Tapinoma sessile

This species is extremely adaptable, nesting in a wide range of sites and being found in numerous habitats. Nests in open soil, under stones and logs or in dead wood, under loose bark, in cavities of plants or in plant galls, under leaves and rubbish piles and even bird nests. The site of the nest is changed often. Nests are populous with size varying with age (from 2,000-10,000 workers) and contain multiple queens. Brood is found in nests from April until September. Reproductives are found in nests from May until October, flights occur in June and July. This species forages singly from trails and are active during both day and night. They tend Homoptera and feed on dead insects or the juices of decaying fruits and vegetables. It is strongly attracted to sweet substances. It is a common house-infesting ant. (Mackay and Mackay 2002)

Identification
A small Dolichoderine that has only the slightest of petiolar nodes and an unusual arrangement of its gastral segments. Unlike other species in this subfamily the final segment of the gaster is directed downward. This leads to the anal pore opening being located ventrally (click on the image below to see details of this character) as opposed to opening at the distal end of the gaster.

Distribution based on Regional Taxon Lists
Nearctic Region: Canada, United States. Neotropical Region: Mexico.

Biology
Tapinoma sessile is one of the most widespread North American ant species. It is abundant in a wide range of habitats. It nests in the ground, under and within objects, in downed wood and will, in some cases, move into human structures. The small dark colored workers move about quickly during warm days but they will also forage during cooler temperatures that many other co-occurring ants avoid. When disturbed the highly excitable workers will swarm out of their nest and emit a distinctive pheromone.

Nests of this species can contain thousands of workers and many queens. Workers tend Homoptera and will opportunistically forage on a wide variety of items, being especially attracted to liquid exudates that are rich in carbohydrates. M.R. Smith (1965), reporting on eastern U.S. pest ants, provides this description of their biology: This common and widely distributed ant is one of our most adaptable species, occurring from sea level to 10,500 feet. It nests in a wide variety of habitats, ranging from sandy beaches, pastures, open fields, woodlands, and bogs, to houses. Most nests in the soil are beneath objects such as stones or logs, but this versatile species also nests under the bark of logs and stumps, in plant cavities, insect galls, refuse, piles, and bird and mammal nests. Nests in the soil are indefinite in form, shallow, and of little permanency. The colonies range in size from a few hundred individuals to many thousands, and contain numerous reproductive females. The individuals of the various colonies are not antagonistic to each other, but are hostile to the introduced Argentine ant. Mating takes place in the nest between males and their sister females, but nuptial flights have also been observed. Although females have been observed to establish colonies independently, it is also highly possible that the ants may form new colonies when one or more fertile females leave the parental colony accompanied by a number of workers. Workers are active and rapid, and normally travel in files. When alarmed, the workers dash around excitedly in an erratic manner, quite often with the posterior part of their abdomen elevated. Workers also emit from theip abdominal glands an odor which has been likened to that of rotten coconut. In Mississippi, male pupae have been noted from April 16 to 30, and males and winged females from May 1 to 15. In Bogs in southeastern Michigan, Kannowski has observed nuptial flights from Jime 26 to July 15. Few ants exceed sessile in their love for honeydew. Not only do workers eat honeydew avidly, but they assiduously attend such honeydew-excreting insects as plant lice, scale insects, mealybugs, and membracids. In some instances, workers have been observed transporting live plant lice. When mealybugs have been disturbed by collectors, the worker ants have tried to pick them up and carry them away. Workers visit the floral and extrafloral nectaries of plants in search of their glandular secretions. Like many species of ants, the workers feed on both dead and live insects.

T. sessile, one of our more important house infesting ants, is capable of invading houses from outdoors, or nesting inside. Although the ants feed on a wide variety of household foods, such as raw and cooked meats, cooked vegetables, dairy products, fruit juices, and pastries, they appear to show a preference for sweets.

Hamm (2010) - Smith’s descriptions were based on specimens from Illinois, from which he reported all colonies as polygynous, with colony size varying from 100 to 10,000 workers. All colonies Smith examined were house infesting, and he did not report collecting ants from natural habitats. There may be a trend within the Dolichoderinae, species that are normally monogynous in their native habitat become polygynous and greatly increase the number of workers when the colony becomes invasive or pestiferous (Giraud et al. 2002, Ingram 2002).

Foraging
VanWeelden et al. (2015) found that food was quickly and thoroughly distributed through a colony when food was located close to the nest. In polydomous colonies food was relatively well distributed within parts of the colony found close to the food source and was less efficiently spread to individuals in more distant nests.

Colongy Founding
Nests are founded by independently, by single queens, and dependently, via fission or budding (Kimball 2016b).

Colorado
Kimball (2016a) - Nests of T. sessile were observed at 9 of the 14 geographic locations. Over the observation periods, temperature ranged from 13.3 to 26.1° Celsius. Forty-four of the approximately 990 rocks examined (4.5%) were found to shelter T. sessile workers and brood. Of the 44 rocks under which T. sessile nests were found, 16 (36.4%) distributed over 8 geographic sites contained visible queens. Only 4 of these 16 nests (25%) were observed to be polygynous. The other 12 (75%) contained only a single visible queen, suggesting monogyny. Observed queen number for each of the four polygynous nests was 2, 2, 5, and 3. These were distributed over three separate sites, all located at higher elevations (2,146 to 2,896 meters). Two of the sites were montane, while one was subalpine. The data show a surprisingly low variation in queen number, when compared to earlier literature from other states (e.g., Smith, 1928; Buczkowski, 2010), as well as an apparent tendency towards monogyny.

Apparent plesiobiosis was also found with some frequency. It was observed in 7 of the 44 nests (15.9% of total observed nests), distributed over three separate sites. Three of these nesting associations were with Solenopsis molesta although these could have been instances of lestobiosis (covert thievery by neighboring ant colonies), given that S. molesta is a well-known brood thief (Holldobler and Wilson, 1990). Three other observed associations were with Lasius species and one was with a Myrmica species. In at least one of these cases (a compound nest between T. sessile and a Lasius sp.), workers of the two species intermingled freely and without aggression or avoidance. In fact, at first glance, they appeared to make up a single colony. Throughout the study, plesiobiosis was found only at elevations of 2057 meters and higher. However, the author has made several informal observations of plesiobiosis in T. sessile at significantly lower elevations during prior years.

One apparent behavioral association deserves special note. On June 1, 2014, at Comanche Reservoir, a rock was lifted and found to shelter a nest of an unidentified Lasius species. Within the exposed galleries, a single T. sessile dealate queen wandered slowly amongst the Lasius workers, eventually disappearing down a tunnel and deeper into the nest. Over this period of time, no aggression or avoidance towards the T. sessile queen was seen to take place. This situation is made stranger by the fact that the queen was alone, without workers of her own, potentially suggesting a parasitic (e.g., xenobiotic) relationship (Buschinger, 2009). Even if a T. sessile nest had been located beneath that of the Lasius, this would have implied an extraordinarily fused nesting arrangement between the two species. Multiple undescribed inquilines of T. sessile have been mentioned in the literature (Fisher and Cover, 2007; Ellison et ai., 2012). However, these have only been reported to parasitize T. sessile, not ants from other genera and subfamilies. Also, the queen observed here did not show the characteristically small size of at least one of these inquiline species (Ellison et al., 2012).

Seven of the T. sessile sampled nests were observed to contain alate reproductives (male or female), distributed over only three geographic locations. When considering colony life cycle, it is important to note that these particular sites were investigated from late-June to mid-July. However, no flight activity was observed.

The data suggest that both polygyny and monogyny are present in northcentral Colorado populations, apparently intermixed. Polygyny was more common at higher (i.e., montane and subalpine) elevations. This could be explained by higher mortality of solitary founding queens when faced with the colder and harsher conditions of higher altitudes (e.g., Heinze, 1992). Colder environments might thus favor young queens that remain in their natal nest rather than disperse, giving rise to polygyny. The study also shows a relatively high occurrence of plesiobiosis in local T. sessile, including associations with multiple other ant species. This could be an adaptation to scarce nesting sites (Czechowski, 2004), but it is difficult to speculate without further study. Finally, also included is a single observation that could indicate socially parasitic (e.g., xenobiotic) behavior in T. sessile. Intriguingly, despite several years of informal searching, the author has never seen T. sessile in Colorado urban or suburban areas and parks. This contrasts with earlier findings that present T. sessile as a successful and abundant pest in human modified habitats (e.g., Smith, 1928; Buczkowski, 2010).

Nomenclature

 *  sessile. Formica sessilis Say, 1836: 287 (w.q.) U.S.A. Emery, 1895c: 333 (q.m.); Wheeler, G.C. & Wheeler, J. 1951: 196 (l.); Crozier, 1970: 119 (k.); Taber & Cokendolpher, 1988: 95 (k.). Combination in Tapinoma: Smith, F. 1858b: 57. Senior synonym of parva, boreale Roger (and its junior synonym boreale Provancher): Mayr, 1886d: 434; Creighton, 1950a: 353; of gracilis: Emery, 1895c: 337; Wheeler, W.M. 1902f: 20; Creighton, 1950a: 353; of dimmocki: Shattuck, 1992c: 153. See also: Smith, M.R. 1928a: 307; Smith, D.R. 1979: 1422; Kupyanskaya, 1990: 156; Shattuck, 1994: 153; Hamm, 2010: 24.
 * boreale. Tapinoma boreale Roger, 1863a: 165 (w.q.) NORTH AMERICA. Subspecies of sessile: Dalla Torre, 1893: 164; Ruzsky, 1925b: 45; Ruzsky, 1936: 95. Senior synonym of boreale Provancher: Dalla Torre, 1893: 164. Junior synonym of sessile: Mayr, 1886d: 434; Creighton, 1950a: 353. [Note. Many publications refer to T. boreale Mayr, 1886d: 434, for example Creighton, 1950a: 342. This reference is to T. boreale Roger as redescribed by Mayr; there is no T. boreale Mayr.]
 * gracilis. Formica gracilis Buckley, 1866: 158 (w.q.) U.S.A. [Unresolved junior primary homonym of Formica gracilis Fabricius, 1804: 405 (now in Pseudomyrmex).] Junior synonym of sessile: Emery, 1895c: 337; Wheeler, W.M. 1902f: 20.
 * parva. Formica parva Buckley, 1866: 159 (w.) U.S.A. Junior synonym of sessile: Mayr, 1886d: 434.
 * boreale. Tapinoma boreale Provancher, 1887: 238 (w.q.) CANADA. Junior primary homonym and junior synonym of boreale Roger: Dalla Torre, 1893: 164.
 * dimmocki. Bothriomyrmex dimmocki Wheeler, W.M. 1915b: 417 (w.q.) U.S.A. Combination in Tapinoma: Emery, 1925e: 19. Junior synonym of sessile: Shattuck, 1992c: 153.

Type Material
Neotype. Worker, with the following measurements (see Table 2 for full character deÞnitions): HL, 0.68 mm; HW, 0.60 mm; SL, 0.60 mm; EL, 0.18 mm; MFC, 0.18 mm; EW, 0.14 mm; FL, 0.50 mm; LHL, 0.40 mm; PW, 0.42 mm; ES, 2.52 mm; SI, 88.2; and CI, 88.20. The neotype resides in the collection of the Museum of Comparative Zoology at Harvard University and bears the following labels: “USA Posey Co. New Harmony, IN 21-VI-09, 110 m 38.130_ N, 87.935_ W Coll by: C. A. Hamm” “Nest in soil next to house 5 m from grave of T. Say”

This concolored black specimen does not differ in any significant way from the escriptions of Say (1836) and Shattuck (1992, 1995). This specimen will carry a label designating it as the neotype. Additional material collected from this series has been deposited at the, MCZ, MSUC, and.

Taxonomic Notes
Hamm (2010) - After being originally described as Formica sessilis, in the Boston Journal of Natural History by Thomas Say in 1836, there has been little taxonomic work on T. sessile apart from its status as a pest. The original description is one paragraph in length and does not adequately describe a taxon that is highly variable in both size and color. The original species description was based on workers and queens from Indiana and many of the specimens from his collection have been lost, including the holotype (Say 1836; R. R. Snelling, personal communication).

In 1928, M. R. Smith revisited the biology of T. sessile. He provided a more complete description of T. sessile and provided measurement data and descriptions of color variation to demonstrate the level of phenotypic variability present in this species. The focus of Smith’s work was related to T. sessile’s ability to infest houses (hence its common name the odorous house ant); as such, his scope was limited. He did not mention any bicolored morphs (Smith 1928).

Description
Hamm (2010):

Worker
(n = 427). HL, 0.68 ±0.06 mm; HW,0.63 ± 0.08 mm; SL, 0.64 ± 0.07 mm; EL, 0.17 ± 0.02 mm; MFC, 0.22 ± 0.03 mm; EW, 0.14 ± 0.02 mm; FL, 0.53 ± 0.09 mm; LHL, 0.46 ± 0.08 mm; PW, 0.43 ± 0.05 mm; ES, 2.33 ± 0.47 mm; SI, 93 ± 3.58; and CI, 91.5 ± 4.39.

Queen
(n = 64). HL, 0.80 ± 0.05 mm; HW, 0.85 ± 0.05 mm; SL, 0.73 ± 0.06 mm; EL, 0.26 ± 0.02 mm; MFC, 0.28 ± 0.04 mm; EW, 0.21 ± 0.02 mm; FL, 0.72 ± 0.08 mm; LHL, 0.61 ± 0.08 mm; PW, 0.83 ± 0.06 mm; WL, 1.17 ± 0.09 mm; WGL, 3.52 ± 0.52 mm; ES, 5.54 ± 1.04 mm; SI, 91.8 ± 5.56; CI, 106 ± 3.18.

Male
(n =59). HL, 0.71 ± 0.05mm; HW, 0.73 ± 0.06 mm; SL, 0.69 ± 0.06 mm; EL, 0.29 ± 0.02 mm; MFC, 0.22 ± 0.02 mm; EW, 0.34 ± 0.02 mm; FL, 0.76 ± 0.07 mm; LHL, 0.65 ± 0.06 mm; PW, 0.75 ± 0.07 mm; MML, 1.09 ± 0.10 mm; WGL, 3.35 ± 0.31 mm; ES, 6.98 ± 1.04 mm; SI, 96.4 ± 4.81; and CI, 103 ± 2.62.

Etymology
Descriptive. sessile translates as "sitting" and this is presumably in reference to the gaster sitting directly on top of the petiole.