Tapinoma incognitum 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 incognitum Cover & Rabeling sp. nov.

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.

Type locality.

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.

Type material.

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.

Additional material.

(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.

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 .

Figures

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).