Tapinoma shattucki Cover & Rabeling, 2024

Cover, Stefan P. & Rabeling, Christian, 2024, Four new inquiline social parasite species in the dolichoderine ant genus Tapinoma (Hymenoptera, Formicidae), ZooKeys 1202, pp. 111-134 : 111-134

Tapinoma shattucki Cover & Rabeling sp. nov.

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 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.

Etymology.

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.

Type locality.

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.

Type material.

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.

Additional material.

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).

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.

Figures

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).