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Ants: Phylogeny and 1758 when the Swedish botanist Carl von Linné Classification published the tenth edition of his catalog of all plant and known at the time. Marek L. Borowiec1, Corrie S. Moreau2 and Among the approximately 4,200 that he Christian Rabeling3 included were 17 species of . The succeeding 1University of Idaho, Moscow, ID, USA two and a half centuries have seen tremendous 2Departments of and Ecology & progress in the theory and practice of biological Evolutionary , Cornell University, Ithaca, classification. Here we provide a summary of the NY, USA current state of phylogenetic and systematic 3Social Research Group, Arizona State research on . University, Tempe, AZ, USA

Ants Within the Tree of Ants are the most ubiquitous and ecologically Life dominant on the face of our Earth. This is believed to be due in large part to the cooperation Ants belong to the order Hymenoptera, which also allowed by their . At the time of writing, includes and bees. ▶ , or true about 13,500 species are described and sociality, evolved multiple times within the named, classified into 334 genera that make up order, with ants as by far the most widespread, 17 subfamilies (Fig. 1). This diversity makes the abundant, and species-rich lineage of eusocial ants the world’s by far the most speciose group of animals. Within the Hymenoptera, ants are part eusocial insects, but ants are not only diverse in of the ▶ , the in which the ovipos- terms of numbers of species. They exhibit an itor has been modified into a ▶ -injecting amazing diversity of form and function exempli- organ, or , and where all known instances fied by a number of charismatic taxonomic or of hymenopteran eusociality are concentrated. functional groups known as ▶ army ants, ▶ trap- Although the ants’ status as aculeates has been jaw ants, ▶ turtle ants, ▶ fungus-growing ants, long known, their affinities within the group exploding ants, and ▶ socially parasitic ants, to have been the subject of different proposals. only name a few. Taxonomic and phylogenetic Application of genome-scale data to phyloge- research helps to discover, describe, and organize netics in the last 5 years reveals that ants are the diversity of organisms, as well as to reveal most closely related to the , the lineage their evolutionary histories. The timeline of mod- that includes bees and sphecoid wasps [14, 21]. ern zoological nomenclature officially begins in

© Springer Nature Switzerland AG 2020 C. Starr (ed.), Encyclopedia of Social Insects, https://doi.org/10.1007/978-3-319-90306-4_155-1 2 Ants: Phylogeny and Classification

Ants: Phylogeny and Classification, Fig. 1 Diversity Paraponerinae (o) (p) (q) Pseudo- of extant ant subfamilies. (a) (b) myrmecinae. All photos by Alex Wild (http://www. (c) (d) Apomyrminae (e) alexanderwild.com), except (a) by Micheal Branstetter, (c (f) (g) (h) & d) from Antweb.org (http://www.antweb.org), and (i) (j) (k) drawing of (k) by Barrett Klein Martialinae (l) (m) (n)

Phylogeny of the Extant Ants (Fig. 2). The leptanilloid clade comprises a single Neotropical species from the subfamily The great diversity of extant ant species is distrib- Martialinae, , and about 70 uted among 17 subfamilies grouped into 3 major described species of the subfamily Leptanillinae, clades, the leptanilloid, poneroid, and formicoid which are confined to the . There is Ants: Phylogeny and Classification 3

Number of species per subfamily Myrmicinae 6678 Ectatomminae 264 Heteroponerinae 33 Formicinae 3146 Dolichoderinae 713 1 Aneuretinae Formicoids 231 Myrmeciinae 94 Dorylinae 692 Ponerinae 1241 Agroecomyrmecinae 2 Paraponerinae 1 Proceratiinae 146 Poneroids Amblyoponinae 140 Apomyrminae 1 Leptanillinae 68

Martialinae M + L 1 225 220 175 150 125 100 75 50 25 0

Ants: Phylogeny and Classification, Fig. 2 Relation- nodes from studies listed in Table 1. Estimates from each ships among extant ant subfamilies and their species study are superimposed so that darker colors indicate more counts. Blue bars show ranges of age estimates for crucial overlap. Age scale in millions of years some uncertainty as to whether leptanilloids are subfamilies remain uncertain, even following indeed a natural, monophyletic group, as application of genome-scale phylogenetic data. Martialis was earlier inferred to be the sister lin- The observed phylogenetic uncertainty probably eage to all other ants and its position has been reflects rapid initial diversification of poneroid sensitive to the way phylogenetic analysis is car- lineages during the late . ried out. Recent studies, however, support a sister- The formicoid clade encompasses the vast group relationship between Martialis and the majority of all ant species (~12,000) and contains Leptanillinae, placing the leptanilloids as sister all remaining ant subfamilies: Aneuretinae, to all remaining ants [11]. Dolichoderinae, Dorylinae, Ectatomminae, The poneroids include more than 1,500 valid Formicinae, Heteroponerinae, Myrmeciinae, described species and are distributed across 6 sub- Myrmicinae, and Pseudomyrmecinae. Although families: Agroecomyrmecinae, Apomyrminae, as a rule these ants are more diverse in (or even Amblyoponinae, Paraponerinae, Ponerinae, and confined to) the tropics at the subfamily level, two Proceratiinae. The Ponerinae harbor more than formicoid subfamilies, the Formicinae and 80% of the poneroid diversity with roughly Myrmicinae, have been particularly successful in 1,250 extant species. Poneroid ants are mostly temperate zones. The majority of ant species confined to the tropics, with only isolated species found in the Nearctic and Palearctic regions in several genera extending into the north temper- belong to these two subfamilies. In contrast to ate regions. While it is almost universally agreed the poneroids, there exists broad consensus that poneroids are a clade, phylogenetic relation- regarding the phylogenetic relationships of sub- ships among poneroid subfamilies are less well families in the formicoid clade. resolved. Apomyrminae and Amblyoponinae are As can be seen in Fig. 2, species diversity of sister groups, but relative placements of other extant ants is unequally distributed across the 4 Ants: Phylogeny and Classification phylogeny. Martialinae plus Leptanillinae contain group Amblyoponinae likely started to diversify only 1% of species diversity, while poneroids and in the mid-Cretaceous, and, despite low species formicoids account for the remaining 11% and diversity, the subfamily is distributed worldwide 88%, respectively. and exhibits a variety of morphologies and life histories. Many species are small in body and colony sizes and are cryptic dwellers of the soil, Overview of Ant Subfamilies leaf litter, or rotting wood. Some are specialized predators of geophilomorph , while the For ease of reference, the following summary of large species castanea is known to the 17 extant and 3 known extinct subfamilies is prey on wood-boring cerambycid beetle larvae. presented in alphabetic order, rather than group- species have an army-ant-like ing them by major clades. life history and wingless queens. Certain species possess queens that are smaller Agroecomyrmecinae than and of conspicuously different color than the A poneroid subfamily consisting of only two liv- workers. Several impression of the ing species, tatusia (Fig. 1a) and extinct Casaleia from the ~47 million years coronacantha. Both are rarely old Messel formation have been assigned to encountered. Tatuidris, found in Central and Amblyoponinae, and two Baltic species , is thought to be a specialized of have been described. predator, for yet undetermined prey in the leaf litter of mid-elevation . Aneuretinae Workers of Ankylomyrma have been collected a This formicoid subfamily is currently represented handful of times in West . It is an arboreal by a single extant species, ▶ Aneuretus simoni, species, but nothing else is known of its natural which is confined to isolated sites in Sri Lanka. history. Agroecomyrmecinae was presumably The Aneuretinae are the to the more diverse in the past, as three species Dolichoderinae. Aneuretus simoni (Fig. 1c)isa have been described from Eocene Baltic and rarely found species nesting predominantly in rot- Rovno of , whose exact dating is ten wood, with polygynous and ▶ polydomous controversial, and from ca. 34 million years old colonies generally containing fewer than 100 North American Florissant shale. workers. The worker caste is polymorphic, and the species exhibits generalized predatory and Amblyoponinae scavenging habits. Aneuretine ants were much A poneroid subfamily of about 140 currently more widespread and speciose in the past. Nine described extant species distributed among 9 gen- extinct genera and 12 species have been described era. Recent phylogenetic work shows two major from Cretaceous and Eocene amber as well as clades, one comprising the genera , from impression fossils from Europe, Asia, and Onychomyrmex, and , and the other . The Cretaceous fossils include the remaining six genera [30]. With more than 50 the genera Burmomyrma and Cananeuretus, species, Stigmatomma (Fig. 1b) is the largest whose assignment to Aneuretinae is uncertain. genus. Species belonging to this small subfamily Napakimyrma paskapooensis is the oldest are known as Dracula ants, because adults bite undisputed aneuretine and the only confirmed their own larvae and consume their hemolymph, a ant fossil known from the . This impres- behavior also observed in Leptanillinae and Pro- sion fossil is known from the 56–60 million-year- ceratiinae. This mode of feeding is thought to be old Paskapoo Formation in Canada. an to unstable supplies affecting specialized predators that are unable to regurgitate Apomyrminae and share food within the colony. As such, it can Apomyrminae is a monotypic poneroid subfam- be seen as an alternative to ▶ trophallaxis. Crown- ily, represented by stygia (Fig. 1d). Ants: Phylogeny and Classification 5

Apomyrminae are sister to Amblyoponinae. impressive battery of chemical defenses and are Apomyrma stygia is known from sub-Saharan known for their advanced chemical communica- Africa, and very little is known of its habits, tion systems. The Neotropical genus ▶ is except for subterranean nesting and indirect evi- known for its diverse ecological ▶ interactions dence of on geophilomorph centipedes. with plants and ▶ scale insects and has been the Fossils remain unknown from the Apomyrminae. focus of many studies on and adapta- tion. The subfamily also has an impressive fossil †Brownimeciinae record comprising 135 fossil species in 20 genera. This extinct monotypic subfamily is known from New and currently undescribed species from ca. Cretaceous dated to approxi- 72 million-year-old Myanmar amber and mately 92 million years ago. The sole species, Chronomyrmex medicinehatensis from 78–79 Brownimecia clavata, is similar to million-year-old Cretaceous amber of Canada sphecomyrmines but distinguished by a long are the oldest known dolichoderines. It appears antennal scape, which is thought to be a derived likely that dolichoderines were more diverse in feature. Brownimecia has been inferred to be the the past and that later arrivals on the ant evolu- sister lineage to all crown-group ants. tionary stage, such as the Myrmicinae and rapidly diversifying lineages within the Formicinae, Dolichoderinae displaced the Dolichoderine on continents other The Dolichoderinae are a highly diverse than . formicoid subfamily of more than 700 extant spe- cies. The classification and phylogeny of this Dorylinae group have been studied extensively, and at pre- This diverse formicoid subfamily contains almost sent the extant taxa are organized into 4 tribes and 700 described extant species distributed among 27 28 genera [25, 28]. One of the tribes, genera, including the charismatic true army ants , contains a clade that encom- [9]. The dorylines have a worldwide distribution, passes a massive Australian radiation that began but the vast majority of species occurs in the some 25 million years ago, making Australia the tropics. The phylogeny of this group has been center of dolichoderine diversity. difficult to infer with certainty, likely because As a large subfamily, the dolichoderines show dorylines experienced an ancient rapid radiation a variety of morphologies and life histories and [10, 13]. Due to a lack of characteristic morpho- therefore defy easy synopsis. Characteristic mor- logical traits that would allow for grouping mul- phological traits include a nonfunctional stinger tiple genera and uncertainty in phylogenetic and thin cuticle devoid (with the exception of the placement of several other genera, no tribal clas- genus ) of much of the sculpture or sification is currently in use. armament that is typical for most other ants. Col- The army ants, the most conspicuous and best- onies range from monogynous and containing a known members of the Dorylinae, are character- few hundred workers to highly populous polygy- ized by a suite of behavioral and morphological nous colonies inhabiting polydomous nests. As characteristics dubbed “the syndrome.” one example, nests of the Australian This includes collective foraging, frequent colony purpureus can house upwards of 300,000 relocation, and highly derived queen and male workers. One of the world’s most significant inva- morphologies. The true army ants are among the sive ants, the ▶ humile most bizarre and impressive social insects. They (Fig. 1e), is also a member of the Dolichoderinae. forage in coordinated swarms that have no scouts The argentine ant and the noninvasive North or leaders; colonies move nesting sites periodi- American odorous house ant sessile cally; and young queens ▶ found new colonies form ▶ supercolonies with no apparent upper by budding. Army ant colonies are among the size limit. Instead of defending themselves by largest insect societies. An estimate of 22 million stinging, dolichoderines are armed with an workers has been made for a single colony of the 6 Ants: Phylogeny and Classification

African army ant wilverthi. Young as well as in and Dorylus queens mate with as many as 20 different Australia. Despite the relatively low species diver- males and are capable of storing more than 800 sity, ectatommines are common and conspicuous million spermatozoa used to lay an estimated 250 elements of tropical New World and Australian million eggs during their lifetimes. ant faunas. The subfamily is absent from Africa New World army ant colonies undergo cycles and from most parts of the northern temperate of alternating phases of foraging and reproduc- regions. tion, during which the queen is either physogastric Most ectatommines appear to be generalized and producing eggs and the colony is not chang- predators and scavengers although specialized ing nesting sites (the so-called statary phase) or predators of fungus-growing ants and the queen ceases egg production, workers forage have been reported in the genus . intensively, and the colony moves between One species of was described as obli- nesting places each day (nomadic phase). In gate ▶ social parasite. Ectatommines often nest in many species the workers and even queens are rotting wood or in the soil. Both ground foraging completely blind, and males have conspicuously and arboreal foraging species are known. robust, large bodies. In spite of the high level of species are generally cryptic and morphological and behavioral specialization, live and forage underground. Colony sizes are molecular phylogenetic studies indicate that relatively small, rarely exceeding several hundred New World army ants (genera , workers. Both monogynous and polygynous col- , , , and onies have been recorded. Many ectatommines, ) and Old World army ants especially in the genus , have lost and Dorylus (Fig. 1f) evolved convergently in the true queen caste and reproduce through mated South America and Africa, respectively. In addi- workers (▶ ). Certain species of tion to the true army ants, the Dorylinae contains a Ectatomma are considered pest control agents number of lineages whose biology, , and for crops in the Neotropics. The subfamily has a phylogeny are poorly known. It is uncertain rather extensive fossil record, and some 14 species whether any species in these non-army ant line- in several genera are known from the ages acquired all components of the army ant Dominican and Eocene amber of Europe, as well syndrome, but each individual trait described as Eocene impression fossils. The affinity of the above has been reported for some species. There 78–79 million-year-old Cretaceous Canadian is one extinct genus, Procerapachys, known from amber fossil Canapone dentata is uncertain, but Eocene Baltic and Bitterfeld ambers. Extant it has been suggested to represent an early doryline genera are also known from Baltic, ectatommine. Pseudectatomma impression fossils Dominican, and Chiapas amber deposits. from ~47 million-year-old European Messel For- mation and Electroponera dubia from Eocene Ectatomminae Baltic amber are the oldest undisputed The Ectatomminae are a medium-sized formicoid ectatommine fossils. subfamily with over 250 described species classi- fied in 4 genera: ▶ Ectatomma (Fig. 1g; 15 extant † species), Gnamptogenys (138 species), The Formiciinae – note the close similarity in ▶ Rhytidoponera (104 species), and name to the next subfamily – are an extinct sub- Typhlomyrmex (7 species). The Ectatomminae family of giant ants currently containing six spe- are the sister group to the Heteroponerinae, and cies in two genera: and . together they form a clade sister to the These are impression fossils known from four Myrmicinae (Fig. 2). Ectatomma and localities in North America and Europe, all dated Typhlomyrmex are confined to the New World, to the Eocene when the two continents were Rhytidoponera occurs in Southeast Asia and Aus- connected by land bridges in the north Atlantic tralasia, and Gnamptogenys is found both in the [1]. Only alates (Titanomyrma) and wing Ants: Phylogeny and Classification 7 impressions (Formicium) have been collected. and in North America. Titanomyrma species are both the largest ants Myrmecocystus is a prime example of the honey- and Hymenoptera known, with queens reaching pot ants, which can store liquid food in highly a body length of 6 cm. The oldest Formiciinae expandable abdomens, thus forming living pan- fossils are 49–54 million years old from the North tries. The food storage behavior of the honeypot American Green River formation. The exact phy- ants evolved independently in desert ants from logenetic placement of the Formiciinae is uncer- several genera, all of the subfamily Formicinae. tain, but they are currently placed within the Formicine ants also engage in obligate symbi- formicoid clade. oses with (genus ) and ant plants (, ). The Formicinae jumping ant destructor is common The Formicinae are the second-largest subfamily throughout the Amazon basin, where its very of ants with 51 extant genera and more than 3,000 large and unusual, highly visually oriented described and many known but undescribed spe- behavior make it conspicuous. Southeast Asian cies. Two genera in one massive radiation, the are an example of trap-jaw ants, or ▶ carpenter ants (Camponotus) and their close ants in which spring-loaded mandible action gen- relatives the ▶ spiny ants (), account erates extremely fast-snapping movement. There for more than 1,700 species. The formicines have is also a high number of socially parasitic species a global distribution and are important compo- in the Formicinae, especially temporary parasites nents of both tropical and temperate faunas. In and ▶ dulotic species in the genera , north temperate regions, at around 45N, the ,andRossomyrmex. The fossil record Formicinae become the most species-rich subfam- of Formicinae is impressive, with 31 extinct gen- ily. The phylogeny and higher classification of the era and 197 fossil species. The Cretaceous New subfamily have recently been revised, dividing the Jersey amber fossil Kyromyrma neffi was dated at subfamily into 11 monophyletic tribes [3]. approximately 92 million years, exhibits a well- Workers can be diagnosed by the acidopore, a developed acidopore, and is the oldest undisputed circular opening formed by a fold of the last formicine ant. abdominal sternite, which is surrounded by a fringe of setae. The Formicinae lack a stinger Heteroponerinae and use this opening to spray formic acid, a The Heteroponerinae are a small formicoid sub- venom unique to the subfamily. family of 3 genera and 33 species. Hetero- The tribe contains Camponotus ponerines have a remarkably disjunct (Fig. 1h) and Polyrhachis, as well as several lesser biogeographic distribution: 28 species of Hetero- known ant genera, such as the large-eyed Austral- (Fig. 1i) are found in Australia, New asian strobe ants , the spectacularly Zealand, and in the Neotropics. Four species of iridescent , the often intricately are Neotropical, and the single sculptured , and one of the world’s larg- species of is known from only two est ants, the Southeast Asian gigas. queens collected in Azerbaijan. Heteroponerinae All Camponotini have a bacterial symbiont, are sister to Ectatomminae (Fig. 2), and the two Blochmannia, found in cells lining the midgut subfamilies can be difficult to separate based on and in the ovaries. Blochmannia is vertically morphological characters. The Heteroponerinae transmitted from generation to generation and can be recognized by a longitudinal carina run- provides essential nutrients to the ants. ning down the top of the head. Little is known of Formicines have successfully radiated into the this group’s biology. Some heteroponerines nest world’s arid zones, with three unrelated genera in or under rotting wood, in the soil, and under occupying this niche in different parts of the stones. Some species were observed to feign death world: the species-rich genus in Aus- when disturbed. Colonies are small and monogy- tralia, ▶ in North Africa and Asia, nous, and both winged and ▶ ergatoid queens are 8 Ants: Phylogeny and Classification known. There are no heteroponerine fossils difficulties associated with establishing the phy- known at the time of writing. logenetic position of Martialis are related to com- positional heterogeneity of molecular sequence Leptanillinae data and the old age of this lineage [11]. Nothing This small leptanilloid subfamily of rarely is known about the biology of Martialis, although encountered subterranean ants currently com- the complete lack of eyes and the pale-yellow prises about 70 described Old World species, cuticle strongly suggest a subterranean life. Elon- although the group’s cryptic habits make it likely gate, forceps-like mandibles suggest specialized that many more species await discovery. predatory habits. Fossils are unknown from the Leptanillines are currently classified in eight gen- Martialinae. era. There are three major lineages within the subfamily: the sister lineage to all other Myrmeciinae leptanillines, represented by the single species This is a formicoid subfamily presently confined Opamyrma hungvuong, species with somewhat to Australia and . Two extant gen- larger workers that are currently classified in the era comprise this group: (Fig. 1l), also genera and , and spe- known as bull ants or bulldog ants, and cies with miniscule workers in the genus , the dawn ants. There are about . The genera Noonilla, , 100 described species of Myrmecia, whereas Scyphodon, and Yavnella are known only from Nothomyrmecia is represented by the single males. extant species Nothomyrmecia macrops [26]. Little is known about the biology of The Myrmeciinae are the closest relatives of the leptanillines, and only a couple of species have big-eyed arboreal ants Pseudomyrmecinae (Fig. been studied in the laboratory. Colonies are prob- 2). Due to their diurnal habits, large body sizes, ably small (not more than several hundred and pugnacity, bulldog ants are the most charac- workers) in most species and either monogynous teristic element of the Australian myrmecofauna. (Leptanilla; Fig. 1j) or polygynous with ergatoid Myrmecia exhibits a variety of reproductive strat- queens (Protanilla). Like the amblyoponine egies. Young queens either establish new colonies Dracula ants that feed on their own larvae, independently or via pleometrosis (dependent col- leptanilline queens and workers engage in larval ony foundation). Colonies can be monogynous or hemolymph feeding. Unlike the amblyoponines, polygynous, and queens can be singly or multiply however, they do not puncture the larval cuticle. mated. Queens of some species are winged, while Instead, they feed through a pair of specialized others are ergatoid, and in a few species the queen duct organs located on the larval abdomen. caste has been replaced by gamergates (reproduc- Leptanilla also appear to be specialized predators tive workers). Colony sizes range from a few of soil centipedes and have been shown to pro- dozen to a couple of thousand workers. Nests are duce brood in reproductive bouts similar to those typically built in the soil but occasionally also in observed in the true army ants. Fossil leptanillines rotting logs, and one species, M. mjobergi, nests are not known. arboreally. The workers are effective solitary for- agers with large eyes and correspondingly good Martialinae vision. Myrmecia deliver exceptionally painful The Martialinae are a monotypic subfamily stings. They are also known for rapid karyotype known from only few specimens collected in the evolution and the high variability in chromosome Brazilian Amazon [22]. The phylogenetic posi- numbers among species. M. croslandi is the only tion of Martialis heureka (Fig. 1k) was initially ant species known to have only a single pair of inferred as the sister lineage to all extant ants. chromosomes, a remarkable condition known However, recent phylogenetic studies suggest a only in one other metazoan, a nematode. sister group relationship between the New World In contrast to many bulldog ants, the dawn ant Martialinae and the Old World Leptanillinae. The Nothomyrmecia macrops is extremely rare and Ants: Phylogeny and Classification 9 known from only a few localities in South Aus- seed-harvesting lineages comprising mostly the tralia. Nothomyrmecia are nocturnal and exhibit north temperate genera , , traits that were historically associated with ances- , , and others. tral ant societies, including small colony size, The are a tribe with global dis- solitary foraging, lack of division of labor, and tribution and two highly speciose genera: queens foraging while raising the first brood. The ▶ and ▶ Solenopsis. Some of the Myrmeciinae are represented by a diverse fossil world’s most notorious invasive ants are the fire record revealing an almost global distribution dur- ants Solenopsis invicta and S. geminata. Other ing the Eocene. Six extinct genera represented by Solenopsis species, known as thief ants, often 16 species are known from Eocene and nest close to larger ants from which they steal impression fossils and ambers of Europe, Asia, resources and even consume brood. Monomorium North America, and South America. includes several seed-harvesting species and the common greenhouse and house pest M. Myrmicinae pharaonis. Monomorium is a large genus that With more than 6,500 described species, the was recently recognized as polyphyletic and is Myrmicinae are by far the largest ant subfamily. currently undergoing a taxonomic reevaluation. According to recent taxonomic rearrangements The Attini are the tribe that originated and based on molecular phylogenetic results, this diversified largely in the New World, with three formicoid subfamily is currently classified into 6 large radiations of some of the world’s most monophyletic tribes and 143 extant genera [29]. remarkable ants: the genera , Their biogeographic distribution is global, and , and fungus-growing ants in the myrmicines are present in every habitat where genus group. At just over 1,000 described ants are found. Because of their great species and many unnamed species, Pheidole competes diversity, the myrmicines exhibit biological and with the formicine genus Camponotus for the title morphological variation that spans most of the of the most speciose ant genus. Pheidole species range observed in the family as a whole. Special- are dimorphic or trimorphic with a well-devel- ized functional groups found in the myrmicines oped major worker caste. Pheidole reached their include ▶ fungus-growing ants, arboreal ▶ turtle exceptional species richness within the last 40 ants, ▶ seed-harvesting ants, ▶ social parasites, million years and colonized all continents except and numerous lineages of independently evolved Antarctica. With more than 800 described cryptic, ▶ trap-jaw ants. Myrmicines gave rise to several leaf-litter dwelling species, Strumigenys is not far spectacular adaptive radiations. Only 5 of the 143 behind. Strumigenys is a genus in which a spring- genera, ▶ Pheidole (Fig. 1m), Strumigenys, loaded (trap-jaw) mandible-closing mechanism ▶ , , and evolved independently several times, each time ▶ , contain more than half of the Strumigenys colonized a new major geographic subfamily’s species. The six myrmicine tribes region. The Atta genus group is endemic to the are well supported by molecular data but poorly New World and comprises the only ants that cul- defined by morphological characters. tivate fungus for food. Leaf-cutting ants in the is the only myrmicine tribe that is endemic to genera and Atta are among the north temperate regions. The two constituent gen- most characteristic Neotropical ants, often seen era, and , are relatively harvesting great amount of vegetation that serves unspecialized scavengers, common in boreal and as substrate for their fungus gardens. Other mem- subboreal habitats. The tribe Pogonomyrmecini bers of the tribe include the Neotropical contains three genera that are restricted to the turtle ants and the invasive little fire New World. Patagonomyrmex and ant auropunctata. The Attini and its are granivores, while core genera originated and diversified mainly in is a likely omnivorous scavenger. The Stenamini the Neotropics but subsequently spread across the are another moderately-sized tribe with several world. 10 Ants: Phylogeny and Classification

All other myrmicines are classified in the tribe as well as . Although Para- [4]. This clade contains 64 gen- ponera does not now occur in the , a era and approximately 40% of the myrmicine single fossil species, P. dieteri, is known from species. The Crematogastrini originated and 15–26 million-year-old Miocene Dominican diversified mostly in the Paleotropics, especially amber. in the Indomalayan region. A few crematogastrine lineages are also present in the New World, Ponerinae including by far the largest northern temperate The Ponerinae are the third largest subfamily of genus, the acorn ants Temnothorax, and the ants with 47 genera and more than 1,200 species mostly tropical , Crematogaster, and [23, 24]. Because of their diversity, ponerines are , among others. Tetramorium is difficult to characterize, although many species another hyperdiverse ant genus that belongs to are large and possess thick cuticle. The subfamily this tribe and is mostly confined to the Old is mostly tropical, with few lineages extending World except for several desert-adapted native into the temperate regions. Ponerines are currently North American species and two recent introduc- divided into two tribes, the Platythyreini and the tions. Certain species of Carebara ants exhibit . The Platythyreini contain a single genus extreme worker-queen or worker . with some 40 species, forming the sister group to In the case of Southeast Asian Carebara diversa all other ponerines. ▶ punctata is and related species, known as ▶ marauder ants, found in and the Caribbean. It the largest workers can weigh 500 times as much shows a remarkable diversity of reproductive as their smallest sisters. Marauder ants forage in strategies, where regular queens, intercastes, and self-organized groups, as do true army ants, and mated workers can reproduce either sexually or their colonies attain sizes of hundreds of thou- via thelytokous ▶ . All other gen- sands of workers. The Myrmicinae have an exten- era are classified within the tribe Ponerini. The sive fossil record with 37 described extinct genera colonies of ponerines are generally small com- and 170 fossil species. The 89–94 million-year- pared to many other ants, and no species is con- old Cretaceous impression fossil Afromyrma sidered an invasive pest. The reproductive petrosa is the earliest putative myrmicine, biology of many ponerines is remarkable, because although its affinities are uncertain. The oldest many species have lost the queen caste in favor of undisputed myrmicines are known from 52 mil- gamergates. The presence of gamergates is also lion-year-old Chinese Fushun amber. known from other subfamilies, although it is most widespread in ponerines. Paraponerinae The Ponerinae are another subfamily in which clavata (Fig. 1n), the infamous ▶ trap-jaw mechanisms evolved convergently. ▶ bullet ant of the Neotropics, is the sole species The sister genera and in this formicoid subfamily. This species has a (Fig. 1o) evolved powerful snapping mandibles remarkably broad distribution, occurring from used for prey capture. Interestingly, the trap-jaw in Central America to Paraguay and mandibles can also be employed to propel indi- Rio Grande do Sul state in Southern , and vidual ants into the air, a behavior that in some it is common throughout most of its range. Para- species may represent active predator avoidance. ponera clavata is famous for its extremely painful The South American ▶ ants have sting. Its Tupí-Guaraní name, tuca-ndy, means arguably the largest workers of any living ants, “the one wounding deeply.” The nests of P. and they rely exclusively on gamergates for repro- clavata are built at the base of trees or arboreally, duction. Dinoponera have, with maximally 60 and colonies can reach a couple of thousand pairs, the highest number of chromosomes workers. Queens forage while raising the first reported in any Hymenopteran. Although primar- brood, and workers forage individually. Their ily scavengers of dead invertebrates, Dinoponera diet consists of invertebrate prey and scavenged Ants: Phylogeny and Classification 11 have been observed to prey on small vertebrates Pseudomyrmecinae such as frogs. This is a medium-sized formicoid subfamily of Southeast Asian are unusual in big-eyed arboreal ants comprising 230 described that jumping is their main mode of locomotion, extant species that are classified into three genera. otherwise known in certain species of (Fig. 1q) and the monotypic genus Odontomachus and Australian Myrmecia ants are endemic to tropical and warm tem- and the Neotropical formicine Gigantiops. Spec- perate regions of the New World, while Tetra- tacular morphological to specialized ponera occurs in the Old World tropics [27]. prey can be seen in Neotropical This subfamily’s closest relatives are the ants, which use their highly modified mandibles to Myrmeciinae (Fig. 2). Pseudomyrmecines are prey on very setose polyxenid millipedes, and known for their mutualistic relationships with some species are extremely robust, plants, which evolved at least a dozen times in presumably as an adaptation to hunting . the subfamily. Perhaps the best-known example is Ponerines also have an extensive fossil record the obligate mutualism between members of the with 12 extinct genera and 86 species. Central American Pseudomyrmex ferrugineus Undescribed species reported from ca. 72 mil- group and swollen-thorn Vachellia acacias. In lion-year-old are the oldest this system, the acacias provide nesting space in undisputed ponerine fossils. swollen thorns and food in the form of extrafloral nectaries and nutritious structures produced at the tips of leaves called food bodies or Beltian bodies. Proceratiinae In exchange the ants aggressively protect acacias The Proceratiinae are a poneroid subfamily of from herbivores. Although the vast majority of some 145 extant species currently classified into pseudomyrmecines are arboreal, a few species three genera: , , and nest in the soil. Colonies are of small to moderate (Fig. 1p). Many species likely size of several hundred workers, except for the remain to be described, as these are cryptic ants ant-plant mutualists, whose colonies can exceed that dwell in the leaf litter and soil. Proceratiines 10,000 workers. Temporary social and belong to the poneroid clade, but their exact phy- workerless social parasitism have been reported logenetic placement remains elusive. Currently from the subfamily. A total of 20 fossil species are they are inferred as the sister group to a monophy- known from Eocene Baltic, Bitterfeld, and Rovno letic clade consisting of Amblyoponinae and ambers of Europe () and 15–26 mil- Apomyrminae (Fig. 2). The three proceratiine lion-year-old Miocene Dominican amber genera are distributed throughout various habitats (Pseudomyrmex). of the tropics and warm temperate regions of the world. Colony sizes are small, often fewer than † 100 workers. Nests are built deep in the soil and in The Sphecomyrminae are considered stem-group rotting wood, and occasionally they nest arbore- fossils of the family Formicidae. Recently the ally. Very few direct observations of Proceratiinae taxonomic scope of Sphecomyrminae was broad- biology have been made, but the few species ened by inclusion of taxa that had previously been studied appear to be predators of eggs. placed in the subfamily Armaniinae, although this The queens of the African Discothyrea oculata taxonomic change could soon be reversed. The start colonies in egg sacs where they find classification of the Armaniinae has been in flux shelter and abundant food for their first brood. and some authors considered this subfamily a Proceratium and Discothyrea are known from stem-group representative of the Formicidae, Dominican and Chiapas ambers, and the single whereas others regarded the Armaniinae as a fam- extinct genus, Bradoponera, consists of four spe- ily outside of the Formicidae. Armaniinae are cies which are known from the Eocene Baltic, Cretaceous impression fossils known exclusively Bitterfeld, and Rovno ambers of Europe. from alates. There are 33 currently recognized 12 Ants: Phylogeny and Classification species of Sphecomyrminae classified in 14 gen- all representatives are extinct and members of the era. The oldest known sphecomyrmines are Sphecomyrminae are not more closely related to Baikuris, , and Haidomyrmodes any particular extant lineage but instead more from ~100 million-year-old French Charentese closely related to other extinct lineages. In parallel amber. to some stem-group ants, however, some Creta- ceous fossils certainly belong to crown-group ants, that is, they can be placed inside the clade Fossil Record and Timeline of Early Ant formed by currently living ant lineages. The most Evolution spectacular example is the ca. 92 million-year-old amber fossil Kyromyrma neffi, which can unmis- Snapshots of the ants’ evolutionary history are takably be recognized as a formicine ant due to its seen in an impressive fossil record. Over 750 well-developed and plainly visible acidopore. fossil ant species have been described, more than No definitive fossil ants were known from the the currently known number of dinosaur species period 55–78 million years ago until very recently [2]. The timeline of ant evolution begins with an when the aneuretine impression fossil early Hymenopteran fossil showing affinities to Napakimyrma paskapooensis was described the lineage containing both ants and Apoidea. One from the Canadian Paleocene Paskapoo Forma- of these early fossils is Cariridris bipetiolata,a tion, which is 56–60 million years old. 112–122 million-year-old impression fossil from By the time Eocene amber deposits formed in Brazil. It was once postulated to be the earliest Europe, most contemporary subfamilies are known ant but more likely represents a relative of known to have been present. Exceptions are the the sphecoid family Ampulicidae. subterranean subfamilies Apomyrminae, Cretaceous amber deposits from 100–125 mil- Leptanillinae, and Martialinae, as well as the lion years ago contain insect fossils but no known monotypic Paraponerinae and species-poor ant inclusions. Ants appear in the fossil record Heteroponerinae. Unfortunately, confident dating around 100 million years ago, and they are at of the Eocene European amber deposits (Baltic, first very uncommon, representing less than 2% Bitterfeld, and Rovno ambers) has been notori- of all Cretaceous insect inclusions from then ously difficult, and various authors have proposed onwards. Despite their low abundance, these either (41–48 million years old) or early ants already exhibited a variety of distinct (34–38 million years) ages for Baltic morphologies. The extinct subfamily and Rovno ambers, and an even wider range of Sphecomyrminae is the most diverse, with genera estimates for Bitterfeld amber, suggested to be as such as Gerontoformica and , young as Upper Oligocene (23–28 million years which have generalized -like mandibles, as ago). Regardless of the exact age of the deposits, well as a number of forms with spectacular man- ants evidently became more abundant in terrestrial dibular and head morphologies not seen in mod- ecosystems and account for 10% or more of the ern ants. Ants in the genera , insect inclusions in Eocene ambers. Although fos- Haidomyrmodes, and Haidoterminus possess sil Eocene ant faunas of Europe and North Amer- remarkable L-shaped or scythe-like mandibles. ica contain extant genera, their taxonomic The tips of these mandibles point toward the composition differs from today’s faunas from the frons and likely functioned as trap-jaw mechanics. same regions. Not only are many lineages cur- Ceratomyrmex and Linguamyrmex are even more rently confined to the tropics, but frequently a extreme, sporting a massive clypeal horn with an mixture of tropical and temperate genera is apical lobe, apparent trigger hairs, and extremely found. This pattern has been observed for other long mandibles that point toward the lobe. No faunas and floras of the Eocene, including fossil such structures are known in extant ants. termites, dragonflies, and floras from Yellowstone Sphecomyrmines are most often considered a National Park, USA. Several explanations have stem lineage of all extant ants, which means that been proposed to explain this pattern, including Ants: Phylogeny and Classification 13 transport and mixing of fossils from places with past. Examples of rapid radiations include early different paleoclimates, time averaging of diversification of the poneromorph clade, the deposits that were actually laid down over periods dorylines, tribe Crematogastrini, and several of of millions of years, and Eocene climates with the hyperdiverse genera. These patterns are fur- reduced seasonality and no modern equivalents. ther discussed below. The study of fossils is crucial for our under- standing of ant evolution. It provides a window into past morphologies, behaviors, and faunas. It Diversification of Modern Ants: is also necessary for modern phylogenetic infer- Geography, Innovation, Dispersal, and ence that has been focused on molecular data and Coevolution sophisticated statistical analysis. Molecular phy- logenies require fossils in order to accurately cal- The majority of ant species live in the tropics: ibrate nodes on the phylogenetic tree and obtain is home to about the same number divergence dates in absolute time. Without a solid (~900) of ant species as Canada and the USA understanding of the phylogenetic position of fos- combined despite occupying a land area almost sils, which can only be obtained through morpho- 400 times smaller. Approximately 79% of ant logical investigations, confident dating of events species and subspecies have ranges that are at in ant evolution is impossible, regardless of theo- least in part within tropical latitudes (23.5 north retical advances in computer modeling. The same and south), despite the fact that the tropics account logic extends to biogeographic inference, where for only 36% of the Earth’s dry land surface. This past species ranges and dispersal events are pattern of species richness is reflected in higher reconstructed from patterns of modern distribu- phylogenetic diversity in the tropics and the fact tions and phylogeny. Because the fossil record that most ant genera and subfamilies are either reveals that distributions of modern taxa have restricted to the tropics or present in the tropics changed over time, often dramatically, such infer- as well as temperate regions. ences are incomplete or biased without consider- Several hypotheses have been put forward to ation of fossils. Ant systematists are becoming explain high tropical diversity, exemplified by increasingly aware of these limitations, fossils ants and many other animal and plant taxa. Com- are being discovered or reevaluated, and ways in parative phylogenetic research suggests that trop- which modeling of the evolutionary past is done ical ant lineages are much older than temperate are being often revised. These factors to lineages and that tropical and temperate lineages constant refinement of our understanding of the give rise to new species at similar rates. Accord- timeline of ant evolution. ingly, most familiar north temperate ant genera, With the above caveats in mind, it is no sur- such as Formica, ,orTemnothorax, must be prise that estimates of the age of crown-group ants regarded as evolutionary newcomers. This sug- and major ant clades have varied among studies gests that there are fewer ants in colder regions (Fig. 2, Table 1). Most phylogenetic work places because these faunas are relatively young [16]. the most recent common ancestor of modern ants Recent molecular phylogenetic studies in the Early Cretaceous period, between 145 and revealed other large-scale geographic patterns in 110 million years ago. This is before the time ant evolution. Many old clades show affinity to when first Cretaceous amber ant fossils become major biogeographic regions, suggesting that available and suggests that the presumed stem- diversification often remains confined to a partic- group sphecomyrmines remained dominant in ter- ular region over long periods. For example, the restrial habitats for some time after crown-group myrmicine tribe Attini of 45 genera and more than ants first appeared on the scene. This timeline also 2,500 species originated in the New World shows that modern ants rose to prominence along- approximately 65 million years ago. Its diversifi- side angiosperms and that bursts of ant diversifi- cation proceeded mainly in the Neotropics until a cation occurred throughout their evolutionary few lineages, most notably the hyperdiverse 14

Ants: Phylogeny and Classification, Table 1 Age range estimates for select major ant clades and five largest subfamilies from major phylogenetic studies. Numbers in square brackets next to references correspond to reference number. Age ranges are reported in millions of years before present Brady et Moreau Ward et Schmidt Moreau Brady et Ward et Blaimer Ward and Blanchard Economo Borowiec al. 2006 et al. al. 2010 2013 and Bell al. 2014 al. 2015 et al. Fisher and Moreau et al. 2018 Borowiec et al. 2019 Clade/reference [12] 2006 [19] [28] [23] 2013 [18] [13] [29] 2015 [3] 2016 [30] 2017 [5] [16] 2019 [10] [11] Formicidae 111–137 141–169 111–152 128–169 145–220 154–169 103–123 Leptanilloids 82–114 Poneroids 100–115 122–134 93–128 69–107 105–147 127–149 81–102 Formicoids 103–120 118–131 95–122 104–118 115–155 129–150 96–106 Amblyoponinae 92–118 113–143 66–114 37–87 93–128 85–131 93–131 62–87 +Apomyrminae Dolichoderinae 87–106 103–113 81–114 62–97 80–85 98–108 100–123 78–89 +Aneuretinae Heteroponerinae 81–92 80–84 79–92 73–115 41–70 +Ectatomminae Pseudomyrmecinae 91–103 72–99 55–60 72–121 95–130 71–92 +Myrmeciinae Dolichoderinae 71–76 86–97 59–74 59–72 60–73 62–92 52–62

Dorylinae 71–94 99–117 68–93 74–101 77–111 95–122 53–101 Classification and Phylogeny Ants: Formicinae 77–83 92–104 56–82 69–90 88–136 89–97 73–116 51–71 Myrmicinae 81–89 100–114 65–88 74–90 88–110 91–100 94–121 51–72 Ponerinae 79–103 111–132 78–112 54–66 86–118 106–130 61–84 Ants: Phylogeny and Classification 15 genera Pheidole and Strumigenys, started dispers- subfamily level (Fig. 2), but it is also present ing out of this realm within the last 30 million within subfamilies. Although currently 334 extant years. A parallel Old World example is another ant genera are recognized, eight of the most myrmicine tribe, the tribe Crematogastrini, with speciose taxa, Camponotus, Crematogaster, 64 extant genera and more than 2,600 species. In Monomorium, Pheidole, Polyrhachis, this case most of the diversification occurred Strumigenys, Temnothorax,andTetramorium within the Indomalayan and Afrotropical regions, together account for over 5,400 species, or about with few lineages eventually dispersing into the 40% of all described ant species. Moreover, these north temperate zone and Neotropics. Other lineages are often sister groups to much less examples of old species-rich clades mostly con- speciose clades and are relatively young, their fined to a single biogeographic region include the crown groups having originated throughout the largely Afrotropical formicine tribes last 40 million years or later. , the north temperate , Much attention has been paid to the causes of and the Australian Melophorini, as well as the this uneven proliferation of species in ants and large clade comprising almost all of New World other taxa. This research is carried out in the doryline species and the ponerine Odontomachus context of understanding that relative rates of genus group, the latter distributed mainly in the both speciation and contribute to over- Paleotropics. Similar patterns of long-term geo- all diversification of a lineage. Common factors graphic affinity of clades can also be seen in less that promote diversification include colonization speciose groups, such as the Amblyoponinae, and of a new region, where competition is reduced, or the exclusively Old World subfamily a key innovation, a trait that promotes diversifica- Leptanillinae. tion or reduces extinction, perhaps by allowing In addition to radiations spanning large bio- expansion into new niches. Identification of any geographic regions, molecular phylogenies com- trait as directly responsible for increased diversi- bined with extensive faunal surveys provide a fication, however, is difficult. In ants, candidates clearer picture of island ant faunas and have for key innovations include, but are not limited to, uncovered numerous examples of island radia- the Blochmannia symbiosis of the Camponotini, tions. The Malagasy ant fauna has been the the dimorphic worker caste of Pheidole, the focus of intensive surveys in the last 25 years. convergently evolved trap-jaw mandibles in dis- Approximately 96% of its 745 species are tantly related myrmicine, formicine, and ponerine endemic to the island, and the fauna includes lineages, as well as the fungus-growing behavior several lineages that represent recent dispersals in the Atta genus group and the specialized hunt- of species-rich genera such as Crematogaster ing behavior in the army ants. The factors respon- and Pheidole, arriving on the island approxi- sible for the ecological success of most species- mately 25 and ten million years ago, respectively. rich ant groups, however, are difficult to know In contrast, the clade of Malagasy-endemic genera with certainty. , Malagidris, Royidris, and Vitsika A contrasting pattern is found in lineages that colonized Madagascar approximately 50 million are remarkably species poor but have persisted years ago, but these endemic genera are currently through long periods of geologic time without species poor, perhaps representing a remnant of significant diversification [20]. The many exam- the original ant radiation now curbed by competi- ples of such lineages among ants include mono- tion from more recent arrivals. Islands are also typic genera such as Aneuretus, Martialis, home to phylogenetically isolated, relict species, Opamyrma, Paraponera, , and such as Aneuretus simoni of Sri Lanka or Tatuidris. Many of these are cryptobiotic, and it besmerus of Madagascar. may be that these lineages experience both low Diversification rates across the tree of life are extinction and low speciation rates as a result of highly uneven, and the ants are no exception. As adaptations to their ecologically stable subterra- already discussed, this pattern is obvious at the nean environments. This does not explain, 16 Ants: Phylogeny and Classification however, cases of Aneuretus or Paraponera, 15 years or so provide a picture of ant relation- which forage above ground. In the case of ships and evolution that often differ from the Aneuretus, at least, it appears that it represents scenarios drawn from morphology-based phylog- the last living lineage of a once more diverse enies, especially at the subfamily and tribe level. It group now in decline. highlights the frequency of morphological con- vergence in ant evolution and allows reexamination of ant morphology in the light of Present State of Ant Systematics independent tests of evolutionary relationships provided by molecular data. Myrmecologists are The publication of the landmark monographs also increasingly taking advantage of micro- Identification Guide to the Ant Genera of the computed tomography, which reveals unprece- World and A New General Catalogue of the Ants dented detail of skeleto-musculature in even the of the World marked a turning point in ant system- smallest animals, living or fossil, and allows ren- atics [6, 7], not long after the comprehensive dering of three-dimensional models of external monograph The Ants [17]. These resources cre- and internal structure. The field of ant systematics ated a major upsurge in , as the is thus ripe for a revival of morphological resulting synergy promoted it as a prominent research. field of entomological inquiry and accelerated The molecular revolution has provided us the pace of taxonomic research. The average num- with a relatively stable and complete picture of ber of new ant taxa described each year more than phylogenetic relationships of ants at the subfam- doubled between the two decades prior to 1995 ily and even genus level [12, 15, 19]. An ongoing and the two decades since, with more than 200 effort of the Global Ant Genome Alliance is new ant taxa described annually in recent years. seeking to understand ant diversity from a new This trend shows no sign of abating, so that the perspective, and providing a genome-based phy- formal description of new taxa, both extant and logeny for all ant genera is likely to further solid- fossil species, is predicted to continue for the ify this framework [8]. However, much foreseeable future. phylogenetic work remains to be done in spe- Since the publication of these landmark books cies-level phylogeny. Most ant genera currently in the 1990s, there has been an effort to move the lack a comprehensive phylogenetic tree, taxonomic catalog and literature online and to although much progress has been made in eluci- foster a more community-driven cataloging effort. dating relationships within hyperdiverse lineages Web portals such as antbase.org and, more such as Crematogaster, Pheidole,orPoly- recently, AntCat.org are providing easy access to rhachis. Efforts targeting other massive genera taxonomic literature and information. Web portals such as Camponotus, Strumigenys,andTetra- such as AntWeb.org and AntWiki.org link these morium are ongoing. resources to specimen-based distribution data and The future of ant systematics will likely con- images, including many specimens from nat- tinue to abound in discoveries of new ant species, ural history collections around the world, contrib- to see a fuller integration and accessibility of uting to further democratization of taxonomic taxonomic literature, cataloging efforts, and spec- information. imen data, as well as bringing exciting new In addition to the recent improvements in insights into the morphology of living and extinct access to taxonomic information and the ants. 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