**2.** *Eciton* **Army Ants**

Several ant genera within the subfamily Dorylinae are referred to as army ants, with the genera *Cheliomyrmex*, *Eciton*, *Labidus*, *Neivamyrmex* and *Nomamyrmex* forming a monophyletic group and found in tropical and subtropical regions in the American continent [43,44]. Army ants are characterised by presenting the "army ant syndrome" which includes the evolution of behavioural and reproductive traits that have allowed them to be key predators in neotropical forests. This suite of traits includes obligate collective foraging, nomadic lifestyle, and highly specialised queens which are characterised by being wingless and having large expanding abdomens capable of lying thousands of eggs [38,45–47]. Colony founding of *Eciton* army ants is by fission. When colonies attain a large size, a sexual brood is produced, containing a small number of new queens and hundreds of males. The parental colony will then divide into two daughter colonies, one of them headed by a new queen and the other one headed either by the old queen or another of the new queens [36,48]. Initial phylogenetic analyses based on data derived from three nuclear genes, one mitochondrial gene, and morphological analyses, suggested that the evolution of the "army ant syndrome" in army ants species in the Americas and Afro-Eurasia was as a result of inheritance of these traits from a unique Gondwanan common ancestor [46]. However, a recent comprehensive phylogenomic study based on 2,166 loci

has indicated multiple origins for the "army ant syndrome" in different continents [44]; therefore, supporting initial views of convergent evolution of this syndrome [38].

The genus is distributed from Mexico to Argentina [38], with 12 *Eciton* species, and several subspecies, described [49]. Genomic analyses using a large number of loci (> 4,000,000) for 146 specimens, collected from a wide distribution range and representing those nine species for which workers and queens are known, strongly supported interspecific relationships congruent with taxonomic classifications [50]. However, further taxonomic revisions are required to delimit species and subspecies boundaries. For example, a phylogeographic study based on mitochondrial DNA sequences indicated that genetic divergence of *E. burchellii foreli* and *E. b. parvispinum* colonies was higher than between other morphologically described species [51].

Foraging and emigration of *Eciton* army ants takes place above ground [36,52]. The species *E. burchellii* and *E. hamatum* are considered truly epigaeic, as they bivouac above ground. Other *Eciton* species such as *E. mexicanum*, *E. vagans* and *E. dulcium* tend to bivouac in more sheltered and darker places, inside or beneath logs, or in underground erosion chambers [36]. In these more hipogaeic species, raids and emigration columns can take place underground but only for short distances and when near the bivouac [53]. Knowledge on *Eciton* army ants is primarily based on research focused on *E. burchellii* and to less extent on *E. hamatum*, as these two species are the most epigaeic diurnal *Eciton* species. Most *Eciton* species are column raiders (e.g. *E. hamatum*, *E. vagans*), with hunting taking place by ants moving initially in narrow columns that later divide into several diverging branches. The size of the raid varies among species, from sparse raid front such as those in *Eciton dulcium,* often narrower than 2 m, to swarm fronts wider than 5 m in in the swarm raider *E. burchellii* [38,53,54].

A large-scale survey conducted at La Selva Biological Station (Costa Rica) examining the diets of sympatric neotropical army ants, including the species *E. burchellii*, *E. dulcium*, *E. hamatum*, *E. lucanoides*, *E. mexicanum* and *E. vagans* revealed high specialisation and small spatio-temporal niche overlap [55]. Using DNA barcoding to identify ant prey to the species level revealed differences in raiding preferences, with *E. dulcium* and *E. mexicanum* preferentially raiding ground-nesting ants and *E. hamatum* and *E. burchellii* arboreal-nesting ants. Furthermore, *E. hamatum* and *E. burchellii* colonies were found to have more diurnal raids whereas most of the other *Eciton* species had primarily nocturnal raids [55].

All *Eciton* species are predominantly predators of other ants (mainly their brood) although they specialise on different ant genera. *E. hamatum* predominantly preys on the brood of *Acromyrmex* leaf-cutting ants, *E. dulcium* on *Pachycondyla* and *Odontomachus* ants, and *E. burchellii* on *Camponotus* ants [14–20]. Although the diet of *E. burchellii* is more varied and includes a diverse range of non-ant litter arthropods, in particular the brood of other social insects [56–61], this non-ant component of the prey might be comparatively low [55]. The large colony size of *Eciton* species, with estimates ranging from 150,000 to 500,000 workers in *E. hamatum* and 500,000 to 2 million workers *in E. burchellii* [38], and the relatively large body size of of some of the workers imply high colony-energy demands [62]. Colonies of *Eciton* army ants have a nomadic-statary cycle, with periods of in which the bivouac (temporary nest) remains in a fixed location where the queen lays thousands of eggs [63]. During this period, the workers hunt prey locally in a regular manner. The statary phase is followed by a nomadic phase coinciding with the eggs hatching into larvae and, during this phase, the colony emigrates daily after the raids over the period of 2–3 weeks [36,64]. In the most studied species, *E. burchellii,* colonies concentrate their raids on invertebrate-rich forest patches [65] and a colony could capture c. 30,000 items of prey in a single day [66]. The nomadic-statary cycle of *Eciton* colonies is interpreted as an adaptation to avoid prey depletion by promoting population recovery of their prey [56]. The pattern of raids, as part of the nomadic lifestyle of *Eciton* army ants, therefore, exerts top-down regulation of leaf litter arthropod communities and creates a mosaic of habitat patches at different ecological succession stages which increases local species diversity [35,65,67].
