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.

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).

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.

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.

U.S.A., Utah, Sevier County, Alumbed Hollow, 8.4 miles west of I-70 (Exit 71) on Salina County Frontage Rd., a dirt road paralleling I-70. GPS: 38.910°N, 111.697°W; elevation 5980’ (1823 m). Small canyon running southwest to northeast with dense, heavily grazed Gambel Oak (Quercus gambelii) thickets to 25’ (8 m) tall on east-facing slope. Collected by SPC (SPC 7749), 16 July 2008. Collection Notes: SPC 7749. Site heavily grazed. Dense Gambel Oak thicket; forest floor protected from grazing by oak stem density. Superficial nest under rock in pale shade of dense Gambel Oak thicket. 2-cm thick oak litter present. Humusy sand soil. Very dry conditions. ~ 500 ants, multiple host queens present. Brood was mostly eggs and young larvae; just a few parasite and host worker pupae present.

Holotype queen (SPC 7749, MCZENT 00806456). Paratype male (SPC 7749, MCZENT 00806457), and the following additional paratypes: 13 queens, 17 males [16-VII-2008, SPC 7749]; 35 queens, 3 males [19-VII-2009, SPC 8077]; 19 queens, 4 males [19-VII-2013, SPC 8656]. Holotype and paratypes deposited in the MCZC. Additional paratypes deposited at CASC, CRC, LACM, and UCDC.

(i) SPC 8077. Same site description as above. Site not as heavily grazed as in 2008. Nest under dead oak branch half buried in oak litter in shade. Very dry conditions. ~ 1500 ants. Multiple host queens present. Eggs and young larvae present plus some host worker pupae. No inquiline pupae seen.

(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.

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.

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.

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).

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.

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).

U.S.A., Utah, Sevier County, Cove Mountain, 13.4 miles south of Glenwood Fish Hatchery on FSR 068. GPS: 38.649°N, 111.950°W; elevation 9350’ (2850 m). Enormous, grazed sagebrush (Artemisia tridentata) meadow around Big Lake. Superficial nest under rock and in adjacent grass clump in open on gentle south facing slope; fine silty sand. Small colony (~ 400 ants). No host queens. Brood, mostly eggs and larvae, plus a few parasite queen pupae. Collected by SPC (SPC 7816), 20 July 2008.

Holotype queen (SPC 7816, MCZENT 00806458). Paratype male (SPC 7816, MCZENT 00806458), and the following paratypes: 63 queens, 4 males [20-VII-2008, SPC 7816]. Holotype and paratypes deposited in the MCZC. Additional paratypes deposited at CASC, CRC, LACM, and UCDC.

(i) U.S.A., Colorado, El Paso County, Black Forest, 4.5 miles south of Hodgen Rd. on Meridian Rd. GPS (from Google Maps): 39.01°N, 104.61°W; elevation 7350’ (2240 m). Open Ponderosa Pine (Pinus ponderosa) forest, 20–40’ (6–12 m) tall with grassy understory and bearberry (Arctostaphylos sp.) in spots; medium sandy soil. Both colonies were under pine branches half buried in soil and litter. Collected by SPC (SPC 4117, 4120), 16 July 1994.

(ii) U.S.A., Colorado, Montrose County, 2.1 miles southeast of junction with Rt. 50 on P77 Road. GPS: 38.420°N, 107.628°W; elevation 8200’ (2500 m). Rich, mixed shrubby northeast facing slope with some sagebrush (Artemisia tridentata), 3–4’ (0.9–1.2 m) tall; fine sandy soil; sparse ground cover. Small colony consisting of two parasite females, ~ 30 host workers and no brood. Collected by SPC (SPC 7388), 16 July 2006.

(iii) One other collection from the type locality: SPC (SPC 8076), collected 18 July 2009, same collection data as type series, under rock in open, ~ 1000 ants, no host brood, a few inquiline pupae only.

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.

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).

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).

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.

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.

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 to 8’ tall. Sandy clay soil. The holotype queen was found in pitfall sample 13F 4,2. 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.

Holotype queen (MCZENT 00806459). Paratype male (MCZENT 00806460; same collecting locality as holotype). Holotype and paratype deposited in the MCZC.

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.

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.

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. 

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).

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 small, scale absent, overhung by first gastric tergite, not visible in dorsal view. In dorsal view, three gastric tergites visible; fourth tergite mostly hidden underneath the third. Body surface covered with micro-sculpture resembling a honeycomb, and short, appressed pubescence. Color medium to dark brown, appendages yellowish to reddish brown. Paratype queens (n = 8): HL 0.62–0.65, HW 0.53–0.59, SL 0.59–0.62, ML 0.85–0.94, CI 86–95, SI 100–111.

Paratype male: HL 0.53, HW 0.53, SL 0.50, ML 0.94, CI 100, SI 94. Medium sized males, head as wide as long (CI). Eyes large, maximum diameter little less than ½ of head width. Ommatidia clearly separated from each other; each ommatidium with a distinctly convex surface. Ocelli slightly elevated above the surface the head, but not forming a raised, triangular turret in side view. Anterior margin of clypeus straight, median impression lacking. Mandibles well developed with a single large apical tooth and ~ 14 denticles on cutting edge of mandible. Antennae with 13 segments, scapes surpassing the posterior corners of head by little less than ½ their length. Palp formula 5,4. Mesosoma with typical modifications related to wing bearing; wings well developed. In lateral view, propodeum forming a single even slope, lacking division into dorsal and posterior surfaces. Metapleural gland orifice pointing backwards, circular in posterior view. Petiole small, scale absent, overhung by first gastric tergite; not visible in dorsal view. In dorsal view, six gastric tergites visible. Integument thin, specimens shrivel when dried. Body surface densely covered with short, appressed pubescence, except for antennal flagellum, which is covered with dense, short, suberect pubescence. Color pale to medium brown, appendages yellowish brown. Paratype males (n = 4): HL 0.53–0.56, HW 0.53, SL 0.50–0.53, ML 0.88–0.94, CI 95–100, SI 94–100.

This species is named to honor our friend Steven O. Shattuck for his pioneering work on the ant genera of the Dolichoderinae, his invaluable contributions to the systematics of Australian ants, his important efforts in establishing the universal online ant research tool AntWiki.org, and for his and his wife Kathy’s invaluable and deeply appreciated help in reorganizing the MCZ ant collection. The species epithet shattucki is used as a Latin noun in the genitive case.

U.S.A., Massachusetts, Middlesex County, Stow, 47 Marlboro Road. GPS: 42.399°N, 71.524°W; elevation 230’ (70 m). Large garden adjacent to Red Maple (Acer rubrum) forest and reforesting wetland. Small colony in flower pot in partial shade at garden edge. Specimens preserved from 26 August to 04 September as pupae eclosed and adults matured. Collected by SPC (SPC 7633), 26 August 2007.

Holotype queen (SPC 7633, MCZENT 00806461). Paratype male (SPC 7633, MCZENT 00806462), and the following additional paratypes: 39 queens, 10 males [26-VIII-2007, SPC 7633]. Holotype and paratypes deposited in the MCZC. Additional paratypes deposited at CASC, CRC, LACM, and UCDC. In addition, there are 26 T. sessile host workers, five host males, and two host worker-queen intermorphs from the same colony, which were also deposited in the MCZC.

Wheeler’s syntype series of “Bothriomyrmex dimmocki” (one winged and four dealate females) comprises the only other known specimens of T. shattucki. The collecting locality of Wheeler’s type series is U.S.A., Massachusetts, Hampden County, Mount Tom, north of Springfield. Collected by George Dimmock, 27 August 1897. The specimens, including the lectotype worker of “Bothriomyrmex dimmocki” are deposited in the MCZC (MCZENT 00021289, MCZENT 00035244).

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.