Prenolepis imparis

Prenolepis imparis, the winter ant, builds deep underground nests. Colonies will seal themselves in their nests during the warmest months of the year and maintain themselves for months in the deepest, coolest chambers. In many parts of its range Prenolepis imparis actively forages during times of the year when the temperature is much cooler than other ant species will tolerate. This helps avoid competition with most co-occuring ant species. Repletes are present in the colony and store fat reserves in their distended gasters.

Identification
Williams and LaPolla (2016) - Obtusely angled propodeum with flat dorsal and posterior faces; entire cuticle smooth and shiny; ectal surface of mandibles with deep longitudinal striations.

This species bears a strong morphological resemblance to Prenolepis nitens but has a different distribution and a more slender mesosoma at the mesonotal constriction. Several key differences can also be seen in male genitalia between P. imparis and P. nitens (see notes under P. nitens).

This species has a wide distribution and shows high morphological variation in the worker caste. As a consequence, many synonyms have been created with descriptions based on variation in color, size, and minor differences in propodeum shape.

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

Biology
Williams and LaPolla (2016) - In P. imparis, colonies are polygynous, but contain relatively small numbers of workers (typically a few thousand workers in a colony) (Wheeler & Wheeler 1986; Tschinkel 1987). The nests of P. imparis are exceedingly deep with no chambers found shallower than 60 centimeters below the ground surface and going down as far as 3.6 m (Tschinkel 1987). False honey ants are peculiar in that the workers are cold-tolerant and forage during the cooler months when most other ant species are inactive. The colony enters an estivation period and becomes inactive above ground for the warmer months, during which time eggs are laid and brood are reared. Reproductives overwinter and emerge on the first warm day of spring for their nuptial flight. One characteristic of P. imparis colonies is the presence of workers with greatly extended gasters. Corpulents were once believed to be true repletes like those of genus Myrmecocystus (Wheeler 1930a; Talbot 1943) until Tschinkel (1987) determined that their enlarged state is actually caused by hypertrophied fat bodies and not the result of crop distention from retained liquid food. Prenolepis imparis is a generalist omnivore (Wheeler 1930a).

Wheeler (1930) - The ant is very timid and retiring and perhaps to some extent crepuscular or even nocturnal. At least it seems to avoid the open sunlight and to be most active in the shade and on cool or cloudy days. I have found the young dealated queens, which closely resemble those of Lasius niger, founding independent incipient colonies in small chambers in the soil.

The food of P. imparis consists of liquids, especially the “honey-dew” of aphids and coccids, the nectar of flowers and extrafloral nectaries, the exudates of living oak-galls, the juices of dead earthworms and those derived from the young tissues of plants. These various liquids are imbibed in such quantities by the foraging workers that their gasters become greatly distended. (Fig. 3) and make their gait very unsteady. In this condition they are really “repletes” like those of the true honey-ants of the Southwestern States and Mexico (Myrmecocystus melliger and mexicanus). The feeding of imparis (var. californica) on young plant-tissues has been recently called to my attention by Mr. H. M. Armitage and indicates that these small timid ants may become of some economic importance. He writes me that in Los Angeles County, Cal., they “were observed feeding extensively not only on the calyx and unopened petals of fruit buds of oranges, but also on the tender new growth. This condition was credited to the fact that the ants become active before the normal citrus bloom and at a time when little natural feeding was available.” Mr. A. C. Burrill has recently made very interesting observations on the feeding habits of imparis and its singular resistance to cold in the Arnold Arboretum at Forest Hills, Mass., and in Missouri, and permits me to quote some of his unpublished notes:

“'I was led to make continuous observations on P. imparis after casually noticing that it appeared at the surface of the soil later in the fall and earlier in the spring than any of our other ants. During the rather warm winter of 1927-28 I selected for daily observation a nest with two entrances in the Arnold Arboretum. The workers were observed to come out onto the surface of the soil nearly every week throughout the winter. The lowest temperatures recorded when the ant came out were on February 6, 1928, when the temperature in the early morning was 6° F. (26° below freezing). At noon, less than eight hours later, a worker appeared at the surface though the soil was still frozen. On February 25 a worker passed from one entrance to the other with a sharp wind blowing at 27° F., as recorded by a tested thermometer at one inch above the ground. The sun, however, induced a thaw, so that the temperature of the ground was really 33.5° F. Tests with honey enticed more of the workers out of doors till the surface cold had fallen to 30° F, when the surface of the soil was frozen and the ants brushed against hoarfrost around their entryway. Honey does not freeze at such a temperature and ants can still lap it up slowly.

“On cool, humid middays below 60° F. workers may remain active above ground all day, but seldom stay in the bright sun or on dry soil. They are at their best during or after cold rains, or a cold, humid period with overcast skies. They were most active outdoors between 35° and 55° F. all winter, but they occasionally crawled over the soil below 30° F. or above 60° F. One case of great activity occurred when a colony moved to a new site during a drizzle and kept excavating the new nest down into the subsoil with air-temperature about 60° F.

“For about five months these ants lived on dead or dying earthworms driven above ground by rains. If other food was available besides honey-dew or honey that had been stored in the crops of the repletes during the fall, I failed to discover it. I often saw imparis leave an earthworm that had been dried up by the sun and return to it as soon as another rain had soaked it up again.

“'Similar observations were made in regard to the winter activities of imparis at Jefferson City, Mo., during the winter of 1928-29. The ants were nesting in trodden, clayey, poorly drained loess soil. The workers were excavating December 14, 15 and 16, which were followed by a heavy freeze-up.”

Mr. Burrill's observations show that the ants do very little excavating during the winter but that this sets in suddenly a few days before the nuptial flight. This, as I have frequently noticed, takes place as early as the latter fortnight of March or the first fortnight of April. That the period must be much the same for the different varieties and over a considerable portion of the United States is indicated by the following dates of nuptial flights recorded in my notebooks, on the labels of mounted males and winged females or in the literature:

P. imparis (typical). Forest Hi11s, Mass., March 15 and 25; April 4, 5 and 6; Bridgetown, N.J., March 28; Albany, N.Y., March; Wauwatosa, Wis., April 20; Jefferson City, Mo., March 19, 20 and 21; Lawrence and Riley Co., Kansas, March, April 2.

var. testacea. Bronxville, N.Y., April 10; Plummer's Island, Maryland, Aprill2.

var. minuta. District of Columbia, April; Cape May, N.Y., March 24.

var. californica. Stanford University, Cal., Jan. 30, Feb. 23, and March 19.

The winged sexes that participate in the nuptial flights mature during the late summer of the previous year and are retained in the nests over winter. I have found males and winged females of the var. testacea at Lakehurst, N. J. in the nests on September 24. This retention of the sexual phases over winter occurs in a few of our other northern ants, e. g. in the various varieties of Camponotus herculeanus and of C. caryce, but in these cases the flight is later, usually in May or June, or even in July. The very early flight of imparis was first observed in Indiana by Say., who says: “They appeared in great numbers on the 2nd of April; the males swarmed around small bushes, alighting on the branches and leaves. The females were few.” I have seen such flights of the males about the Japanese barberry bushes in the Arnold Arboretum on fine days in late March and early April. The males dance up and down in rather compact swarms like the late summer swarms of male bees of the genus Chloralictus.

Both Burrill’s observations on its activity during the winter months and those on its very early nuptial flight indicate that P. imparis may be regarded as negatively thermotropic or adapted to cold.

Replete Workers
One characteristic of Prenopepis imparis colonies is the presence of workers with greatly extended gasters. These enlarged workers were once believed to be true repletes like those found in Myrmecocystus (Wheeler 1930a; Talbot 1943). However, Tschinkel (1987) determined that their enlarged state is actually caused by hypertrophied fat bodies and not the result of crop distention from retained liquid food.

Castes
Williams and LaPolla (2016) - Microgynous queens were observed by Alex Wild (personal communication, September 9, 2015) in the Sierra Nevadas, California in October, 2001. Two specimens were collected by Wild, one of which was examined for this study (USNMENT00755072). The other can be found on www.AntWeb.org (CASENT0005430). These queens appear to be P. imparis, but are much smaller and were observed flying in the fall rather than early spring. While it is possible that this is an entirely new species, perhaps even an inquiline, based on its size, it is also possible that these individuals are microgynous (Rüppell & Heinze 1999) P. imparis queens. With only two specimens available and little morphological difference from P. imparis observed more evidence must be found in order to describe them as a new species.

Nomenclature

 *  imparis. Formica imparis Say, 1836: 287 (q.m.) U.S.A.
 * Wheeler, W.M. 1930b: 16 (w.); Wheeler, G.C. & Wheeler, J. 1953c: 142 (l.); Hauschteck, 1962: 219 (k.).
 * Combination in Lasius: Roger, 1863b: 12.
 * Combination in Prenolepis: Mayr, 1886d: 431.
 * Status as species: Mayr, 1863: 416. Roger, 1863b: 12; Emery, 1893i: 635; Dalla Torre, 1893: 178; Ruzsky, 1905b: 262; Wheeler, W.M. 1916m: 591; Wheeler, W.M. 1917a: 523; Wheeler, W.M. 1930b: 15 (redescription); Creighton, 1950a: 413; Smith, D.R. 1979: 1445; Bolton, 1995b: 364; Mackay & Mackay, 2002: 409; Coovert, 2005: 118; Ellison, et al. 2012: 220.
 * Senior synonym of americana: Emery, 1893i: 635; Dalla Torre, 1893: 178.
 * Senior synonym of wichita: Dalla Torre, 1893: 178; Wheeler, W.M. 1930b: 15.
 * Senior synonym of minuta, pumila, testacea: Creighton, 1950a: 414.
 * Senior synonym of californica: Wheeler, G.C. & Wheeler, J. 1986g: 14.
 * Senior synonym of arizonica, colimana, coloradensis, veracruzensis: Williams & LaPolla, 2016: 223.
 * arizonica. Prenolepis imparis var. arizonica Wheeler, W.M. 1930b: 22 (w.q.m.) U.S.A.
 * Subspecies of imparis: Creighton, 1950a: 414; Smith, D.R. 1979: 1444; Bolton, 1995b: 363; Mackay & Mackay, 2002: 410.
 * Junior synonym of imparis: Williams & LaPolla, 2016: 223.
 * colimana. Prenolepis imparis var. colimana Wheeler, W.M. 1930b: 24 (w.) MEXICO.
 * Subspecies of imparis: Bolton, 1995b: 363.
 * Junior synonym of imparis: Williams & LaPolla, 2016: 223.
 * coloradensis. Prenolepis imparis var. coloradensis Wheeler, W.M. 1930b: 22, fig. 4 (w.) U.S.A.
 * Subspecies of imparis: Creighton, 1950a: 415; Smith, D.R. 1979: 1445; Bolton, 1995b: 363; Mackay & Mackay, 2002: 410.
 * Junior synonym of imparis: Williams & LaPolla, 2016: 223.
 * veracruzensis. Prenolepis imparis var. veracruzensis Wheeler, W.M. 1930b: 24 (w.) MEXICO.
 * Subspecies of imparis: Bolton, 1995b: 364.
 * Junior synonym of imparis: Williams & LaPolla, 2016: 223.
 * wichita. Formica (Tapinoma) wichita Buckley, 1866: 169 (w.) U.S.A.
 * Combination in Prenolepis: Mayr, 1886d: 431.
 * Junior synonym of nitens: Mayr, 1886d: 431.
 * Junior synonym of imparis: Dalla Torre, 1893: 178; Wheeler, W.M. 1930b: 15.
 * americana. Prenolepis nitens var. americana Forel, 1891b: 94, pl. 3, fig. 4 (m.) U.S.A.
 * Junior synonym of imparis: Emery, 1893i: 635; Dalla Torre, 1893: 178.
 * minuta. Prenolepis imparis var. minuta Emery, 1893i: 636 (w.m.) U.S.A.
 * Wheeler, W.M. 1930b: 21 (q.).
 * Subspecies of imparis: Wheeler, W.M. 1906b: 11; Wheeler, W.M. 1916m: 591; Wheeler, W.M. 1930b: 21 (redescription).
 * Junior synonym of imparis: Creighton, 1950a: 414.
 * testacea. Prenolepis imparis var. testacea Emery, 1893i: 636 (q.m.) U.S.A.
 * Wheeler, W.M. 1905f: 390 (w.).
 * Subspecies of imparis: Wheeler, W.M. 1904e: 304; Wheeler, W.M. 1930b: 20 (redescription).
 * Junior synonym of imparis: Creighton, 1950a: 414.
 * californica. Prenolepis imparis var. californica Wheeler, W.M. 1930b: 23, fig. 4 (w.q.m.) U.S.A.
 * Subspecies of imparis: Creighton, 1950a: 414; Smith, D.R. 1979: 1445.
 * Junior synonym of imparis: Wheeler, G.C. & Wheeler, J. 1986g: 14.
 * pumila. Prenolepis imparis var. pumila Wheeler, W.M. 1930b: 21 (w.m.) U.S.A.
 * Junior synonym of imparis: Creighton, 1950a: 414.

Type Material
Williams and LaPolla (2016) - Syntype queen(s?) and male(s?), (no specific locality provided) (depository unknown) [not examined].

Worker
Williams and LaPolla (2016) - (n=105): CMC: 16–18; EL: 0.23–0.33; EW: 0.19–0.24; HL: 0.74–0.98; HLA: 0.43–0.48; HLP: 0.25–0.34; HW: 0.71–0.98; IOD: 0.48–0.57; LF1: 0.22–0.29; LF2: 0.11–0.15; LHT: 1.01– 1.15; MMC: 1–5; MTW: 0.40–0.52; MW: 0.24–0.36; PDH: 0.28–0.39; PMC: 3–6; PrCL: 0.41–0.52; PrCW: 0.26– 0.33; PrFL: 0.78–0.98; PrFW: 0.17–0.21; PTH: 0.37–0.42; PTL: 0.33–0.40; PTW: 0.24–0.34; PW: 0.48–0.61; SL: 0.88–1.22; TL: 2.94–4.51; WF1: 0.06–0.09; WF2: 0.06–0.08; WL: 0.95–1.42; BLI: 122–153; CI: 90–101; EPI: 140–179; FLI: 185–208; HTI: 120–133; PetHI: 121–130; PetWI: 83–90; PrCI: 58–70; PrFI: 20–24; REL: 27–35; REL2: 28–36; REL3: 46–59; SI: 115–140.

Light to dark brown with head and gaster sometimes darker than mesosoma; entire cuticle smooth and shiny; abundant decumbent setae on scapes and legs; erect macrosetae on head, mesosoma, and gaster; head about as broad as long and square in shape with rounded posterolateral corners and a straight posterior margin; compound eyes moderately large and convex, but do not surpass the lateral margins of the head in full-face view; torulae overlap with the posterior border of the clypeus; anterior border of clypeus with a pair of prominent anterolateral lobes; mandibles with 5–7 teeth (usually 6) on the masticatory margin; ectal surface of mandibles with deep longitudinal striations; in profile view, propodeum is obtusely angled with a flat dorsal face; dorsal apex of petiole scale is sharply angled and forward-inclined.

Wheeler (1930) - Length 3-4 mm.

Head as broad as long, slightly narrower in front than behind, with straight posterior border and feebly rounded sides and posterior corners. Eyes moderately convex, nearly one-fourth as long as the sides of the head, and situated a little behind its middle. Mandibles rather flat, with •convex external borders, their apical borders feebly oblique, 6-toothed, the apical and basal tooth largest, the former strongly curved, the third tooth from the tip minute. Maxillary palpi very long, reaching to the occipital foramen. Clypeus convex in the middle, subcarinate behind, depressed on the sides, its anterior border broadly rounded and entire. Frontal area large but indistinctly defined; frontal groove absent; frontal carinae feeble, short and subparallel. Antennae slender; scapes extending about two-fifths their length beyond the posterior corners of the head; first funicular joint as long as the subequal second and third together; joints 2-8 nearly twice as long as broad, remaining joints somewhat shorter, except the last, which is as long as the two penutimate joints. Thorax small and slender, divided into two portions by a deep constriction of the posterior part of the mesonotum; the promesonotum somewhat longer than broad, evenly convex and subhemiopherical above, the dorsal outline not interrupted at the promesonotal suture, the mesonotum anteriorly as long as broad, subtrapezoidal, a little broader in front than behind, sloping downward and passing posteriorly into the constricted portion which bears on its dorsal surface the pair of somewhat projecting metathoracic spiracles close to the base of the epinotum. The latter is subrectangular from above and nearly as broad as long, in profile with subequal base and declivity, the former slightly convex and rising posteriorly where it has a faint longitudinal impression and forms a distinct but rounded angle with the declivity. This is broad and flat, with its projecting spiracles near the middle of its sides. Petiole as long as high, its node strongly inclined forward, in profile compressed and cuneate, with feebly convex anterior and posterior surfaces and rather acute border;• seen from behind it is trapezoidal, broader above than below, with straight sides and a transverse superior border, which is straight or very slightly concave in the middle. Gaster proportionally large, broad anteriorly, tapering behind to a point, convex above, the basal segment concave anteriorly where it overlies the petiole, its anterior border above straight and transverse in the middle and forming a distinct angle on each side. Legs rather slender.

Very smooth and shining; mandibles glossy, very finely longitudinally striate; head and thorax with small, sparse, piligerous punctures; gaster very finely, superficially and tranversely shagreened, with coarser piligerous punctures, and along the posterior borders of the segments with minute, hair-bearing tubercles.

Hairs and pubescence whitish or pale yellowish, the former rather coarse, erect or suberect, pointed, of unequal length, more abundant on the head and gaster than on the thorax, longest on the gaster. Cheeks, gula and front also with conspicuous short, sparse, appressed hairs, or very coarse pubescence. Antennae with abundant fine, oblique hairs or pubescence, longest and most conspicuous towards the tips of the scapes. Legs with very short, sparse, inconspicuous, appressed or subappressed pubescence.

Varying from pale castaneous to dark piceous brown, the thorax and anterior portion of the head usually paler, the gaster darker and more blackish; mandibles, antennae, legs, including coxae and posterior edges of gastric segments, brownish yellow or yellowish brown. Palpi pale yellow, mandibular teeth black.

Queen
Williams and LaPolla (2016) - (n=9): EL: 0.42–0.48; HL: 1.26–1.44; HW: 1.38–1.55; SL: 1.36–1.55; TL: 7.05–8.33; WL: 2.49–3.08; BLI: 175–199; CI: 105–117; REL: 31–38; REL2: 30–34; SI: 97–103. Much larger and distinctly lighter in color than male; light to medium brown; abundant short, erect macrosetae on head, mesosoma, and gaster; entire cuticle covered in dense pubescence; head broader than long and square in shape; three ocelli present; compound eyes large and convex, barely surpassing the lateral margins of the head in full-face view; antennae with 12 segments; scapes long, surpassing the posterior margin of the head; mandibles with 6 teeth on the masticatory margin; ectal surface of mandibles with longitudinal striations; mesosoma large to accommodate flight muscles and without a constriction; small collar-like pronotum; large and strongly convex shelf-like mesonotum; petiole is forward-inclined and triangular, as seen in worker; dorsal apex of petiole scale is sharply angled.

Wheeler (1930) - Length 7.5-8.5 mm.; wings 7-7.5 mm.

Head broader than long and more narrowed anteriorly than in the worker. Eyes rather large; ocelli small and close together. Thorax massive, broader than the head, from above broadly elliptical, dorsally somewhat flattened in profile; mesonotum as long as broad; scutellum large; epinotum small, subperpendicular, rounded, without base or declivity. Petiolar node broad, thick below, strongly anteroposteriorly compressed above, its superior border deeply excised in the middle. Gaster large, oblong-elliptical, its basal segment angulate on each side anteriorly as in the worker.

Less shining than the worker, with the mandibles more coarsely striate and the head, thorax and gaster much more densely punctate.

Erect hairs shorter and more numerous than in the worker. Head, thorax and gaster covered with yellowish appressed pubescence which, however, is not sufficiently dense to conceal the shining and punctate integument. A similar but somewhat shorter pubescence covers the antennae and legs.

Reddish brown; mandibles, antennae and legs slightly paler and more reddish. Wing membranes uniformly yellowish brown; veins and pterostigma resin-colored.

Male
Williams and LaPolla (2016) - (n=10): EL: 0.31–0.45; HL: 0.66–0.82; HW: 0.72–0.89; SL: 0.54–0.75; TL: 3.16–4.16; WL: 1.30–1.65; BLI: 165–191; CI: 102–110; REL: 47–57; REL2: 43–54; SI: 75–89.

Much smaller and distinctly darker than queen; dark brown; abundant long, erect macrosetae on head, mesosoma, and gaster; entire cuticle covered in dense pubescence; head broader than long and oval in shape; three large ocelli present; compound eyes very large and convex, surpassing the lateral margins of the head in full-face view; antennae with 13 segments; scapes very short, barely surpassing the posterior margin of the head; mandibles with a single apical tooth on the masticatory margin; ectal surface of mandibles with longitudinal striations; mesosoma large to accommodate flight muscles and without a constriction; small collar-like pronotum; large and strongly convex shelf-like mesonotum; petiole is forward-inclined and triangular, as seen in worker; dorsal apex of petiole scale is sharply angled; genitalia oriented ventrally; parameres elongate, roughly triangular, and curved medially; ectal surface of parameres flattened; digiti are long and slender and follow the penis valve as it curves ventrally; cuspi are broad, triangular, and very short relative to the rest of the genitalia; parameres are covered in dense very long, erect macrosetae; edges of cuspi are covered in short, erect macrosetae; 9th sternite is large and broad.

Wheeler (1930) - Length 3.5-4 mm.

Head, including the eyes, broader than long, broadly rounded behind, without posterior corners, narrowed anteriorly, with short, straight cheeks. Mandibles rather large and overlapping, but with only the apical tooth developed. Clypeus large, less convex than in the worker. Antennal scapes slender, about one-third as long as the long funiculus; first joint of the latter small, of the usual shape, about one and one-half times as long as broad; remaining joints broader, of uniform width, twice as long as broad, the penultimate joints somewhat shorter, the last as long as the two preceding joints together. Thorax not broader than the head; mesonotum convex anteriorly, distinctly broader than long; epinotum evenly rounded and sloping in profile, without distinct base and declivity. Petiolar node shaped somewhat as in the worker, but much thicker, inclined forward, with blunt apical border, straight and transverse but not impressed in the middle. Gaster elongate-elliptical; external genital appendages long, somewhat curved, uniformly tapering and blunt at the tip. Legs slender; hind tibiae feebly bent near the middle.

Shining and finely punctate; antennae and legs more finely and densely than the remainder of the body; mandibles very finely striate as in the worker.

Hairs grayish, rather soft and flexuous, long and abundant on the head and thorax and very conspicuous on the tip and venter of the gaster. Pubescence long, sparse and appressed, most distinct on the gaster, very fine and short on the legs, slightly longer and more oblique on the scapes.

Piceous black; antennae and mandibles dark brown; femora black, the trochanters, tips of femora, the tibiae and tarsi brownish yellow. Wings varying in color from whitish to grayish hyaline, with colorless or pale yellowish veins and pterostigma.

Etymology
Say described this ant from a queen and male in copula. The two sexes were quite different in color hence he named the species imparis.

References based on Global Ant Biodiversity Informatics

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