A

Ants: Phylogeny andClassification

Marek L. Borowiec1, Corrie S. Moreau2 andChristian Rabeling31University of Idaho, Moscow, ID, USA2Departments of Entomology and Ecology &Evolutionary Biology, Cornell University, Ithaca,NY, USA3Social Insect Research Group, Arizona StateUniversity, Tempe, AZ, USA

Ants are the most ubiquitous and ecologicallydominant insects on the face of our Earth. This isbelieved to be due in large part to the cooperationallowed by their sociality. At the time of writing,about 13,500 ant species are described andnamed, classified into 334 genera that make up17 subfamilies (Fig. 1). This diversity makes theants the world’s by far the most speciose group ofeusocial insects, but ants are not only diverse interms of numbers of species. They exhibit anamazing diversity of form and function exempli-fied by a number of charismatic taxonomic orfunctional groups known as ▶ army ants, ▶ trap-jaw ants, ▶ turtle ants, ▶ fungus-growing ants,exploding ants, and ▶ socially parasitic ants, toonly name a few. Taxonomic and phylogeneticresearch helps to discover, describe, and organizethe diversity of organisms, as well as to revealtheir evolutionary histories. The timeline of mod-ern zoological nomenclature officially begins in

1758 when the Swedish botanist Carl von Linnépublished the tenth edition of his catalog of allplant and animal species known at the time.Among the approximately 4,200 animals that heincluded were 17 species of ants. The succeedingtwo and a half centuries have seen tremendousprogress in the theory and practice of biologicalclassification. Here we provide a summary of thecurrent state of phylogenetic and systematicresearch on the ants.

Ants Within the Hymenoptera Tree ofLife

Ants belong to the order Hymenoptera, which alsoincludes wasps and bees. ▶Eusociality, or truesociality, evolved multiple times within theorder, with ants as by far the most widespread,abundant, and species-rich lineage of eusocialanimals. Within the Hymenoptera, ants are partof the ▶Aculeata, the clade in which the ovipos-itor has been modified into a ▶ venom-injectingorgan, or stinger, and where all known instancesof hymenopteran eusociality are concentrated.Although the ants’ status as aculeates has beenlong known, their affinities within the grouphave been the subject of different proposals.Application of genome-scale data to phyloge-netics in the last 5 years reveals that ants aremost closely related to the Apoidea, the lineagethat includes bees and sphecoid wasps [14, 21].

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

Phylogeny of the Extant Ants

The great diversity of extant ant species is distrib-uted among 17 subfamilies grouped into 3 majorclades, the leptanilloid, poneroid, and formicoid

clades (Fig. 2). The leptanilloid clade comprises asingle Neotropical species from the subfamilyMartialinae, Martialis heureka, and about 70described species of the subfamily Leptanillinae,which are confined to the Old World. There is

Ants: Phylogeny and Classification, Fig. 1 Diversityof extant ant subfamilies. (a) Agroecomyrmecinae (b)Amblyoponinae (c) Aneuretinae (d) Apomyrminae (e)Dolichoderinae (f) Dorylinae (g) Ectatomminae (h)Formicinae (i) Heteroponerinae (j) Leptanillinae (k)Martialinae (l) Myrmeciinae (m) Myrmicinae (n)

Paraponerinae (o) Ponerinae (p) Proceratiinae (q) Pseudo-myrmecinae. All photos by Alex Wild (http://www.alexanderwild.com), except (a) by Micheal Branstetter, (c& d) from Antweb.org (http://www.antweb.org), anddrawing of (k) by Barrett Klein

2 Ants: Phylogeny and Classification

some uncertainty as to whether leptanilloids areindeed a natural, monophyletic group, asMartialis was earlier inferred to be the sister lin-eage to all other ants and its position has beensensitive to the way phylogenetic analysis is car-ried out. Recent studies, however, support a sister-group relationship between Martialis and theLeptanillinae, placing the leptanilloids as sisterto all remaining ants [11].

The poneroids include more than 1,500 validdescribed species and are distributed across 6 sub-families: Agroecomyrmecinae, Apomyrminae,Amblyoponinae, Paraponerinae, Ponerinae, andProceratiinae. The Ponerinae harbor more than80% of the poneroid diversity with roughly1,250 extant species. Poneroid ants are mostlyconfined to the tropics, with only isolated speciesin several genera extending into the north temper-ate regions. While it is almost universally agreedthat poneroids are a clade, phylogenetic relation-ships among poneroid subfamilies are less wellresolved. Apomyrminae and Amblyoponinae aresister groups, but relative placements of other

subfamilies remain uncertain, even followingapplication of genome-scale phylogenetic data.The observed phylogenetic uncertainty probablyreflects rapid initial diversification of poneroidlineages during the late Cretaceous.

The formicoid clade encompasses the vastmajority of all ant species (~12,000) and containsall remaining ant subfamilies: Aneuretinae,Dolichoderinae, Dorylinae, Ectatomminae,Formicinae, Heteroponerinae, Myrmeciinae,Myrmicinae, and Pseudomyrmecinae. Althoughas a rule these ants are more diverse in (or evenconfined to) the tropics at the subfamily level, twoformicoid subfamilies, the Formicinae andMyrmicinae, have been particularly successful intemperate zones. The majority of ant speciesfound in the Nearctic and Palearctic regionsbelong to these two subfamilies. In contrast tothe poneroids, there exists broad consensusregarding the phylogenetic relationships of sub-families in the formicoid clade.

As can be seen in Fig. 2, species diversity ofextant ants is unequally distributed across the

Agroecomyrmecinae

Amblyoponinae

Aneuretinae

Apomyrminae

Dolichoderinae

Dorylinae

Ectatomminae

Formicinae

Heteroponerinae

Leptanillinae

Martialinae

Myrmeciinae

Myrmicinae

Paraponerinae

Ponerinae

Proceratiinae

Pseudomyrmecinae

Form

icoi

dsPo

nero

ids

6678

264

33

3146

713

1

231

94

692

1241

2

1

146

140

225 220 175 150 125 100 75 50 25 0

1

68

1

Number of species per subfamily

M +

L

Ants: Phylogeny and Classification, Fig. 2 Relation-ships among extant ant subfamilies and their speciescounts. Blue bars show ranges of age estimates for crucial

nodes from studies listed in Table 1. Estimates from eachstudy are superimposed so that darker colors indicate moreoverlap. Age scale in millions of years

Ants: Phylogeny and Classification 3

phylogeny. Martialinae plus Leptanillinae containonly 1% of species diversity, while poneroids andformicoids account for the remaining 11% and88%, respectively.

Overview of Ant Subfamilies

For ease of reference, the following summary ofthe 17 extant and 3 known extinct subfamilies ispresented in alphabetic order, rather than group-ing them by major clades.

AgroecomyrmecinaeA poneroid subfamily consisting of only two liv-ing species, Tatuidris tatusia (Fig. 1a) andAnkylomyrma coronacantha. Both are rarelyencountered. Tatuidris, found in Central andSouth America, is thought to be a specializedpredator, foraging for yet undetermined prey inthe leaf litter of mid-elevation rainforests.Workers of Ankylomyrma have been collected ahandful of times in West Africa. It is an arborealspecies, but nothing else is known of its naturalhistory. Agroecomyrmecinae was presumablymore diverse in the past, as three fossil specieshave been described from Eocene Baltic andRovno ambers of Europe, whose exact dating iscontroversial, and from ca. 34 million years oldNorth American Florissant shale.

AmblyoponinaeA poneroid subfamily of about 140 currentlydescribed extant species distributed among 9 gen-era. Recent phylogenetic work shows two majorclades, one comprising the genera Amblyopone,Onychomyrmex, and Prionopelta, and the otherthe remaining six genera [30]. With more than 50species, Stigmatomma (Fig. 1b) is the largestgenus. Species belonging to this small subfamilyare known as Dracula ants, because adults bitetheir own larvae and consume their hemolymph, abehavior also observed in Leptanillinae and Pro-ceratiinae. This mode of feeding is thought to bean adaptation to unstable food supplies affectingspecialized predators that are unable to regurgitateand share food within the colony. As such, it canbe seen as an alternative to▶ trophallaxis. Crown-

group Amblyoponinae likely started to diversifyin the mid-Cretaceous, and, despite low speciesdiversity, the subfamily is distributed worldwideand exhibits a variety of morphologies and lifehistories. Many species are small in body andcolony sizes and are cryptic dwellers of the soil,leaf litter, or rotting wood. Some are specializedpredators of geophilomorph centipedes, while thelarge species Myopopone castanea is known toprey on wood-boring cerambycid beetle larvae.Onychomyrmex species have an army-ant-likelife history and wingless queens. CertainMystrium species possess queens that are smallerthan and of conspicuously different color than theworkers. Several Eocene impression fossils of theextinct genus Casaleia from the ~47 million yearsold Messel formation have been assigned toAmblyoponinae, and two Baltic amber speciesof Stigmatomma have been described.

AneuretinaeThis formicoid subfamily is currently representedby a single extant species, ▶Aneuretus simoni,which is confined to isolated sites in Sri Lanka.The Aneuretinae are the sister group to theDolichoderinae. Aneuretus simoni (Fig. 1c) is ararely found species nesting predominantly in rot-ten wood, with polygynous and ▶ polydomouscolonies generally containing fewer than 100workers. The worker caste is polymorphic, andthe species exhibits generalized predatory andscavenging habits. Aneuretine ants were muchmore widespread and speciose in the past. Nineextinct genera and 12 species have been describedfrom Cretaceous and Eocene amber as well asfrom impression fossils from Europe, Asia, andNorth America. The Cretaceous fossils includethe genera Burmomyrma and Cananeuretus,whose assignment to Aneuretinae is uncertain.Napakimyrma paskapooensis is the oldestundisputed aneuretine and the only confirmedant fossil known from the Paleocene. This impres-sion fossil is known from the 56–60 million-year-old Paskapoo Formation in Canada.

ApomyrminaeApomyrminae is a monotypic poneroid subfam-ily, represented by Apomyrma stygia (Fig. 1d).

4 Ants: Phylogeny and Classification

Apomyrminae are sister to Amblyoponinae.Apomyrma stygia is known from sub-SaharanAfrica, and very little is known of its habits,except for subterranean nesting and indirect evi-dence of predation on geophilomorph centipedes.Fossils remain unknown from the Apomyrminae.

†BrownimeciinaeThis extinct monotypic subfamily is known fromCretaceous New Jersey amber dated to approxi-mately 92 million years ago. The sole species,Brownimecia clavata, is similar tosphecomyrmines but distinguished by a longantennal scape, which is thought to be a derivedfeature. Brownimecia has been inferred to be thesister lineage to all crown-group ants.

DolichoderinaeThe Dolichoderinae are a highly diverseformicoid subfamily of more than 700 extant spe-cies. The classification and phylogeny of thisgroup have been studied extensively, and at pre-sent the extant taxa are organized into 4 tribes and28 genera [25, 28]. One of the tribes,Leptomyrmecini, contains a clade that encom-passes a massive Australian radiation that begansome 25 million years ago, making Australia thecenter of dolichoderine diversity.

As a large subfamily, the dolichoderines showa variety of morphologies and life histories andtherefore defy easy synopsis. Characteristic mor-phological traits include a nonfunctional stingerand thin cuticle devoid (with the exception of thegenus Dolichoderus) of much of the sculpture orarmament that is typical for most other ants. Col-onies range from monogynous and containing afew hundred workers to highly populous polygy-nous colonies inhabiting polydomous nests. Asone example, nests of the Australian Iridomyrmexpurpureus can house upwards of 300,000workers. One of the world’s most significant inva-sive ants, the ▶ argentine ant Linepithema humile(Fig. 1e), is also a member of the Dolichoderinae.The argentine ant and the noninvasive NorthAmerican odorous house ant Tapinoma sessileform ▶ supercolonies with no apparent uppersize limit. Instead of defending themselves bystinging, dolichoderines are armed with an

impressive battery of chemical defenses and areknown for their advanced chemical communica-tion systems. The Neotropical genus ▶Azteca isknown for its diverse ecological ▶ interactionswith plants and ▶ scale insects and has been thefocus of many studies on coevolution and adapta-tion. The subfamily also has an impressive fossilrecord comprising 135 fossil species in 20 genera.New and currently undescribed species from ca.72 million-year-old Myanmar amber andChronomyrmex medicinehatensis from 78–79million-year-old Cretaceous amber of Canadaare the oldest known dolichoderines. It appearslikely that dolichoderines were more diverse inthe past and that later arrivals on the ant evolu-tionary stage, such as the Myrmicinae and rapidlydiversifying lineages within the Formicinae,displaced the Dolichoderine on continents otherthan Australia.

DorylinaeThis diverse formicoid subfamily contains almost700 described extant species distributed among 27genera, including the charismatic true army ants[9]. The dorylines have a worldwide distribution,but the vast majority of species occurs in thetropics. The phylogeny of this group has beendifficult to infer with certainty, likely becausedorylines experienced an ancient rapid radiation[10, 13]. Due to a lack of characteristic morpho-logical traits that would allow for grouping mul-tiple genera and uncertainty in phylogeneticplacement of several other genera, no tribal clas-sification is currently in use.

The army ants, the most conspicuous and best-known members of the Dorylinae, are character-ized by a suite of behavioral and morphologicalcharacteristics dubbed “the army ant syndrome.”This includes collective foraging, frequent colonyrelocation, and highly derived queen and malemorphologies. The true army ants are among themost bizarre and impressive social insects. Theyforage in coordinated swarms that have no scoutsor leaders; colonies move nesting sites periodi-cally; and young queens ▶ found new coloniesby budding. Army ant colonies are among thelargest insect societies. An estimate of 22 millionworkers has been made for a single colony of the

Ants: Phylogeny and Classification 5

African army ant Dorylus wilverthi. YoungDorylus queens mate with as many as 20 differentmales and are capable of storing more than 800million spermatozoa used to lay an estimated 250million eggs during their lifetimes.

New World army ant colonies undergo cyclesof alternating phases of foraging and reproduc-tion, during which the queen is either physogastricand producing eggs and the colony is not chang-ing nesting sites (the so-called statary phase) orthe queen ceases egg production, workers forageintensively, and the colony moves betweennesting places each day (nomadic phase). Inmany species the workers and even queens arecompletely blind, and males have conspicuouslyrobust, large bodies. In spite of the high level ofmorphological and behavioral specialization,molecular phylogenetic studies indicate thatNew World army ants (genera Cheliomyrmex,Eciton, Labidus, Neivamyrmex, andNomamyrmex) and Old World army ants Aenictusand Dorylus (Fig. 1f) evolved convergently inSouth America and Africa, respectively. In addi-tion to the true army ants, the Dorylinae contains anumber of lineages whose biology, taxonomy, andphylogeny are poorly known. It is uncertainwhether any species in these non-army ant line-ages acquired all components of the army antsyndrome, but each individual trait describedabove has been reported for some species. Thereis one extinct genus, Procerapachys, known fromEocene Baltic and Bitterfeld ambers. Extantdoryline genera are also known from Baltic,Dominican, and Chiapas amber deposits.

EctatomminaeThe Ectatomminae are a medium-sized formicoidsubfamily with over 250 described species classi-fied in 4 genera: ▶Ectatomma (Fig. 1g; 15 extantspecies), Gnamptogenys (138 species),▶Rhytidoponera (104 species), andTyphlomyrmex (7 species). The Ectatomminaeare the sister group to the Heteroponerinae, andtogether they form a clade sister to theMyrmicinae (Fig. 2). Ectatomma andTyphlomyrmex are confined to the New World,Rhytidoponera occurs in Southeast Asia and Aus-tralasia, and Gnamptogenys is found both in the

New World as well as in Southeast Asia andAustralia. Despite the relatively low species diver-sity, ectatommines are common and conspicuouselements of tropical New World and Australianant faunas. The subfamily is absent from Africaand from most parts of the northern temperateregions.

Most ectatommines appear to be generalizedpredators and scavengers although specializedpredators of fungus-growing ants and millipedeshave been reported in the genus Gnamptogenys.One species of Ectatomma was described as obli-gate▶ social parasite. Ectatommines often nest inrotting wood or in the soil. Both ground foragingand arboreal foraging species are known.Typhlomyrmex species are generally cryptic andlive and forage underground. Colony sizes arerelatively small, rarely exceeding several hundredworkers. Both monogynous and polygynous col-onies have been recorded. Many ectatommines,especially in the genus Rhytidoponera, have lostthe true queen caste and reproduce through matedworkers (▶ gamergates). Certain species ofEctatomma are considered pest control agentsfor crops in the Neotropics. The subfamily has arather extensive fossil record, and some 14 speciesin several genera are known from the MioceneDominican and Eocene amber of Europe, as wellas Eocene impression fossils. The affinity of the78–79 million-year-old Cretaceous Canadianamber fossil Canapone dentata is uncertain, butit has been suggested to represent an earlyectatommine. Pseudectatomma impression fossilsfrom ~47 million-year-old European Messel For-mation and Electroponera dubia from EoceneBaltic amber are the oldest undisputedectatommine fossils.

†FormiciinaeThe Formiciinae – note the close similarity inname to the next subfamily – are an extinct sub-family of giant ants currently containing six spe-cies in two genera: Titanomyrma and Formicium.These are impression fossils known from fourlocalities in North America and Europe, all datedto the Eocene when the two continents wereconnected by land bridges in the north Atlantic[1]. Only alates (Titanomyrma) and wing

6 Ants: Phylogeny and Classification

impressions (Formicium) have been collected.Titanomyrma species are both the largest antsand Hymenoptera known, with queens reachinga body length of 6 cm. The oldest Formiciinaefossils are 49–54 million years old from the NorthAmerican Green River formation. The exact phy-logenetic placement of the Formiciinae is uncer-tain, but they are currently placed within theformicoid clade.

FormicinaeThe Formicinae are the second-largest subfamilyof ants with 51 extant genera and more than 3,000described and many known but undescribed spe-cies. Two genera in one massive radiation, the▶ carpenter ants (Camponotus) and their closerelatives the ▶ spiny ants (Polyrhachis), accountfor more than 1,700 species. The formicines havea global distribution and are important compo-nents of both tropical and temperate faunas. Innorth temperate regions, at around 45�N, theFormicinae become the most species-rich subfam-ily. The phylogeny and higher classification of thesubfamily have recently been revised, dividing thesubfamily into 11 monophyletic tribes [3].Workers can be diagnosed by the acidopore, acircular opening formed by a fold of the lastabdominal sternite, which is surrounded by afringe of setae. The Formicinae lack a stingerand use this opening to spray formic acid, avenom unique to the subfamily.

The tribe Camponotini contains Camponotus(Fig. 1h) and Polyrhachis, as well as several lesserknown ant genera, such as the large-eyed Austral-asian strobe ants Opisthopsis, the spectacularlyiridescent Calomyrmex, the often intricatelysculptured Echinopla, and one of the world’s larg-est ants, the Southeast Asian Dinomyrmex gigas.All Camponotini have a bacterial symbiont,Blochmannia, found in cells lining the midgutand in the ovaries. Blochmannia is verticallytransmitted from generation to generation andprovides essential nutrients to the ants.

Formicines have successfully radiated into theworld’s arid zones, with three unrelated generaoccupying this niche in different parts of theworld: the species-rich genusMelophorus in Aus-tralia, ▶Cataglyphis in North Africa and Asia,

and Myrmecocystus in North America.Myrmecocystus is a prime example of the honey-pot ants, which can store liquid food in highlyexpandable abdomens, thus forming living pan-tries. The food storage behavior of the honeypotants evolved independently in desert ants fromseveral genera, all of the subfamily Formicinae.

Formicine ants also engage in obligate symbi-oses with mealybugs (genus Acropyga) and antplants (Cladomyrma, Myrmelachista). Thejumping ant Gigantiops destructor is commonthroughout the Amazon basin, where its verylarge eyes and unusual, highly visually orientedbehavior make it conspicuous. Southeast AsianMyrmoteras are an example of trap-jaw ants, orants in which spring-loaded mandible action gen-erates extremely fast-snapping movement. Thereis also a high number of socially parasitic speciesin the Formicinae, especially temporary parasitesand ▶ dulotic species in the genera Formica,Polyergus, and Rossomyrmex. The fossil recordof Formicinae is impressive, with 31 extinct gen-era and 197 fossil species. The Cretaceous NewJersey amber fossil Kyromyrma neffi was dated atapproximately 92 million years, exhibits a well-developed acidopore, and is the oldest undisputedformicine ant.

HeteroponerinaeThe Heteroponerinae are a small formicoid sub-family of 3 genera and 33 species. Hetero-ponerines have a remarkably disjunctbiogeographic distribution: 28 species of Hetero-ponera (Fig. 1i) are found in Australia, NewZealand, and in the Neotropics. Four species ofAcanthoponera are Neotropical, and the singlespecies of Aulacopone is known from only twoqueens collected in Azerbaijan. Heteroponerinaeare sister to Ectatomminae (Fig. 2), and the twosubfamilies can be difficult to separate based onmorphological characters. The Heteroponerinaecan be recognized by a longitudinal carina run-ning down the top of the head. Little is known ofthis group’s biology. Some heteroponerines nestin or under rotting wood, in the soil, and understones. Some species were observed to feign deathwhen disturbed. Colonies are small and monogy-nous, and both winged and ▶ ergatoid queens are

Ants: Phylogeny and Classification 7

known. There are no heteroponerine fossilsknown at the time of writing.

LeptanillinaeThis small leptanilloid subfamily of rarelyencountered subterranean ants currently com-prises about 70 described Old World species,although the group’s cryptic habits make it likelythat many more species await discovery.Leptanillines are currently classified in eight gen-era. There are three major lineages within thesubfamily: the sister lineage to all otherleptanillines, represented by the single speciesOpamyrma hungvuong, species with somewhatlarger workers that are currently classified in thegenera Anomalomyrma and Protanilla, and spe-cies with miniscule workers in the genusLeptanilla. The genera Noonilla, Phaulomyrma,Scyphodon, and Yavnella are known only frommales.

Little is known about the biology ofleptanillines, and only a couple of species havebeen studied in the laboratory. Colonies are prob-ably small (not more than several hundredworkers) in most species and either monogynous(Leptanilla; Fig. 1j) or polygynous with ergatoidqueens (Protanilla). Like the amblyoponineDracula ants that feed on their own larvae,leptanilline queens and workers engage in larvalhemolymph feeding. Unlike the amblyoponines,however, they do not puncture the larval cuticle.Instead, they feed through a pair of specializedduct organs located on the larval abdomen.Leptanilla also appear to be specialized predatorsof soil centipedes and have been shown to pro-duce brood in reproductive bouts similar to thoseobserved in the true army ants. Fossil leptanillinesare not known.

MartialinaeThe Martialinae are a monotypic subfamilyknown from only few specimens collected in theBrazilian Amazon [22]. The phylogenetic posi-tion of Martialis heureka (Fig. 1k) was initiallyinferred as the sister lineage to all extant ants.However, recent phylogenetic studies suggest asister group relationship between the New WorldMartialinae and the Old World Leptanillinae. The

difficulties associated with establishing the phy-logenetic position ofMartialis are related to com-positional heterogeneity of molecular sequencedata and the old age of this lineage [11]. Nothingis known about the biology ofMartialis, althoughthe complete lack of eyes and the pale-yellowcuticle strongly suggest a subterranean life. Elon-gate, forceps-like mandibles suggest specializedpredatory habits. Fossils are unknown from theMartialinae.

MyrmeciinaeThis is a formicoid subfamily presently confinedto Australia and New Caledonia. Two extant gen-era comprise this group: Myrmecia (Fig. 1l), alsoknown as bull ants or bulldog ants, andNothomyrmecia, the dawn ants. There are about100 described species of Myrmecia, whereasNothomyrmecia is represented by the singleextant species Nothomyrmecia macrops [26].The Myrmeciinae are the closest relatives of thebig-eyed arboreal ants Pseudomyrmecinae (Fig.2). Due to their diurnal habits, large body sizes,and pugnacity, bulldog ants are the most charac-teristic element of the Australian myrmecofauna.Myrmecia exhibits a variety of reproductive strat-egies. Young queens either establish new coloniesindependently or via pleometrosis (dependent col-ony foundation). Colonies can be monogynous orpolygynous, and queens can be singly or multiplymated. Queens of some species are winged, whileothers are ergatoid, and in a few species the queencaste has been replaced by gamergates (reproduc-tive workers). Colony sizes range from a fewdozen to a couple of thousand workers. Nests aretypically built in the soil but occasionally also inrotting logs, and one species, M. mjobergi, nestsarboreally. The workers are effective solitary for-agers with large eyes and correspondingly goodvision. Myrmecia deliver exceptionally painfulstings. They are also known for rapid karyotypeevolution and the high variability in chromosomenumbers among species. M. croslandi is the onlyant species known to have only a single pair ofchromosomes, a remarkable condition knownonly in one other metazoan, a nematode.

In contrast to many bulldog ants, the dawn antNothomyrmecia macrops is extremely rare and

8 Ants: Phylogeny and Classification

known from only a few localities in South Aus-tralia. Nothomyrmecia are nocturnal and exhibittraits that were historically associated with ances-tral ant societies, including small colony size,solitary foraging, lack of division of labor, andqueens foraging while raising the first brood. TheMyrmeciinae are represented by a diverse fossilrecord revealing an almost global distribution dur-ing the Eocene. Six extinct genera represented by16 species are known from Eocene and Oligoceneimpression fossils and ambers of Europe, Asia,North America, and South America.

MyrmicinaeWith more than 6,500 described species, theMyrmicinae are by far the largest ant subfamily.According to recent taxonomic rearrangementsbased on molecular phylogenetic results, thisformicoid subfamily is currently classified into 6monophyletic tribes and 143 extant genera [29].Their biogeographic distribution is global, andmyrmicines are present in every habitat whereants are found. Because of their great speciesdiversity, the myrmicines exhibit biological andmorphological variation that spans most of therange observed in the family as a whole. Special-ized functional groups found in the myrmicinesinclude ▶ fungus-growing ants, arboreal ▶ turtleants, ▶ seed-harvesting ants, ▶ social parasites,and numerous lineages of independently evolved▶ trap-jaw ants. Myrmicines gave rise to severalspectacular adaptive radiations. Only 5 of the 143genera, ▶Pheidole (Fig. 1m), Strumigenys,▶Crematogaster, Tetramorium, and▶ Temnothorax, contain more than half of thesubfamily’s species. The six myrmicine tribesare well supported by molecular data but poorlydefined by morphological characters. Myrmiciniis the only myrmicine tribe that is endemic tonorth temperate regions. The two constituent gen-era, Myrmica and Manica, are relativelyunspecialized scavengers, common in boreal andsubboreal habitats. The tribe Pogonomyrmecinicontains three genera that are restricted to theNew World. Patagonomyrmex andPogonomyrmex are granivores, while Hylomyrmais a likely omnivorous scavenger. The Stenaminiare another moderately-sized tribe with several

seed-harvesting lineages comprising mostly thenorth temperate genera Aphaenogaster, Messor,Stenamma, Veromessor, and others.

The Solenopsidini are a tribe with global dis-tribution and two highly speciose genera:▶Monomorium and ▶ Solenopsis. Some of theworld’s most notorious invasive ants are the fireants Solenopsis invicta and S. geminata. OtherSolenopsis species, known as thief ants, oftennest close to larger ants from which they stealresources and even consume brood.Monomoriumincludes several seed-harvesting species and thecommon greenhouse and house pest M.pharaonis. Monomorium is a large genus thatwas recently recognized as polyphyletic and iscurrently undergoing a taxonomic reevaluation.

The Attini are the tribe that originated anddiversified largely in the New World, with threelarge radiations of some of the world’s mostremarkable ants: the genera Pheidole,Strumigenys, and fungus-growing ants in theAtta genus group. At just over 1,000 describedand many unnamed species, Pheidole competeswith the formicine genus Camponotus for the titleof the most speciose ant genus. Pheidole speciesare dimorphic or trimorphic with a well-devel-oped major worker caste. Pheidole reached theirexceptional species richness within the last 40million years and colonized all continents exceptAntarctica. With more than 800 described cryptic,leaf-litter dwelling species, Strumigenys is not farbehind. Strumigenys is a genus in which a spring-loaded (trap-jaw) mandible-closing mechanismevolved independently several times, each timeStrumigenys colonized a new major geographicregion. The Atta genus group is endemic to theNew World and comprises the only ants that cul-tivate fungus for food. Leaf-cutting ants in thegenera Acromyrmex and Atta are among themost characteristic Neotropical ants, often seenharvesting great amount of vegetation that servesas substrate for their fungus gardens. Other mem-bers of the tribe include the NeotropicalCephalotes turtle ants and the invasive little fireant Wasmannia auropunctata. The Attini and itscore genera originated and diversified mainly inthe Neotropics but subsequently spread across theworld.

Ants: Phylogeny and Classification 9

All other myrmicines are classified in the tribeCrematogastrini [4]. This clade contains 64 gen-era and approximately 40% of the myrmicinespecies. The Crematogastrini originated anddiversified mostly in the Paleotropics, especiallyin the Indomalayan region. A few crematogastrinelineages are also present in the New World,including by far the largest northern temperategenus, the acorn ants Temnothorax, and themostly tropical Carebara, Crematogaster, andNesomyrmex, among others. Tetramorium isanother hyperdiverse ant genus that belongs tothis tribe and is mostly confined to the OldWorld except for several desert-adapted nativeNorth American species and two recent introduc-tions. Certain species of Carebara ants exhibitextreme worker-queen or worker polymorphism.In the case of Southeast Asian Carebara diversaand related species, known as ▶marauder ants,the largest workers can weigh 500 times as muchas their smallest sisters. Marauder ants forage inself-organized groups, as do true army ants, andtheir colonies attain sizes of hundreds of thou-sands of workers. The Myrmicinae have an exten-sive fossil record with 37 described extinct generaand 170 fossil species. The 89–94 million-year-old Cretaceous impression fossil Afromyrmapetrosa is the earliest putative myrmicine,although its affinities are uncertain. The oldestundisputed myrmicines are known from 52 mil-lion-year-old Chinese Fushun amber.

ParaponerinaeParaponera clavata (Fig. 1n), the infamous▶ bullet ant of the Neotropics, is the sole speciesin this formicoid subfamily. This species has aremarkably broad distribution, occurring fromHonduras in Central America to Paraguay andRio Grande do Sul state in Southern Brazil, andit is common throughout most of its range. Para-ponera clavata is famous for its extremely painfulsting. Its Tupí-Guaraní name, tuca-ndy, means“the one wounding deeply.” The nests of P.clavata are built at the base of trees or arboreally,and colonies can reach a couple of thousandworkers. Queens forage while raising the firstbrood, and workers forage individually. Theirdiet consists of invertebrate prey and scavenged

arthropods as well as nectar. Although Para-ponera does not now occur in the Caribbean, asingle fossil species, P. dieteri, is known from15–26 million-year-old Miocene Dominicanamber.

PonerinaeThe Ponerinae are the third largest subfamily ofants with 47 genera and more than 1,200 species[23, 24]. Because of their diversity, ponerines aredifficult to characterize, although many speciesare large and possess thick cuticle. The subfamilyis mostly tropical, with few lineages extendinginto the temperate regions. Ponerines are currentlydivided into two tribes, the Platythyreini and thePonerini. The Platythyreini contain a single genuswith some 40 species, forming the sister group toall other ponerines. ▶Platythyrea punctata isfound in Central America and the Caribbean. Itshows a remarkable diversity of reproductivestrategies, where regular queens, intercastes, andmated workers can reproduce either sexually orvia thelytokous▶ parthenogenesis. All other gen-era are classified within the tribe Ponerini. Thecolonies of ponerines are generally small com-pared to many other ants, and no species is con-sidered an invasive pest. The reproductivebiology of many ponerines is remarkable, becausemany species have lost the queen caste in favor ofgamergates. The presence of gamergates is alsoknown from other subfamilies, although it is mostwidespread in ponerines.

The Ponerinae are another subfamily in which▶ trap-jaw mechanisms evolved convergently.The sister genera Anochetus and Odontomachus(Fig. 1o) evolved powerful snapping mandiblesused for prey capture. Interestingly, the trap-jawmandibles can also be employed to propel indi-vidual ants into the air, a behavior that in somespecies may represent active predator avoidance.The South American ▶Dinoponera ants havearguably the largest workers of any living ants,and they rely exclusively on gamergates for repro-duction. Dinoponera have, with maximally 60pairs, the highest number of chromosomesreported in any Hymenopteran. Although primar-ily scavengers of dead invertebrates, Dinoponera

10 Ants: Phylogeny and Classification

have been observed to prey on small vertebratessuch as frogs.

Southeast Asian Harpegnathos are unusual inthat jumping is their main mode of locomotion,otherwise known in certain species ofOdontomachus and Australian Myrmecia antsand the Neotropical formicine Gigantiops. Spec-tacular morphological adaptations to specializedprey can be seen in Neotropical Thaumatomyrmexants, which use their highly modifiedmandibles toprey on very setose polyxenid millipedes, andsomeCentromyrmex species are extremely robust,presumably as an adaptation to hunting termites.Ponerines also have an extensive fossil recordwith 12 extinct genera and 86 species.Undescribed species reported from ca. 72 mil-lion-year-old Burmese amber are the oldestundisputed ponerine fossils.

ProceratiinaeThe Proceratiinae are a poneroid subfamily ofsome 145 extant species currently classified intothree genera: Discothyrea, Probolomyrmex, andProceratium (Fig. 1p). Many species likelyremain to be described, as these are cryptic antsthat dwell in the leaf litter and soil. Proceratiinesbelong to the poneroid clade, but their exact phy-logenetic placement remains elusive. Currentlythey are inferred as the sister group to a monophy-letic clade consisting of Amblyoponinae andApomyrminae (Fig. 2). The three proceratiinegenera are distributed throughout various habitatsof the tropics and warm temperate regions of theworld. Colony sizes are small, often fewer than100 workers. Nests are built deep in the soil and inrotting wood, and occasionally they nest arbore-ally. Very few direct observations of Proceratiinaebiology have been made, but the few speciesstudied appear to be predators of arthropod eggs.The queens of the African Discothyrea oculatastart colonies in spider egg sacs where they findshelter and abundant food for their first brood.Proceratium and Discothyrea are known fromDominican and Chiapas ambers, and the singleextinct genus, Bradoponera, consists of four spe-cies which are known from the Eocene Baltic,Bitterfeld, and Rovno ambers of Europe.

PseudomyrmecinaeThis is a medium-sized formicoid subfamily ofbig-eyed arboreal ants comprising 230 describedextant species that are classified into three genera.Pseudomyrmex (Fig. 1q) and the monotypic genusMyrcidris are endemic to tropical and warm tem-perate regions of the New World, while Tetra-ponera occurs in the Old World tropics [27].This subfamily’s closest relatives are theMyrmeciinae (Fig. 2). Pseudomyrmecines areknown for their mutualistic relationships withplants, which evolved at least a dozen times inthe subfamily. Perhaps the best-known example isthe obligate mutualism between members of theCentral American Pseudomyrmex ferrugineusgroup and swollen-thorn Vachellia acacias. Inthis system, the acacias provide nesting space inswollen thorns and food in the form of extrafloralnectaries and nutritious structures produced at thetips of leaves called food bodies or Beltian bodies.In exchange the ants aggressively protect acaciasfrom herbivores. Although the vast majority ofpseudomyrmecines are arboreal, a few speciesnest in the soil. Colonies are of small to moderatesize of several hundred workers, except for theant-plant mutualists, whose colonies can exceed10,000 workers. Temporary social parasitism andworkerless social parasitism have been reportedfrom the subfamily. A total of 20 fossil species areknown from Eocene Baltic, Bitterfeld, and Rovnoambers of Europe (Tetraponera) and 15–26 mil-lion-year-old Miocene Dominican amber(Pseudomyrmex).

†SphecomyrminaeThe Sphecomyrminae are considered stem-groupfossils of the family Formicidae. Recently thetaxonomic scope of Sphecomyrminae was broad-ened by inclusion of taxa that had previously beenplaced in the subfamily Armaniinae, although thistaxonomic change could soon be reversed. Theclassification of the Armaniinae has been in fluxand some authors considered this subfamily astem-group representative of the Formicidae,whereas others regarded the Armaniinae as a fam-ily outside of the Formicidae. Armaniinae areCretaceous impression fossils known exclusivelyfrom alates. There are 33 currently recognized

Ants: Phylogeny and Classification 11

species of Sphecomyrminae classified in 14 gen-era. The oldest known sphecomyrmines areBaikuris, Gerontoformica, and Haidomyrmodesfrom ~100 million-year-old French Charenteseamber.

Fossil Record and Timeline of Early AntEvolution

Snapshots of the ants’ evolutionary history areseen in an impressive fossil record. Over 750fossil ant species have been described, more thanthe currently known number of dinosaur species[2]. The timeline of ant evolution begins with anearly Hymenopteran fossil showing affinities tothe lineage containing both ants and Apoidea. Oneof these early fossils is Cariridris bipetiolata, a112–122 million-year-old impression fossil fromBrazil. It was once postulated to be the earliestknown ant but more likely represents a relative ofthe sphecoid family Ampulicidae.

Cretaceous amber deposits from 100–125 mil-lion years ago contain insect fossils but no knownant inclusions. Ants appear in the fossil recordaround 100 million years ago, and they are atfirst very uncommon, representing less than 2%of all Cretaceous insect inclusions from thenonwards. Despite their low abundance, theseearly ants already exhibited a variety of distinctmorphologies. The extinct subfamilySphecomyrminae is the most diverse, with generasuch as Gerontoformica and Sphecomyrma,which have generalized wasp-like mandibles, aswell as a number of forms with spectacular man-dibular and head morphologies not seen in mod-ern ants. Ants in the genera Haidomyrmex,Haidomyrmodes, and Haidoterminus possessremarkable L-shaped or scythe-like mandibles.The tips of these mandibles point toward thefrons and likely functioned as trap-jawmechanics.Ceratomyrmex and Linguamyrmex are even moreextreme, sporting a massive clypeal horn with anapical lobe, apparent trigger hairs, and extremelylong mandibles that point toward the lobe. Nosuch structures are known in extant ants.

Sphecomyrmines are most often considered astem lineage of all extant ants, which means that

all representatives are extinct and members of theSphecomyrminae are not more closely related toany particular extant lineage but instead moreclosely related to other extinct lineages. In parallelto some stem-group ants, however, some Creta-ceous fossils certainly belong to crown-groupants, that is, they can be placed inside the cladeformed by currently living ant lineages. The mostspectacular example is the ca. 92 million-year-oldamber fossil Kyromyrma neffi, which can unmis-takably be recognized as a formicine ant due to itswell-developed and plainly visible acidopore.

No definitive fossil ants were known from theperiod 55–78 million years ago until very recentlywhen the aneuretine impression fossilNapakimyrma paskapooensis was describedfrom the Canadian Paleocene Paskapoo Forma-tion, which is 56–60 million years old.

By the time Eocene amber deposits formed inEurope, most contemporary subfamilies areknown to have been present. Exceptions are thesubterranean subfamilies Apomyrminae,Leptanillinae, and Martialinae, as well as themonotypic Paraponerinae and species-poorHeteroponerinae. Unfortunately, confident datingof the Eocene European amber deposits (Baltic,Bitterfeld, and Rovno ambers) has been notori-ously difficult, and various authors have proposedeither Lutetian (41–48 million years old) orPriabonian (34–38 million years) ages for Balticand Rovno ambers, and an even wider range ofestimates for Bitterfeld amber, suggested to be asyoung as Upper Oligocene (23–28 million yearsago). Regardless of the exact age of the deposits,ants evidently becamemore abundant in terrestrialecosystems and account for 10% or more of theinsect inclusions in Eocene ambers. Although fos-sil Eocene ant faunas of Europe and North Amer-ica contain extant genera, their taxonomiccomposition differs from today’s faunas from thesame regions. Not only are many lineages cur-rently confined to the tropics, but frequently amixture of tropical and temperate genera isfound. This pattern has been observed for otherfaunas and floras of the Eocene, including fossiltermites, dragonflies, and floras from YellowstoneNational Park, USA. Several explanations havebeen proposed to explain this pattern, including

12 Ants: Phylogeny and Classification

transport and mixing of fossils from places withdifferent paleoclimates, time averaging ofdeposits that were actually laid down over periodsof millions of years, and Eocene climates withreduced seasonality and no modern equivalents.

The study of fossils is crucial for our under-standing of ant evolution. It provides a windowinto past morphologies, behaviors, and faunas. Itis also necessary for modern phylogenetic infer-ence that has been focused on molecular data andsophisticated statistical analysis. Molecular phy-logenies require fossils in order to accurately cal-ibrate nodes on the phylogenetic tree and obtaindivergence dates in absolute time. Without a solidunderstanding of the phylogenetic position of fos-sils, which can only be obtained through morpho-logical investigations, confident dating of eventsin ant evolution is impossible, regardless of theo-retical advances in computer modeling. The samelogic extends to biogeographic inference, wherepast species ranges and dispersal events arereconstructed from patterns of modern distribu-tions and phylogeny. Because the fossil recordreveals that distributions of modern taxa havechanged over time, often dramatically, such infer-ences are incomplete or biased without consider-ation of fossils. Ant systematists are becomingincreasingly aware of these limitations, fossilsare being discovered or reevaluated, and ways inwhich modeling of the evolutionary past is doneare being often revised. These factors lead toconstant refinement of our understanding of thetimeline of ant evolution.

With the above caveats in mind, it is no sur-prise that estimates of the age of crown-group antsand major ant clades have varied among studies(Fig. 2, Table 1). Most phylogenetic work placesthe most recent common ancestor of modern antsin the Early Cretaceous period, between 145 and110 million years ago. This is before the timewhen first Cretaceous amber ant fossils becomeavailable and suggests that the presumed stem-group sphecomyrmines remained dominant in ter-restrial habitats for some time after crown-groupants first appeared on the scene. This timeline alsoshows that modern ants rose to prominence along-side angiosperms and that bursts of ant diversifi-cation occurred throughout their evolutionary

past. Examples of rapid radiations include earlydiversification of the poneromorph clade, thedorylines, tribe Crematogastrini, and several ofthe hyperdiverse genera. These patterns are fur-ther discussed below.

Diversification of Modern Ants:Geography, Innovation, Dispersal, andCoevolution

The majority of ant species live in the tropics:Costa Rica is home to about the same number(~900) of ant species as Canada and the USAcombined despite occupying a land area almost400 times smaller. Approximately 79% of antspecies and subspecies have ranges that are atleast in part within tropical latitudes (23.5� northand south), despite the fact that the tropics accountfor only 36% of the Earth’s dry land surface. Thispattern of species richness is reflected in higherphylogenetic diversity in the tropics and the factthat most ant genera and subfamilies are eitherrestricted to the tropics or present in the tropicsas well as temperate regions.

Several hypotheses have been put forward toexplain high tropical diversity, exemplified byants and many other animal and plant taxa. Com-parative phylogenetic research suggests that trop-ical ant lineages are much older than temperatelineages and that tropical and temperate lineagesgive rise to new species at similar rates. Accord-ingly, most familiar north temperate ant genera,such as Formica, Lasius, or Temnothorax, must beregarded as evolutionary newcomers. This sug-gests that there are fewer ants in colder regionsbecause these faunas are relatively young [16].

Recent molecular phylogenetic studiesrevealed other large-scale geographic patterns inant evolution. Many old clades show affinity tomajor biogeographic regions, suggesting thatdiversification often remains confined to a partic-ular region over long periods. For example, themyrmicine tribe Attini of 45 genera and more than2,500 species originated in the New Worldapproximately 65 million years ago. Its diversifi-cation proceeded mainly in the Neotropics until afew lineages, most notably the hyperdiverse

Ants: Phylogeny and Classification 13

Ants:P

hylog

enyan

dClassification,T

able

1Age

rang

eestim

ates

forselectmajor

antcladesandfive

largestsub

families

from

major

phylog

eneticstud

ies.Num

bersinsquare

bracketsnext

toreferences

correspo

ndto

referencenu

mber.Age

rang

esarerepo

rted

inmillions

ofyearsbefore

present

Clade/reference

Brady

etal.2

006

[12]

Moreau

etal.

2006

[19]

Wardet

al.2

010

[28]

Schmidt

2013

[23]

Moreau

andBell

2013

[18]

Brady

etal.2

014

[13]

Wardet

al.2

015

[29]

Blaim

eretal.

2015

[3]

Wardand

Fisher

2016

[30]

Blanchard

andMoreau

2017

[5]

Econo

mo

etal.2

018

[16]

Borow

iec

2019

[10]

Borow

iec

etal.2

019

[11]

Formicidae

111–13

714

1–16

9111–15

212

8–16

914

5–22

015

4–16

910

3–12

3

Leptanillo

ids

82–114

Pon

eroids

100–

115

122–

134

93–128

69–1

0710

5–14

712

7–14

981–1

02

Formicoids

103–

120

118–13

195

–122

104–

118

115 –

155

129–

150

96–1

06

Amblyo

poninae

+Apo

myrminae

92–118

113–14

366

–114

37–8

793

–128

85–1

3193

–131

62–8

7

Dolicho

derinae

+Aneuretinae

87–1

0610

3–113

81–114

62–97

80–8

598–1

0810

0–12

378–8

9

Heterop

onerinae

+Ectatom

minae

81–9

280

–84

79–9

273

–115

41–7

0

Pseud

omyrmecinae

+Myrmeciin

ae91–1

0372

–99

55–6

072–1

2195

–130

71–9

2

Dolicho

derinae

71–7

686–9

759

–74

59–7

260–7

362

–92

52–6

2

Dorylinae

71–9

499–117

68–9

374–1

0177–111

95–1

2253– 1

01

Formicinae

77–8

392–1

0456

–82

69–9

088–1

3689–9

773

–116

51–7

1

Myrmicinae

81–8

910

0–114

65–88

74–9

088–110

91–1

0094

–121

51–7

2

Pon

erinae

79–1

03111–13

278

–112

54–6

686–118

106–

130

61–8

4

14 Ants: Phylogeny and Classification

genera Pheidole and Strumigenys, started dispers-ing out of this realm within the last 30 millionyears. A parallel Old World example is anothermyrmicine tribe, the tribe Crematogastrini, with64 extant genera and more than 2,600 species. Inthis case most of the diversification occurredwithin the Indomalayan and Afrotropical regions,with few lineages eventually dispersing into thenorth temperate zone and Neotropics. Otherexamples of old species-rich clades mostly con-fined to a single biogeographic region include thelargely Afrotropical formicine tribesPlagiolepidini, the north temperate Formicini,and the Australian Melophorini, as well as thelarge clade comprising almost all of New Worlddoryline species and the ponerine Odontomachusgenus group, the latter distributed mainly in thePaleotropics. Similar patterns of long-term geo-graphic affinity of clades can also be seen in lessspeciose groups, such as the Amblyoponinae, andthe exclusively Old World subfamilyLeptanillinae.

In addition to radiations spanning large bio-geographic regions, molecular phylogenies com-bined with extensive faunal surveys provide aclearer picture of island ant faunas and haveuncovered numerous examples of island radia-tions. The Malagasy ant fauna has been thefocus of intensive surveys in the last 25 years.Approximately 96% of its 745 species areendemic to the island, and the fauna includesseveral lineages that represent recent dispersalsof species-rich genera such as Crematogasterand Pheidole, arriving on the island approxi-mately 25 and ten million years ago, respectively.In contrast, the clade ofMalagasy-endemic generaEutetramorium,Malagidris, Royidris, and Vitsikacolonized Madagascar approximately 50 millionyears ago, but these endemic genera are currentlyspecies poor, perhaps representing a remnant ofthe original ant radiation now curbed by competi-tion from more recent arrivals. Islands are alsohome to phylogenetically isolated, relict species,such as Aneuretus simoni of Sri Lanka orPilotrochus besmerus of Madagascar.

Diversification rates across the tree of life arehighly uneven, and the ants are no exception. Asalready discussed, this pattern is obvious at the

subfamily level (Fig. 2), but it is also presentwithin subfamilies. Although currently 334 extantant genera are recognized, eight of the mostspeciose taxa, Camponotus, Crematogaster,Monomorium, Pheidole, Polyrhachis,Strumigenys, Temnothorax, and Tetramoriumtogether account for over 5,400 species, or about40% of all described ant species. Moreover, theselineages are often sister groups to much lessspeciose clades and are relatively young, theircrown groups having originated throughout thelast 40 million years or later.

Much attention has been paid to the causes ofthis uneven proliferation of species in ants andother taxa. This research is carried out in thecontext of understanding that relative rates ofboth speciation and extinction contribute to over-all diversification of a lineage. Common factorsthat promote diversification include colonizationof a new region, where competition is reduced, ora key innovation, a trait that promotes diversifica-tion or reduces extinction, perhaps by allowingexpansion into new niches. Identification of anytrait as directly responsible for increased diversi-fication, however, is difficult. In ants, candidatesfor key innovations include, but are not limited to,the Blochmannia symbiosis of the Camponotini,the dimorphic worker caste of Pheidole, theconvergently evolved trap-jaw mandibles in dis-tantly related myrmicine, formicine, and ponerinelineages, as well as the fungus-growing behaviorin the Atta genus group and the specialized hunt-ing behavior in the army ants. The factors respon-sible for the ecological success of most species-rich ant groups, however, are difficult to knowwith certainty.

A contrasting pattern is found in lineages thatare remarkably species poor but have persistedthrough long periods of geologic time withoutsignificant diversification [20]. The many exam-ples of such lineages among ants include mono-typic genera such as Aneuretus, Martialis,Opamyrma, Paraponera, Phalacromyrmex, andTatuidris. Many of these are cryptobiotic, and itmay be that these lineages experience both lowextinction and low speciation rates as a result ofadaptations to their ecologically stable subterra-nean environments. This does not explain,

Ants: Phylogeny and Classification 15

however, cases of Aneuretus or Paraponera,which forage above ground. In the case ofAneuretus, at least, it appears that it representsthe last living lineage of a once more diversegroup now in decline.

Present State of Ant Systematics

The publication of the landmark monographsIdentification Guide to the Ant Genera of theWorld and A New General Catalogue of the Antsof the Worldmarked a turning point in ant system-atics [6, 7], not long after the comprehensivemonograph The Ants [17]. These resources cre-ated a major upsurge in myrmecology, as theresulting synergy promoted it as a prominentfield of entomological inquiry and acceleratedthe pace of taxonomic research. The average num-ber of new ant taxa described each year more thandoubled between the two decades prior to 1995and the two decades since, with more than 200new ant taxa described annually in recent years.This trend shows no sign of abating, so that theformal description of new taxa, both extant andfossil species, is predicted to continue for theforeseeable future.

Since the publication of these landmark booksin the 1990s, there has been an effort to move thetaxonomic catalog and literature online and tofoster a more community-driven cataloging effort.Web portals such as antbase.org and, morerecently, AntCat.org are providing easy access totaxonomic literature and information. Web portalssuch as AntWeb.org and AntWiki.org link theseresources to specimen-based distribution data andimages, including many type specimens from nat-ural history collections around the world, contrib-uting to further democratization of taxonomicinformation.

In addition to the recent improvements inaccess to taxonomic information and theincreased pace of description of new taxa,advances in DNA sequencing technologies andphylogenetic methodology have sparked a revo-lution in our understanding of evolutionary rela-tionships among ants. The molecularphylogenetic studies produced in the last

15 years or so provide a picture of ant relation-ships and evolution that often differ from thescenarios drawn from morphology-based phylog-enies, especially at the subfamily and tribe level. Ithighlights the frequency of morphological con-vergence in ant evolution and allowsreexamination of ant morphology in the light ofindependent tests of evolutionary relationshipsprovided by molecular data. Myrmecologists arealso increasingly taking advantage of micro-computed tomography, which reveals unprece-dented detail of skeleto-musculature in even thesmallest animals, living or fossil, and allows ren-dering of three-dimensional models of externaland internal structure. The field of ant systematicsis thus ripe for a revival of morphologicalresearch.

The molecular revolution has provided uswith a relatively stable and complete picture ofphylogenetic relationships of ants at the subfam-ily and even genus level [12, 15, 19]. An ongoingeffort of the Global Ant Genome Alliance isseeking to understand ant diversity from a newperspective, and providing a genome-based phy-logeny for all ant genera is likely to further solid-ify this framework [8]. However, muchphylogenetic work remains to be done in spe-cies-level phylogeny. Most ant genera currentlylack a comprehensive phylogenetic tree,although much progress has been made in eluci-dating relationships within hyperdiverse lineagessuch as Crematogaster, Pheidole, or Poly-rhachis. Efforts targeting other massive generasuch as Camponotus, Strumigenys, and Tetra-morium are ongoing.

The future of ant systematics will likely con-tinue to abound in discoveries of new ant species,to see a fuller integration and accessibility oftaxonomic literature, cataloging efforts, and spec-imen data, as well as bringing exciting newinsights into the morphology of living and extinctants. New molecular techniques and theoreticaladvances will continue to build on this foundationto achieve higher phylogenetic resolution towardthe tips of the ant tree of life, enhance stability inant taxonomy and classification, and decipher antevolution in a comprehensive framework

16 Ants: Phylogeny and Classification

integrating genetic, morphological, behavioral,ecological, and evolutionary information.

References

1. Archibald, S. B., Johnson, K. R., Mathewes, R. W., &Greenwood, D. R. (2011). Intercontinental dispersal ofgiant thermophilic ants across the Arctic during earlyEocene hyperthermals. Proceedings of the Royal Soci-ety B: Biological Sciences, 278(1725), 3679–3686.

2. Barden, P. (2017). Fossil ants (Hymenoptera:Formicidae): Ancient diversity and the rise of modernlineages. Myrmecological News, 24, 1–30.

3. Blaimer, B. B., Brady, S. G., Schultz, T. R., Lloyd, M.W., Fisher, B. L., &Ward, P. S. (2015). Phylogenomicmethods outperform traditional multi-locusapproaches in resolving deep evolutionary history: Acase study of formicine ants. BMC Evolutionary Biol-ogy, 15(1), 271.

4. Blaimer, B. B., Ward, P. S., Schultz, T. R., Fisher, B.L., & Brady, S. G. (2018). Paleotropical diversificationdominates the evolution of the hyperdiverse ant tribeCrematogastrini (Hymenoptera: Formicidae). InsectSystematics and Diversity, 2(5), 3.

5. Blanchard, B. D., & Moreau, C. S. (2017). Defensivetraits exhibit an evolutionary trade-off and drive diver-sification in ants. Evolution, 71(2), 315–328.

6. Bolton, B. (1994). Identification guide to the ant generaof the world. Cambridge,MA: Harvard University Press.

7. Bolton, B. (1995).A new general catalogue of the ants ofthe world. Cambridge, MA: Harvard University Press.

8. Boomsma, J. J., Brady, S. G., Dunn, R. R., Gadau, J.,Heinze, J., Keller, L., Moreau, C. S., Sanders, N. J.,Schrader, L., Schultz, T. R., Sundström, L., Ward, P.S., Wcislo, W. T., & Zhang, G. (2017). The global antgenomics alliance (GAGA). Myrmecological News,25, 61–66.

9. Borowiec, M. L. (2016). Generic revision of the antsubfamily Dorylinae (Hymenoptera, Formicidae).ZooKeys, 608, 1–280.

10. Borowiec, M. L. (2019). Convergent evolution of thearmy ant syndrome and congruence in big-data phy-logenetics. Systematic Biology, 68(4), 642–656.

11. Borowiec, M. L., Rabeling, C., Brady, S. G., Fisher, B.L., Schultz, T. R., &Ward, P. S. (2019). Compositionalheterogeneity and outgroup choice influence the inter-nal phylogeny of the ants. Molecular Phylogeneticsand Evolution, 134, 111–121.

12. Brady, S. G., Schultz, T. R., Fisher, B. L., &Ward, P. S.(2006). Evaluating alternative hypotheses for the earlyevolution and diversification of ants. Proceedings ofthe National Academy of Sciences of the USA, 103(48), 18172–18177.

13. Brady, S. G., Fisher, B. L., Schultz, T. R., &Ward, P. S.(2014). The rise of army ants and their relatives:Diversification of specialized predatory doryline ants.BMC Evolutionary Biology, 14(1), 93.

14. Branstetter, M. G., Danforth, B. N., Pitts, J. P., Faircloth,B. C.,Ward, P. S., Buffington,M. L., Gates,M.W., Kula,R. R., &Brady, S. G. (2017). Phylogenomic insights intothe evolution of stinging wasps and the origins of antsand bees. Current Biology, 27(7), 1019–1025.

15. Branstetter, M. G., Longino, J. T., Ward, P. S., &Faircloth, B. C. (2017). Enriching the ant tree of life:Enhanced UCE bait set for genome-scale phyloge-netics of ants and other Hymenoptera. Methods inEcology and Evolution, 8(6), 768–776.

16. Economo, E. P., Narula, N., Friedman, N. R., Weiser,M. D., & Guénard, B. (2018). Macroecology andmacroevolution of the latitudinal diversity gradient inants. Nature Communications, 9(1), 1778.

17. Hölldobler, B., & Wilson, E. O. (1990). The ants.Cambridge, MA: Harvard University Press.

18. Moreau, C. S., & Bell, C. D. (2013). Testing themuseum versus cradle tropical biological diversityhypothesis: Phylogeny, diversification, and ancestralbiogeographic range evolution of the ants. Evolution,67(8), 2240–2257.

19. Moreau, C. S., Bell, C. D., Vila, R., Archibald, S. B., &Pierce, N. E. (2006). Phylogeny of the ants: Diversifi-cation in the age of angiosperms. Science, 312(5770),101–104.

20. Pie, M. R., & Feitosa, R. M. (2016). Relictual antlineages (Hymenoptera: Fatinicidae) and their evolu-tionary implications. Myrmecological News, 22,55–58.

21. Peters, R. S., Krogmann, L., Mayer, C., Donath, A.,Gunkel, S., Meusemann, K., Kozlov, A.,Podsiadlowski, L., Petersen, M., Lanfear, R., Diez, P.A., Heraty, J., Kjer, K. M., Klopfstein, S., Meier, R.,Polidori, C., Schmitt, T., Liu, S., Zhou, X., Wappler,T., Rust, J., Misof, B., & Niehuis, O. (2017). Evolu-tionary history of the Hymenoptera. Current Biology,27(7), 1013–1018.

22. Rabeling, C., Brown, J. M., & Verhaagh, M. (2008).Newly discovered sister lineage sheds light on earlyant evolution. Proceedings of the National Academy ofSciences of the USA, 105(39), 14913–14917.

23. Schmidt, C. (2013). Molecular phylogenetics ofponerine ants (Hymenoptera: Formicidae: Ponerinae).Zootaxa, 3647(2), 201–250.

24. Schmidt, C. A., & Shattuck, S. O. (2014). The higherclassification of the ant subfamily Ponerinae (Hyme-noptera: Formicidae), with a review of ponerine ecol-ogy and behavior. Zootaxa, 3817(1), 1–242.

25. Shattuck, S. O. (1992). Generic revision of the antsubfamily Dolichoderinae (Hymenoptera:Formicidae). Sociobiology, 21, 1–181.

26. Taylor, R. W. (1978). Nothomyrmecia macrops: Aliving-fossil ant rediscovered. Science, 201(4360),979–985.

27. Ward, P. S. (1990). The ant subfamily Pseudo-myrmecinae (Hymenoptera: Formicidae): Genericrevision and relationship to other formicids. System-atic Entomology, 15(4), 449–489.

Ants: Phylogeny and Classification 17

28. Ward, P. S., Brady, S. G., Fisher, B. L., & Schultz, T. R.(2010). Phylogeny and biogeography of dolichoderineants: Effects of data partitioning and relict taxa onhistorical inference. Systematic Biology, 59(3),342–362.

29. Ward, P. S., Brady, S. G., Fisher, B. L., & Schultz, T. R.(2015). The evolution of myrmicine ants: Phylogeny

and biogeography of a hyperdiverse ant clade (Hyme-noptera: Formicidae). Systematic Entomology, 40(1),61–81.

30. Ward, P. S., & Fisher, B. L. (2016). Tales of draculaants: The evolutionary history of the ant subfamilyAmblyoponinae (Hymenoptera: Formicidae). System-atic Entomology, 41(3), 683–693.

18 Ants: Phylogeny and Classification