, finely transversely striate only along masticatory margin. Preocular carina fading posterior to eye, 1–3 isolated, short, thin, supraocular carinae sometimes present, never reaching posterior cephalic corner. Eye large (OI=20 ± 1), convex, 15–17 ommatidia across largest diameter. Frontal lobe as in worker, antennal scape not reaching posterior cephalic corner.

Mesosoma. Lateral pronotal tubercles low and obtuse. Scutum in dorsal view with notauli and median mesoscutal line reduced, parapsidal lines thin. Scutellum small and short, with posterior notch shallow to absent and with median impression sometimes separating scutellum in two lateral halves. Propodeal carinae low, each with low posterodorsal denticle.

Metasoma. Petiole with two dorsal and two lateral low and obtuse denticles, best seen in frontodorsal view. Postpetiole with two short, low carinae dorsally and two low denticles laterally. First gastral tergite with lateral carinae well developed, dorsal carinae absent, anteromedian groove visible.

Male
HWe 0.66–0.74 HW 0.46–0.58 IFW1 0.20–0.27 IFW2 0.13–0.18 HL1 0.53–0.61 SL 0.49–0.62 EL 0.22– 0.28 Om 19–28 EW 0.09–0.14 WL 1.20–1.44 PL 0.27–0.38 PPL 0.16–0.22 GL 0.96–1.12 HFL 1.20–1.50 PW 0.50–0.80 IOD 0.36–0.50 FWg 3.48–4.10 HWg 2.20–2.84 CI 115–125 FLI 30–38 SI 71–88 OI 29–39 [N=7]

Head in full-face view longer than broad (CI=121 ± 4). Eye large (OI=34 ± 3), convex, 19–28 ommatidia across largest diameter. Preocular carina fading posterior to eye, medially curved before fading. Notauli and mesoscutal line faint, parapsidal lines thin, groove between axillae smooth, sometimes weakly transversely costate. Propodeum without denticles or carinae. Petiole and postpetiole with lateral denticles very reduced, dorsal denticles absent.

Larva
Approximately eight setae on each side of dorsal and lateral body surfaces (i.e., approximately 16 total). Supra-antennal setae absent. Four genal setae on each side. Mandibular apical tooth undivided. Labial denticles not visible. First thoracic segment ventrally with multiple multidentate spinules, arranged in transverse rows. Numbers of ventral hairs: six on each thoracic segment, eight on the abdomen (not including anal setae). Single pair of sensilliform setae anterior to anal opening.

Type Material
Lectotype worker (here designated): (“Brazil”)* Mexico, Veracruz, Córdoba, [18.8808, -96.9272], E. Norton (: 1w, CASENT0915956) Paralectotypes: Same data as lectotype: (NHMW: 1w, CASENT0915955) (: 1w, USNM00924096).

The original description of S. opacus by G. L. Mayr in 1865, which is also the original description of the genus Sericomyrmex, lists “Brasilien” as the type locality. Based on the route of the Novara-Expedition, which is the expedition from which Mayr’s specimens supposedly originated, the Brazilian type locality is most likely Rio de Janiero (Mayr, 1865). However, the locality labels of the type specimens (CASENT0915955, CASENT0915956, USNMENT00924096) of Sericomyrmex opacus that we studied indicate “Mexico, Cordoba,” and “Norton” as the collector. The labels look original when compared to the labels of other Mayr type specimens, both in terms of handwriting and in resemblance to the labels of other specimens collected by Norton. Indeed, Mayr described other species based on specimens collected by E. Norton in Mexico, so he clearly had access to Norton’s Mexican material. Importantly, the type specimen of S. opacus is identical to the type specimen of Sericomyrmex aztecus, a Mexican species described by Forel (1885), which in turn is identical to numerous specimens collected in Mexico and elsewhere in Central America. S. aztecus is the name most commonly applied to such specimens. Given these facts, the most likely explanation is that the locality of the type specimen of S. opacus (“Brasilien”) given in the original description is incorrect and that the locality labels affixed to the syntype specimens are correct. The alternative explanation is that the published locality is instead correct, the specimen labels are incorrect, and that the type specimen of S. opacus originated in Brazil. We judge this alternative to be highly unlikely, because S. opacus (under which we have synonymized S. aztecus, Sericomyrmex diego, and Sericomyrmex zacapanus) does not occur anywhere near Rio de Janeiro.