﻿Four new inquiline social parasite species in the dolichoderine ant genus Tapinoma (Hymenoptera, Formicidae)

﻿Abstract Four new inquiline social parasites are described in the dolichoderine ant genus Tapinoma from the Nearctic region, and keys are provided for queens and males of the Nearctic Tapinoma species. The new social parasite species represent the first inquiline species in the genus Tapinoma and the first confirmed inquilines known from the ant subfamily Dolichoderinae. The four new species appear to be workerless inquilines that exploit a single host, Tapinomasessile (Say), and they represent at least two distinct life history syndromes. Tapinomaincognitum Cover & Rabeling, sp. nov. is highly derived morphologically and is a host-queen-tolerant inquiline. In contrast, T.inflatiscapus Cover & Rabeling, sp. nov. shows a lesser degree of morphological modification and appears to be a host-queen-intolerant social parasite. The life history of T.pulchellum Cover & Rabeling, sp. nov. is presently unknown, but its close similarity to T.incognitum suggests that it is also a host-queen-tolerant inquiline. The life history of T.shattucki Cover & Rabeling, sp. nov. is still uncertain. Our findings provide novel insights into the complex biology of ant inquiline life history syndromes.


Introduction
Social parasitism, the dependence of one social insect species on another during colony founding and/or during its complete life cycle, is one of the most intriguing life history phenomena found in the eusocial Hymenoptera (see Hölldobler and Wilson 1990;Bourke and Franks 1991;Buschinger 2009;Rabeling 2021 for general reviews).Most ant social parasites belong to one of three basic life history syndromes.Temporary social parasites require the assistance of a host species during colony founding only, and established colonies of the parasite produce numerous workers and are fully independent (Wheeler 1904;Borowiec et al. 2021).Dulotic social parasites establish new colonies as ZooKeys 1202: 111-134 (2024), DOI: 10.3897/zookeys.1202.120478 Stefan P. Cover & Christian Rabeling: Four new inquiline social parasite species in the ant genus Tapinoma temporary social parasites, but in mature colonies the parasite workers raid host nests and steal their brood, some of which is eaten.The remaining brood complete development to become host workers that feed and maintain the parasite colony (Hölldobler and Wilson 1990).The vast majority of dulotic parasites are completely dependent on their host workers.
The third major life history syndrome of ant social parasites is permanent inquilinism (Wilson 1971).Inquiline ants live within the nests of their hosts for their entire life cycle, excepting a brief period during which dispersal (and mating in some species) occurs.In most cases the inquiline queen coexists with one or more host queens but "castrates" the colony by suppressing or greatly reducing the production of host sexual forms and substituting the production of parasite sexual offspring (Kutter 1968;Rabeling and Bacci 2010).In others, the inquiline somehow eliminates the host queen(s) or is adopted successfully only in host colonies that have lost their queen (Wilson 1971;Rabeling et al. 2019).The inquiline worker caste is usually absent or produced in very small numbers.Inquilines seem to be extremely rare, and their populations appear to be few in number, small in size, and sporadically distributed (Wilson 1963).In addition, inquiline ants are usually small and inconspicuous relative to their hosts and are thus seldom noticed or collected, reinforcing the impression of rarity.As a result, our knowledge of the ecology, life history, and distribution of most inquiline ant species is fragmentary.
Inquilinism has evolved at least 57 times convergently across the formicoid clade of the ant tree of life, and 88 inquiline species have been confirmed to date (Gray and Rabeling 2023).The majority of inquiline species are concentrated in the ant subfamilies Myrmicinae and Formicinae.Curiously, true inquilines are unknown from the subfamily Dolichoderinae, though temporary social parasites occur with some frequency.Temporary social parasites have been reported from the dolichoderine genera Arnoldius, Azteca, Bothriomyrmex, Chronoxenus, and Dorymyrmex (Trager 1988;Dubovikoff 2005;Longino 2007;Guerrero et al. 2010).Based on the morphology of the queens, inquilinism has been suggested for Azteca nanogyna and Azteca diabolica (Longino 2007;Guerrero et al. 2010).Both are known from only a handful of specimens collected by canopy fogging or in flight-interception traps (Longino 2007;Guerrero et al. 2010), and their hosts and life-histories remain unknown.
Until the present, social parasites of any kind were unknown in the dolichoderine genus Tapinoma.Currently, 72 extant species are recognized in the genus Tapinoma and the genus is distributed globally (Bolton 2024).The North American ant fauna contains only four described species (Fisher and Cover 2007).Tapinoma litorale Wheeler, Tapinoma schreiberi Hamm, and Tapinoma sessile (Say) are native, whereas Tapinoma melanocephalum (Fabricius) is a global tramp species established in Florida (Hamm 2010;Deyrup 2017).Here, we report the first inquilines discovered in the genus Tapinoma.Curiously, the new Tapinoma inquilines all parasitize a single host species, Tapinoma sessile.The evolution of several inquilines against the complex population genetic backdrop of a single, widespread host species makes this host-parasite complex unique in the context of ant social parasitism and deserving of further examination.

Specimens examined
In addition to new collections, specimens from the insect collections listed below were examined for this study:

Morphometric measurements
Specimens were examined and measured using a Leica MS5 stereomicroscope fitted with a stage micrometer.Measurements were taken at 25× magnification.Morphometric conventions and indices follow Bolton (1987) and modifications described in Rabeling et al. (2007).Morphometric measurements and indices are defined as follows in Table 1.

HL Head Length
Length of the head in full face view, excluding mandibles, measured in a straight line from the midpoint of the anterior clypeal margin to the midpoint of the posterior margin of the head.In species where the posterior margin or the clypeal margin (or both) is concave, the measurement is taken from the midpoint of a transverse line spanning the anteriormost or posteriormost projecting points, respectively.

HW Head Width
Maximum width of head, not including the eye.

CI
Cephalic Index HW*100/HL SL Scape Length Maximum straight-line length of the antennal scape excluding the basal constriction or neck close to the condylar bulb.

ML Mesosoma Length
Diagonal length of the mesosoma in profile from the point at which the pronotum meets the cervical shield to the posterior base of the metapleuron.

Species accounts
Tapinoma incognitum Cover & Rabeling, sp.nov.https://zoobank.org/6E385B56-D69F-4EB7-A75C-88C2D7D23A67 Diagnosis.A workerless, host-queen-tolerant inquiline social parasite of Tapinoma sessile showing morphological and life history traits of the inquiline syndrome.Both females and males are miniaturized (i.e., smaller than the host workers), alate, and morphologically complete (Fig. 1, Table 2).Females eclose with intact wings, but the wings are fragile and quickly deciduous.Males are brachypterous.Females have a reduced 4,3 palp formula, anterior clypeal border with weak median concavity, denticulate mandibles with only 2-4 denticles.In side view petiole with low, rounded node; viewed from the rear dorsal margin concave (rarely flat).Males similar in size and overall habitus to females but often darker in color and easily recognized by their extruding genitalia.Males have a reduced 5,3 palp formula and only 12 antennal segments.Females of T. incognitum are closely similar in habitus to those of T. pulchellum sp.nov., but can be easily distinguished by palp count, concave anterior clypeal border, mandibular dentition, and propodeal profile.Description.Holotype queen: HL 0.53, HW 0.53, SL 0.44, ML 0.82, CI 100, SI 83.Head in full-face view nearly square, dorsal margin straight with corners evenly rounded.Anterior margin of clypeus with shallow median impression; posterior border rounded, not projecting forward between antennal insertions.Mandibles reduced, barely touching each other when mandibles are fully closed; apical tooth well developed, cutting edge of mandible with 2-4 small denticles.Antennae with 12 segments, scapes relatively short, surpassing the dorsal margin of head by less than their own maximum width.Palp count 4,3.Mesosoma with typical modifications related to wing bearing.In lateral view, propodeum forming an evenly rounded convexity and lacking distinct dorsal and posterior surfaces.Orifices of propodeal spiracles slightly elevated and conspicuous.Metapleural gland orifice significantly reduced.Petiole in side view with low, blunt node; viewed from the rear trapezoidal with concave dorsal margin.Petiolar spiracles located on top of laterally extended tubercles.In dorsal view, four gastric tergites visible.Integument thin, specimens can shrivel when dried.Body surface covered with short, appressed pubescence; posterior margin of all gastric sternites and fourth gastric tergite with sparse, long setae.Color pale brown to yellowish brown, appendages pale yellow.Paratype queens (n = 8): HL 0.50-0.53,HW 0.50-0.53,SL 0.41-0.47,ML 0.76-0.88,CI 94-100, SI 78-94.
Paratype male: HL 0.50, HW 0.47, SL 0.47, ML 0.76, CI 94, SI 100.Males small, approximately the same size as the queen, brachypterous, closely similar to the conspecific queen in habitus.Head in full-face view almost square.Eyes small, maximum diameter ~ ¼ of head length; individual ommatidia partly fused, lacking the distinct convex surface of each ommatidium; compound eyes appear to be coated with a translucent resin.Ocelli slightly elevated above the surface the head.Anterior clypeal margin with a broad, median, shallow impression.Mandibles reduced, with a single large apical tooth; denticles on cutting edge of mandible indistinct.Antennae with 12 segments, scapes shorter than the head (CI), surpassing the dorsal margin of head by twice their maximum width.Palp count 5,3.Mesosoma enlarged with typical modifications related to wing bearing.In lateral view, propodeum rounded, convex, with dorsal and posterior of approximately equal length.Metapleural gland orifice absent/reduced.Petiole small, overhung by first gastric tergite, not entirely visible in dorsal view.In dorsal view, five gastric tergites visible.Hind wings reduced to wing remnants lacking venation.Body surface covered with short, appressed pubescence, except for the antennal scapes and flagellum, which are covered by a dense, short, suberect pubescence.Color medium brown to black.Paratype males (n = 3): HL 0.50-0.56,HW 0.47-0.53,SL 0.44-0.47,ML 0.76, CI 94-100, SI 83-100.
Etymology.When first seen in the field, the collector's initial impression was of a Tapinoma sessile colony infested by tiny diapriid wasps of some kind.
A second look made it clear they were, in fact, tiny inquilinous ants.Hence, the species name is the nominative neuter of the Latin adjective incognitus, meaning unknown, unrecognized, in disguise.(ii) SPC 8656.Same site description as above.Not as heavily grazed as in 2008.In 9 cm diameter hard, dead oak stump in shade.Very dry conditions.~ 1000 ants.Multiple host queens present.Eggs, larvae, and a few inquiline and host worker pupae present.
Discussion and biology.Tapinoma incognitum is known from three collections that were made at the type locality on separate occasions.All were mixed colonies containing T. incognitum and its host T. sessile.Each colony contained multiple fertile host queens, numerous host workers, and some host worker pupae.In addition, each nest contained males and females of T. incognitum, and, in two collections, parasite pupae.No T. incognitum workers were found.In each colony, several T. incognitum queens were observed with enlarged metasomas, implying that multiple parasite queens were reproductively active (i.e., functional polygyny of social parasite; Table 2).A striking feature of this species is the strong convergence in size and habitus between females and males (i.e., gynaecomorphism; Table 2).Males, however, are easily recognizable by their externally visible genitalia and because they are brachypterous; the wing remnants are small, crumpled, distorted, and persistent.In addition, T. incognitum also displays other morphological characters typical of the inquiline syndrome (Fig. 1, Table 2).Hallmark characters include reduced body size, the reduction of antennal segments in the males, and the reduction of palp segments in both queens (palp formula 4,3) and males (palp formula 5,3).The wings of queens are extremely fragile, easily deciduous, and almost certainly non-functional, and the males cannot fly.Thus, mating must take place in or around the nest.
We kept a colony alive for a few days and made some behavioral observations.It was eye-catching that the host workers carried social parasite queens as if they were pupae and the parasites retracted their appendages against their bodies and became pupae-like when carried.The host workers also regurgitated to and groomed the social parasite queens.Host and social parasite queens encountered one another often but seemed to ignore each other.This suggests that T. incognitum is well integrated in the host society.Collectively these morphological and life history traits indicate that T. incognitum is a workerless, host-queen-tolerant inquiline social parasite of T. sessile.

Tapinoma inflatiscapus
Cover & Rabeling, sp.nov.https://zoobank.org/6DB40425-2AF1-48D3-9D3A-E1B47FBB0F76 Diagnosis.A unique, workerless, host-queen-intolerant inquiline social parasite of Tapinoma sessile with relatively few morphological adaptations to its parasitic lifestyle.Inquiline females and males are equal in size, smaller than the host females and males, and approximately the size of host workers (Fig. 2, Table 2).Both females and males are winged and seem capable of flying.Females have a reduced 5,4 palp formula whereas males have the same palp formula (6,4) as host males.Both sexes of T. inflatiscapus are easily distinguished from those of all other North American congeners by the presence of short, erect hairs on the dorsal surface of the head, the mesosomal dorsum, and the first gastric tergite.
In addition, the antennal scapes are covered by short, dense, suberect pubescence and may have one or two erect hairs near the distal end.Lastly, in females, the scape reaches its maximum diameter between the mid-point and the antennal insertion, not posterior to the mid-point as in many other Tapinoma species.
Description.Holotype queen: HL 0.76, HW 0.76, SL 0.74, ML 1.15, CI 100, SI 96.Head distinctly heart shaped, approximately as wide as long.Clypeus broad, anterior margin flat or slightly convex, median impression absent; posterior margin rounded, projecting between antennal insertions.Mandibles well developed, overlapping when closed; first three apical teeth well developed, continually decreasing in size from apex to base.Eleven teeth and denticles present, most denticles on cutting edge of mandible ill defined.Antennae with 12 segments, scapes relatively long, easily surpassing the dorsal margin of head; scape somewhat dorsoventrally flattened, basal third slightly curved.Scape widest between the insertion and the mid-point.Palp count 5,4.Mesosoma robust with typical modifications related to wing bearing.In side view, propodeum with dorsal surface approximately one third as long as posterior surface.Metapleural gland orifice large and rounded in oblique view; orifice guarded by long setae pointing inwards.Petiole reduced, overhung by first gastric tergite.In dorsal view, four gastric tergites visible.Integument thin.Body surfaces with micro-sculpture resembling a honeycomb.Entire body with dense, short, suberect to erect pubescence, including head and antennal scapes.Short erect hairs present on dorsal surface of head and mesosoma.Color pale to medium brown, legs and antennae paler, yellowish brown.Paratype queens (n = 8): HL 0.65-0.76,HW 0.65-0.76,SL 0.68-0.76,ML 1.03-1.15,CI 92-100, SI 96-113.
Paratype male: HL 0.62, HW 0.62, SL 0.62, ML 1.12, CI 100, SI 100.Males similar in size to the females, head as wide as long (CI).Eyes large, maximum diameter ~ ⅓ of head width.Ommatidia clearly separated from each other; each ommatidium with a distinct convex surface.Ocelli slightly elevated above the surface of the head, but forming a raised, triangular turret in side view.Anterior margin of clypeus flat, median impression absent; posterior margin rounded, projecting between antennal insertions.Mandibles well developed with a single large apical tooth and ~ 18 denticles on cutting edge of mandible.Antennae with 13 segments, scapes surpassing the posterior border of the head by a bit less than ½ their length.Palp formula 6,4.Mesosoma robust with typical modifications related to wing bearing.In side view, propodeum with dorsal surface ~ ⅓ as long as the posterior surface.Metapleural gland orifice pointing backwards, circular in posterior view.Petiole visible in dorsal view.In dorsal view, six gastric tergites visible.Front and hind wings well developed.Dorsal body surfaces with short, suberect to erect pubescence, including head and antennal scapes.Short, erect hairs present on dorsal surfaces of head and mesosoma, long erect hairs irregularly dispersed over the body.Color uniformly pale to yellowish brown.Paratype males (n = 6): HL 0.53-0.71,HW 0.53-0.71,SL 0.59-0.65,ML 0.91-1.12,CI 90-100, SI 92-117.
Etymology.In T. inflatiscapus females, the antennal scape reaches its maximum diameter between the mid-point and the antennal insertion instead of posterior to the mid-point as in other Tapinoma.This unique, diagnostic morphological character is emphasized in the species epithet, which is a compound Latin noun in the nominative case used in apposition (inflati is the participle in the genitive case of inflatus + scapus, the noun in the nominative singular case).Discussion and biology.Tapinoma inflatiscapus is a host-queen-intolerant inquiline that parasitizes T. sessile colonies in mid to high elevation habitats in the mountains of Utah and Colorado.So far it has been found in sagebrush meadows (Artemisia tridentata), mixed shrub and sagebrush, and Ponderosa Pine (Pinus ponderosa) woodlands.Morphologically, T. inflatiscapus is most similar to T. shattucki from Massachusetts from which it can be easily distinguished by the unique shape of the antennal scape, the presence of short, erect hairs on the dorsal body surfaces, and its comparatively robust habitus.In addition, the male palp formula is 6,4.All other Tapinoma inquiline males have reduced palp formulae (Table 2).
Tapinoma inflatiscapus has been collected from two localities in Colorado and a single locality in Utah.On all occasions, T. inflatiscapus was found in mixed colonies with its host, T. sessile.In every case these colonies lacked a host queen and any host brood, suggesting that T. inflatiscapus either kills the host queen(s) or can only colonize queenless host colonies.Parasite workers have not been observed, so T. inflatiscapus appears to be a workerless inquiline.We could not observe whether T. inflatiscapus is mono-or polygynous, although collection SPC 7388 contained two dealate queens.
The morphology of T. inflatiscapus queens and males reflects some characteristics of the inquiline syndrome (Fig. 2, Table 2), but the degree of specialization is not nearly as pronounced as in T. incognitum and T. pulchellum.Tapinoma inflatiscapus alates are smaller than those of the host and approximately the size of host workers.The number of antennal segments is not reduced, and the palp count is reduced in the queens (palp formula 5,3), but not in males.The mandibles are normal in size and dentation.Both queens and males are winged and mesosomal development is robust, thus the wings appear to be functional and dispersal by flight probable.Mating may take place outside of the nest.If so, there may be less inbreeding and much better dispersal than in inquilines where mating takes place inside the host nest and where flight is problematic or impossible.
Tapinoma pulchellum Cover & Rabeling, sp.nov.https://zoobank.org/E469ABF5-240D-4F47-B950-1FB131C8407E Diagnosis.An apparently workerless, inquiline social parasite of Tapinoma sessile exhibiting morphological traits of the inquiline syndrome.Queens and males are tiny, much smaller than the host workers, and are very similar to each other in size and habitus (Fig. 3, Table 2).Females are apparently alate, but males are brachypterous.Both sexes have a reduced (5,4) palp formula and twelve antennal segments.Females have a flat anterior clypeal border, edentate mandibles, and a petiole with a small, dorsally rounded node in side view.Females most similar to but are readily distinguished from T. incognitum by differing palp count, edentate mandibles, anterior clypeal border, petiole shape, and propodeal profile (subangulate with short dorsal face and long, weakly concave posterior face versus rounded convexity in T. incognitum).
Description.Holotype queen: HL 0.44, HW 0.50, SL 0.44, ML 0.76, CI 113, SI 88.Parasite queen notably smaller than host workers.Head in full-face view nearly square, dorsal margin straight with corners evenly rounded.Anterior margin of clypeus flat, median impression absent, posterior margin rounded, not projecting forward between antennal insertions.Mandibles reduced, apical tooth well developed, barely touching each other when mandibles are closed; other mandibular teeth absent.Antennae with 12 segments, scapes relatively short, surpassing the dorsal margin of head by less than their own width.Palp count 5,4.Mesosoma fully developed with typical modifications related to wing bearing.Propodeum in side view rounded, with short dorsal face and longer, slightly concave posterior face.Propodeal spiracles slightly elevated and conspicuous.Metapleural gland orifice reduced.In side view petiole with low, blunt node; viewed from the rear subrectangular in shape with dorsal margin flat to slightly convex.In dorsal view, four gastric tergites visible.Integument thin.Body surface covered with short, appressed pubescence; posterior margin of all gastric sternites and fourth gastric tergite with long erect setae.Color dark brown to yellowish brown, appendages pale yellow.
Paratype male: HL 0.44, HW 0.50, SL 0.44, ML 0.76, CI 100, SI 113.Specimen damaged, but largely intact.Head glued on to point next to body.Male small, approximately the same size as the queen, brachypterous, closely similar to the conspecific queen in habitus.Head square.Eyes small, maximum diameter ~ ¼ of head length.Individual ommatidia partly fused, lacking the distinct convex surface of each ommatidium; compound eyes appear as if coated with a translucent resin.Ocelli slightly elevated above the surface the head.Anterior margin of clypeus with a broad, median, shallow impression.Mandibles reduced in size, with a single large apical tooth but lacking other teeth or denticles.Antennae with 12 segments, scapes short.Palp count 5,4.Mesosoma well-developed with typical modifications related to wing bearing.In sideview, propodeum rounded, divided into dorsal and posterior surfaces of approximately equal length.Metapleural gland orifice absent/reduced.Petiole small, overhung by first gastric tergite, not entirely visible in dorsal view.In dorsal view, five gastric tergites visible.Wings vestigial, distorted, lacking venation.Body surface densely covered with short, appressed pubescence, except for the antennal scapes and flagellum, which are covered by a dense, very short, suberect pubescence.Color medium brown, appendages yellowish brown.
Etymology.Tapinoma pulchellum is a beautiful ant (Fig. 3) and also beautifully embodies the hallmark morphological traits of the inquiline syndrome.The specific epithet pulchellum is the singular nominative neuter of the Latin adjective pulchellus, which is the diminutive of pulcher, meaning pretty or beautiful.The paratype male was found in pitfall sample 13F 5,1.There is also a worker of the potential host from pitfall sample 13F 5,3.Collected by Amy Arnett in June 1997.Type material.Holotype queen (MCZENT 00806459).Paratype male (MCZENT 00806460; same collecting locality as holotype).Holotype and paratype deposited in the MCZC.
Discussion and biology.Tapinoma pulchellum is known from only two specimens: a dealate female and a damaged male, which both exhibit typical characters of the morphological inquiline syndrome (Fig. 3, Table 2).Tapinoma pulchellum is closely similar to T. incognitum, from which it can be readily distinguished by the palp formula of both queens and males, the edentate mandibles, the flat anterior clypeal border, the petiole shape, and the unique propodeal profile.This striking similarity makes it highly probable that T. pulchellum is a workerless inquiline, similar in its life-history to T. incognitum.
Both specimens were recovered from adjacent pitfall traps at the type locality in Eno River State Park in North Carolina.Accordingly, T. pulchellum has not been observed in mixed colonies with its host.However, T. sessile is the only Tapinoma species at the type locality, so T. sessile is almost certainly the host of T. pulchellum.Visits to the type locality in 2011 and 2012 failed to turn up additional specimens of T. pulchellum but allowed observations of the putative host at that site.The field contained a dense population of Tapinoma sessile, and nests were located at the base of grass clumps or in the dense grassy thatch that covered the ground under the living vegetation.The T. sessile population was unusual.Colonies were large, with more than 4,000-5,000 ants, and uniformly monogynous.Both workers and queens were larger than the average size for T. sessile.Sexuals were not present, indicating that the mating flights had taken place already in early July.
Tapinoma shattucki Cover & Rabeling, sp.nov.https://zoobank.org/573C25EE-4870-459E-82B0-C54DD83F4990 Diagnosis.An apparently workerless inquiline social parasite of Tapinoma sessile showing relatively few morphological indications of its parasitic lifestyle.Tapinoma shattucki queens and males superficially resemble those of the host, except for their smaller size, more delicate habitus, and notably reduced size of the metasoma relative to the mesosoma (Fig. 4, Table 2).Both sexes are fully alate and seem capable of flying.Female and male palp formulae reduced to 5,4.In lateral view, propodeum of female and male forming a single, flat posterior surface, and the petiolar scale is vestigial or absent.In the male, sides of head strongly convex, head nearly circular exclusive of the compound eyes in full-face view.In T. shattucki, males exhibit morphology similar to their free-living congeners.Females may be easily distinguished from other Tapinoma species by their palp count, propodeal profile, small body size, and reduced metasoma size relative to the mesosoma.
Description.Holotype queen: HL 0.65, HW 0.59, SL 0.59, ML 0.88, CI 91, SI 100.Tapinoma shattucki superficially resembles a miniature form of T. sessile.Head parallel sided, slightly longer than wide (CI), dorsal margin of head straight with corners evenly rounded.Anterior margin of clypeus with broad, shallow median impression; posterior margin rounded, projecting between antennal insertions.Mandibles well developed, overlapping when closed, with 10 or 11 teeth or denticles; apical and preapical teeth well developed, distinctly larger than remaining teeth; apical tooth slightly larger than the subapical.Antennae with 12 segments, scapes long, clearly surpassing the dorsal margin of head by more than their own width.Palp formula 5,4.Mesosoma with typical modifications related to wing bearing; wings fully developed.In side view, propodeum lacking clear division into dorsal and posterior surfaces; appears as a single long, sloping posterior face.Metapleural gland orifice large and round in oblique view; orifice guarded by erect, long setae pointing inwards.Petiole  Discussion and biology.This taxon has a convoluted taxonomic history.Wheeler (1915: 418) described Bothriomyrmex dimmocki based on "two workers, one winged, and four dealate females taken by Dr. George Dimmock August 27, 1897, from a single colony on Mt.Tom, near Springfield, Mass."Based primarily on the unusually small size of the queens, Wheeler placed the ants in Bothriomyrmex, a genus of temporary social parasites that exploits Tapinoma hosts during colony founding, which was then thought to be exclusively Old World in distribution (Santschi 1906; but see Dubovikoff and Longino 2004;Prebus and Lubertazzi 2016).Curiously, Wheeler's description centered on the two workers, which he was at pains to distinguish from those of the Mediterranean Bothriomyrmex meridionalis (Roger).The four small, reproductive females were described afterwards and in much less detail.Emery (1925) correctly transferred B. dimmocki to Tapinoma, almost certainly without seeing the types.Creighton (1950) noted the close similarity of the T. dimmocki worker to that of T. sessile, but maintained provisional species status for T. dimmocki, given the problematic small size of the females.Shattuck, as part of his important re-assessment of the dolichoderine genera (Shattuck 1992), examined the types of T. dimmocki in the MCZC.He was able to affirm what Creighton suspected, namely the worker types of T. dimmocki did not belong in the genus Bothriomyrmex; instead, they were ordinary workers of Tapinoma sessile.Accordingly, Shattuck (1992) synonymized T. dimmocki with T. sessile and designated one of the two worker syntypes as the lectotype of B. dimmocki.This was an appropriate designation, as Wheeler's description and discussion centered primarily on the worker caste.As Shattuck remarked at the time, his taxonomic action neatly disposed of a problematic name, leaving the significance of the minute females an open question.
In late August 2007, a new collection clarified the identity of the minute females.One of us (SPC) collected a colony of Tapinoma sessile in his garden in Stow, Massachusetts, in the bottom of a flowerpot.Surprisingly, alates of both sexes were present, though this was in late-August, a month or more later than the normal mating flights of T. sessile in eastern Massachusetts.In addition, the alates were notably smaller than normal T. sessile queens and males.A detailed examination revealed that the alates represented an inquiline parasite and that the females matched the miniature females in Wheeler's syntype series of "Bothriomyrmex dimmocki."Shattuck's (1992) designation of a host worker as the B. dimmocki lectotype makes this name a junior synonym of T. sessile.Therefore, we describe Tapinoma shattucki as a new species.Tapinoma shattucki females may be readily distinguished from host queens by palp count, propodeal profile, small body size, and reduced metasoma size relative to the mesosoma.
Both times T. shattucki was collected, it was found in mixed colonies with its host.The type colony from Stow, Massachusetts, consisted of adult and pupal parasite females and males plus host workers and several adult host males.This suggests the possibility that, similar to T. inflatiscapus, T. shattucki might be a host-queen-intolerant inquiline that either kills the host queen(s) or preferentially exploits queenless host colonies.Two of the parasite females had swollen metasomas and were almost certainly reproductively active indicating parasite polygyny (Table 2).We did not observe any T. shattucki workers but instead numerous winged queens and males.Hence, our observations suggest that T. shattucki is a workerless inquiline social parasite of T. sessile and not a temporary social parasite as Wheeler (1915) suggested.Similar to T. inflatiscapus, T. shattucki queens and males show morphological differences between sexes comparable to the sexual dimorphism observed in the host (Table 2).The morphology of T. shattucki queens and males shows some characteristics of the inquiline syndrome (Fig. 4, Table 2) but the degree of morphological specialization is similar to that in T. inflatiscapus and not nearly as pronounced as seen in T. incognitum and T. pulchellum.Tapinoma shattucki alates are smaller than host sexuals and approximately the size of host workers.In addition, the number of maxillary palps is reduced in both queens and males (palp formula 5,4).As in T. inflatiscapus, the mandibles are normal in size and dentition, and both queens and males are fully winged and seem capable of flight.Hence, T. shattucki queens and males may mate outside the host nest and/or could disperse on the wing.

Discussion
In this study, we describe four new species of inquiline social parasites in the dolichoderine ant genus Tapinoma from the Nearctic region.These social parasites represent the first inquiline species in the genus Tapinoma as well as the first confirmed inquilines in the ant subfamily Dolichoderinae.All four Tapinoma inquiline species appear to be workerless and represent at least two very different life histories (Table 2).Tapinoma incognitum is polygynous and the parasite queens co-exist with host queens, revealing T. incognitum to be host-queen-tolerant.In contrast, T. inflatiscapus colonies contain neither host queens nor host brood, strongly suggesting that T. inflatiscapus is host-queen-intolerant.
Tapinoma shattucki may also be host-queen-intolerant but we need additional collections to be certain.The life history of T. pulchellum is undocumented at present.However, close similarity of T. pulchellum to T. incognitum (Table 2) suggests that T. pulchellum might be a host-queen-tolerant inquiline as well.
For a host-queen-tolerant inquiline ant, the primary advantage of retaining the host queen(s) in the colony is the ongoing production of host workers.This secures the potential longevity of the host colony and allows the continuing reproduction of the parasite.Host queen intolerance is a more complex phenomenon.In some species, the parasite attacks and eliminates host queens some time after her acceptance by the host colony (Hölldobler and Wilson 1990;Brandt et al. 2005).Such aggressive replacement of the host queen has been observed in inquilines, such as Leptothorax goesswaldi, Leptothorax wilsoni, Monomorium santschii, and Pseudomyrmex leptosus (Forel 1906;Kutter 1968;Klein 1987;Buschinger and Klump 1988;Heinze 1989), and also in some inquilines that likely evolved from a dulotic ancestor, such as Temnothorax adlerzi, T. birgitae, T. brunneus, and T. corsicus (Buschinger 1989;Heinze et al. 2015).
Other so-called host-queen-intolerant inquilines do not aggressively replace the host queen but instead can only gain acceptance in host colonies that already lack a host queen (Hölldobler and Wilson 1990;Buschinger 2009).Examples include Tetramorium atratulum, Pseudoatta argentina, and Formica talbotae (Bruch 1928;Kutter 1968;Talbot 1977;Rabeling et al. 2015).Host queen intolerance is a remarkably specialized life history strategy, and in the absence of host queens, no new host workers are produced.Thus, the lifespan of the parasitized colony must be short, limited to the time that existing host workers survive, and thus limiting the parasite to one or perhaps two annual reproductive cycles.Such life histories must be sustainable only where there are large, relatively stable host populations and where there is a rapid turnover among colonies, meaning that queenless host nests occur somewhat frequently.Note that at this point, we do not know whether T. inflatiscapus eliminates host queens aggressively or colonizes only queenless host nests, but either way, colonies are likely comparatively short lived.
The Tapinoma inquiline social parasites are especially interesting because they all exploit only a single host species, T. sessile.Systems like this, in which a single host is exploited by multiple species of inquilines, are rare, and their evolutionary ecology is not well understood.Tapinoma sessile is an extremely widespread and ecologically successful ant found throughout North America from central Canada to Florida and south into the Mexican highlands (Fisher and Cover 2007).Tapinoma sessile occurs in a wide variety of habitats, nests almost anywhere, frequently moves its nest in response to changing conditions, and is often a nuisance in buildings.As one might expect, T. sessile exhibits variation in size, coloration, and colony structure over its enormous range.A phylogenetic analysis of T. sessile samples from across North America, identified four well-supported "T.sessile" clades (Menke et al. 2010).The analysis of DNA sequence variation in the mitochondrial marker COI revealed within clade genetic distances of less than 2.3% but between clade genetic distances of 7.5-10%.Accordingly, Menke and colleagues (2010) suggested that T. sessile might consist of a complex of cryptic species.Using genomic markers, we are revisiting these findings with a special emphasis on the evolutionary origin of the inquiline social parasites.Recent studies of inquiline social parasite evolution have revealed several convergent pathways by which inquilinism evolves.These include inquiline parasites speciating directly from their future hosts in sympatry, inquilines originating in allopatry and via hostshift speciation, as well as inquilines transitioning secondarily from a different parasitic life history, such as temporary social parasitism and dulosis, to inquilinism (e.g., Rabeling et al. 2014;Heinze et al. 2015;Messer et al. 2020;Borowiec et al. 2021;Degueldre et al. 2021;Ward and Branstetter 2022;Mera-Rodríguez et al. 2023).We are confident that further studies of host-parasite co-evolution in Tapinoma will reveal novel insights into the complex mosaic evolution of ant inquiline social parasites.

Figure 1 .
Figure 1.Morphological comparison of the Tapinoma incognitum holotype queen A, C, E and a paratype male B, D, F in lateral A, B dorsal C, D and full-face E, F view.The type series was collected in Alumbed Hollow in Utah and belongs to a single nest series with the collection code SPC 7749.Scale bars: 0.5 mm (A-D); 0.2 mm (E, F).

Figure 2 .
Figure 2. Morphological comparison of the Tapinoma inflatiscapus holotype queen A, E and a paratype male B, F in lateral A, B and full-face E, F view.For the dorsal views of T. inflatiscapus C, D dealate queen C and male D paratypes were photographed.The type series was collected at Cove Mountain in Utah and belongs to a single nest series with the collection code SPC 7816.Scale bar: 0.5 mm (A-F).
Type locality.U.S.A., North Carolina, Orange County, Eno River State Park, 8 miles northwest of downtown Durham; open field adjacent to the Eno Trace trailhead.GPS: 36.073°N,79.008°W; elevation 460' (140 m).Large, maintained open field surrounded by mature secondary oak-hickory forest.The field was dominated by scattered Juniperus virginiana to 30' tall plus a few young Pinus virginiana.Dense, grassy-herbaceous vegetation plus young Sweetgum (Liquidambar styraciflua) up

Figure 3 .F
Figure 3. Morphological comparison of the Tapinoma pulchellum holotype queen A, C, E and a paratype male B, D, F in lateral A, B dorsal C, D and full-face E, F view.The type series was collected in two adjacent pitfall traps at Eno River State Park in North Carolina.Scale bars 0.5 mm (A, C-E); 0.25 mm (B, F).

Figure 4 .
Figure 4. Morphological comparison of the Tapinoma shattucki holotype queen A, E and a paratype male B, D, F in lateral A, B dorsal D and full-face E, F view.For the dorsal view of the T. shattucki queen C, a dealate paratype was photographed.The type series was collected in Stow, Massachusetts and belongs to a single nest series with the collection code SPC 7633.Scale bar: 0.5 mm (A-F).

Figure 5 .
Figure 5. Morphological comparison of a Tapinoma sessile queen A, C, E and a male B, D, F in lateral A, B, dorsal C, D, and full-face E, F view.Individuals were collected in the Black Forest in Colorado and belong to a single nest series with the collection code SPC 4118.Scale bar: 0.5 mm (A-F).
CASC California Academy of Sciences, San Francisco, CA, U.S.A. CRC C. Rabeling Collection, University of Hohenheim, Stuttgart, Germany LACM Los Angeles County Museum of Natural History, Los Angeles, CA, U.S.A. MCZC Museum of Comparative Zoology, Harvard University, Cambridge, MA, U.S.A. UCDC Bohart Museum of Entomology Collection, University of California at Davis, CA, U.S.A.