Ponera pennsylvanica

A common member of most ant communities in deciduous forests of the eastern United States.

Identification
Taylor (1967) - A member of the coarctata species group, endemic to eastern North America. Distinguished from other species of the genus, including Ponera coarctata, by the characters listed on the latter species page. The larger size, presence of a distinct dorsal mesometanotal suture in the worker, and the undifferentiated antennal club in the female castes, distinguish pennsylvanica from Ponera exotica M. R. Smith, the only other known New World species of Ponera. Exotica is evidently not closely related to pennsylvanica, and is likely introduced from the Indo-Australian area, though it is not yet known from there.

Distribution
In Florida this species is known from north Florida south into Marion Co. The general distribution is from southern Nova Scotia south into northern Florida, west into eastern Minnesota and south into northeastern Texas (Taylor 1967). West of this area there are a few scattered records from North and South Dakota, Colorado and New Mexico (Taylor 1967), with an outlier in Michoacan in west-central Mexico (Mackay 1991). Like Ponera exotica, P. pennsylvanica presents a biogeographic riddle. It is unlikely to be a boreal lineage, even though its closest known relative occurs in Europe. A reasonable hypothesis is that a lineage moved north from the Indo-Australian region and expanded around the world in the band of Arctotertiaiy forest, which has vestiges in Europe and a better representation in eastern North America and the temperate Far East. Perhaps related species will turn up in China or Japan to support this hypothesis.

Distribution based on Regional Taxon Lists
Afrotropical Region: Uganda. Nearctic Region: United States. Neotropical Region: Mexico.

Biology
Taylor (1967) - P. pennsylvanica is characteristically found nesting in deciduous forests of various types, although it may penetrate more open habitats or coniferous forests. In the Chicago region Talbot (1934), and Gregg (1944) have reported it from stands of Black, Red, or White Oak, and mixed oak forest on sandy soil and from mixed oak forest on clay. The greatest number of colonies were found in such situations, but the species was often taken at woods margins, in fields, or along roadsides. Dennis (1938) found pennsylvanica widespread and common in Tennessee, in oak-pine and oak-chestnut forests over a wide elevational range (90 to 1515 m). It was found to be the most abundant of 15 species nesting in locust woods in Seneca County, Ohio, by Headley (1952). This author sampled the ants nesting in 15 square-meter plots during August 1958, and in 12 adjacent, similar sized plots in June, July and September 1949. In all 299 ant colonies were taken in the 27 square meters thus studied, with P. pennsylvanica by far the most common species, with 109 (36 %) of the colonies collected. The next most abundant species (Stenamma brevicorne Mayr) had only 35 nests (12% of the total). P. pennsylvanica has been listed as one of the most common ants in North Carolina by Carter (1962b). He found it especially abundant on the Piedmont Plateau in various forest associations, most notably in upland mature pine and oakhickory climax stands; the elevational range being 600 to 1030 m. P. pennsylvanica was stated by Carter to be "predictable" in coastal forest, but scarcer than on the Plateau. All of his records, except one, were from well shaded mesic forests. The extreme northwest record from Saganaga Lake, Minnesota, was made in coniferous forest (Gregg 1946).

Nests in forest areas are usually constructed in and under rotting logs, or in rotting stumps, at all but the earliest stages of decay (Talbot 1934); in small fragments of wood, acorns or other such objects; or in soil or leafmold. In drier habitats, especially non-forested areas, the colonies are usually found in soil under stones or other material, and at times the species may be common under stones in pastures (Gregg 1944). Workers forage in or on the surface of the ground, and may be readily collected using the Berlese funnel. Headley (1952) has given details concerning nest depth and proportions, and the size and proportion of colonies. He found 217 workers in 86 nests, undoubtedly a low count. During his August 1948, Ohio, survey he found no eggs and few larvae in 86 colonies. Pupae were, however, fairly abundant, with a mean of 11.2 per nest. The largest colony contained 36 pupae of all castes, one included 15 workers and 18 pupae, 10 of which were alates. The species apparently does not overwinter any brood (Talbot 1957).

Twenty-two sexual imagos were present in 4 of Headley's August colonies, but no flights were observed by him. Mating flight activity may extend from mid-August to early October, according to dates on the specimens I have seen, and those published by Headley (1943), Gregg (1944), and Kannowski (1959). Colony foundation is independent and non-claustral (Kannowski loco Cit.), and dealate females may be taken in rotting wood, soil, moss, or curled leaves. Some very incomplete mating flight observations are given by Haskins and Enzmann (1938).

Ecology
Taylor (1967) - The range of pennsylvanica corresponds remarkably well with the area carrying the Eastern Deciduous Forest Formation. The western and southern extensions of which are indicated on the map (see also Braun 1950); the western limit of the forest formation proper is shown as a dotted line; that of the forest-prairie transition belt as a solid line. Tongues of transition forest extend westwards into prairie grassland carrying with them forest pockets in which pennsylvanica occurs. The area of forest or transition country from which pennsylvanica has been recorded is shaded on the map; apparently this is the main range of the species. The few records known from outside the forest zones are indicated as dots.

The records appear to indicate a northwestern limit at about the latitude of northern Iowa. However, there are more northerly records from Saganaga Lake in extreme NE Minnesota (Gregg 1946), and from the Porcupine Mts in the upper peninsula of Michigan. These must be close to the limits of the range in this area.

Further east, in the southern Great Lakes area and New England, pennsylvanica seems fairly common at latitudes where it is reportedly rare in the west. The northernmost known records are from the vicinity of Quebec. The Digby, Nova Scotia, record may indicate present distribution at least as far as the "neck" of that province, however, the species is unknown from northern Maine and New Brunswick. P. pennsylvanica may be taken at low elevations in northern New England, but it is less common there than in the southern part of the area. Sturtevant (1931) noted that it was much less common on Cape Cod than near New York City.

The northern distributional limits are almost certainly set by deteriorating climatic conditions, and in fact correspond fairly well with the 40° Fahrenheit (4SC) mean annual temperature isotherm (see Vischer 1954: map 3). pennsylvanica apparently declines in abundance northwards, from a line running approximately through Milwaukee and Ottawa, on the map projection used in fig. 22. This corresponds almost exactly with the 45° Fahrenheit (7.4°C) mean annual isotherm as shown by Vischer. Ignoring the two extreme northwestern records, the main northern limit of pennsylvanica trends southwards as it moves inland towards the central plains. This trend is perhaps correlated with the increase in severity of climate at a given latitude moving westwards across the Great Lakes area.

The western limit of the main distributional range is clearly marked. All records available from South Dakota, Nebraska, Kansas, and Oklahoma, being towards their eastern areas; with the extreme western ones falling almost exactly in a north-south line. Details of the distribution in these States and in Iowa are interesting, for most records fall within the area of the Eastern Deciduous Forest Formation or its prairie-transition belt. The Iowa records of Smith and Buren are significant in this connection. This state is occupied mainly by country floristically part of the midwestern grassland-prairie association. Two areas are within the range of the forest-prairie transition belt: (1) the valleys of the Mississippi and Des Moines Rivers and (2) the drainage system of the Missouri River, which forms a long tongue extending northwards from Missouri, along the western Iowa border. All of the Iowa records listed above fall within the transitional forest zones, and not in the prairie zones. The single South Dakota record is also from transition country, and was made in the relatively small extension of the Missouri River valley "transition forest tongue in the extreme south east of the state. The Nebraska records, too, both lie within the limits of this transition forest extension, at the extreme east of the state. The only known Kansas records are from Riley and Dickinson Counties, which lie outside the broad limits of the forest transition belt; unfortunately no ecological details are available.

All Oklahoma records are from the east of the state, in transition country. Professor W. G. Carter, the collector, has provided more precise ecological details than are usually available. The Payne County specimens were taken in mesic forests of oaks and other hardwoods, growing in stream floodplains and deep ravines. The area in general is characterized by large tracts of tall or mixed grassland and there are also xeric scrub oak forests in certain highland areas. McCurtain County records were also from similar mesic ravine forest in primarily grassland areas. Professor Carter (in litt.) recently found pennsylvanica common in eastern Oklahoma.

South of Oklahoma the known distributional limit has a strong easterly trend. Records from this area are scarce, but, it is notable that the 50% line of soil moisture saturation, as mapped by Vischer (1954: map 954, p. 359), follows about the same path. Considering that soil moisture values depend on interaction of numerous ecological parameters this may be significant, especially as pennsylvanica is soil inhabiting.

In Florida pennsylvanica is known only from Gainsville, in the north. It was not taken by Van Pelt (1956) in his careful survey of Welaka Reserve, slightly further south. The apparent southern limit indicated by these data almost exactly coincides with the limit of deciduous forest, and lies across an area where "one after another of the deciduous (forest) species reaches the southern limits of its range" (Braun 1950) exact limiting factors acting here on pennsylvanica are not clear, but considering its decline southward in Mississippi (M. R. Smith 1956) and possibly in other Gulf Coast states and Georgia (teste E. O. Wilson), climatic factors are probably involved. The possibility of competitive exclusion by other ant species should be considered.

In general the marginal limiting factors for P. pennsylvanica appear to be "density independent" ones-climatic deterioration in the north, decreasing humidity in the west and southwest, and high temperature (and humidity?) in the south and southeast. These factors seem to be effective even in areas where suitable forest occurs, such as the northwest and southwest of its range.

Although the main limits of P. pennsylvanica are approximately co-extensive with those of the Eastern Deciduous Forests, the ant ranges far to the west, where it is obviously very scarce. As Gregg (1963) has noted, it might be expected to occur sporadically in river valley forests throughout the central plains area.

Nomenclature

 *  pennsylvanica. Ponera pennsylvanica Buckley, 1866: 171 (w.) U.S.A. Emery, 1895c: 267 (q.m.); Wheeler, G.C. & Wheeler, J. 1952c: 631 (l.). Subspecies of coarctata: Emery, 1895c: 267; Dennis, 1938: 277; Creighton, 1950a: 48. Revived status as species: Taylor, 1967a: 29.

Type Material


No types are known to exist. This species is unlikely to be confused with any other, and usage of the name has been unambiguous; I have not, therefore, designated a neotype. (Taylor 1967)

Worker
Smith (1936) - Length: 3-3.75 mm. Head, excluding the mandibles, very much longer than broad, sides moderately convex, posterior border faintly but definitely excised in the middle. Eyes very small, not convex, composed of approximately 6 ommatidia, placed toward the front of the head about one-fourth the distance from the base of the mandibles. Ocelli absent. Clypeus convex medianally. No distinct frontal area. Frontal furrow extending approximately half the distance from the frontal carinae to the posterior border of the head. Mandibles with two or three prominent teeth anteriorly, and many small denticulae posteriorly. Scape robust, not attaining the posterior border of the head; funiculus clavate, the terminal segment almost, or about as long as the three preceding segments together. Thorax laterally compressed, moderately convex dorsally, wider anteriorly than posteriorly; pro-mesonotal and meso-epinotal sutures distinct. Epinotal declivity faintly concave with weakly marginate sides, meeting the base in a well rounded obtuse angle. Petiole thick antero-posteriorly, scarcely narrowed dorsally, convex anteriorly, concave posteriorly, beneath with a prominent rectangular tooth, the front of which bears a spiracular opening, the posterior of which terminates in a rather sharp tooth. Tibiae of legs each with a well developed pectinated spur.

Mandibles smooth and shining, with sparse, scattered punctures. Head and thorax subopaque, covered with dense and coarse, but not confluent, punctures; petiole and gaster more shining, finely and densely punctate or shagreened.

Entire body covered with rather dense, yellowish gray, appressed pubescence. Erect hairs on the head, thorax, petiole and gaster; most numerous on the latter. Hairs longest on the anterior and posterior ends of the body.

Typical specimens black; mandibles, clypeus, frontal carinae, antennae, legs, and tip of gaster reddish or yellowish. Occasionally individuals are reddish brown with varying amounts of infuscation.

Queen
Smith (1936) - Length: 4 mm. Resembling the worker in general appearance but larger and more robust. Head with large compound eyes which are placed slightly less than their greatest diameter from the base of the mandibles. Vertex with three ocelli in the form of a triangle. Petiole smaller, and more slender (anterio-posterially) than with the worker. Wings hyaline, ciliated, with yellowish veins and darker stigma; with one submarginal and a discoidal cell.

Taylor (1967) - 17 queens from all parts of the distributional range have the following measurements: HL 0.37-0.41 mm; HW 0.30-0.35 mm; SL 0.24-0.29 mm; CI 82-86; SI 80-84; PW 0.27-0.31 mm; PNL 0.12-0.15 mm; PH 0.28-0.31 mm; DPW 0.22-0.25 mm; PNI 76-80; maximum diameter of compound eye 0.09-0.11 mm; ocular index 27-31. Palpal formula (2 specimens dissected): Maxillary 2: Labial 2, as in the worker. Wing venation as in P. coarctata. Differing from the workers in the usual characters of full sexuality. Similar in general to P. coarctata, but larger in size. There is no overlap in the ranges for PW, PH, DPW, SI and PNI in queens of these 2 species. The petiolar node is distinctly broader in pennsylvanica (compare the PNI values), and thinner when viewed from above (cf. figs. 3 and 4). The sculpturation is much more heavily developed than in queens of coarctata; the diagnostically important sculptural characters being: mandibles, clypeus, frons, and scapes as in worker. Entire mesosoma opaque, with a close cover of punctures about 0.01 mm in diameter. Node and gaster feebly shining, with a finer more scattered puncturation than mesosoma-the punctae of the disc of the first gastric segment, for example, about 0.007 mm in diameter, separated by distances about equal to 1.5 to 2.5 X their maximum diameter. Color as in worker.

Male
Taylor (1967) - HL 0.60-0.63 mm; HW (across eyes) 0.64-0.70 mm; CI 107-113; WL 1.04-1.22 mm; PNL 0.20-0.23 mm; PH. 0.35-0.41 mm; DPW 0.28-0.31 mm; maximum diameter of eye 0.27-0.32 mm; ocular index 43-46; palpal formula (6 specimens dissected): Maxillary 5: Labial 3, as in P. coarctata. Wing venation as in queen. Color medium to rather dark brown; eyes black; clypeus, mandibles, legs and gastric apex lightly infuscated; wing veins yellowish.

Immature Forms
Taylor (1967) - The larva has 4 pairs of dorsal abdominal "mushroom" tubercles. A detailed description with figures has been published by G. C. & J. Wheeler (1952). The pupae of all castes are enclosed in cocoons.