STRANGE WASPS INDEEDSPECULATIONS ON THE EVOLUTION OF EUSOCIALITY IN ANTS
Robert W. Taylor
Email: bob.taylor@homemail.com.au

ABSTRACT
Following review of formicid characters a new model for evolution ofant eusociality is presented. This “semisocial ancestry hypothesis” has
the following steps: (1) Ants are descended from a solitary wasp whichaccumulated and collectively reared its brood in acellular nests (without
individual brood cells). Metapleural glands delivering antibiotic
compounds evolved in adults as a correlated adaptation for nest
sanitation. (2) Such wasps gathered to form cooperating family groups ofcongenerational alate females and their offspring. (3) Female dealation
evolved, correlated with pedestrian foraging. Colonies now comprised
coeval dealate females rearing alate daughters and sons. Competition
generated reproductive and non-reproductive castes in the parental
generation, resulting in semisocial colonies, eventually with one dealate
mated reproductive queen per nest, and a force of congenerational nonreproductive dealate “workers”. These semisocial formicids progressively
acquired petiolar nodes, elbowed antennae, increased longevity and antlike larvae. (4) Eusociality began when daughter females dealatedprematurely and joined the parental-generation worker force. (5)Daughter (eusocial) workers adaptively superceded parental-generation(semisocial) workers and, with the evolution of mechanisms for caste
determination, lost mesosomal complexity to constitute eusocial colonies
like those of modern ants, with alate/dealate female reproductives andmesosomally-reduced daughter workers. (6) Some of these gave rise tomodern ants. (7) Competition between eusocial and semisocial formicids
resulted in extinction of the latter, leaving eusocial ants transcendent.
These events occurred early in formicid history. The fossil record
indicates that eusocial ants referable to several modern subfamilies were
present by late Cretaceous times.
The semisocial formicids could have included derived species eitherwith morphologically undifferentiated apterous mesosomally-reduced
females, or with alate/dealate queens and congenerational apterous
workers. The extinct upper Cretaceous Sphecomyrma freyi is possibly
the female or worker of one of these.

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It is proposed that the worker caste in modern ants has evolved morethan once, and that eusociality arose several times in the semisocial
ancestral group, producing separate lineages of eusocial ants, several ofwhich survived to constitute the modern fauna.
The unusual reproductive biology of the aberrant ponerine genus
Diacamma is reviewed.
Key words: Diacamma, Formicidae, Hymenoptera., Sociality,
Sphecomyrma.
INTRODUCTION
Ants are classified in the family Formicidae assigned to suborder
Apocrita of order Hymenoptera. Within Apocrita they are allocated tosuperfamily Vespoidea. The several other vespoid families comprise the“true” wasps. Three apocritan superfamilies, Vespoidea, Chrysidoidea
and Apoidea, are referred to collectively as aculeates. Superfamily
Apoidea includes bees and sphecid wasps. This classification represents
ants as specialized vespoid wasps.
This essay reviews salient characteristics of the Formicidae,
especially their synapomorphies. It considers the evolutionary origins
and adaptive features of relevant characters and the possible sequenceof their phylogenetic acquisition, their possible evolutionary interactions,the possible effects of each on options for future evolution, and theirroles in the evolution of formicid sociality. Some propositions are new tothe scientific discussion of ant evolution. It is hoped that they might
encourage further discussion and research.
Nomenclature here follows Bolton (2003). “Ponerinae” (with quotes) or“Ponerinae sens. lat.“ refer to that subfamily in the sense of the previous
Emery/Wheeler classification. Bolton’s terms “mymeciomorph”,“poneromorph” etc, refer to sets of putatively related subfamilies. The
words “formicid” and “formicids” refer to taxa which have contributed
directly to ant phylogeny. The term “standard reproductives” refers toalate females which later dealate (these are also referred to as
alate/dealate females).
Many general statements about ants are not been referenced below.
They are well covered by Wilson (1971) and Hölldobler and Wilson(1990).
Symplesiomorphies of the Formicidae
Several prominent symplesiomorphies of ants are synapomorphies oforder Hymenoptera. Thus (1) sex is genetically determined by
haplodiploidy (males haploid and impaternate, females diploid) as in all
Hymenoptera (Crozier and Pamilo, 1996), (2) ant mouthparts, apart fromthe labrum and mandibles, form a labiomaxillary complex with anassociated infrabuccal pouch, and (3) mature ant larvae, prior to
pupation, enclose themselves in cocoons of silk spun from their labial

3
glands (the many formicid exceptions result from repeated secondaryloss of this character).
Other formicid characteristics are synapomorphies of suborder
Apocrita: (4) The first true abdominal segment is fused with the thorax toconstitute the propodeum, configuring a novel 4-segmented thoracic
tagma, the mesosoma, and (5) a deep constriction separates the
propodeum and the second true abdominal segment forming a waist
between the mesosoma and the segmentally reduced abdomen (termed
the metasoma). A small anterior exoskeletal section of the firstmetasomal segment forms a ball-joint within the propodeum. Thisfacilitates rotation of the sting-bearing metasoma. The operative
musculature is largely in the propodeum. Abdominal articulation from themetathorax would be compromised by its leg-bearing function, hence thepropodeum has evolved.
Some symplesiomorphies of ants are synapomorphies of the aculeate
Apocrita. These include (6) modification of the ovipositor to function as avenom-injecting sting, (7) apodus larvae, and (8)(probably) antennae
primitively with 12 articles in females, 13 in males.
Further putative formicid symplesiomorphies relevant here are: (9)ocelli present, (10) a univoltine life cycle with one new generation
produced each year, (11) sperm in mated females retained in the
spermatheca and used to fertilise eggs for long periods following mating,(12) adults are liquid feeders, (13) larvae are entomophagus, (14)females collect prey to provision larvae, (15) prey is delivered to brood
whole, without dissection, (16) males take no part in the lives of females
or offspring following copulation, and (17) navigation is visually mediated.
The points of phylogenetic origin of these attributes in formicid evolution
are less obvious than for those features mentioned previously. All are
acceptable as putative formicid symplesiomorphies (Carpenter, 1991).
Formicid synapomorphies
Synapomorphies are the particular features by which we recognize
and characterize groups of organisms. The truly homologous
synapomorphic characters of any taxon are represented nowhereoutside that taxon. Also, where several synapomorphies are recognized,
any one of them, on its own (with due recognition of its tansformations),
can distinguish a subject taxon from its sister taxa.
Putative formicid synapomorphies relevant here include: (1) female
antennae elbowed, with an unusually elongate basal segment (thescape), (2) wing bases in reproductive females structured to facilitate
wing-shedding (the action “dealation”, the process “allectomy”), (3)presence of a permanently apterous (wingless) non-reproductive worker
caste, (4) metapleural glands present in both adult sexes, (5) firstmetasomal segment (2nd true abdominal) forming a posteriorly
constricted waist node, the petiole, and (6) nests acellular, without
individual brood cells. (See Bolton, 2003, for other formicid
synapomorphies).

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Synapomorphic character states are uniquely and separately acquiredin phylogeny. It follows that when several synapomorphies arerecognized within a taxon they would usually have arisen in temporal
evolutionary succession. The succession can, nonetheless, be so rapidthat sets of characters may appear almost simultaneously in phylogeny
when organisms are adapting rapidly to extreme selection pressures.
Indeed, separate adaptively-related characters probably arise in concert
under some circumstances (this might have been the case with coevolution of acellular nesting and metapleural glands in ants – see
below).
The first appearance of each eventual synapomorphy as a newly
evolved character would often represent some sort of adaptive breakthrough, contributing to relative success and diversification of the newly
equipped species. Acquisition of a further such character in theaccumulation of a set of successively derived future synapomorphies,
being a unique event, could occur in only one of the lineages within thesubject clade, leaving the remainder as a paraphyletic rump exhibiting
the first acquired character but not the second.
Thus, the evolutionary history of acquisition of the formicid
synapomorphies listed above must have involved a chain of successive
steps, in which taxa equipped with newly evolved characters survived,
while their sister groups lacking those characters did not. This process
must have been repeated at least often enough to match the number ofcharacter acquisitions, and the loss of sister groups must have been
complete at each step – how else could so many characteristics ofmodern formicids be considered synapomorphies. In effect a grouppossessing multiple synapomorphies could have not just one common
ancestor, but a chain of common ancestors, and its lineage might havehistorically experienced a sequence of extreme diversity bottlenecks.
Characters homoplasious (evolved convergently, and analogous but
not homologous) with any of the synapomorphies recognized in a current
natural taxon would occur only in separate taxa. These would lack someor all of the remaining synapomorphic character set
Importantly, the most recently acquired apparently synapomorphic
character observed in a contemporary taxonomic group could in reality
comprise a set of successfully deceptive homoplasious charactersseparately evolved in the immediately ancestral stock. This is much more
likely than with previously evolved characters. We will revisit this
proposition below.
Characters resembling most of the above formicid synapomorphiesand symplesiomorphies are present as homoplasies elsewhere in nature.
Examples include haplodiploidy, production of pupal cocoons, elbowed
antennae, univoltinism, visual navigation etc. This is to be expected, anddoes not diminish the arguments presented here.

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Metapleural glands – the classic formicid synapomorphy
Metapleural glands are characteristic of family Formicidae. No
antecedent or similar structures are known in other taxa. Their absence
in several genera, e.g. Oecophylla, Dendromyrmex, Polyrhachis, mostCamponotus (not C. gigas (Taylor, 1992) and at least two other species(S.O. Shattuck & G.D. Alpert, pers. coms)) and various males (perhaps
those of the majority of ants) is clearly due to secondary loss (Brown,
1968; Hölldobler and Engel-Siegel, 1985). Their cuticular components
can often be clearly discerned in fossils. Wheeler (1928: 112) first drewattention to the presence of metapleural glands as a character defining
the Formicidae.
The glands are bilateral, situated posterolaterally on the mesosoma.
Each has an accessory cuticular chamber, usually marked by an external
swelling, with a small opening to the exterior. Internally a specialized
section of the inner chamber wall receives unicellular ducts from acluster of relatively large cells lying in the body cavity immersed inhaemolymph (Hölldobler and Engel-Siegel, 1985). The structure isalmost uniform throughout the Formicidae, with the specialized section ofchamber wall usually a flat sieve-like disc, but in Nothomyrmecia,
Myrmecia and some other ants a microperforate vermiform cuticularcollecting sac receives the cell ducts, the cell mass is relatively large andthe cells relatively small (pers. obs.; Tullock et al., 1962; Hölldobler andEngel-Siegel, 1985)). There seems no reason to doubt that metapleuralglands were uniquely evolved and are a genuine formicid synapomorphy.
The glandular fluid contains antibiotic molecules which are proof
against various soil bacteria and fungi (Beattie et al., 1986; Veal, Trimble
and Beattie, 1992; Mackintosh et al., 1995). Its function is clearlyprotection of ants, especially their larvae, from contamination or infection.
Adult ants groom more actively than other aculeates, and this spreads
metapleural gland secretion on their bodies and elsewhere in the nests.
The gland does not manufacture the chemicals it secretes, but extracts
them from the haemolymph. Some of the compounds involved are foundin other insects as components of defensive arrangements equivalent to,but very different from, the vertebrate immune system (Kaaya, 1993;
Casteels,1998).
Metapleural glands are important in the social biology of ants. Their
adaptive significance is obvious, and their first appearance was
doubtless profoundly significant in ant evolution
The powerfully antibiotic metapleural gland secretions are lethal topollen grains. This explains why ants are barely involved in pollination of
flowering plants, and why many flowers are adapted to exclude or repel
ants (Beattie et al., 1986). While the acquisition of metapleural glands
was a major step in ant evolution, the prospect of a formicid role inpollination was simultaneously closed. Pollination is, of course, aprominent activity among other aculeates, especially in bees

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The acellular nest as a synapomorphy of family Formicidae
Various taxa of non-formicid aculeates form reproductive socialgroups ultimately comprising colonies of one or several coeval femaleswith their brood and adult offspring. There is sometimes also a nonreproductive, exclusively female worker caste. Colonies are typicallyhoused in nests ranging from simple excavations to elaborate
constructions. Many of the between-group similarities seen in aculeatesociality and nesting behavior are homoplasious.
Comparative studies of living aculeates with varying levels of sociality
and nest complexity reveal that both features are derived ultimately fromstocks in which solitary, winged, mated non-social females excavatedand provisioned simple nests, each accommodating a single offspring.
As more offspring came to be assembled in nests they were, apparently
from the beginning, typically housed in separate individual brood cells
constructed by the parental females, even when only two or three
immatures were involved. Aculeate larvae are primarily entomophagous,
and the likelihood of cannibalism among ravenous larvae clustered inundivided nest chambers with immobilized prey is thought to explain theinitial adaptiveness of larval segregation. Isolation of individual larvae insuch “cellular” nests would likely also impede the spread of infectious
disease vectors and parasites, and reduce the risk that all of the broodwould be lost in a predatory attack.
Modern nests, no matter how elaborate, are indubitably cellular invirtually all non-formicid aculeates – specifically constructed to segregate
each developing individual from its siblings. Cellular nests are
remarkably persistent in relevant lineages, and the structural integrity ofnests with sections assembled from close-packed hexagonal cells
providing strong architectural elements has contributed to the evolutionof large, intricate nests such as those of the honeybees and vespine
wasps.
On the other hand, the colonies of all ants reside in continuous nest
spaces, which house the various brood stages in company with resident
adults. The nest space is often a simple chamber, but it may beelaborately divided; either randomly, as when colonies nest in leaf litter;or structurally, as when large nest mounds contain numerous
interconnected chambers. Brood is often sorted in stage-specific groups,
but no ant is known to house individual immatures in individual cells
equivalent to those of other aculeates. Ant nests are “acellular”.
Accommodation of brood in continuous nest spaces is rare among
aculeates. Reported examples in solitary sphecid wasps (three species
insubfamily
Sphecinae,
one
each
in
Larrinae
andCrabroninae)(Krombein, 1967) are barely exceptions. Loosely partitioned
cells are constructed successively in restricted linear spaces by parental
females, and chambers with several larvae result when partitions areomitted or later break-down. Krombein reviewed three related Isodontia
species (Sphecini) “one of which makes individual cells, another with a7single brood chamber but sometimes with individual cells, and one which
always makes a single brood chamber”.
Females of the solitary bee Megachile policaris provision individual
brood chambers (of which there may be several) with a pollen mass inwhich eggs are laid, eventually producing a congregation of larvae.
According to Krombein this species “differs markedly from other known
megachilid bees and, in fact, from almost all aculeate Hymenoptera inthat several larvae develop amicably in a single large brood cell with noapparent cannibalism”. All species of Lithurgus (Megachilidae) have
similar behavior (C.D. Michener, pers. com.). Bumble bees (Apidae:
Bombini) lay up to 16 eggs in distendable cells. The larvae congregate
initially, but later separate themselves with silk partitions (Michener,
1974).
Colonies of the incipiently eusocial allodapine bees (Xylocopinae:
Allodapini) and their brood are housed, like ants, in acellular nests.
These frequently occupy the slender tubular pith cavities of herbaceous
plants or other similar spaces, where the accommodation of numerous
individual brood cells would be physically constrained. The adaptive
rationale for these nesting arrangements is clear. Allodapines are
believed to be descended from ancestors which constructed loosely
partitioned cells not unlike the sphecids discussed above. Their acellular
nests are to that degree a secondary development. In evolutionary terms
the loss of brood cells occurred in correlation with the development ofnovel nesting arrangements whereby otherwise inaccessible tubularspaces were able to provide accommodation for populous colonies(Michener, 1974). Acellular nesting is an allodapine synapomorphy
acknowledged to have arisen uniquely from ancestral stock. Its origin
establishes a single exception to the aculeate norm (M.P. Schwarz, pers.
com.).
These putative exceptions all involve descent from stock with few
larvae and little evident commitment to cellular nesting. There is noevidence of reversals from cellular to non cellular nesting amongarchitecturally more advanced aculeates, suggesting that cellular nesting
behavior might be functionally irreversible, particularly when nests aresubstantially complex.
The above considerations imply that cellular nests might never havefeatured in the ant lineage (or were never developed to a level precluding
evolutionary reversal). Formicids are therefore likely to have originatedfrom a solitary flying wasp which accumulated its larvae in a single nest
space without cellular partitions. This behavioral acquisition marked theinception of the formicid phylogenetic clade, and the acellular nest has
persisted through millions of years to stand eventually as a synapomorpyof the family Formicidae.
The existence of ants today could have depended on the absence of
cannibalism in the larvae of that ancestor. Evolution is like that – very
chancy! There would, of course, be extreme selective pressure against
larval cannibalism once such nesting arrangements were established.

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The earliest formicids
The first formicids would, by virtue of their ancestry, have displayed
haplodiploidy; a labiomaxillary complex; ocelli; a propodeum, constricted
waist and venom-injecting sting. Adults would have used liquid food andfertilized their eggs with spermathecally-stored sperm. They would havepracticed general brood care and foraged for insect or other similar prey
returned to the nest as larval food without dissection. Reproduction
would have been univoltine, the pupae enclosed in cocoons, and broodoverwintered in the nests, probably as larvae. There would be strong
selective pressure for overwintering by adults in nests, which would
provide protection for hibernating brood. Navigation would have beenvisually mediated.
The proclivities of modern ants imply that nests would have been
constructed in the soil, and that mature larvae within the nests would
have been banked with soil by workers prior to spinning their pupal
cocoons (as in Nothomyrmecia, Myrmecia, Amblyopone and other“primitive” ants). Observations on living solitary wasps indicate that
foraging would likely have been epigaeic and diurnal, with the preyencountered above ground, on vegetation and other exposed surfaces,
but not retrieved from within the soil, leaf litter or fallen rotting wood.
The brood would be less protected from disease or fungal infection
than in cellular nests. For this reason the evolution of a precursor to themetapleural gland, capable of antibiotic delivery, is likely to haveoccurred in close correlation with acellular nesting. The near uniformity of
metapleural gland accessory cuticular structures implies that the glands
would probably have rapidly developed to resemble the modern organs.
Recognition of early gland developmental stages might be difficult infossils because the cells would probably have opened minutely to thebody surface prior to evolution of cuticular reservoir chambers.
Cooperation of two or more females in nest foundation, egg laying,brood care, nest defense and foraging would improve brood protection,
since the nest could be permanently guarded. Communal aggregation
would also support evolutionary increase in colony populations andreproductive productivity, and have an additive effect on efficiency of
biocidal protection from metapleural gland secretions, supporting
increased nest size. Aggregation would almost certainly have been
selectively favored. It would have assembled “primitively social”“polygynous family groups”, as discussed by West-Eberhard (1978).
The evolution of progressive provisioning of the brood would occur asa consequence of the association of adults and larvae in nests. Thisbehavior is found elsewhere among vespoids only in some highly social
Vespidae – all other vespoids aside from ants are mass-provisioning(J.M. Carpenter, pers. com.).
Joint colony foundation (pleometrosis) by recently mated females ispracticed today by Nothomyrmecia, some Myrmecia species and otherants (Hölldobler and Taylor, 1983; Wheeler, 1933, Wilson, 1971). The9characteristics of this behavior might be homologous with those of theancestral nesting arrangements hypothecated here.
The ancestral formicids would have generally resembled modern,flying solitary aculeate wasps, except that they would have possessed
metapleural glands or antecedent structures.
The univoltine life cycle, brood synchronicity and longevity
Aculeates are generally univoltine, producing one brood each year.
Eggs are usually laid in spring. Adults are short-lived, few surviving morethan about a year. In most cases colonies die out each winter, with only
a few individuals surviving in hibernation to continue the cycle thefollowing spring. In advanced species these are mated females, often
found claustral in excavated or preexisting hibernacular cavities (Evans
and West-Eberhard, 1973). Overwintering as prepupae or cocoonenclosed adults is practiced in solitary taxa or those with low levels ofsocial development (Krombein, 1967).
Constructed nests may be perennial but most species construct newnests each year. Reuse of nests is not common, even in sociallyadvanced groups like polistine wasps (Matsuura and Yamane, 1984). Ina few cases, mainly in the bee subfamily Apinae, nests are occupied at
all seasons by queens, workers, and brood, with in-nest hibernation inareas of severe climate. Even in these, adults are not long-lived and few
individuals apart from queens, which tend to live longer than workers,
survive more than one active season (Michener, 1974).
Extreme longevity has been frequently reported in queen ants, andworkers may survive for several years at least (Hölldobler and Wilson,
1990). Thus at some point in the ant lineage selection favored increased
longevity, probably among the semisocial formicids hypothecated below.
There is a general misconception that egg-laying by queen ants is
continuous and that all brood stages are present in ant nests at allseasons. Even the writings of myrmecologists can indicate a lack ofunderstanding that ants are primitively univoltine and that the life-cyclesof all ants are derived from univoltine ancestry.
Univoltinism is a significant trait of ants. It is demonstrated in coloniesof the Australian bulldog ant Myrmecia gulosa (Myrmeciinae), in whichqueens lay eggs for a period of only a few weeks each spring. As
hatching proceeds eggs are progressively replaced by small hatchlinglarvae until only larvae remain. They grow throughout summer andautumn and overwinter as large, incompletely grown individuals. The
following spring larval growth resumes and mature larvae begin to spin
cocoons. This occurs at about the time egg laying is again initiated to
produce the following generation. Emergence of callow adults from thecocoons is complete by early to mid summer. Overwintering larvae do
not feed (and might be diapaused, though this has not been investigated),
and the workers cease foraging for insect prey during winter, when they
seldom depart the nests (pers. obs.).

10Transformation of this life history has been frequent among ants.
Myrmecia tarsata does not overwinter brood. Eggs are laid in spring andemergence complete before the following winter. During winter larvae
are absent and foraging for larval food unnecessary. The nest entrances
are then sealed with soil, enclosing the overwintering broodless colony.
Other Myrmecia species seem to be bivoltine, producing a “fast” summer
brood maturing in late summer from spring-laid eggs, and a “slow”
overwintering brood maturing in spring from eggs laid the previous
summer. A survey of the brood contents of many preserved colony
series of Australian Sphinctomyrmex species (Cerapachyinae) has
demonstrated that they overwinter no brood (pers. obs.).
There is evident seasonal synchronicity of brood in the nests of mostant colonies encountered in the field. Winter nests of many species
contain only large larvae, while in spring eggs, hatchling larvae andpupae are present. This applies to a range of taxa in both temperate andtropical habitats. Dacetinops species in Borneo and lowland New Guinea
are typical (Taylor, 1985). These observations imply that the univoltine
life cycle with overwintering larvae is common among modern ants.
Despite literature search I am unable to say with certainty whether thequeens of any ants at all are continuous egg layers. Reporting of
seasonal brood contents in ant nests has been inadequate for suchanalysis
Ant taxa with unusual derived life cycles include the North American
primitive formicine Prenolepis imparis. It has a seasonally reversed
univoltine life cycle, with actively foraging workers and feeding larvae in
winter and summer closure of the nests (Tschinkel, 1987). The
Australian formicine genus Myrmecorhynchus, judging from its winterforaging activity, could be similar.
The so called “Schneirla cycle” of ecitononine and aenictine army ants
involves a number of brood cycles each year, and these ants are
multivoltine. Brood development is highly synchronized in each colony,
and cycles of colony migration are correlated with developmental statusof the brood. If the brood events of a single Schneirla cycle arerepresented on a clock diagram and compared with a similarly derived
diagram of the annual cycle of a univoltine ant like Myrmecia gulosa thetwo figures are almost identical, except for their time scales, one of about
6 weeks duration, the other of 12 months. It seems likely that thismultivoltine system originated by temporal compression and repetition of
the annual life cycle of a univoltine ancestor. The Australian
amblyoponine army ant Onychomyrmex hedleyi has 3 cycles per year
homoplasious with those of the Ecitoninae. The ancestral condition isprobably that exemplified by the related Amblyopone australis, which is
univoltine, with a brood cycle almost exactly resembling that of Myrmecia
gulosa (pers. obs.).
Univoltine life cycles have almost certainly been the norm throughout
ant phylogeny, and the various derived life cycles among living ants
probably all evolved following the acquisition of eusociality.

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Allectomy as a formicid synapomorphy
Allectomy is a synapomorphy of family Formicidae. Dealation ispracticed by fertilized winged female ants after mating and aerial
dispersal prior to colony foundation. Wing retention is then no longer
adaptive so the wings are discarded and formerly “alate” females
become “dealate”. Dealation is universal in winged female ants.
Flight for mating and dispersal prior to dealation is clearly adaptive,
since wings are developed in the majority of formicid species. Many ants
have evolved secondarily worker-like apterous (ergatoid) queens, or thequeens have been evolutionarily superceded by mated workers(gamergates)(Peeters and Ito, 2001). These usually mate outside thenests using sexual calling pheromones to attract males, as in
Rhytidoponera metallica (Hölldobler and Haskins, 1977).
Nothomyrmecia queens have very reduced wings, barely longer thanthe mesosoma (Taylor, 1978), and cannot fly. These vestiges might
facilitate recognition of nubile virgin queens by males. It is likely thatsome foraging, mated dealate foundress queens and numerous workers
would be abroad at the time of mating activity. Mistakes by males inmating with either of these would involve gamete wastage, and would besociobiologically maladaptive. Also, if queens dealated immediately after
copulation this would forestall undesirable supplementary matings. Some
species of Myrmecia have similarly vestigial wings (Clark, 1951).
Allectomy is rare elsewhere among Hymenoptera. The only reported
examples appear to be in the parasitic genera Leaiopria and Mimopria(Proctroproidea: Diapriidae) which are parasites of termites (Naumann
and Masner, 1985). Females shed their wings upon entering establishedtermite nests after flight from parental termitaria. The wings are used fordispersal (and perhaps in mating flight), but are no longer functional in
the confines of host termite colonies, so they are discarded. The
adaptive rationale is clear.
Allectomy in the ant lineage could have evolved only in females ofstock which, though winged, had largely or fully dispensed with flightwhile foraging, and were therefore already capable of successfully
provisioning their nests on foot. This important truism seems seldom tohave impressed myrmecologists, who have usually assumed thatdealation first evolved because wings would be obstructive in confinednest galleries (as in the aforementioned diapriids). This might be true, butestablished prior pedestrian foraging would be necessary in any case.
The primary adaptive function of allectomy is more likely related toevolutionary loss through wing redundancy in foraging.
The fossil record and other considerations suggest that ants arose aspart of the late Mesozoic radiation of insects following the Gondwanic
origin and early diversification of flowering plants beginning in the lateJurassic, about 130 million years ago (Grimaldi, Agosti and Carpenter,
1997; Schultz, 2000). Potential prey organisms, especially relatively
sedentary phytophagous insects in dense populations, would have been

12
readily accessible as prey for ancestral formicids during those times (asis the case today with Nothomyrmecia and various species of Myrmecia).
The proposition that pedestrian foraging would be functional in suchcircumstances is entirely plausible. If an ant ancestor with biology similar
to that reviewed above was able to forage successfully without flight,
natural selection would act to favor stocks where wing-shedding waspresent, and to improve its efficiency. That scenario is accepted here.
Pedestrian foraging, nocturnality and hypogaeic behavior
The model proposed here posits that pedestrian foraging originated informicid phylogeny as an inevitable correlate of allectomy by matedfemales, prior to appearance of the “true” eusocial worker caste. Thismay be considered the single most influential event in ant phylogeny,
notwithstanding the acquisition of metapleural glands. Some of thereasons for that assertion relate to the activities of modern worker ants,
and will be deferred (since eusocial workers have not yet entered this
discussion). Others are appropriate for review now.
The development of pedestrianism positioned the ancestral formicids
for two profoundly important further developments in foraging behavior
and other out-of-nest activities: (1) nocturnality, and (2) hypogaeic
foraging. Both of these assertions are justified by biological comparisonof modern-day ants with contemporary wasps, among which these
activities are barely represented.
Nocturnality
The first pedestrian formicids, perhaps alone among their aculeatecompetitors, had incidentally acquired the capacity to forage nocturnally– a far from trivial event in their evolutionary success. This would havefacilitated activity at times when diurnally active potential competitors,
predators and parasites were inactive. Predatory success would begreatly enhanced, since nocturnally torpid resting diurnal insects would
be relatively easily captured.
Nocturnal activity is frequently observed among modern epigaeic ants.
In many habitats foragers are far more common and diverse at night thanin the day. Diurnal activity levels can be high in many habitats, involving
an array of species. Suites of other species are abroad at night, withdiversity and biomass almost always exceeding those of the day. Ants ofmany genera like Nothomyrmecia, Myrmecia, numerous Ponerinae sens.
lat., Myrmicinae like Meranoplus and Podomyrma, and many reported
dacetines, are nocturnal epigaeic foragers. Numerous formicines like
Myrmecorhynchus, Notostigma and many Polyrhachis and Camponotus
species are also nocturnal. Colony migration and foraging in the various
taxa of army ants is an almost entirely nocturnal activity. In thesecharacteristics ants stand in strong exception to other aculeates.
Nocturnal foraging is prominent in only two vespid wasp genera, theneotropical Apoica (Polistinae - with 9 species) and the S. E. Asian
Provespa (Vespinae - with 3 species)(Schremmer, 1972; Matsuura,

131991). These are proportionately large-eyed compared to relatives, andtheir nocturnality is crepuscular and otherwise dependent on moon light.
They appear to have no special adaptations for this life style, apart fromextended visual acuity, and explosive departure of foragers from nests at
dusk in Apoica (Hunt, Jeanne and Keeping, 1995)(a phenomenon also
observed in Nothomyrmecia). Both genera are sister to diurnal relatives.
Nocturnality is a synapomorphy in each, so only two evolutionary eventswere required to originate their unusual behavior.
Nocturnality is also exceptional among bees. Species of Caupolicana(Zikanapis)(Colletidae) have enlarged ocelli and fly when the moon is out.
Lasioglossum (Sphecodogastra) texanum is crepuscular but requires
flight on moonlight nights to obtain sufficient pollen for cell provisioning(C.D. Michener, pers. com.). Species of Megalopta (Halictidae:
Augochlorini) and Perdita (Xerophasma)(Andrenidae: Panurginae) arenight-flying, and the former genus includes a subgenus of nocturnal
cleptoparasites, Megalopta (Noctoraptor)(M.S. Engel, pers. com.).
The repeated evolution of nesting behavior among the aculeate
Apocrita required the correlated development of precise navigational
capacities. Female aculeates have the ability to depart from and navigate
back to their nests. Research demonstrates that their navigation isalmost exclusively visual, and apparently for that reason overwhelmingly
diurnal. Chemical navigation using scattered scent marks is present insome winged species as an unusual exception not necessarily correlatedwith nocturnality (it is used for example in tribe Epiponini (swarm
founding Polistinae) during colony foundation – J.M. Carpenter, pers.
com.).
Nocturnal visual navigation by winged aculeates must be sufficiently
difficult that it has seldom evolved in flying, nest-building species. Someparasitic wasps fly at night, but they have no requirement for precisely
navigated return flights, and within appropriate habitats their flights andresting stations are essentially random.
Visual epigaeic navigation derived from wasp ancestry is clearly theancestral condition in ants. Much has been written on the subject. The
classic study of Wehner, Harkness and Schmid-Hempel (1983) is a wellreferenced source. Such navigation by pedestrian epigaeic ants at night,
using local landmarks over relatively short distances, is clearly moreeasily effected than its equivalent in flying visual navigators. It is themodus operandi of Nothomyrmecia macrops, of several observed
Myrmecia species, and many other ants (Taylor, 1978; Hölldobler andTaylor, 1983).
Unpublished field experiments involving canopy manipulation or
screening, and the removal or duplication of local tree trunks,
demonstrate that Nothomyrmecia navigates visually, using the overhead
tree canopy silhouetted against the night sky as a map. Similar canopy
navigation occurs in the African ponerine Paltothyreus tarsatus(Hölldobler, 1980). There is no detectable evidence that Nothomyrmecia
uses scent trails.

14
In conclusion it is clear that the aculeate night belongs to the ants, andthat the correlated evolution of pedestrian foraging and nocturnality
greatly advanced evolutionary success in the formicid lineage.
Hypogaeic foraging
Hypogaeic foraging involving the search for prey in the soil or leaf litter,in or under rotting wood lying on the ground, or under stones, is rareamong winged vespoid or sphecid wasps. It is, however, the usual
predatory mode of numerous ant species, some of which are so deeplycommitted bionomically to life underground that they are seldom
observed abroad, even on the soil surface. Such hypogaeic elements are
typified by most Amblyoponinae and many (but not all) species of theother poneromorph subfamilies, the dorylomorphs, leptanillomorphs, andsome myrmicines. Their prey includes an array of soil-dwelling animals(mainly arthropods), often including eggs or larvae. The distinction
between nocturnal and diurnal activities seems not to be significant in themost hypogaeic of these species.
Alternatively, several major ant lineages predominately include
epigaeically foraging species. These seek prey largely on vegetation or
the ground surface, without penetration or excavation of soil, litter orrotting wood. Many of them are nocturnal They include themyrmeciomorph subfamilies Myrmeciinae and Pseudomyrmecinae, allspecies of which are epigaeic, and the formicomorph subfamilies
Dolichoderinae and Formicinae, most of which are epigaeic.
The prey of vespoid and sphecid wasps rarely includes specialist soildwelling animals (Krombein, 1967). These insects hunt epigaeically,usually on vegetation, or on the ground or other exposed surfaces. Soildwelling arthropods, such as subterranean insects and their larvae, or
subsoil-inhabiting spiders and centipedes are seldom encountered, if at
all.
These distinctions between ants and wasps appear to be related tothe presence of wings in foraging apterous aculeates, and their absence
in foraging ants. It may reasonably be surmised that penetration of thesoil, litter or rotting wood, and the obstructed retrieval of immobilized
prey encountered there, would be maladaptively risky for winged
foragers (especially regarding potential damage to their wings), and that
this explains the near-absence of such behavior in winged aculeates.
Some groups of parasitic (as opposed to predatory) Apocrita include
species with permanently apterous, somewhat worker-ant-like females.
Among aculeates these include members of the chrysidoid families
Dryinidae, Bethylidae and Chrysididae, and the vespoid families
Mutillidae and Typhiidae. Many of these females enter the soil or litter insearch of hosts (usually insect larvae), prior to oviposition. The hosts are
not usually later withdrawn from cover. The aptery of these species
seems likely to be adaptively correlated with this hypogaeic lifestyle. It
has clearly evolved convergently and frequently. These exceptions

15
support the assertion that the presence of wings is not conducive tohypogaeic foraging.
The opportunities furnished by the evolution of apterous pedestrianism
for the subsequent acquisition of nocturnality and/or hypogaeic foraging
activity were clearly variously exploited in later formicid evolution. This isevidenced by the diversity among modern ants with respect to thesebehaviors. It seems clear that there must have been early separation of
lineages, some of which were primarily epigaeic (and frequently
nocturnal) in foraging behavior, and others which were primarily
hypogaeic predators. Myrmecia and Amblyopone today exemplify
eventual eusocial products of two such alternative groups. Haskins andHaskins (1951) pointed out that these genera represent “widely divergent
groups of ants” despite resemblance in many aspects of their colony
foundation and general social organization. The differences between
them can readily be explained by the present hypothesis that their distantMesozoic ancestors responded differently to the opportunities associatedwith pedestrianism - one lineage retaining wasp-like epigaeic behavior,leading to Myrmecia (in which many species are also nocturnal) - theother adopting a hypogaeic lifestyle, leading to Amblyopone. The
similarities between them are primarily derived from their distantcommon ancestry.
The petiole and elbowed antennae
A posteriorly constricted petiolar node comprising the first metasomal
segment (2nd true abdominal) is a synapomorphy of family Formicidae.
The large remaining section of the metasoma is termed the gaster. The
3rd abdominal segment often has its tergite and sternite fused or firmly
joined laterally. Its anterior part forms a ball-joint enclosed by the petiole(a configuration serially analogous to the propodeal/petiolar joint in
Apocrita, and with a similar set of composite structures). The petiole isvery narrowly constricted posteriorly in almost all Formicinae andDolichoderinae, but broadly articulated in some other ants, notably in
subfamily Amblyoponinae. The extra articulation clearly increases gastral
rotation. Stinging ants probe prey with the mandibles, apparently seeking
chinks in the exoskeletal armor. When the jaws find purchase the sting isswung far forwards under the body and applied near the mandibular tips.
The eggs of queens, and trophic eggs in workers, are removed from theabdominal tip in similar manner. Venom-spraying formicines like Formica
spray forwards with the gaster held under the mesosoma (hence theancient name “pismires”). The grooming behavior termed “abdominal tip
cleaning” in which the gastral apex is swung ventrally forwards to themouthparts might, incidentally, be an ant synapomorphy, apparently
absent in other aculeates.
The antennae are elbowed in female ants with the scape notably
elongate, often nearly as long as the other articles together. This
increases antennal mobility and reach, especially in confined nest
chambers. Scape length is functionally correlated with mandibular length,

16
and is considered an adaptation supporting antennation (olfactoryscanning) of brood and potential prey, and other workers, as introphallaxis (Dlussky, 1975).
The non-eusocial, semisocial formicids
This argument has now reached a stage where the pivotal formicid
stock hypothecated here has been prescribed. Its species are resident incolonies each comprising a group of coeval dealate, fertilized females,
variously and seasonally with their eggs, larvae and pupae, and alsoseasonally with second-generation alate females and males. The winged
virgin adults leave the nests to mate and males afterwards disperse.
Newly mated females typically seek potential nest sites by flying andpedestrian search and join with other coeval females jointly to foundfurther colonies. Alternatively, they might enter established colonies(perhaps without first mating) to supplement their personnel, contributing
to the development of larger colonies than would otherwise be possible.
There would likely be strong adaptive support for this behavior. Dealationin each female occurs after the mating flight, or upon entry to a new nest.
It seems probable that the mode of colony assembly would limit theprospects for development of very large colonies, even considering thatalternative strategies might be evolved.
Polygynous family groups like the above are common in social bees
(Michener, 1974). Twenty nine sphecoid and vespoid wasp species inwhich non-reproductive workers are lacking and adult females sharenests were listed by West-Eberhard (1978), who estimated the existenceof at least 39 such species.
These insects would initially have had wasp-like characteristics, butwould become increasingly ant-like as they diversified. Ocelli would
initially be present in all individuals, and elbowed antennae and apetiolate waist would be present in some species, though perhaps not
fully ant-like. Selection would probably favor increased adult longevity,
perhaps to levels like those known in living ants. The larvae would
become progressively more ant like, some probably with the crook-necksquash shape seen in Myrmeciinae and Amblyoponinae. Foraging inmany species would be nocturnal, with dealate pedestrian foragers andvisual navigation.
Comparative research on social wasps and bees demonstrates that
reproductive competition between the parental females in such
communal nests would result in the evolution of more-or-lessmorphologically undifferentiated reproductive and non-reproductive
female castes, even (in the early stages) if the non-reproductives had
mated. Also, some virgin females might dealate without mating tobecome workers. The resulting social arrangements (with 1-generational
matrifilial colonies, a worker caste and division of labor) are defined as“semisocial”. These would be “caste-containing groups” in the sense ofWest-Eberhard (1978).

17
The asymmetries of relationship between the parental and daughter
generations, related to haplodiploid sex determination (which will not be
explored here), would selectively favor the presence in each colony of asingle mated reproductive individual, the Queen.
It will be argued below that this pre-eusocial stage would likely havebeen temporary in ant phylogeny, but its significance would have been
profound because, among other things, it would have facilitated theevolution of efficient behavioral relationships and communication
between cooperating workers and between larvae and workers. Most
importantly for this model, these would include the communication ofhunger by larvae to workers, in order to stimulate foraging and preyseeking behavior.
Considering its unique past acquisition of several highly adaptive
major characteristics, this lineage of “semisocial formicids” would haveexperienced great evolutionary success. It would almost certainly haveundergone adaptive radiation and taxonomic diversification, possibly withthe evolution of many species, genera and even higher taxa. There
would be a diversity of general lifestyles and degrees of crypsis innesting and foraging, preferred nest sites, prey preferences, mating flightparameters, modalities of reproductive behavior etc. The structure of themandibles in particular would have diversified in adaptive response torequirements for dealing with different prey organisms in different groups,
and some taxa would doubtless have specialized on particular types ofprey. Adult size would likely vary greatly between taxa. Interspecificcompetition would be profound, as usual among related, similar species.
This would generate biological and ecological displacement andconsequent adaptive shifts, which would contribute to further
diversification and radiation.
It is likely that some of the hypothecated semisocial formicids would
have evolved fully apterous females like those seen in other apocritan
taxa (notably in Mutillidae, Tiphiidae and Rhopalomastidae). Thus
species with mesosomally-reduced workers and morphologicallyundifferentiated reproductive females could have occurred. They wouldin some cases have resembled workers of modern eusocial ants. Thisevolution could follow if the alate females abandoned flight in mating,and instead attracted males using sexual calling pheromones. The wings
would thus lose adaptive function and natural selection would eliminate
them. This would probably be an all-or-nothing situation, with females
either all standard (alate/dealate) or all permanently apterous, dependingon the species.
The evolution of high-level female polymorphism, with congenerational
standard reproductives and fully apterous ant-like workers (andappropriate
caste-determining
physiologicalorsociobiologicalprocesses) would possibly have evolved in some taxa. The main lineage,
leading to eusociality must have retained winged females and allectomy,
for that is the synapomorphous condition in modern ants.

18
Untransformed non-eusocial formicids seem no longer to exist, but inevolutionary terms they could well once have constituted a diverse,
vigorous group.
Feeding, foraging and larval communication
Adult ants, like those of other aculeates, are liquid feeders. This isreinforced by use of the labiomaxillary complex and infrabuccal pouch to
remove large particulate inclusions from the food, and by removal of fineparticles from the digestive tract by the proventriculus, which functions as
a valve-dam. The proventriculus is variously and elaborately specialized
for this purpose among ants (Eisner, 1957)1.
Ant larvae, however, are primitively entomophagus and feed on thesolid masticated flesh of prey (the many exceptions, including
graminivory and liquid feeding by larvae, are derived transformations of
this character).
Thus, foraging adult ants are motivated by two behavioral drives: (1)to collect insect or other prey for their larvae; (2) to locate and utilize
liquid food for personal nourishment.
Unpublished field studies on Nothomyrmecia demonstrate that thesedrives are separately satiated. Nocturnally-foraging workers delayed
experimentally at honey baits early in their progress imbibe the liquid
avidly. They do not return to the nests when the drive for selfnourishment is satiated, but proceed to hunt for insect prey. Their preygetting drive is satiated when predation has succeeded, stimulating
return to the nest by foragers carrying booty. Otherwise that drive
subsides at dawn when numbers of unsuccessful foragers return home.
The self-feeding drive is doubtless activated by worker hunger, but the
prey-getting drive might need to be triggered by larvae communicating
their need for food to attendant workers, which would be thus stimulated
to leave the nest and to forage.
Such arrangements might be indicated by the fact that colonies ofvarious ants are known periodically briefly to cease foraging, later toresume. This is pronounced in the nocturnal Nothomyrmecia macrops,
where workers from previously active foraging colonies have been
observed not to depart their nests on nights when those from adjacentcolonies were foraging (and climatic conditions therefore not inimical),and to resume full activity the following night. It is reasonable to suggestthat larvae in these resting colonies were then well fed, were notcommunicating hunger, and thus were not stimulating worker foraging
behavior.

1

Eisner emphasised those taxa with the most derived proventricular structures (mainly genera of
subfamilies Formicinae, Dolichoderinae and Aneuretinae). There is need for further research among theponeromorph subfamilies and those not yet studied. Amblyopone was shown to have a uniquely 6-partite
proventriculus while all other species studied were 4-partite, including Odontomachus, the only otherponeromorph reviewed. The two myrmicines and sole doryline examined had “degenerate” proventricular
structure, while that of Cerapachys (Cerapachyinae) was more complex.

19
I have frequently maintained and studied colonies of Australianbulldog ants (Myrmecia) in observation nests. On several occasions
workers which would normally attack introduced live insect prey havecompletely ceased such activity and ignored potential prey. This occurs
following pupation of the last larvae present in relevant nests. The
enclosure of larvae in pupal cocoons seems effectively to have removed
larval influences, so the workers were no longer motivated for preycollection.
Authors have suggested that head waving and other body movements
might be used by larvae to communicate with workers. The
demonstration by Cassill and Tschinkel (1995) of olfactorycommunication of hunger by larvae to tending adults suggests a much
more precise mechanism. It raises the question that this communication
might generally be pheromonal. More research is desirable.
The evolution of formicid eusociality
The term “eusociality” refers to the level of social development
attained by modern ants. It requires the presence of overlapping
generations, extreme division of reproductive labor, and cooperative
brood-care by non-reproductive adults. The workers of eusocial ants are
not coeval with their queens, but members of the following generation.
The queen (there is usually one per colony) is normally the mother of
all other individuals resident in a nest – all eggs, larvae, pupae andadults are her offspring - though males, which are genetically haploid,
may sometimes be derived from unfertilized, worker-laid eggs (Crozier
and Pamilo, 1996). Reproduction is the queen’s prerogative. She is the
only member of the colony to have previously mated. The sperm bank on
which her reproduction depends is carried in her spermatheca.
The workers are specialized brood-carers, and have in most cases
lost the physical capacity for reproduction. The ability to develop wings
has been lost in their evolutionary derivation, and the pterothoracic
mesosomal sclerites specialized for flight have been deleted or fused,
resulting in drastically simplified mesosomal structure.
Eusociality has evolved homoplasiously several times elsewhere in
the Aculeata. Classic examples are wasps of subfamilies Stenogastrinae,
Polistinae and Vespinae, members of the bee subfamilies Meliponinae
and Apinae and the remarkable sphecid Microstigmus comes. No othereusocial species have fully apterous workers like those of ants.
Evolutionary transformation of the semisocial formicids posited above
to fully eusocial ants would require the daughter females to became nonreproductive workers. These second-generation workers would
eventually supplant the phylogenetically antecedent dealate queencoeval workers. Natural selection would act to favor individuals withdiminished reproductive organs and reduced alary and mesosomal
complexity, ultimately to produce fully apterous non-reproductive workers
with simplified mesosomal structure. Physiological or sociobiologicalmechanisms effecting caste determination would evolve in concert.

20
The first step to eusociality would require little more than the presenceof unmated dealate daughter workers in nests. If their supplementation of
the worker force was adaptive, natural selection would work to formalize
the process. The other components of eusociality are already latent in
the lineage. Details, such as loss of the dealate queen-coeval worker
caste and reduction of worker mesosomal complexity would later be
effected by natural selection, to produce all the defining characteristics ofmodern formicid eusociality.
The hypothecated semisocial formicids can be considered neatly
predisposed for evolution of the daughter worker caste. The necessary
first step is elegantly simple. It requires premature dealation. There areat least two models by which this could occur:(1) The larval manipulation hypothesis:
All females in semisocial formicid societies, including alate daughters,
would inevitably be programmed by evolution with the instincts (1)eventually to perform dealation and (2) to respond to stimulation fromhungry larvae by foraging. All that is required is for some daughter alates
to respond prematurely to the imperatives of larval stimulation by
foraging, and to lose their wings by premature dealation without having
mated.
This process can be envisaged as larval manipulation because it
would be in the interests of larvae to increase the force of workers
dedicated to their care.
(2) The parental manipulation hypothesis of Alexander (1974):
In this case the reproductive female(s) would impose the worker roleon alate daughters by forcibly removing their wings, as happens (ineffect) in contemporary Diacamma species (see below).
In either case, because mating requires flight, dealation of thedaughter workers would preclude them from future reproduction. The
asymmetries of relationship to their mothers and sisters, occasioned by
haplodiploid sex determination (Crozier and Pamilo, 1996), would favor
their assumption of the worker role, to assist in the production of
reproductive siblings from eggs laid by their fertilized mothers. In thisway daughter workers would appear almost instantaneously. Assumingthat the arrangement was competitively successful natural selection
would rapidly improve the functionality of this newly acquired eusociality.
These scenarios are presented as models for the evolution of formicid
eusociality. Dealation is a systematic component, and because of its
presence the evolution of eusociality in ants has followed an entirely
different course from those hypothecated to have produced eusociality
elsewhere among winged eusocial aculeates.
The initial step is so simple that it could be considered an almost
inevitable prospect (though certainly not in any way pre-ordained) forformicids with the hypothecated semisocial lifestyle suggested above.
Either scenario could easily have occurred more than once in theformicid clade.

21
Semisocial formicids with standard females and mesosomally-reduced
workers, as formulated above, could similarly evolve eusociality. They
would have the advantage of pre-existing caste determination
mechanisms. Those with monomorphic, fully apterous females would beunable to initiate eusociality, at least not along the lines suggested here.
Did the ant worker caste evolve more than once?
One of the “givens” of myrmecology is that evolution of the worker
caste and resultant eusociality was unique in ant phylogeny. This
proposition is rarely mentioned, far less questioned (though see Wilson,
1971: 74, for a suggested separate origin of amblyoponine sociality). Itspervasiveness is evident. Consider that every published formicid “familytree” is rooted in assumptions that (1) the ancestor of all ants was itself
eusocial, with an apterous daughter worker caste, and (2) all of thediversity of modern ants is derived from that single ancestor, and (3) thatits appearance predated the complex evolutionary branching which
established the major taxa of ants.
The possibility that the worker caste is not a synapomorphy of family
Formicidae, but that workers and eusociality might have been
homoplasiously derived more than once from a group of non-eusocial
ancestral formicids, seems never to have challenged the entrenched
position.
I propose that the worker caste in ants might not be uniquely evolved,
but homoplasious. This hypothesis has the following components: (1)that the daughter worker caste and eusociality arose more than oncefrom an ultimately monophyletic but diverse group of pre-eusocial,semisocial, formicids of the kind hypothecated above; (2) that multiple
derivation of eusocial workers among the semisocial formicids would
have been unexceptional, given the simplicity of such evolution along thelines suggested in this essay; (3) that some of the major features ofmodern ant diversity were already present among the antecedent noneusocial formicids and survive today to define some of the major
lineages of family Formicidae. Each relevant lineage, according to thismodel, originated from a taxon within the ancestral pre-eusocial groupwhich had acquired a daughter worker caste and eusociality separatelyfrom other such taxa; (4) That the hypothecated non-eusocial formicids,
although argued here likely to have been a diverse and successful group,
have become extinct as failed competitors of eusocial ants; (5) that theprocess of worker mesosomal simplification proceeded separately witheach acquisition of eusociality and that these separate origins havecomprehensively produced deceptively similar worker mesosomal
structures across the diversity of modern ants.
It is arguable that myrmecologists have been consistently deceived by
the convergences in worker mesosomal structure. As long as theobserved reductions were accepted as being homologous the worker
caste and eusociality were accepted to be monophyletic. In fact, of22
course, there is much diversity of mesosomal structure among worker
ants.
This hypothesis is posited as a challenge to others to consider, andperhaps to test its propositions and implications. Some further comments
are in order.
It implies that the latest common ancestor of all extant ants was asemisocial wasp-like formicid. It does not challenge the family
Formicidae as a natural clade. If accepted it would have little impact on
the taxonomic classification of ants, except that some current taxa might
be raised in rank if their eusociality is considered to be independently
derived. Bolton’s elevation of the former “ponerine” tribes Amblyoponini
and Ectatommini to subfamilies (the Ectatommini to four: Ectatomminae,
Heteroponerinae, Paraponerinae and Proceratiinae) might already havepartly accomplished this. Bolton’s formicomorph, myrmeciomorph, andleptanillomorph subfamily groups and the subfamily Ponerinae could be
separately derived lineages, and the myrmicomorphs might be derived
severally from stock shared with the poneromorphs.
Many ants have a second waist node, termed the postpetiole, formedby posterior constriction of the 3rd true abdominal segment, and therelated formation of an articular exoskeletal ball-joint anteriorly on the 4th
segment. This arrangement seems likely to have been derived more thanonce.
Greater understanding of the processes of mesosomal reduction andof evolution of the postpetiole is needed to identify cladistically usefulcharacters. Is tubulation or some similar modification of the 4th true
abdominal segment a necessary precursor to the evolution of a
postpetiolar node and its articular condyle?2 If so, is this configuration
uniquely evolved, and what does the answer to that question say about
affinities among the various postpetiolate subfamilies of ants? Brown(1954) recognized separate “poneroid” and “myrmeciioid” lineages. If histreatment of the myrmeciomorph subfamilies (sensu Bolton), including
the Myrmeciinae and Pseudomyrmecinae, is set aside, Brown’s
hypothesis basically separates the abdominally tubulate (and frequently
postpetiolate) ants from the remainder (i.e. the Aneuretinae,
Dolichoderinae and Formicinae). In effect it posits two clades within theFormicidae. Ward and Brady (2003) rigorously estimated that the
subfamily Myrmeciinae diverged from other ant lineages about 100mya.
That lineage might have been established as a third major clade before it
evolved eusociality. This would imply separate evolution of postpetiolar
2

The term tubulation (in the sense of Taylor, 1978) refers to the configuration of the 4th true abdominal
segment. It is “tubulate” when the lateral margins of its tergite and sternite are aligned and more-or-lessfused to assemble a strong cylindrical structure. This is partly inserted into the exoskeleton of the preceding
segment 3 to furnish a rotatable joint. The inserted section is set off from the remainder of the segmental
exoskeleton by a girdling constriction. When segment 3 constitutes a postpetiolar node the anterior sectionof segment 4 can be very small, forming a ball-joint articulating the large terminal abdominal tagma(termed the “gaster”, but here 1 segment shorter than the formicomorph gaster). This arrangement is ananalog of the constricted preceding propodeal/petiolar and petiolar/postpetiolar joints.

23
structures, as seems likely on anatomical and phylogenetic grounds (P.S.
Ward, pers. com.).
Some modern taxa seemingly could have evolved only from eusocial
stock. The dorylomorph army ant subfamilies (Ecitoninae, Aenictinae,
Dorylinae and Leptanilloidinae) are pertinent. Models for their derivation,
culminating with the work of Brady (2003), have consistently implied
descent from unexceptional eusocial non-migratory primitive
poneromorphs (see Wilson, 1958). Brady’s analysis has them sharing
ancestry with the Cerapachyinae (which were also placed as
dorylomorphs by Bolton, 2003).
The Diacamma enigma
The ponerine genus Diacamma has social organization strikingly
different from those of all other ants. It could be an informative exception,and must be considered in any broad discussion of ant eusociality.
Diacamma is Indo-Australian, with perhaps 30 species. The bionomics
of an Okinawan species (the sole Japanese taxon, identified then as D.
rugosum) were reported by Fukumoto, Abe and Taki (1989). A second
species (D. australe, Queensland, Australia), with essentially identicalbiology, was reviewed by Peeters and Higashi (1989). The following
account is based on these reports and other research detailed below.
The genus typically forms small colonies in surface soil and leaf litter. Its
females are uniformly apterous, and there is no general morphologicaldifferentiation between the sole parental reproductive in each nest andassociated workers.
All Diacamma females at eclosion carry a pair of tiny bilateral lobate
appendages housed in small dorsolateral mesonotal pits, which areconsidered to be vestigial wings (Tulloch, 1934). Their presence
determines reproductive status. Females with appendages are able tofunction as reproductives, to mate and produce diploid offspring. Those
without function as non-reproductive workers. Reproductive status (i.e.
caste) is controlled by the parental female removing these appendages
from prospective workers as they emerge from their pupal cocoons. In
this way vigorous resident reproductives are able totally to prevent theirdaughters from becoming reproductives.
Colonies reproduce by fission. A group including the parental
reproductive and workers departs to establish a new nest. Cocoons
containing pupae remain with workers in the original nest. The females
eclosing from these compete among themselves to retain their vestigial
wings which others attempt to remove. Eventually one (usually the first to
emerge) remains intact – she becomes the prospective new reproductive.
She leaves the nest to mate nearby after sexually calling a male partner.
Following return she begins to lay fertilized eggs. Reproductives alone
retain wing vestiges in the nests (i.e. there is no equivalent of dealation
after mating). The cycle continues.

24
Note that the new colony is initially semisocial3, with the reproductive
and workers all daughters of the same female. These ants are clearly
eusocial, though with a distinctive caste-determination system, and amajor semisocial phase in their colony cycle. If the species was
univoltine the semisocial phase could last for almost a year until the first
offspring of the new queen emerged.
Both the Fukumoto and Peeters groups demonstrated that
reproductives by their presence are able to inhibit the production of
haploid (male) eggs by workers (a not unusual trait in queen ants), andboth considered their subject species to be “queenless”. Peeters andHigashi identified the reproductive females as gamergates (matedreproductive workers).
The mesonotal appendages were termed “gemmae” by Peeters andBillen (1991) and shown to be highly glandular by Billen and Peeters(1991), who presumed this to be related to “the peculiar mechanism ofreproductive regulation found in this queenless ant”. Mesosomal
anatomy of D. australe was discussed and illustrated by Bitsch andPeeters (1992) who confirmed that the mesonotal appendages werevestigial mesothoracic wings. Gronenberg and Peeters (1993),
concluded that workers lacking gemmae can never replace a gamergate.
Peeters, Billen and Hölldobler (1992) discovered that the gemmae were
not removed in workers of the Indian D. vagans, and that mated topranked individuals dominated the nests and functioned as reproductives.
Similar reproductive dominance occurs in orphaned groups of theJapanese species, with an alpha dominant worker monopolizing haploid
egg production (Peeters and Tsuji, 1993). Sommer, Hölldobler andRembold (1993) described a Malaysian species with bionomics similar to
the Japanese model. Following comparison of four Indian species,
Veuille et al. (1998) demonstrated that retention of gemmae is thederived condition versus gemma removal in Diacamma. Recent literature,
too extensive for citation here, has shown that other Diacamma species
have worker castes determined by the removal of gemmae.
There are two possible origins for this remarkable behavior. Either it isprimary, derived directly from a non-eusocial ancestor, or it is secondary,derived from a eusocial ancestor. Two possible secondary models are:(1) That Diacamma was derived from a conventionally eusocial
ancestor through acquisition of gemmae and associated mesosomal
structures by its workers, followed by replacement of the queen castewith gamergates. This seems improbable because gemmae, as wingvestiges, would need to be developed on an already structurally-reduced
mesosoma, and it is not easy to understand why this would happen ineusocial ants with a pre-existing caste-determining mechanism.
A shift in gene expression could theoretically reassign “queen”
characters to the workers, producing partial resurrection of an unreduced
3

This is the case also in all but the first departure in a series of honey bee swarms and presumably in otherswarm-founding aculeates, including ants with apterous reproductives and colony budding.

25
mesosoma and wing vestiges in a lineage formerly with mesosomallyreduced workers. Apart from the possibility that ocelli were restored inthis way to the workers of Harpegnathos (which are uniquely ocellate
among ponerine workers) there seem to be no other examples of thisphenomenon in poneromorph ants.
(2) That a secondary round of worker-caste evolution occurred fromconventional eusocial ancestry, starting with groups of coeval
alate/dealate females, the descendents of which eventually enacted ascenario like that outlined below. Potential startup groups could perhaps
originate from assemblies of pleometrotic colony foundresses.
Neither alternative seems very plausible and both models require
extinction of conventionally eusocial ancestral stock.
One hesitates to suggest the primary option, but, consider thefollowing evolutionary scenario: (1) A monomorphic alate/dealate
semisocial formicid has small colonies comprising congenerational
dealate workers and a single dealate reproductive female. They rearalate females and males from eggs laid by the reproductive female. (2)The reproductive female co-opts some of her alate daughters as workers
by forcibly removing their wings. Unmutilated daughters are able to fly
and mate. They are the future reproductives. (3) The daughter (eusocial)
workers adaptively supercede the queen-coeval (semisocial) workers,
and the evolving lineage attains eusociality. Worker determination
continues to be controlled by removal of wings from virgin alates by thequeen. (4) A descendent lineage abandons the mating flight andreproductive females use pedestrian sexual calling to attract males.
Wings are no longer adaptive for flight and the lineage moves towards
monomorphic aptery, undergoing evolutionary reduction in wing size andsimplification of mesosomal structure. (5) The wings are eventually notentirely lost, but tiny lobate vestiges of the forewings remain, housed insmall dorsolateral mesonotal pits. Caste is now determined by theparental reproductive female removing these vestigial wings fromprospective workers etc.
Of these alternatives the first secondary model seems the least
implausible. The second might satisfy those who believe that Diacamma
females are phylogenetically degenerate queens. The primary model
would gain some support if Diacamma stood taxonomically apart fromother ants, but all relevant cladistic and other analyses place the genus
firmly in the subfamily Ponerinae as presently constituted (though thisremains a probably artificial group likely destined for moredismemberment such as that by Bolton, 2003). The primary model would
not be implausible if the taxonomic difficulties could be resolved. If
accepted it would firmly demonstrate that modern ant eusociality evolved
at least twice from non-eusocial stock, falsifying the common assumptionthat it is uniquely derived.
Pedestrianism in eusocial ants

26
The evolution of worker aptery and pedestrianism expedited both
nocturnality and hypogaeic foraging among the semisocial formicids
posited here, contributing to their adaptive success. Pedestrianism
remains influential among formicids. One of its major advantages is thatmuch larger, heavier, prey can be carried or dragged by individuals thanis possible in flight, and foragers can readily cooperate in the transport oflarge prey.
The evolution of worker subcastes, including large-headed majors or“soldiers” would otherwise be nearly impossible. They are unknown
among winged social aculeates, except for Australian halictine bees ofgenus Lasioglossum, which have a caste of large-headed, small-winged
males which stay on the nests, fight for possession of females and repel
parasites (Houston, 1970). The aerodynamic constraints ofmacrocephaly seem likely to have constrained its development in winged
aculeates. Ants, being pedestrian, suffer no such constraint, andmacrocephaly clearly does not inadaptively inhibit progression by largeheaded soldiers, which have evolved repeatedly and are present inseveral subfamilies.
The absence of wings has allowed ants to nest in a diversity ofsituations barely exploited by flying aculeates.
Permanent substrate contact has occasioned repeated evolution ofscent trail-laying techniques important in foraging, nest migration andworker recruitment to food. These utilize a range of chemicals andspecialized glands unknown among other aculeates (Hölldobler, 1978,and subsequent papers). Trail pheromones are produced by a richdiversity of glands, and their taxonomic distribution could provide
characters useful for phylogenetic analysis.
What happened to the ant ancestors?
The adage “extinction is the common lot, survival the exception”
applies to the history hypothecated above. Extinction of taxa related tothe crown group is a necessary component at each step in thephylogenetic assembly of modern ants, establishing the status ofcharacters now accepted as formicid synapomorphies.
The pre-eusocial formicids seem unlikely to have been very effective
competitors of eusocial ants. Their lifestyle would likely have been lessefficient than eusociality. Colony size is probably inherently limited, andpotentials for behavioral and biological diversification more restricted
than in true ants.
Various ants are known to be highly effective biological competitors ofother ants and to locally eliminate other ant species. Consider the
success of “tramp” species introduced by human commerce to new
localities. They regularly eliminate or restrict resident endemic ants orother introduced species from their habitats. Their overwhelming
competitive abilities involve not only ecological success, but direct,
targeted violent aggression (Holway et al., 2002). The vagaries of theArgentine ant, Linepithema humilis and the Madeira ant Pheidole

27
megacephala in Bermuda (Haskins and Haskins, 1965, 1988), and theneotropical Little fire ant Wasmannia auropunctata in New Caledonia(Fabres and Brown, 1978), Cameroon and the Galapagos Islands areprime examples.
It is accepted here that the hypothecated non-eusocial formicids could
have been eradicated by competitive assault from true ants. Similar
apocalyptic extinction has not occurred in the other eusocial aculeates.
Wasps (with some exceptions like Vespa) seem less prone than ants toassault their relatives, and flying adult wasps can more readily escapeattack than pedestrian formicids. Even if their brood was lost they could
survive, possibly to reestablish their nests. Bees have moved away fromlarval entomophagy. As pollen collecting herbivores they are not given to
attacking other bees. I suggest that for these reasons a diversity of social
life-styles ranging from solitary to eusocial has survived among thewasps and bees, but not among ants.
The timing of events
The earliest phylogenetic events posited above would probably haveoccurred in rapid evolutionary succession following acquisition ofacellular nesting and metapleural glands. It is now accepted from fossil
and other evidence that eusocial ants first appeared in the Mesozoic
perhaps 140-100 million years ago (mya). Their rise by the late
Cretaceous is evidenced by several recently described fossils, which
demonstrate that eusocial ants of modern appearance, with close
modern taxonomic affinities, were present in those times.
The fossil Kryomyrma neffi Grimaldi and Agosti (2000a) from late
Cretaceous (Turonian) New Jersey amber (estimated 92 mya) is anindubitable formicine worker, with detailed resemblance to modern taxa.
The genus Afropone Dlussky, Brothers and Rasnitsyn (2004) with two
species represented by compression fossils of late Cretaceous age(Turonian, 93.5-89.3 mya) from Orapa, Botswana was assigned by its
authors to subfamily “Ponerinae”. Its metasomal structure, with adifferentiated postpetiole, is derived when compared to the usual
ponerine configuration, and resembles that of the modern
myrmeciomorph subfamilies Myrmeciinae and Pseudomyrmecinae, towhich Afropone could be annectant. Its large eyes support thisassignment.4
Brownimecia clavata Grimaldi, Agosti and Carpenter (1997)(White
Oaks New Jersey amber; Turonian, est. 94-90 mya) was considered by
its authors to be a “ponerine”. The term “poneromorph” is appropriate,
since the former subfamily Ponerinae now comprises 6 modern
poneromorph subfamilies. No assessment is given as to whether the 4th4

In modern poneromorphs the postpetiole is not usually strongly constricted posteriorly to form a distinct
node, but poneromorphs are engineered for postpetiolar development because of their tubulate 4th
abdominal exoskeleton and its insertion into the 3rd segment. A few poneromorphs are quite strongly
postpetiolate, including Paraponera (Paraponerinae) and some species of Cerapachys (Cerapachyinae) andProceratium (Proceratiinae).

28
abdominal segment is tubulate or not. Such indication is required if
fossils are claimed to be poneromorph. Constriction of the 4th abdominal
segment alone is insufficient for confident identification – the defining
character is the presence of its tubular structure. That said, the published
Brownimecia illustrations do evidence tubulation. Bolton (2003) placed
Brownimecia in a monotypic subfamily Brownimeciinae, and consideredthat “It may belong, speculatively, in a basal group that is sister to theponeromorphs, or perhaps to the poneromorphs and leptanillomorphs
together”. Both of these subfamily groups have tubulate abdomens, andmany species are postpetiolate.
Canopone dentata Dlussky (1999)(Medicine Hat, Alberta; Campanian,late Cretaceous, est. 83.5 – 70.6 mya) has a clearly tubulate abdomen
and somewhat constricted postpetiole. Dlussky was unsure of its
placement as a “ponerine” or myrmeciine. I consider it to be
poneromorph, with some similarities to Paraponera. The presence ofworker ocelli is unusual in poneromorphs, but that character is present inliving Harpegnathos.
A posteriorly constricted postpetiolar node is present in Afromyrma
Dlussky, Brothers and Rasnitsyn (2004)(Orapa, Botswana; Turonian, est.
94-90 mya). Its compression fossil is poorly preserved but seems
certainly to be myrmicomorph. Its was originally assigned to subfamily
Myrmicinae.
Eotapinoma macalpinei Dlussky (1999) from Canadian amber(Medicine Hat, Alberta; Campanian, late Cretaceous, est. 83.5 – 70.6mya) was considered a likely dolichoderine by its author and a “putative
dolichoderine” by Grimaldi and Agosti (2000b).
Gerontoformica cretacica Nel, Perrault, Perrichot and Neraudeau(2004)(Archingeay-les-Nouillers,Charente-Maritime,
SW France;
Uppermost Albian amber, est. 100 mya) is not well preserved, but the
published photographs of its petiole and gaster closely recall
Nothomyrmecia. Nothing in the original paper challenges thisresemblance. Gerontoformica is described as having peg-like teeth on its
clypeus (reminiscent of Amblyoponinae and Anomalomyrmini(Leptanillinae)), small eyes and long mandibles. Its authors remarked
that its first gastral (i.e. 3rd true abdominal ) segment had a “pretergite”,
though their fig 2c seems to illustrate this character for the 4th abdominal
segment. They did not indicate whether abdominal segment 4 wastubulate or not, but their photographs imply the latter. The fossil was said
to lack a sting, but Fig. 2a, seems to depict an unexserted sting shaft. Fig.
1c does not represent the head in detail sufficient to illustrate the texturaldescription of cephalic, mandibular and antennal particulars. The
discovery of more specimens from Archingeay is eagerly awaited. The
authors are urged to publish line illustrations backing-up theirobservations and photographs.
Using Bayesian dating analysis Ward and Brady (2003) indicate thatthe most recent common ancestor of a clade containing Nothomyrmecia
and Myrmecia is 74 my old (95% confidence limits 53-101my).

29
Derivation of the several living dorylomorph army ant subfamilies fromancestral stock considered antecedent also to the Cerapachyinae has
recently been rigorously estimated to have occurred in mid Cretaceous
times (i.e. around 100mya), possibly in Gondwana (Brady, 2003),
providing further evidence that true ants of modern appearance andtaxonomic affinity, with behavior like that of highly specialized modern
ants, were likely present by the late Cretaceous
These examples all imply that evolution from solitary wasp ancestry to
ant eusociality could have been complete within the first few million years
of formicid history, with tens of millions of years available to play out thedetails, including the extinction of non-eusocial formicids, which probably
disappeared promptly anyway following the rise of eusocial ant
competitors. It also seems likely that the earliest events in formicid
evolution would have occurred within a limited geographical range,
perhaps entirely in Gondwana or one of its drifting continental fragments,
so the loss of groups standing as sister taxa to those of the emerging
main formicid lineage would have been local geographical events.
It appears that mid-Cretaceous aculeates would have included acluster of related lineages among which the prospective formicid
synapomorphies first evolved and began successively to accumulate. At
the point of each character acquisition two sister taxa would be
generated, one lacking the new character, but possessing thosepreviously evolved. There would probably at times have been several
contemporaneous taxa carrying incomplete sets of the relevantcharacters. Also, some of the characters could have arisen more thanonce, later to be secured as potential formicid synapomorphies by thesurviving lineage. One lineage alone, according to this model, followed
the course prescribed here and that lineage is represented today only by
the living ants – the remaining taxa significant to this history have in themeantime become extinct.
It also appears prima facie that the fossil record has not yet yielded
possible pre-eusocial ant ancestors with incomplete sets of the relevantcharacters reviewed above. The hypothecated first solitary protoformicid
would be a strong candidate for recognition as a fossil. It would probably
have the general facies of a modern solitary vespid wasp, withmetapleural glands or antecedent structures, but probably not a petiolar
waist node or elbowed antennae (though many modern solitary vespids
have somewhat elongated scapes). A fossilized dealate, queen-coeval
forager from an ancestral semisocial formicid colony of the type positedhere would probably have a more generally (perhaps substantially) antlike facies but with some wasp-like attributes. It would very likely be
identified as a somewhat wasp-like dealate female eusocial ant, unless
its fossilization preceded acquisition of the petiolar node and/or elbowed
antennae, or it had those characters incompletely developed comparedto modern ants. The extinct subfamily Armaniinae Dlussky (reviewed by
Bolton, 2003: 73, 259) requires consideration in these regards (P.S.

30
What is Sphecomyrma?
The extinct genus Sphecomyrma (type species S. freyi Wilson,
Carpenter and Brown, 1967) was based on two worker specimens
enclosed in a piece of Turonian New Jersey amber of estimated age 92
mya. Its characteristics are summarized in the original paper, and by
Grimaldi, Agosti and Carpenter (1997). They will not be reiterated here.
Further specimens from New Jersey are now known (of Turonian age,
mid Cretaceous, ca 92 mya. – Grimaldi, Agosti and Carpenter, 1997). A
second closely similar worker-based species S. canadensis Wilson has
been described from Alberta, Canada (78-79 mya), and a worker-based
related genus Cretomyrma Dlussky from the Taymyr peninsula, Siberia(of Santonian age, 85.5-83.5 mya). Two male-based genera Dluskyidris
Bolton and Baikuidris Dlussky (both from Siberian Santonian amber) are
believed to be relatives of Sphecomyrma. All are currently classified infamily Formicidae, subfamily Sphecomyrminae. There have been noreports of fossil alate or dealate females.
Sphecomyrma has also been reported from late Albian Burmese
amber (ca 100-105 mya)(Grimaldi, Engel and Nascimbene, 2002), alongwith an enigmatic poneroid fossil which represents the oldest known
definitive ant. A further new species has recently been discovered in
New Jersey amber (Engel and Grimaldi, in prep.). These authors willreport multiple workers of S. freyi from a single piece of New Jersey
amber, implying that the species was social. Engel (pers com.) “would
argue that Sphecomyrma was a likely social (perhaps even eusocial)early group of ants”, a view with which I concur.
There has been much discussion concerning the taxonomic andphylogenetic status of Sphecomyrma. It has been considered a taxon
sister to the Formicidae, with the two families comprising superfamily
Formicoidea (Wilson, Carpenter and Brown,1967), assigned with theother taxa indicated above to subfamily Sphecomyrminae in family
Formicidae (Grimaldi, Agosti and Carpenter, 1977; Bolton, 2003), or
alleged perhaps not to be an ant at all, and to require a separate family(Dlussky, 1975, and his other references in Grimaldi, Agosti andCarpenter 1997). Baroni Urbani in 1988 considered it to be a nonsocial
wasp (reported in Grimaldi and Agosti, 2000b). In the morphologicalphylogenetic analysis of Ward and Brady (2003) Sphecomyrma is sisterto the extant ants sampled, with strong bootstrap support.
The argument centers on whether or not the relevant fossils possess
metapleural glands (which were identified with reservation in the original
Wilson, Carpenter and Brown paper). Recent researchers using recently
collected specimens agree that metapleural glands are present, and theaffirmative evidence of Grimaldi, Agosti and Carpenter (1997) isconvincing. It has also been suggested that the antennae, which haveshorter scapes than in all living female ants, could indicate that
Sphecomyrma might be neither eusocial nor an ant (see Grimaldi, Agosti
and Carpenter (1997) for summary). The last hypothesis is probably
unfalsifiable.

31
Wilson (1971: 31) considered the wasp-like mandibles of
Sphecomyrma an anomaly in comparison with the very different
mandibles of Amblyopone, which was presented as an archetypicalprimitive ant. This difficulty is resolved by the model for ant evolution
presented here. It envisages a range of mandibular specializations
among the hypothecated semisocial formicids, easily accommodating
both Sphecomyrma and Amblyopone. Note that broadly “wasp-like”
mandibles are present in virtually all male ants, despite the great rangeof mandibular specializations seen among ant workers.
At the risk of introducing a further probably unfalsifiable hypothesis, I
represent that the various Sphecomyma fossils could be females orworkers of a semisocial formicid, like those hypothecated here.
It was suggested above that these insects would likely have retained
strong wasp-like characteristics along with emerging resemblances tomodern ants, that ocelli would be present, and that the antennae andwaist nodes in some species might have transitional rather than fully antlike structure. Also that some species might either (1) like mutillids, have
evolved monomorphic wingless females with mesosomal reductions
homoplasiously resembling those of modern worker ants, or (2) have
evolved a eusocial-ant-like apterous worker caste while retaining
standard reproductives (the two castes being congenerational withinnests). All these possibilities are plausible, and all could apply to
Sphecomyrma. It could represent a species meeting either of the two
options numbered above.
Those interested in this proposition should consider it while re-reading
the classic Wilson, Carpenter and Brown paper. Its authors believed
Sphecomyrma to be “truly intermediate between primitive ants and theaculeate wasps” and stated that “if we are mistaken about the presenceof the metapleural gland, it would be possible to consider Sphecomyrma
a wasp rather than an ant”. In other words they considered themetapleural glands to be essentially the only definitively formicid attributeof S. freyi.
Such observations regarding wasp/ant intermediacy would also apply
exactly to the semisocial formicids hypothecated here.
The “true” nature of Sphecomyrma might in fact be intractablebecause either: (1) the discovery of alate or dealate female
Sphecomyrma fossils would not falsify the above hypothesis, since they
could be related semisocial formicids of another species which had not
lost flight capacity. Neither would they prove that the Sphecomyrma
fossils were eusocial workers; or (2) alate or dealate fossils with“complete” mesosomal structure, collected in indubitable conspecificassociation with mesosomally-reduced specimens like the known
Sphecomyrma fossils, could represent either a semisocial species withcongenerational alate/dealate females and apterous workers, or a fully
eusocial ant with apterous mesosomally-reduced daughter workers.
They would indicate at best only that Sphecomyrma was either asemisocial or a eusocial formicid.

32
The mesosoma in Sphecomyrma is unusual compared to those of all
modern ants, but the suggestion by Wilson, Carpenter and Brown (1967)
that the suture between the meso- and metathorax is “possibly movable”
seems implausible.
Concluding remarks
Past discussions on the origins and evolution of formicid eusociality
have reiterated the Wheeler/Malyshev hypothesis (Wheeler, 1928: 63;
Malyshev, 1968). It was reviewed without significant support by Wilson(1971) and Hölldobler and Wilson (1990), and will not be further
considered here. Wilson’s suggestion (1971) that amblyoponine antsmight have evolved in the Wheeler/Malyshev manner via Sclerodermalike ancestry is untenable (Hölldobler and Wilson, 1990).
Various authors, notably Wilson (1971: 72), Hölldobler and Wilson(1990: 27) and Bourke and Franks (1995: 73) have espoused the“subsocial route” for evolution of ant eusociality, largely inspired by theseminal analysis of wasp sociality by Evans (1958). This model derives
formicid eusociality ultimately from groups comprising a parental female
caring for her nondispersing daughters. The model implies that suchancestral females would have initiated their colonies alone, not inpleometrotic foundress groups. It is argued that this is the usual mode of
development of modern ant colonies, which is held in effect to reflect
phylogeny and to support the model. It is also held that semisocialgroups of the kind posited in this essay would lack kinship bonds
sufficient for the semisocial model to operate under natural selection.
Bourke and Franks considered the semisocial option “a poor model forthe origin of altruism in early ants”.
The first assumption is simplistic. Pleometrotic colony foundation by
partially claustral groups of coeval mated females could just as well behomologous with the ancestral condition, as proposed above.
Pleometrosis is known largely among “primitive” ants (Wheeler, 1933,
Wilson, 1971, Hölldobler and Taylor, 1983), and its general absence in“higher” ants, versus solitary colony foundation, could reflect aphylogenetic succession.
The ability of queens to found colonies alone without foraging isanother feature of ants based on the presence of allectomy, which
provided the opportunity for metabolization of the redundant postdealation flight muscles in order to sustain claustral, solitary foundress
females and their initial brood. The larvae in such founding colonies are
fed by the queen with surplus eggs, or trophic egg material, or nutrient
provided by trophallaxis (processes broadly analogous with milk feedingin mammals, by which female biomass is converted to larval biomass).
Solitary, claustral colony foundation is in fact commonly practiced
mainly among the formicomorph and myrmicomorph subfamilies. Their
queens are usually much larger than conspecific workers. The
myrmeciomorphs, many poneromorphs and several known myrmicines,
are partially claustral, frequently with pleometrosis (see Wilson, 1971:

3372). The dorylomorphs and leptanillomorphs (possibly excluding thebarely studied Anomalomyrmini and a few cerapachyines) and someother “army ants”, like Onychomyrmex (Amblyoponinae), have apterous
queens and found new colonies by budding, as do those Ectatomminae,
Heteroponerinae and Ponerinae with apterous reproductives orgamergates. Most of these, except the dorylomorph and leptanillomorpharmy ants, have relatively small queens, little larger than workers.
Solitary foundation is thus broadly correlated with the secondarypresence of strong size dimorphism between flying queens and theirworkers, where the ratio of available bulk of maternal mesosomal flightmusculature to biomass of the foundation brood is high. It cannot be
represented as the likely situation in the earliest formicids.
The initial groups of winged communal wasps posited in the current
model would draw their personnel from local populations, so would likely
have significant relationship ties. Natural selection would work upon suchgroups whatever their composition, and if individuals joining positively
selected groups were more likely than non-joiners to reproduce, theirgroup-joining behavior would be positively selected. Note that, in theearliest stages of group formation discussed above, all joiners would
have the opportunity to contribute eggs to the collective cache, and theiraverage chances for survival and reproduction could be better than ifthey had not joined. Even if some individuals lost out in the reproductive
stakes the group could still be relatively adaptive, and the tendency bymated females to join groups could become selectively established.
Furthermore, if those groups with the most closely related personnel
tended to be the best survivors, selection would favor individuals whichgrouped with close relatives such as sisters. This process could furnish
the allegedly deficient kinship bonds.
Wheeler (1928: 107) considered that “The Ponerinae represent theprimitive stock from which the other ant subfamilies…have radiated”. He
asked “Are the various subfamilies of ants really derived from a common
stock or are they polyphyletic like the various social subfamilies of wasps
and bees?” Careful reading indicates that he was talking about the“various subfamilies of ants” as possible separate social lineages. He
answered (p. 108) “I deem it…probable...that the family (i.e. theFormicidae as a social lineage) as a whole is monophyletic”.
In reaching that conclusion Wheeler specifically rejected aphylogenetic scheme diagrammatically proposed by Emery (1920: 370;
the diagram reproduced by Wheeler, 1928: 107). This posited several
lines of evolution from an “unknown hypothetical formicid stock” out of
which the following lineages were derived: (1) a “Prodorylinae” (i.e.
Cerapachyinae) – Dorylinae line, (2) a line comprising a hypotheticalancestral group the “Proponerinae”, (3) a lineage labeled“?Dolichoderinae”, and (4) a lineage labeled “?Formicinae”. The“Proponerinae” gave rise to several lines labeled: (1) Euponerinae; (2)Amblyoponini, Ectatommini, Proceratiini; (3) Myrmeciini, Platythyreini;and (4) Myrmicinae. Wheeler expressed the view that this model

34“indicated a polyphyletic origin of the subfamilies”, and that was his main
reason for rejecting it. Emery’s proposal was ultimately monophyletic, inthat he clearly considered the hypothetical stock to have beenmonophyletic, but it is not clear whether he considered the lineages hedepicted to have represented separate origins of sociality as Wheeler
concluded.
Emery’s phylogenetic concept was in fact structurally close to that
proposed here. His “unknown hypothetical formicid stock” isschematically equivalent to the semisocial formicids of the current model(and his “Proponerinae” is close to the “poneroid lineage” of Brown;1954).
Wheeler (p. 108) rejected Emery’s model because: “if we take
Emery’s point of view…the hypothetical ancestors must have been very
primitive and in all probability solitary or subsocial Aculeates”. Thus
Emery and Wheeler between them, came close to the phylogenetic
hypothesis developed in this essay (even though the latter opted for the
subsocial route). A shift of emphasis by Wheeler almost 80 years ago
might have generated ideas similar to those expressed here. Ideas, likeevolution, can be very chancy!
The early stages of the evolutionary path to formicid eusociality
proposed above relate well to the “polygynous family hypothesis” ofWest-Eberhard (1978). The presence of allectomy, however, projected
the ant lineage on a later course completely different from those of othersocially advanced aculeates. The fact that the simple act of dealation candetermine the potential for future worker versus reproductive status infemale ants provided a mechanism for ready establishment of the worker
caste and eusociality. That is the most important component of thismodel!
Several recent studies have investigated the internal phylogeny offamily Formicidae cladistically, or by DNA sequence analysis andcomparison, most notably Baroni Urbani, Bolton and Ward (1992);
Shattuck (1992); Grimaldi, Agosti and Carpenter (1997); Grimaldi andAgosti (2000a), Brady (2003); Ward and Brady (2003); and Saux et al.
(2004). More work in this area is desirable, especially with the possibilityof multiple evolution of eusociality in mind. Possibly significant worker
mesosomal and postpetiolar characters need to be better understood,
along with features related to the many pheromone-producing glands ofants, especially those involved in the laying of scent trails. Further workon proventricular structure, especially in the poneromorph, dorylomorph,
leptanillomorphandmyrmicomorph
subfamilies
might
yield
phylogenetically useful characters.
There is a degree of prospective adaptation at each phylogenetic levelin the model proposed here. It is almost as if each step was
automatically destined to be followed by the next. This is especially trueof the semisocial formicids with colonies comprising dealate reproductive
females and workers of one generation, and alate, potentially
reproductive, females of the next. Evolution of a eusocial worker caste

35
following premature dealation of daughter alates in the manner describedabove is so simple a process that its occurrence could be consideredalmost predictable. Perhaps this should not amaze hymenopterists, forthe various major groups of aculeates have produced many separatesocial lineages, with a number culminating in eusociality (in several
groups of bees, vespine wasps, a sphecoid wasp and ants). This musthave been, in effect, preordained by the unique set of relevantcharacteristics phylogenetically assembled in the ancestor common to allof these taxa.
Whether or not this model is considered plausible, it is clear that ants
as aculeates are endowed with a number of extraordinary characteristics.
They have metapleural glands, allectomy in alate reproductive females,
permanently apterous workers and strong worker dimorphism, often withlarge-headed soldiers; they lay scent trails, have acellular nests, andmay practice nocturnal and/or hypogaeic foraging behavior. In each casethese characteristics are unique to ants or very unusual, even rare,among other aculeates.
In conclusion ants, as my title implies, are very strange wasps indeed!
Acknowledgments
Sincere thanks are due to colleagues who gave advice on this paper
or helped to polish the arguments and presentation. All are gratefully
acknowledged: Andrew Beattie, Denis Brothers, James M. Carpenter,
Ross Crozier, Michael S. Engel, Charles D. Michener, Ian Naumann,
Michael Schwarz, Steve Shattuck, Roy Snelling, John Trueman andPhilip S. Ward, and my wife Wendy Taylor. Michelle Hearn and Margaret
Thornton provided much appreciated library support. Newly reported fieldstudies on Nothomyrmecia macrops were conducted with the late Roger
Bartell. Many of the ideas expressed here were discussed in my plenary
address “Strange Wasps Indeed” delivered to the XIII International
Congress of IUSSI, Adelaide, Australia, 3 January, 1999.

36
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