Amoimyrmex

A small genus of leaf-cutter ants that occur in southern South America.

Identification
Distinguished from other leaf‐cutting ants (Acromyrmex, Atta) by the combination of the following: dorsum of promesonotum with 3 to 4 pairs of spines; first tergum of gaster smooth, without tubercles; integument covered by irregular striae, mostly on head and mesosoma.

Biology
Cristiano, Cardoso and Sandoval (2020) - A small southern South American genus of ground-nesting leaf-cutter ants. They occur in open habitats of the Chaco, Pampas, and Campos Sulinos, and sand dune areas of the Atlantic Coast in Brazil (Restingas), where they may be locally abundant.

Nests of Amoimyrmex have several entrances. Each has a bare perimeter of exposed soil at ground level, never with a mound. The soil nest consists of several subspherical to subelliptical chambers, connected by tunnels, containing the fungus garden, which is often suspended from the roof of the chamber (Weber 1972). Some biological data and life history observations have been recorded for the individual species of the genus. Collectively, this information is to scant to afford any general interpretations.

Species of Amoimyrmex were previously considered to be part of the Acromyrmex subgenus Moellerius, due to the lack of supraocular spines in workers, their short and stout mandibles and their use of grass (monocots) as substrates on which to grow fungus (Emery 1905; Gonçalves 1961; Weber 1972). These species were always placed apart from the other species of Acromyrmex due to their lack of tubercles on the first gastral tergite (Mayhé‐Nunes 1991; Cristiano et al. 2013).

They are further distinguished from other leaf‐cutting ants by their biology (nest, number of individuals per colony, caste and subcaste), chromosome counts and cytogenetical markers and behaviour.

Based on our molecular and morphological analyses, Amoimyrmex was determined to be the sister group of the remaining leaf‐cutting ants, having diverged simultaneously from their shared common ancestor. This phylogenetic position is our strongest argument for transferring the species studied here to a new genus because species of Amoimyrmex are not more closely related to either Atta or Acromyrmex. This is also reflected in the mosaic of characters that are shared with species in Atta and in Acromyrmex. These characters are probably plesiomorphic character states within the leaf‐cutting ants: there are three to four pairs of spines on the promesonotum in Amoimyrmex and Acromyrmex, while Atta has two pairs of spines, and the smooth gastral tergum occurs in Amoimyrmex and Atta, whereas Acromyrmex has a tuberculate gaster (Brandão et al. 2011; Cristiano et al. 2013). We highlight the diploid chromosome number of 22 for the new genus, the same as for Atta spp., while the Acromyrmex species for which we have data have a diploid chromosome number of 38, with the exception of the social parasite Acromyrmex ameliae with 2n = 36 (reviewed in Cardoso et al. 2018). These combined data indicate that the species studied here do not accord with any described genus of fungus‐farming ants.

Based on their fungus‐farming agricultural system, Amoimyrmex belongs to the informal group of fungus‐farming ants called ‘higher attine’. Among the ‘higher attine’, Trachymyrmex sensu stricto – the clade composed of most of the North American species (Solomon et al. 2019) – is the sister group of Amoimyrmex + Atta + Acromyrmex. Ants of the genus Trachymyrmex have spines or tubercles on their posterior cephalic margins, several mesonotal spines and tuberculated gasters (Rabeling et al. 2007). However, the spines are rough and microscopically multituberculate, which are character states not observed among leaf‐cutting ants (Mayhé‐Nunes & Brandão 2002).

Worker Morphology
These characters are collated in the table "WorkerMorphology". View table.

Nomenclature

 *  [Myrmicinae: Attini]
 * Amoimyrmex Cristiano, Cardoso & Sandoval, 2020: 647. Type-species: Atta striata, by original designation.

Worker
General appearance. Polymorphic fungus‐farming ant. Total length varying from 3.36 to 8.82 mm. Weber’s length varying from 0.90 to 2.85 mm. Body yellowish‐brown, reddish‐brown, dark brown or almost black. Some specimens with darker areas at head apex, occipital lobe and frontal regions. Gaster always darker than rest of the body. Shiny integument, covered by irregular striae and non‐decumbent pilosity, hairs with apex hook shaped. Gaster smooth, without tubercles, shinier and with fewer striae than the rest of body.

Head. In full‐face view, posterior cephalic margin medially emarginate. Posterior cephalic corner angle acute to rounded, with a small spine or tubercle directed upwards (faintly rising on dorsum of head in lateral view) and a small tubercle laterad. Integument covered by roughly parallel irregular striae. Anterior clypeal border convex. Clypeal apron with long setae reaching to almost half of mandibular length, originating closer to its posterior margin. Mandible triangular, striate to smooth, with 8–11 teeth, the apical being larger and more prominent than others. Edge of masticatory margin with short light‐coloured hairs, directed medially. Short hairs sparsely distributed over mandibular dorsum. Palp formula 4, 2. Preocular carina located anterior to eye, almost reaching the posterior cephalic corner, forming a semicircle. Eye convex and protruding laterally, placed on the anterior half of head. Number of ommatidia across largest eye diameter 16–24. Supraocular spine absent. Frontal lobe partially covering antennal insertions, with a hook shape. Frontal carina present (may be confused with striae) conspicuously extending to vertex, in some cases almost reaching posterior cephalic corner. Antennal scrobe absent. Antenna 11‐segmented, lacking distinct antennal club. Antennal scape slightly surpassing posterior cephalic margin, less than 1/3 of its length (SI = 73.02–117.65).

Mesosoma. Pronotum armed with two large lateral pronotal spines and, in some cases, minute median pronotal projections as tubercles or spines, best seen in frontal view. Mesonotum with two lateral mesonotal spines and two posterior mesonotal spines, smaller than lateral pronotal spines. Metanotal groove distinct. Propodeum bearing two large spines, longer than lateral pronotal spines. Propodeal spiracular carinae present, propodeal spiracle opening rounded, with a shiny and gilded border, projecting diagonally with apex pointing slightly up and backwards (relative to the longitudinal axis of the body, in lateral view). Metapleural gland bulla golden, glossy, devoid of hairs, conspicuous, projecting parallel with apex pointing backwards. Integument covered by roughly parallel irregular striae, yellowish non‐decumbent setae, varying in length and reaching the surface of the spines, and decumbent pilosity in the pronotal region and at the base of the propodeal spines. Legs with conspicuous reticulation, in some cases with coxa and femur darker than the rest of body. Mesotibial and metatibial spurs absent. Tarsal claws with arolium present.

Metasoma. Petiole with short peduncle, similar to postpetiole in lateral view, with small subpetiolar process. Petiolar node with four spine‐like projections directed up and backwards and two lateral longitudinal carinae. Postpetiole in dorsal view broader than long and broader than petiole. Postpetiolar node with two small teeth on lateral margins, located in the medial portion, projected to the sides and slightly backwards, best seen in dorsal view, and two lateral longitudinal carinae. Dorso‐posterior region of node in some cases with a series of two to eight small tubercles or spines. Well‐developed subpostpetiolar process. First gastral tergite larger and longer than first sternite and sometimes with a pair of reddish to yellowish spots. Integument of petiole and postpetiole covered by irregular striae. Gaster with smooth and shiny integument, devoid of tubercles. Gastral pilosity and striae vary within species.

Queen
General appearance. Larger than workers (e.g. ‘’silvestrii’’ worker (W) and (Q) queen, mean values: HW = 1.83 W, 2.28 Q; L = 2.20 W, 3.05 Q; GL = 1.71 W, 3.05 Q; TL = 7.07 W, 10.30 Q), colour reddish‐brown to yellowish‐brown, with gaster darker than rest of the body. Non‐decumbent pilosity less dense and shorter than in workers. Shiny integument covered by striae. Gaster smooth, without tubercles, brighter and with less striae than rest of the body and bearing two yellowish well‐defined spots.

Head. In full‐face view, posterior cephalic margin medially emarginate but less accentuated than worker. Posterior cephalic corner angular to rounded, with a small spine or tubercle directed upwards. Integument covered by roughly parallel irregular striae, as in worker. Vertexal tumulus bearing three conspicuous, glossy, light yellow ocelli of the same size. Frontal carina present (may be confused with striae), shorter than in worker. Eye larger and less convex than in worker. Number of ommatidia across largest eye diameter 24–30. Mandibles with apical tooth larger and more prominent than in worker. Antenna very similar to that of worker.

Mesosoma. Pronotum with a pair of inferior tuberculiform pronotal spines and a pair of lateral pronotal spines, directed forwards and slightly to the sides, best seen in dorsal view. Scutum in dorsal view with reduced notauli, forming a shallow impression, sometimes completely absent. Median mesoscutal line (may be confused with striae) with only anterior portion visible as a slight mark on the integument. Parapsidal lines thin, forming a superficial impression, more visible on the back of the scutum. Axillae in dorsal view converging laterally, narrowing medially, partially separated in the middle by a groove. Scutellum strongly convex in lateral view, narrowing posteriorly with a trapezoidal outline in dorsal view, bearing two tuberculiform denticles at posterior margin. Propodeum with a pair of protruding long spines, directed upwards, in lateral view. Integument with well‐defined longitudinal and parallel striae, non‐decumbent pilosity with straight or hook‐shaped apex. Fine decumbent pilosity present on pronotum and at the base of propodeal spines.

Wings. Light to dark brown, opaque and covered with minute pilosity. Veins darker than the surface of the cells. Forewing (length: 8.10–7.83 mm) with following veins: costa (C), Sc + R, media (M), M + Cu, cubitus (Cu), cu‐a, anal vein (A), radius (R1), radial sector (Rs), r‐rs and rs‐m. The following closed cells are formed: costal (Co), radial (R), cubital (Cu), first radial 1 (1R1) and first radial 2 (1R2). Cubitus and media towards wing margin, anal vein not extending distad of cu‐a and cubitus with two spur veins distally. Pterostigma well developed. Hindwing (length: 5.85– 5.40 mm) with the following veins (Fig. 7h): Sc + R, media (M), M + Cu, cubitus (Cu), cu‐a, anal vein (A), radius (R1) and radial sector (Rs). The following closed cells are formed: costal (Co), radial (R) and cubital (Cu). Radial sector vein and cubitus almost reaching wing margin, cubitus pointing upwards. Anal vein with a small extension after cu‐a, not reaching wing margin. Anterior margin with 12 to 14 hamuli. Metasoma. Petiole with short peduncle, in lateral view, and with a well‐developed spiniform subpetiolar process. Petiolar node with two dorsal spine‐like projections and two smaller lateral denticles. Postpetiole in dorsal view broader than long, with two small teeth on the lateral margins, located in the medial portion, projecting to the sides and slightly backwards. Subpostpetiolar process present, smaller than in worker. Integument of petiole and postpetiole covered by irregular striae. Gaster with smooth and shiny integument, without tubercles. First gastral tergite with two yellowish spots located anterolaterally in dorsal view. More dense pilosity on gastral tergites two to four. Semicircularstriae present on the first gastral tergite, mainly near the gastral base. Parallel striae in subsequent tergites.

Male
(Genus‐level description of Amoimyrmex male based on Amoimyrmex silvestrii and Amoimyrmex striatus. Males of Amoimyrmex bruchi unknown) General appearance. Larger than workers(e.g. silvestrii worker (W) and (M) male, mean values: HW = 1.83 W, 1.40 M; WL = 2.20 W, 2.51 M; GL = 1.71 W, 2.72M; TL = 7.07 W, 7.90M), colour dark brown, with gaster slightly lighter than the rest of body. Antennae with apical six antennomeres ranging from reddish‐brown to yellowish‐brown, approaching apical segment of flagellum. Body covered with fine pubescence, sparser and shorter than in worker and queen. Integument reticulated, dull and covered by striae. Gaster smooth, without tubercles, brighter and with less striae than the rest of body.

Head. In full‐face view subquadrate, posterior cephalic margin straight, without emargination. Posterior cephalic corner rounded with a small spine or tubercle directed upwards. Vertex with three large, dull, light yellow ocelli of the same size. Integument opaque, reticulated and covered by irregular striae, as in worker and queen. Parallel longitudinal striae arising from the vertexal tumulus and curving towards the posterior cephalic corner. Anterior clypeal border convex. Clypeal apron with five long setae reaching at least half of mandibular length. Long, curved mandible lacking distinct basal and masticatory margins, with several prolonged parallel striae. Mandible varying from dark brown to light brown, with only two teeth at apex, the apical larger and more prominent. Palp formula 4, 2. Preocular carina located anterior to eye, almost reaching posterior cephalic corner, forming a semicircle. Eye larger and more convex than worker and queen. Number of ommatidia across largest eye diameter 34–36. Frontal lobe partially concealing condylar bulb, with defined margins. Frontal carina present (may be confused with striae), less conspicuous and shorter than worker and queen, extending to height of median ocellus. Antennal scrobe absent. Antenna 13‐segmented, without distinct club. Antennal scape long, straight, extending well beyond posterior cephalic corner (SI = 111.50–113.33).

Mesosoma. Pronotum with a pair of inferior tuberculiform pronotal spines, without lateral pronotal spines. Scutum in dorsal view without notauli. Median mesoscutal line (may be confused with striae) with only posterior portion visible as a slight mark on integument. Impressed parapsidal lines, more visible and strong on the back of the scutum. Axillae in dorsal view straight laterally, narrowing medially, entirely separated from one another by a transversely costate groove. Scutellum convex, less than queen in lateral view, narrowing posteriorly with a trapezoidal outline in dorsal view, bearing two tuberculiform denticles in the posterior margin. Propodeum with a pair of protruding long spines, directed backwards. Integument with well‐defined longitudinal and parallel striae, non‐decumbent pilosity with straight or hook‐shaped apex in scutum. Fine decumbent pilosity scarce in the pronotum and denser at the base of the propodeal spines.

Wings. Venation and cells similar to queen, wings smaller and slightly lighter in colour. Forewing (length: 6.93–7.10 mm) with a rounded upper angle and in one specimen with an additional closed medial cell (1M). Hindwing (length: 4.59–4.77 mm) with 8 hamuli on its anterior margin. Metasoma. Petiole with short peduncle, subpetiolar process smaller than queen and similar to worker. Petiolar node with two small tubercles on lateral margins, located in the medial portion, projecting to the sides. Postpetiole in dorsal view broader than long, with two small teeth on the lateral margins, projecting to the sides and slightly backwards. Subpostpetiolar process present, smaller than worker and larger than queen. Integument of petiole and postpetiole smooth, with scarce irregular striae, less than those found in the queen. Gaster with smooth and shiny integument, without tubercles and striae. Non‐decumbent pilosity less dense and shorter than in the queen.

Etymology
Amoimyrmex is a neologism formed by the joining the world ‘amoi’ from the Guarani, Mbyá dialect, which means ‘distant ancestor’ or ‘grandparent’, and the Greek term ‘myrmex’, which means ant, thus referring to the position of the clade within the group of leaf‐cutting ants. Defined here, Amoimyrmex is recognised as the first genus to diverge within the evolutionary history of leaf‐cutting ants.