Amoimyrmex striatus

Locally abundant in some areas within its range, colonies of Amoimyrmex striatus live in relatively small subterranean nests in areas of low vegetation cover.

Identification
Cristiano, Cardoso and Sandoval (2020) - Distinguished from congeners by the combination of: body yellowish‐brown to reddish‐brown, with dark brown gaster; pronotum with a pair of minute median pronotal projections as tubercles, best seen in frontal view; shiny integument covered by parallel irregular striae and scarce yellowish non‐decumbent setae, varying in length; posterior cephalic corner acute; anterior portion of the first gastral tergite almost smooth, with few short irregular striae, discernible in medium magnification (~50×)

A. striatus has distinctive irregular parallel striation, a shiny integument (more than A. silvestrii), comparatively less pilosity in the metasoma, and shorter irregular striae in the first gastral tergite, restricted to the insertion of the postpetiole (two‐thirds of the tergite in A. silvestrii).

Distribution
Cristiano, Cardoso and Sandoval (2020) - The species has been found in Argentina, Brazil and Uruguay, three bordering countries of South America (Kusnezov 1963; Delabie et al. 2011; Cristiano et al. 2016; Simões‐Gomes et al. 2017). We consider that the Paraná River Basin delimits the species distribution and influenced the diversification of the species of Amoimyrmex, as well as providing the dispersal route during past geomorphological and climatic changes. Its known distribution in Brazil is restricted to the southern region, in the states of Santa Catarina and Rio Grande do Sul, on the coastal plains, reaching as far north as Florianópolis Island (Cristiano et al. 2016; Simões‐Gomes et al. 2017). In Rio Grande do Sul, the species occurs further inland, with high abundance across the Central Depression of the State (Loeck et al. 2003). This geomorphological region is the likely dispersal route of A. striatus to the southern Brazilian coast (Cristiano et al. 2016). Cristiano et al. (2016) also suggested that A. striatus populations are geographically and genetically structured and estimated the coastal Brazilian populations to be younger than the inland populations. The distribution of the species in Argentina and Uruguay is still not well known, but its distribution in the former seems to be restricted to the provinces of Misiones, Corrientes and Entre Rios, which circumscribe the Paraná River Basin.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina, Bolivia, Brazil, Paraguay, Uruguay.

Biology
Jofre et al. (2018) - Amoimyrmex striatus is one of the most common leaf-cutting ant species in Argentina (Farji-Brener & Ruggiero, 1994). This species constructs relative small subterranean nests in areas of low vegetation cover. Workers forage at close distances from the nest without a defined network trail system (Boneto, 1959; Diehl-Fleig, 1995). Amoimyrmex striatus consumes a high percentage of plant species available in the environment (Armani & Quirán, 2007; Nobua-Behrmann, 2014), and their degree of selectivity or opportunism depends on the availability of resources within the proximity of their colonies (Farji-Brener & Protomastro, 1992; Pilati et al., 1997; Franzel & Farji-Brener, 2000; Armani & Quirán, 2007; Nobua-Behrmann, 2014).

Nesting Biology
Cristiano, Cardoso and Sandoval (2020) - Amoimyrmex striatus nests in open and well‐drained soils and prefers soils poor in organic matter (Diehl‐Fleig & Rocha 1998). The nests can be easily recognized by the bare soil around the nest entrances, which consist of entrance holes without craters or mounds (Diehl‐Fleig 1995). It is suspected that the workers clean the soil surface above the subterranean chambers by removing all vegetation, as if it had been weeded and swept. The nest architecture, as presented as a figure by Weber, consists of several small and interconnected chambers. The nest openings are 5–7 mm wide and 49–150 mm deep, with the first chamber beginning lateral to the tunnel (Della‐ Lucia & Moreira 1993). The fungus garden is suspended from the roof of the chamber, although in our field expeditions, we found that the fungus garden may also be sessile on the chamber floor.

Foraging/Diet
Cristiano, Cardoso and Sandoval (2020) - Amoimyrmex striatus is described as a leaf‐cutting ant specialising on monocots (Gonçalves 1961). However, when studying plant resources used by A. striatus in Florianópolis, Brazil, Lopes (2005) observed that the species used different resources seasonally and could cut both monocot and dicot leaves. Indeed, the author also suggested that A. striatus could be an opportunistic scavenger, using dead exhausted materials for fungus cultures. This behaviour is similar to that observed for other fungus‐farming ants, such as Trachymyrmex, whose workers can facultatively cut fresh material to cultivate their fungus but rely mainly on insect frass and organic debris (Weber 1972). This information improves our understanding of the evolutionary transitions to higher attine agriculture because relying on freshly cut vegetation is congruent with the phylogenetic position of Amoimyrmex.

Nobua-Behrmann et al. (2017) studied temporal and thermal aspects of Amoimyrmex striatus foraging behavior in the Monte Desert in Argentina where this species co-occurs with Acromyrmex lobicornis. Amoimyrmex striatus colonies were active during spring and summer, with no signs of foraging activity during winter. Foraging intensity was highest during spring and summer, with up to 90 ants (foragers returning to the nest) / 5 minutes. During spring and summer, foraging activity was exclusively diurnal with 4 - 6 hours of foraging during the morning and 4 hours in the afternoon, avoiding the hotter midday. This changed to a single period of low activity concentrated during the hottest part of the day in autumn. During the hottest months, the overall daily foraging pattern appears to be complementary to its similar co-occurring congener: ''Am. striatus daily foraging activity begins in the mornings when Ac. lobicornis'' colonies are ceasing their foraging activity cycle. Furthermore, the ending of ''Am. striatus colonies foraging activity in the evenings also coincides with the beginning of Ac. lobicornis'' foraging. During autumn, colonies showed very scarce activity (< 15 ants / 5 minutes in any single colony). Daily and seasonal variations in nest maintenance activity were broadly similar to foraging activity patterns, though spanning larger time ranges than their foraging activity (starting earlier and finishing later).

The ants foraged within a particular temperature range that was relatively constant throughout the year and differed from its congener: ''Am. striatus colonies foraged at higher temperatures than Ac. lobicornis'' in all seasons (26 - 45 °C vs. 16 - 35 °C, respectively).

Jofre et al. (2018) - Foraging behavior was studied in a natural reserve of San Luis, Argentina. The chaco vegetation found within the reserve had in the past been affected by overgrazing, fire, and logging. In addition to quantifying the plants selected by foragers, it was found that small nests of ''Am. striatus'' harvested a greater percent of the plant species available in their foraging area and showed a higher level of selectivity than larger nests.

Reproduction
Cristiano, Cardoso and Sandoval (2020) - Colonies of the species may be founded by a single fertilized female (haplometrosis) or by the association of more than one fertilised female (pleometrosis), depending on density and physical distances between founders (Diehl‐Fleig & de Araújo 1996). The production of sexuals is suggested to occur from the end of October to mid‐January and to be male biased (Diehl‐Fleig 1993). Nuptial flights seem to occur in small and patchy daily swarms (Diehl‐ Fleig 1993).

Ecology
Barrera et al. (2015) studied the diversity of leaf cutting ants along a forest-edge-agriculture habitat gradient. Their study site, in Chaco Serrano of Central Argentina, had forest remnants of various sizes within an agriculture area with wheat, soy and maize. A. striatus was moderately abundant (17% of the 162 colonies sampled). Along the forest edge it was similar in abundance to Acromyrmex lundii, with Acromyrmex crassispinus also present but occurring at a slightly lower abundance. A few colonies of Acromyrmex heyeri and Amoimyrmex silvestrii were also found along the forest edge. Ten nets were found within 5m of the forest edge but none were sampled 25m from the forest edge in the croplands. A. striatus was not found in the forest interior but was the most common species in the cropland matrix close to the forest edge. This suggests this species may do well in disturbed areas. In some regions A. striatus is known as a pest species (Diehl-Fleig, 1993).

Reproductive Castes
Diploid males are known to occur in this species (found in 0.7% of 140 examined nests) (Araújo & Cavalli-Molina, 2001; Cournault & Aron, 2009).

Nomenclature

 * . Atta striata Roger, 1863a: 202 (w.q.m.) URUGUAY.
 * Combination in Atta (Oecodoma): Mayr, 1863: 458;
 * combination in Atta (Acromyrmex): Forel, 1885a: 361; Emery, 1888c: 357;
 * combination in Atta (Moellerius): Emery, 1905c: 42;
 * combination in Acromyrmex: Bruch, 1914: 217;
 * combination in Acromyrmex (Moellerius): Gallardo, 1916d: 338; Emery, 1924d: 351.
 * combination in Amoimyrmex: Cristiano, Cardoso & Sandoval, 2020: 667.
 * Status as species: Roger, 1863b: 35; Mayr, 1863: 458; Forel, 1885a: 361 (in key); Emery, 1888c: 357; Emery, 1888e: 690; Dalla Torre, 1893: 154; von Jhering, 1894: 388; Forel, 1895b: 139; Emery, 1905c: 42; Emery, 1906c: 166; Forel, 1912e: 181; Bruch, 1914: 217; Gallardo, 1916d: 338; Santschi, 1916e: 389; Emery, 1924d: 351; Santschi, 1925a: 389 (in key); Borgmeier, 1927c: 136; Kusnezov, 1953b: 338; Kusnezov, 1956: 34 (in key); Gonçalves, 1961: 129; Kempf, 1972a: 16; Zolessi & Abenante, 1977: 78; Zolessi, et al. 1988: 5; Fowler, 1988: 290; Cherrett & Cherrett, 1989: 52; Brandão, 1991: 323; Bolton, 1995b: 57; Wild, 2007b: 30.
 * Senior synonym of laeviventris: Fowler, 1988: 290; Brandão, 1991: 323; Bolton, 1995b: 57.
 * laeviventris. Acromyrmex (Moellerius) striatus var. laeviventris Santschi, 1925a: 388.
 * [First available use of Acromyrmex (Moellerius) striatus st. silvestrii var. laeviventris Santschi, 1920d: 380 (w.) ARGENTINA (Jujuy, Santa Fe); unavailable (infrasubspecific) name.]
 * Subspecies of striatus: Kempf, 1972a: 16.
 * Junior synonym of striatus: Fowler, 1988: 290; Brandão, 1991: 323; Bolton, 1995b: 55.

Type Material
Cristiano, Cardoso and Sandoval (2020) - We could not locate the type specimen Roger used to describe A. striatus. Roger questioned if the reproductive caste and worker specimens were the same species (Roger 1863). The types were also from different countries – the queen and male from Uruguay and the workers from Argentina. Thus the taxonomic history of the species is centered around a series of uncertain identification, as suggested by Klingenberg and Brandão (2009) in the cases of Mycetophylax, Cyphomyrmex and Myrmicocrypta.

Karyotype


Amoimyrmex striatus shares the characteristics of both Acromyrmex and Atta, as it presents peculiarities such as its karyotype formula 2K = 20M + 2SM, indicating that A. striatus may be better classified as a genus distinct from its sibling leafcutter ants (Cristiano et al., 2013; de Castro et al., 2020). Cristiano, Cardoso & Sandoval (2020) transferred this species to the newly established genus Amoimyrmex.

Worker
Cristiano, Cardoso and Sandoval (2020) - Range (specimen used for redescription): TL 3.36–8.82 (7.07), HL 0.72–1.94 (1.64), HW 0.70–2.30 (1.83), ML 0.30–0.88 (0.55), SL 0.54–1.82 (1.52), EL 0.10– 0.35 (0.29), WL 0.90–2.80 (2.14), PL 0.24–0.74 (0.55), PW 0.22–0.63 (0.43), PPL 0.24–0.65 (0.48), PPW 0.34–1.12 (0.81), GL 0.96–2.12 (1.71), CI 92.86–141.54 (111.59), MI 29.33–50.00 (33.33), OI 12.96–19.23 (15.58), SI 73.02– 110.34 (92.75) [N = 156].

Head. In full‐face view, posterior cephalic margin medially emarginate. Posterior cephalic corner acute, with a small spine or tubercle directed upwards, faintly visible on dorsum of head in lateral view, and a small tubercle laterad. Integument covered by parallel irregular striae and scarce yellowish non‐decumbent setae, varying in length. Frontal carina, that may be confused with striae, extending to vertex and almost reaching posterior cephalic corner. Mandible with 8–11 teeth (some can be considered denticles, varying in size and position), dorsally glossy with superficial striae and covered by sparse light‐coloured hairs, with larger and thicker ones at masticatory margin. Eye convex, 19–24 ommatidia across largest diameter. Frontal lobe partially covering antennal insertions, with a hook shape. Antennal scrobe absent. Antennal scape slightly surpassing posterior cephalic margin, less than 1/3 of its length (SI = 73.02– 110.34). Colour yellowish‐brown to reddish‐brown. Mesosoma. Pronotum armed with two large lateral pronotal spines and minute median pronotal projections as tubercles, best seen in frontal view. Mesonotum with two lateral mesonotal spines and two posterior mesonotal spines, with similar size and approximately half of the size of the lateral pronotal spines. Propodeum bearing two large spines, moderately larger than lateral pronotal spines. Integument covered by parallel irregular striae and scarce non‐decumbent pilosity, reaching up the spines. Legs with conspicuous reticulation, in some cases with coxa and femur darker than the rest of body. Metasoma. Petiolar node with four spine‐like projections directed up and backwards and two lateral longitudinal carinae. Postpetiole node with two small teeth on lateral margins, located in medial portion, projecting laterally and slightly backwards, best seen in dorsal view, and two lateral longitudinal carinae. Dorso‐posterior region of node with two small tubercles. Integument of petiole and postpetiole covered by parallel striae and non‐decumbent pilosity. Gaster darker than rest of body with smooth and shiny integument and scarce yellowish non‐decumbent setae of two sizes, being the longest ones twice the size of the small ones. Anterior portion of first gastral tergite almost smooth, with few short irregular striae, discernible in medium magnification (~50×), close to base of gaster.

Queen
Cristiano, Cardoso and Sandoval (2020) - TL 9.50–10.70, HL 1.75–1.95, HW 2.25–2.30, ML 0.80–0.90, SL 1.50–1.65, EL 0.35–0.40, WL 2.65–3.25, PL 0.73–0.75, PW 0.65–0.70, PPL 0.75–0.78, PPW 1.05–1.30, GL 2.70–3.10, CI 115.38–111.43, MI 43.24– 46.15, OI 15.56–17.39, SI 81.08–88.57 [N = 9].

Head. In full‐face view, posterior cephalic margin medially emarginate but less accentuated than worker. Posterior cephalic corner angular, with a small spine directed upwards and a small tubercle laterad. Integument covered by parallel irregular striae but denser than in worker. 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 28–30. Mandibles with 9 teeth (two can be considered denticles) and apical tooth larger and more prominent than in worker. Mesosoma. Integument with well‐defined and longitudinal and parallel striae and sparse non‐decumbent pilosity but with denser striae than in worker. Scutellum strongly convex in lateral view, narrowing posteriorly with a trapezoidal outline in dorsal view, bearing two denticles in the posterior margin. Propodeum with a pair of protruding long spines, directed upwards, in lateral view. Metasoma. Petiole with a well‐developed subpetiolar process armed with a sharp spine. Integument of postpetiole covered by parallel transversalstriae. First gastral tergite with two yellowish spots located anterolaterally in dorsal view. Dense semicircularstriae present on the first gastral tergite, mainly at the base. Dense pilosity present on gastral tergites two to four; tergite one with sparse setae as in workers. Parallel striae on subsequent tergites.

Male
Cristiano, Cardoso and Sandoval (2020) - TL 8.22–8.36, HL 1.11–1.13, HW 1.25–1.28, ML 0.71–0.72, SL 1.25–1.26, EL 0.32–0.34, WL 2.68–2.70, PL 0.49–0.51, PW 0.51–0.52, PPL 0.64–0.67, PPW 0.78–0.79, GL 2.59–2.63, CI 112.61–113.27, MI 63.72– 63.96, OI 28.82–30.09, SI 111.50–112.61 [N = 3].

Head. In full‐face view subquadrate, posterior cephalic margin straight, without emargination. Posterior cephalic corner rounded with a small spine directed upwards. Integument opaque, reticulated and covered by irregular striae, as in worker and queen. Eye bigger and more convex than worker and queen. Number of ommatidia across largest eye diameter 34. Mesosoma. 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, in lateral view. Integument with well‐defined longitudinal and parallel striae and scarce non‐decumbent pilosity. Metasoma. Petiole with short peduncle, subpetiolar process smaller than queen, similar with worker and less developed than in Amoimyrmex silvestrii. Postpetiole in dorsal view broader than long, with two small teeth on the lateral margins, located in medial portion, projecting laterally and slightly backwards. Postpetiolar process more developed than in queen and worker, and sharper than in A. silvestrii. Integument of petiole and postpetiole with scarce striae. Gaster with smooth and shiny integument, and non‐decumbent pilosity, less dense and shorter than in queen and its congener Amoimyrmex silvestrii.

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