Sericomyrmex

A genus of fungus growing ants.

Identification
Jesovnik and Schultz (2017) - Ants of the genus Sericomyrmex are small- to medium-sized, larger than ants of most of the lower-attine ant genera (e.g., Cyphomyrmex, most Apterostigma) and similar in size to most Trachymyrmex species and to the media workers of leaf-cutter ants (Atta and Acromyrmex). They are monomorphic to slightly polymorphic and can easily be separated from other attine ant genera by the following combination of characters: a silky, woolly appearance due to dense pilosity and hair; cordate head shape; well-developed frontal lobes; robust and short, moderately tuberculate mesosoma; and a smooth gaster, lacking tubercles. Members of the Trachymyrmex iheringi clade are similar to Sericomyrmex in size, general appearance, and the presence of long, flexuous hairs in some species (e.g., Trachymyrmex opulentus and Trachymyrmex dichrous), but Sericomyrmex can be distinguished from them by the following: posterior cephalic corners in full-face view smooth (most Trachymyrmex with multiple small to large denticles, tubercles, or spines, with exception of T. dichrous); antennal scape short, in full-face view almost reaching cephalic margin but never surpassing it (scape longer in Trachymyrmex, always surpassing cephalic margin); frontal lobes wide, concealing the base of the scape, with three distinct margins (in Trachymyrmex often rounded or with just two margins, smaller and narrower, often not concealing the base of the scape); tubercles on mesosoma smooth, simple, not bearing minute tubercles (tubercles on tubercles are very common in Trachymyrmex, giving the mesosoma a spiny appearance); gaster smooth, unsculptured, if mildly tuberculate this is visible only under SEM (in Trachymyrmex the gaster often visibly tuberculate). Because of the absence of denticles and spines on the posterior cephalic corners, dense hair, and tubercles on the mesosoma without additional, minute tubercles, T. dichrous can be mistaken for a Sericomyrmex species. It is easily identified and separated from Sericomyrmex by its bicolored integument (head dark brown, rest of body light ferrugineous brown), narrow and rounded frontal lobes, and strongly convex eyes protruding from the sides of the head.

Ants of the genus Apterostigma Mayr can be separated from Sericomyrmex as follows: head long, narrowing posterad of the eyes (except for Apterostigma megacephala); posterior cephalic border evenly rounded, never medially emarginate (median emargination always present in Sericomyrmex); frontal lobes frequently rounded (rectangular or triangular with three distinct margins in Sericomyrmex); mesosoma and general appearance slender, similar to some non-attine ant Myrmicinae genera (e.g., Aphaenogaster) (Sericomyrmex habitus more robust).

Males of Sericomyrmex can be separated from most other attine males by the presence of 12 antennal segments, deviating from the usual 13 segments in the males of most attine species, and from the 11 segments found in males of some social parasites (Gallardo 1916, Schultz et al. 1998, Rabeling and Bacci 2010). However, 12 antennal segments occur in parallel in Mycetophylax faunulus, Mycetophylax auritus, Mycetophylax conformis, Mycetagroicus inflatus, and Trachymyrmex opulentus (Mayr 1887, Wheeler 1925a, Ješovnik et al. 2013, Klingenberg and Brandão 2009). Males of Sericomyrmex can be separated from the other attine ant males with 12-segmented antennae by the following: presence of long, flexuous hairs on most of the body (no hair or very short and sparse hair in M. inflatus, Cyphomyrmex, Mycetophylax, and Trachymyrmex); dull, light brown integument, faintly reticulate and not strongly sclerotized (strongly sclerotized and dark brown to black in C. faunulus and M. conformis, light brown but costate on head and mesosoma in Trachymyrmex), scutellum with flat to slightly notched posterior margin, but never with sharp and long posterior processes (as in Trachymyrmex, C. faunulus), smooth posterior cephalic corners (with a small sharp denticle in C. faunulus and C. auritus).

The larvae of Sericomyrmex can easily be recognized as those of attine ants because of the following synapomorphies: short, narrow, monolobate labrum; fleshy subconical mandibles; leg vestiges present as open slits in the integument; and a reduced number of supra-antennal setae (one to no setae in Sericomyrmex). The larvae of Sericomyrmex can be separated from larvae of closely related Trachymyrmex species by anal hairs in a typical Sericomyrmex pattern (sensu Schultz and Meier 1995): a single pair of hairs anterior to the anus (5–8 extremely short to sensilliform setae in the Trachymyrmex iheringi group) and the presence of setae on the lateral and dorsal surfaces of the body in most species (setae absent in Trachymyrmex).

There are some characters that Sericomyrmex larvae share with the closely related M. explicatus, in particular labial spinules that are present only on the anterior surface dorsal to sericteries (ventral side of labium hidden) and the absence of supra-antennal setae (in some species of Sericomyrmex). The larva of M. explicatus differs from Sericomyrmex larvae by the anal setae pattern: M. explicatus with ~eight extremely short setae anterad and 0–2 setae dorsad of the anus, whereas Sericomyrmex species we examined have two setae anterad, 0–2 setae dorsad; and by the number of genal setae: three in M. explicatus, 4–6 in Sericomyrmex.

Distribution
Jesovnik and Schultz (2017) - Like the majority of attine-ant genera, Sericomyrmex has a wide Neotropical distribution, ranging from Mexico (Sánchez-Peña 2010) southward to Bolivia, Paraguay, and Paraná, Brazil (Kempf 1972, Brandão 1991, Mayhé-Nunes and Jaffé 1998, Fernández and Sendoya 2004). Sericomyrmex species can be found in a variety of habitats, from dry savanna to tropical wet forest, as well as in disturbed, open, and urban habitats (Mehdiabadi and Schultz 2010).

Species richness
Species richness by country based on regional taxon lists (countries with darker colours are more species-rich). View Data



Biology
Jesovnik and Schultz (2017) - Sericomyrmex belongs to the so-called “higher”-attine ants, a clade of fungus-farming ants that farms highly specialized, higher fungal cultivars, which are obligate symbionts of ants and never found outside ant nests (Schultz and Brady 2008, Ješovnik et al. 2017). The higher-attine ants include five other genera: the paraphyletic Trachymyrmex, the leaf-cutting Atta and Acromyrmex, a monotypic social parasite Pseudoatta, and a sister lineage to Sericomyrmex with a single known representative, the species Xerolitor explicatus.

A moderate degree of within-colony polymorphism, as well as other natural history traits, are shared by Sericomyrmex and Trachymyrmex species. This separates them from both the monomorphic lower-attine ants, which have smaller colonies and collect insect frass and organic debris, and from the highly polymorphic leaf-cutters, which have enormous colonies and forage for fresh vegetation (Leal et al. 2011).

When encountered they move slowly and when disturbed they react like most other attine ants: by curling into a ball and becoming immobile. Because of their slow movements and opaque integuments, they are difficult to notice on the forest floor. The nest entrances of some species can be recognized by their raised cylindrical craters consisting of excavated soil particles, but those of other species often consist of just a simple hole in the ground, difficult to notice on the forest floor (Urich 1895, Forel 1912, Wheeler 1925b). Sericomyrmex species nest underground and the nest consists of a minimum of one but usually of at least a few subspherical to subelliptical chambers connected by narrow tunnels (Urich 1895, Forel 1912, Weber 1969, Weber 1972, Sosa-Calvo et al. 2015). Each chamber is filled with a fungus garden, a spongy, yellow-brown or yellow-grey mass of mycelium and substrate containing eggs, brood, and workers. The fungus garden can be sessile on the chamber floor (Leal et al. 2011) but is more often suspended from the roof of the chamber by small rootlets (Weber 1972, Fernández- Marín et al. 2004, Mehdiabadi and Schultz 2010). A study in Panamá found that 40 out of 44 colonies of Sericomyrmex amabilis had a single foundress queen and that the remaining nests had two to four queens (Fernández-Marín et al. 2004), while a study of Brazilian cerrado species found one to two queens in colonies of Sericomyrmex scrobifer (Leal et al. 2011). Colony sizes vary from medium (several hundred workers) to large (~6000 workers), but Sericomyrmex colonies never become as large as the colonies of Atta leaf-cutter ants (Weber 1972). Sericomyrmex workers mostly forage alone, but in species with larger colonies (e.g., Sericomyrmex mayri Forel and Sericomyrmex bondari Borgmeier) they sometimes form short, dense foraging columns in the immediate area of the nest entrance. They collect organic material, mostly freshly fallen leaflets, sometimes moss, grass, leaves, and fruits (Leal and Oliveira 1998, De Fine Licht and Boomsma 2010). They can also feed on seeds but will usually not bring them back to the nest (Feldmann et al. 2000). They cut leaves on some occasions (Weber 1967, 1972) but do not climb vegetation in order to cut leaves as leaf-cutter ants do (Leal et al. 2011). Local occurrences of Sericomyrmex are often patchy, with colonies abundant in some areas but absent or sparse in adjacent areas. In areas where they are abundant they can be very dense, with nest entrances very close to each other. The species 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).

Morphology
Ješovnik & Schultz (2017) - Interestingly, the white layer on the eyes of saussurei is exceptionally consistent compared to character-state distributions in other Sericomyrmex species. In fact, it is among the most consistent of all morphological characters across all Sericomyrmex species. In all specimens of S. saussurei examined, from across a large geographic range, the eyes are convex and covered with a thick white layer. Similar white layers are also seen in parvulus and opacus. In those species, however, the layer itself is thinner and often incomplete, and the eyes are generally smaller and flat, creating a distinctly different appearance. Also, in both parvulus and opacus the layer is completely absent in some individuals or populations. In the remaining species of Sericomyrmex the eyes are uncoated, without a white layer. It would be interesting to determine the biological significance of this layer and to analyze its chemical properties. Based on our SEM images it seems to be an extension of the waxy, crystal-like cuticular layer found on the integuments of workers and queens in all Sericomyrmex species, but which is absent in males and in callow workers, as well as in some individuals of Sericomyrmex maravalhas. Why this layer extends to and completely covers the eyes in some species but not others remains unknown.

Nomenclature

 *  SERICOMYRMEX [Myrmicinae: Attini]
 * Sericomyrmex Mayr, 1865: 18. Type-species: Sericomyrmex opacus, by monotypy.

All the material under this section is based on: Jesovnik and Schultz (2017)

Mayr described the genus Sericomyrmex and its first species, Sericomyrmex opacus, based on specimens collected by the American naturalist E. Norton in Córdoba, Mexico (Mayr 1865).

The genus Sericomyrmex is considered taxonomically difficult mostly because of its substantial morphological homogeneity, further complicated by considerable variation within species and sometimes within colonies, because some species are mildly polymorphic. The differences between some species (e.g., between S. parvulus]] and S. opacus) can be very subtle, lacking discrete character states, and not much greater than the differences sometimes observed between two workers of the same species. The original species descriptions tend to be short and are often based on just a few collected foragers, both common practices at the time in which they were written, instead of on complete nest series taken from several geographical locations. As a consequence, some of the characters cited in those descriptions are not useful for distinguishing between Sericomyrmex species, since they are now known to vary within species.

Worker
General appearance. Small to medium-sized (mean WL: parvulus=1.04 mm, mayri=1.71 mm), body color evenly light yellow or yellowish-brown to deep, ferrugineous-brown. Some individuals with darker areas on frontal lobe and along frontal carina. Integument dull and opaque, in adult workers covered with apparent waxy, crystal-shaped cuticular layer, absent in callow workers and males, and completely or partially absent in some workers of Sericomyrmex maravalhas (see maravalhas notes for details).

Entire body covered with dense pubescence: short, thin, appressed to decumbent, light yellow hairs. Entire body also with thicker, longer, often flexuous hairs, yellow to gray or black, darker in color at base, appressed to erect. Dorsum of head, mesosoma, and metasoma with hair denser and longer than remaining body. Mandible and metapleural gland bulla devoid of hairs, glossy. Head. In full-face view cordate, tapering anteriorly, lateral margin slightly convex, posterior cephalic corner acute to rounded, never with tubercles or spines. Posterior cephalic margin medially emarginate. Vertex medially impressed in full-face view, low tumuli laterad of vertexal impression, one on each side, sometimes distinct and sometimes barely visible. Anterior clypeal border broadly convex. Clypeal apron with shallow median notch, median clypeal seta arising from its middle. Mandible triangular, with 7–9 teeth, dorsally glossy, variably striate or smooth. Lateral margin of mandible in full-face view straight in basal two-thirds, curving at apex. Edge of masticatory margin with 2–6 short, decumbent hairs, directed medially. Slender, light-colored, short hairs evenly but sparsely distributed over entire mandibular dorsum, directed apically. Palp formula 4, 2. Preocular carina directed posterad, fading posterior to eye, never reaching posterior cephalic corner. Eye distinct, placed laterally on anterior half of head, variable in size and shape, from almost flat to convex and protruding laterally, in some species covered with silvery-white layer of unknown, waxy substance, hereafter referred to as “white layer.” Number of ommatidia across largest eye diameter 7–14. Frontal lobe always completely covering antennal condylar bulb; with three distinct margins: posterior, lateral, and medial; and with dorsal fenestra, i.e., thinner, sometimes almost translucent area just above condylar bulb, circular in shape and darker in color on edges. Lobe size and shape vary. Frontal carina straight to slightly curved laterally, diverging toward posterior cephalic corner, in some species reaching cephalic corner, in others not. Area laterad of frontal carina (incomplete antennal scrobe) with less hair than rest of head, mostly appressed pubescence. Antenna 11-segmented, lacking distinct antennal club. Antennal scape relatively short, in most species not reaching posterior cephalic corner, slightly narrower basally, angled in first third, slightly curved distally. Antennal pedicel longer than funicular segments two and three combined.

Mesosoma. Anteroventral pronotal corner obtuse, sometimes bearing small, anterad-directed denticle, best seen in lateral view. Pronotum anteriorly with single low median tumulus and two larger lateral pronotal tubercles, both best seen in frontodorsal view. Mesonotum with two lateral mesonotal tubercles (most prominent feature on mesosoma) and two smaller, lower, posterior mesonotal tubercles. Metanotal groove distinct, best seen in lateral view, with reduced hairs. Propodeum dorsally with two longitudinal, low, posteriorly diverging carinae, sometimes weakly serrate, each often with small and blunt posterodorsal denticle. Propodeal spiracular carinae absent, propodeal spiracle opening oval, directed posterad, mounted on small tubercle. Metapleural gland bulla transparent, glossy, devoid of hairs. Mesotibial and metatibial spurs absent. Arolium present.

Metasoma. Petiole with short peduncle, in lateral view longer than postpetiole, lacking subpetiolar process. Petiole and postpetiole each with pair of low, short, sometimes serrate, longitudinal dorsal carinae, sometimes reduced to low denticles, best seen in dorsolateral view. Postpetiole in some species with another pair of carinae laterally, sometimes reduced to low denticles. Postpetiole in dorsal view broader than long and broader than petiole. Ventral sternite of postpetiole protruding anteriorly, sometimes forming lobe in lateral view. First gastral tergite (A4) larger and longer than first sternite, dorsally overhanging remaining segments (A5–A8). First gastral tergite laterally impressed on both sides, with weakly to strongly developed lateral longitudinal carinae along anterior two-thirds, sometimes also with weakly to strongly developed anteromedian dorsal carinae.

Queen
General appearance. Larger than worker (e.g., Sericomyrmex amabilis worker and queen, mean values: HW=1.06 w, 1.32 q; WL=1.37 w, 2.05 q; GL=0.96 w, 1.77 q), color ferrugineous-brown, often darker than worker. Pilosity as in worker or somewhat denser.

Head. In full-face view cordate, cephalic emargination distinct. Vertexal tumulus more pronounced than in worker, bearing glossy, light yellow to dark grey ocellus, integument surrounding ocellus sometimes darker than elsewhere. Ocelli half embedded in integument and covered with hair so in full-face view usually only median anterior ocellus visible. Preocular carina in some species fading posterior to eye, as in worker. In one species, preocular carina extending beyond eye, becoming thinner posterad and almost meeting frontal carina to form complete scrobe. In third state preocular carina fades posterior to eye but one to several short, weak, isolated supraocular carinae are visible, not reaching posterior cephalic corner. Eye larger and more convex than in worker, without white layer (except partially in eye of Sericomyrmex saussurei queen). Frontal lobe as in worker or more robust.

Mesosoma. Lateral pronotal tubercles conical, short and blunt, best seen in dorsal view. Anapleural suture (=median episternal groove) wide, shallow, curved in lateral view, anepisternum inflated. Scutum in dorsal view with notauli reduced, not converging medially, forming faint, shallow impression, sometimes entirely absent. Median mesoscutal line absent or with only anterior portion visible, sometimes forming weak costa, posteriorly with shallow longitudinal impressions at each side. Parapsidal lines thin, slightly curved. Axillae in dorsal view laterally rounded, narrowing medially, entirely separated from one another by shallow groove, groove sometimes transversely costate. Scutellum slightly convex in lateral view, in dorsal view narrowing posteriorly, posterior margin with wide, shallow, median V-shaped notch, notch sometimes continuing into median impression that divides scutellum in two lateral parts. Propodeum in dorsal view with two low, posteriorly diverging carinae, often reduced to laterally flattened, obtuse denticles.

Wings. Terminology follows Goulet and Huber (1993) and Yoshimura and Fisher (2012). Light to dark brown, opaque, covered with minute pilosity, veins brown. Forewing (length: 4.85–8.03 mm) with following veins: costa (C), Sc+R, media (M), cubitus (Cu), anal vein (A), radius (R1), radial sector (Rs), M+Cu, r-rs, and cu-a. Radial sector reaching costal margin, cubitus and media weaker towards wing margin, anal vein not extending distad after cu-a, cubitus in S. mayri sometimes with 1–2 short spur veins distally. Five closed cells: costal (C), radial (R), cubital (Cu), first radial 1 (1R1), and first radial 2 (2R1). Pterostigma weakly developed. Hindwing (Length: 6–5.28 mm) with reduced venation: Sc+R, radial sector (Rs), M+Cu, cubitus (Cu), media (M), anal vein (A), and cu-a. Two enclosed cells: radial and cubital, cubital much smaller. Radial sector vein and media not reaching wing margin, cubitus in S. mayri sometimes with 1–3 short spur veins distally. Anterior margin of hindwing with 7–9 hamuli, varying within species.

Metasoma. Petiole compact, without subpetiolar process, petiolar peduncle short. Petiole with two dorsal denticles, often acute, more prominent than in worker, and with two smaller lateral denticles, best seen in frontodorsal view. Postpetiole broader than long and broader than petiole in dorsal view, with two dorsal and two lateral short longitudinal carinae, sometimes reduced to low denticles, best seen in frontodorsal view. Size and sharpness of petiolar denticles usually correlated with body size. First gastral tergite (A4) larger and longer than first sternite, laterally impressed on both sides, with pair of strongly developed lateral longitudinal carinae, dorsally with shallow longitudinal anteromedian groove, anteromedian dorsal carinae on each side of groove moderately developed to absent.

Male
General appearance. Body color pale yellow-brown to dark brown, head darker, antennae and legs lighter than rest. Body covered with fine pubescence and thicker hairs, sparser than in worker and queen. Area around ocellus and petiolar and postpetiolar dorsum with more hair; mesosoma and gaster, dorsally and laterally, with less hair. Integument relatively dull, with reticulate sculpture, but shinier than very opaque integument of worker and queen. SEM images indicate males lack waxy, crystal-shaped cuticular layer, present in workers and queens.

Head. In full-face view obovate to subquadrate, as long as wide to longer than wide (CI=100–133), posterior cephalic margin straight, without emargination, lateral margin straight or slightly convex. Vertex with three large, white to grey ocellus, mounted on sides of small tumuli. Clypeus in full-face view broadly convex. Clypeal apron with shallow median notch, median clypeal seta arising from its middle. Mandible triangular, with lateral margin straight, except curving at apex, dorsal surface finely reticulate near the basal angle and with thin, light yellow, sparse hairs, directed apically. Masticatory margin with 5–7 teeth, usually 4–5 teeth in distal two-thirds, gap between single basal tooth and rest; sometimes this gap absent and teeth distributed more or less evenly. Palp formula: 4, 2. Preocular carina varying in length, directed posterad, curved medially before fading. Eye convex, large, protruding laterally, 20–28 ommatidia across largest diameter. Frontal lobe small and narrow, not completely concealing condylar bulb, without clearly defined medial, lateral, and posterior margin. Frontal carina absent. Antennal scape long, straight, extending well beyond posterior cephalic corner (SI=74–95). Antenna 12-segmented, without distinct club, pedicel thicker and longer than funicular segments two or three.

Mesosoma. Anteroventral pronotal corner obtuse, never denticulate. Lateral pronotal tubercles absent. Mesopleuron with shallow anapleural suture. Katepisternum inflated and rounded ventrally, best seen in lateral view. Scutum in dorsal view with notauli distinct, almost converging posteriorly. Median mesoscutal line faint, best visible in antero-dorsal view, fading posteriorly. Parapsidal lines thin, slightly curved. Notauli, mesoscutal, and parapsidal lines sometimes lighter than surrounding integument and sometimes darker; this variation seen within males from same nest. Axillae rounded laterally, entirely separated from one another by shallow groove, groove sometimes transversely costate. Scutellum inflated, convex in lateral view, in dorsal view narrowing posteriorly, posterior margin straight or with shallow medial notch. Propodeum dorsally completely smooth or with two low, short propodeal carinae. Propodeal spiracle directed posterad, mounted on small tubercle, best seen in dorsal view, propodeal spiracular carina absent.

Wings. Venation and cells same as in queen, wings smaller and lighter in color. Forewing length: 3.48–6.25 mm, hindwing length: 2.20–4.29 mm. Anterior margin of hindwing with 7–10 hamuli.

Metasoma. Petiole with short peduncle, without subpetiolar process, longer than postpetiole in lateral view, with pair of small denticles laterally and sometimes also dorsally. Postpetiole in dorsal view broader than long, broader than petiole, laterally with pair of very reduced denticles, dorsally smooth. Gaster in dorsal view elliptical, without any carinae. First gastral tergite and sternite equal in length and size, gastral tergites and sternites 2–5 (i.e. A5 to A8) visible in dorsal view and with rows of long, decumbent to suberect hairs on posterior borders, hairs on dorsal side slightly longer than ventral hairs.

Genitalia. Abdominal sternum IX with long and thin spiculum, lateral margins extending anteriorly about length of spiculum, tapering posteriorly, posterior margin straight, without apical triangle or with apical triangle low, surface weakly reticulate and with simple hairs posteriorly. Basimere smooth, longer than broad, telomeres short, medially curved and bluntly rounded apically, with sparse simple hairs. Volsella with strongly medially curved, clubbed digitus, cuspis ventrally produced into rounded lobe, sometimes with 1–2 teeth, with thin, sparse, medially pointed hairs. Volsella basally with two additional processes, best seen in ventral view, here named proximal basivolsellar and distal basivolsellar process, previously undescribed structures of male ant genitalia. Valviceps of penisvalve with 10–12 long, pointed denticles along ventral edge, distally broadly notched.

Larva
Description based on SEM study of 23 prepupal larvae from nine different nests of Sericomyrmex amabilis, S. bondari, S. mayri, S. opacus, S. parvulus, S. saramama]], and S. saussurei. Larvae for S. lutzi]], S. radioheadi]], S. scrobifer, and S. maravalhas unknown. Terminology follows Schultz and Meier (1995), including “seta” (with visible basal setal socket) and “hair” (without visible socket). We only use the term seta, since the socket is always visible. When setal base is visible but seta is either absent or very short, we use sensilliform or papilliform, respectively.

Body profile “attoid” sensu Wheeler and Wheeler (1948), i.e., longitudinally curved, bean-shaped, and with ventral profile shorter than dorsal. Thoracic-abdominal articulation absent, thoracic intersegmental constrictions superficial, deep lateral depressions associated with abdominal spiracles absent, and leg vestiges visible as open slits ventrally on thorax. Dorsal and lateral body surfaces from bare (S. parvulus) to with more than 15 setae on each side (S. amabilis). Setae simple, some long and flexuous, some (e.g., on clypeus and labrum) reduced, papilliform, with only setal socket and very short bristle.

Head. Genal lobe present. One long supra-antennal seta posterior to each antenna (“Sericomyrmex condition” sensu Schultz and Meier 1995) in some species (S. amabilis, S. bondari, S. mayri), absent in others (S. opacus, S. parvulus, S. saramama, S. saussurei). No setae between antennae, four setae on each gena, except in S. mayri with six. Two supraclypeal setae in all species, reduced to papillae, except S. bondari with two long supraclypeal setae. Spinules on head restricted to clypeus, genae, and vertex; in S. opacus almost completely absent, restricted to few on clypeus.

Mouthparts. Labrum monolobate, narrow, inflated, with anterior setae absent or reduced to sensilla or papillae. Mandible fleshy and subconical, covered with spinules. Subapical mandibular tooth absent, mandibular apical tooth distinct, divided in some species, undivided in others. Mandibular gnathobases absent. Basal portion of maxilla fused with head capsule, maxillary palp digitiform, widely removed laterad from galea. Maxillary accessory palpal sensillum present. Area between maxillary galea and palp with two reduced setae. Labium short, only feebly protruding, lateral sericteral protuberances absent, labial palps reduced to sensilla. Spinules on labium completely absent or sparse, present only anterior to sericteries. Ventral part of labium (posterior to sericteries) usually not visible. Hypopharyngeal spinules densely distributed and predominantly multidentate.

Thorax and abdomen. Thoracic segment one (T1) ventrally with transverse rows of sparsely distributed multidentate spinules (except S. saussurei). Thoracic segments two and three (T2 and T3) without multidentate spinules, except in S. bondari, which has sparse spinules on T2. Number of setae on ventral thoracic segments varies from 0–5 on each side of each segment, in general T1 with more setae than T2 or T3. Less than ten setae on T1 in most species, except S. mayri with 10–14 setae on T1, the number of setae considered sufficient to form, in combination with genal setae (six in S. mayri), “feeding basket” of attine ant larvae (Schultz and Meier 1995). Abdominal segments lacking any ventromedian protuberances, with variable numbers of long and simple setae, except in S. parvulus, which has no ventral abdominal setae. Anal setal pattern: single pair of papilliform to long setae anterior to anal opening. Additional pair of setae sometimes ventrolaterally on segment nine (e.g., in S. bondari), but widely removed. Ventral anal lip absent.

Etymology
Sericomyrmex ants are light yellow to deep ferrugineous brown and densely covered with long, flexuous hairs, which, to the naked eye, give them a silky, velvety appearance and which earned them their name: “sericeus” means “silky” in Latin.