Monomorium minimum

The most common, abundant and wide ranging Monomorium species in North America.

Identification
DuBois (1986) - Queen Winged; scutum and scutellum not depressed; metanotum (in side view) projecting to level of propodeum and scutellum; propodeum angular. Worker Propodeum angular; mesopleuron not punctate; PI 33-40 (37).

Since Monomorium minimum queens are winged, they can easily be separated from other species occurring in the same area, most of which have Wingless queens. Only Monomorium viridum queens are winged. Nests of M. viridum appear to be restricted to pure sand while nests of M. minimum only occur in soils containing some clay (W. L. Brown pets. observ.) Workers of this Species can usually be separated by the combination of characters listed in the diagnosis, particularly the smooth and shining mesopleuron.

Distribution
DuBois (1986) - This species ranges from Pennsylvania and District of Columbia, south to Georgia, west to Texas and New Mexico, and north to Idaho, Montana, and North Dakota. Although this species has been collected in northern Wisconsin and North Dakota, no collections have been made in southern Canada; this species undoubtedly occurs there. In the eastern United States, this species occurs sporadically and is apparently replaced by Monomorium emarginatum in the northeastern states and by Monomorium viridum along the Atlantic coast in New Jersey, Georgia, and Florida. In the southern United States, M. minimum is known from collections in northern Georgia (Athens), Alabama (northern Alabama south to Mobile), Mississippi (only two collections, Amory and State College), and numerous collections in Louisiana and Texas. This species overlaps the range of Monomorium cyaneum in Texas and New Mexico. Monomorium cyaneum apparently prefers drier habitats and usually nests under rocks while M. minimum prefers moisture habitats usually near the edges of woods. Most of my records of M. minimum from Colorado are from the eastern plains. I have only examined one worker which was collected from western Colorado (Mesa Verde National Park). However, Gregg (1963) includes many localities from the mountainous regions of the state (his M. viridum peninsulatum records should be included with his records for M. minimum). The furthest western records for M. minimum are three collections from southern Idaho (Rupert, 6.4 km W of St. Anthony, and Holbrook). These three collections consist of workers only. It is possible these are Monomorium wheelerorum although they appear to be M. minimum workers.

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

Habitat
Essentially all habitats, ranging from arid grasslands and shrubs to pinyon-juniper forests to ponderosa pine-riparian sites. It is found most often in open areas. (Mackay and Mackay 2002)

Biology
DuBois (1986) - Recorded as nesting in soil, under rocks, in logs, man-made structures, and living trees (mesquite, Prosopir glandulosu) (Dennis, 1938; Buren, 1944; Kannowski, 1956; Hess, 1958; Wheeler and Wheeler, 1963). Talbot (1934) and Gregg (1944) found M. minimum most frequently in sandy soil, and Gregg (1963) discovered it in a number of microhabitats representing both Upper Sonoran and Transition life zones: foothills meadow, deciduous canyon forest, oak woodland, ponderosa pine-oak woodland, pinyon-cedar woodland, pinyon-cedar-oak woodland, mixed grass prairie, short grass prairie, sagebrush desert, sagebrush-greasewood desert, sandhills grassland, roadside, and exposed bedrock.

Nests in soil appear to have a characteristic structure; most are shallow (less than 10 cm deep, Kannowski, 1956) with most brood chambers located from just under the soil surface to depths of 5 cm (Wheeler and Wheeler, 1963). Although this description referred to nests in North Dakota, nests in Kansas and Illinois are of a similar internal construction. However, tumuli vary considerably depending upon environmental conditions. In Kansas, most nests are located near edges of woodlands and have tumuli with a typical crater structure. Nests in areas of scant vegetation usually lack conspicuous tumuli, since soil particles are presumably blown away as soon as they are deposited on the surface. Nests in sheltered areas, such as at the base of grass clumps, possess mounds with many crenulations causing the structure to resemble a lump of brain coral.

In addition to the wide environmental tolerance of this species, there is a wide altitudinal tolerance. In Kansas, collection elevations range from 244 m to over 1220 m (pers. observ.) In Colorado, Gregg (1963) found collection elevations to range from 1067 m through 2591 In. Similar elevational extremes were found in Tennessee by Dennis (1938). This species also occurs down to sea level.

Monomorium minimum queens live approximately 1 year in laboratory colonies while workers live approximately 4 months. There appear to be three larval instars (pers. observ.).

Most nests contain multiple queens. For example, Gregg (1944) found that nests near Chicago, Illinois usually contained 12-14 queens and Dennis (1938) found a similar number of queens per nest in Tennessee. Presumably these are functioning queens since most are associated with small, separate piles of eggs and first instar larvae (pers. observ.). Recent preliminary allozyme Studies have demonstrated that several queens produce eggs at the same time in nests near Lawrence, Kansas (pers. observ.). Although it is not known how many queens establish a nest, most nests are probably established by one or two queens since a number of small nests with a small number of offspring and one or two queens are found each autumn in the Lawrence, Kansas, vicinity. Since nuptial flights occur in July in this vicinity, most newly mated queens should have new nests established by autumn.

Production of sexuals occurs in late spring and summer; most nests in Kansas contain larvae which will develop into sexuals by late May. Most sexual adults eclose by mid-July. Nuptual flights have not been observed, although they probably occur, since both males and gynes are winged.

Monomorium minimum is one of two species reported to have inquilines (Monomorium ergatogyna is the other species). Van Pelt and Van Pelt (1972) reported larvae of Microdon baliopterus Loew (Diptera: Syrphidae) from nests of M. minimum in Big Bend National Park, Texas.

Monomorium minimum nests have been discovered in close association with nests of other ant species. Wheeler and Wheeler (1963) found M. minimum associated with nests of the following species: Lasius claviger, Camponotus vicinus, Formica fusca, Lasius crypticus, and Pogonomyrmex occidentalis. I have found nests of M. minimum associated with nests of an additional species, Solenopsis (Diplorhoptrum) molesta (Say), in Kansas.

Roeder et al. (2018) in an Oklahoma study found the CTmax (critical thermal maximum) for this species was 55.1 ± 0.2 and the average worker mass was 0.047 ± 0.002 mg.

Castes

 * Worker

Nomenclature

 * atra. Myrmica (Monomorium) atra Buckley, 1867: 342 (q.) U.S.A. Junior synonym of minimum: Emery, 1895c: 274.
 *  minimum. Myrmica (Monomorium) minima Buckley, 1867: 338 (w.q.) U.S.A. Emery, 1895c: 274 (m.); Wheeler, G.C. & Wheeler, J. 1955c: 122 (l.); Crozier, 1970: 116 (k.). Combination in Monomorium: Emery, 1895c: 274. Subspecies of minutum: Emery, 1895c: 274; Wheeler, W.M. 1904e: 301; Wheeler, W.M. 1906b: 2. Revived status as species: Wheeler, W.M. 1910g: 562; Wheeler, W.M. 1913c: 115; Wheeler, W.M. 1914b: 42; Dennis, 1938: 279. Senior synonym of atra: Emery, 1895c: 274; of metoecus: Brown, in Ettershank, 1966: 90. See also: Wheeler, G.C. & Wheeler, J. 1973a: 30; DuBois, 1986: 76.
 * metoecus. Monomorium metoecus Brown & Wilson, 1957b: 239, fig. 1 (q.) U.S.A. Wilson & Brown, 1958: 33 (w.). Junior synonym of minimum: Brown, in Ettershank, 1966: 90.

Worker
DuBois (1986) - Head: (representing different localities; N: 10) HL 0.46-0.56 (0.52), HW 0.38-0.48 (0.42), SL 0.28-0.40 (0.33), EL 0.06-0.10 (0.09), MOD 0.05-0.08 (0.06). Structure - Head a little longer than broad, CI 76-86 (81), distinctly longer than scape SIL 56-71 (64); SIW 71-90 (79). In full frontal view, head broadest at or slightly above eyes; side convex; occiput rounded laterally, summit flat. Eye small in size. Scape never reaching occiput. Mandible with 4 teeth; maxillary palp 2-segmented; labial palp 2-segmented. Clypeal teeth sharp, of moderate length. Frontal carinae diverging slightly posteriorly rarely, parallel. Pilosity—Setae suberect to subdecumbent on clypeus, frons, gular region, mandible, and occiput, decumbent to appressed elsewhere. Setae of scape subdecumbent to decumbent, of pedicel and flagellum decumbent to appressed. Sculpture—All surfaces smooth and shining with small to moderate piliferous punctures. Alitrunk. Measurements PW 0.22-0.32 (0.28), PL 0.18-0.25 (0.21), WL 0.50-0.65 (0.57). Structure - Mesonotum lacking notal furrow. Anterior propodeal suture of moderate depth. Propodeum angular, basal face 1.5-2X as long as declivitous face. PI 33-40 (37). Pilosity—Dorsal outline of alitrunk with more than 10 erect to suberect setae. Leg pilosity as in queen. Sculpture—Entire surface smooth and shining except as follows: small to moderate piliferous punctures on dorsum; lower 1/2 of side of propodeum (below spiracle) with 1-3 small, parallel, longitudinal rugae. Petiole. Dorsum of node convex. Subpetiolar process of moderate to reduced size, anterior to node. Setae erect to suberect on dorsum, absent elsewhere. All surfaces smooth and shining with small piliferous punctures on dorsum. Postpetiole. Dorsum of node convex. Anterior subpostpetiolar process reduced, located medially on venter. Setae suberect to erect on dorsum, absent elsewhere. All surfaces smooth and shining with small piliferous punctures on dorsum. Gaster. Setae erect to suberect with percentage of erect setae increasing towards posterior end of gaster. Setae of first gastral tergite not exceeding level of dorsum of postpetiolar node. All surfaces smooth and shining with small to moderate piliferous punctures evenly distributed. Color: Head brown to dark brown with trace of bluish reflection, mandible yellow brown to yellow. Alitrunk brown to dark brown with trace of bluish reflection, legs yellow brown with tibiae and tarsi yellow. Petiole, postpetiole, and gaster brown. All setae white.

Queen
DuBois (1986) - Head: (representing different localities; N= 10) {Neotype measurements in brackets} HL 0.69-0.86 (0.77) {0.79}, HW 0.65-0.81 (0.74) {0.66}, SL 0.40-0.60 (0.50) {0.58}, IOD 0.12-0.20 (0.17) {0.17}, OD 0.04-0.06 (0.05) {0.06}, EL 0.15-0.20 (0.17) {0.20}, MOD 0.11-0.18 (0.13) {0.15}. Structure—Head slightly longer than broad or slightly broader than long, CI 84-106 (95) {84}, distinctly longer than scape, SIL 56-79 (65) {73}; SIW 53-88 (68) {88}. In full frontal view, head broadest slightly above eyes; side convex; occiput rounded laterally, summit flat. Eye moderate in size. Scape reaching but never surpassing occiput. Mandible with 4 teeth; maxillary palp 2-segmented; labial palp 2-segmented. Clypeal teeth sharp, moderate in length. Frontal carinae diverging slightly posteriorly. Pilosity—Setae erect to suberect on clypeus, frons, gular region, mandible, and occiput, decumbent to appressed elsewhere. Setae of scape suberect to decumbent, of pedicel and flagellum decumbent to appressed with percentage of latter increasing distally. Sculpture—Head smooth and shining with small to moderate piliferous punctures evenly distributed except as follows: small, parallel, longitudinal rugosities beginning all along lateral margin of clypeus, extending past antennal insertion, converging with rugosities from frontal carinae and extending to level of anterior ocellus. Other small, parallel, longitudinal rugosities beginning between frontal carinae, sometimes extending to level of anterior ocellus. Alitrunk: PW 0.50-0.72 (0.61) {0.62}, PL 0.31—0.42 (0.38) {0.40}, WL 1.34-1.81 (1.61) {1,37}. Structure—Mesonotum usually lacks notal furrow rarely, with small emargination. Pronotal-scutal suture on dorsal 1/3 of alitrunk. Scutum and scutellum not depressed. Mesopleural suture straight or deflected ventrally on posterior end and deflected dorsally on anterior end (resulting in S-shaped suture). Small pit usually present on posterior end of suture. Metanotum (in lateral view) reaching or exceeding level of propodeum and scutellum. Propodeum angular, basal face approximately 2/3 length of declivitous face. PI 21-29 (24) {29}. Wings present; venation typical; 6-8 hamuli on hindwing; stigma located directly above vannal emargination {neotype lacks wings but condition of basal sclerites indicate queen once possessed wings}. Pilosity—Dorsal outline of alitrunk with more than 30 erect to suberect setae. Setae erect to suberect on coxae and trochanters, erect to suberect on flexor surfaces of femora (decumbent to appressed elsewhere), decumbent to appressed on tibiae and tarsi. Sculpture—Smooth and shining except as follows: moderate to large piliferous punctures on dorsum of alitrunk; lower 1/2 of side of propodeum (below spiracle) with several moderate, parallel, longitudinal rugae rarely, some rugae occur above spirac1e. Petiole: Dorsum of node convex to flat. Subpetiolar process moderate in size, anterior to node. Setae decumbent to appressed on anterior surface, erect to suberect on dorsum, posterior surface, and side, absent from venter. Dorsum of node smooth and shining with moderate piliferous punctures; side with moderate, parallel, longitudinal rugae becoming transverse near posterior surface of node; posterior surface of node with moderate, semicircular, concentric rugae. Postpetiole: Dorsum of node flat to slightly emarginate. Anterior subpostpetiolar process reduced, located medially on venter. Setae appressed to decumbent on anterior surface of node, suberect to erect on remaining surfaces. Dorsum of node smooth and shining with moderate to small piliferous punctures; remainder of node covered with dense, moderate to small, non-piliferous punctures. Gaster: Setae erect to decumbent on all surfaces with percentage of erect to suberect setae increasing towards posterior end of gaster. Setae of first gastral tergite exceeding level of dorsum of postpetiolar node. All surfaces smooth and shining with small to moderate piliferous punctures. Color: Head brown to dark brown with mandible and lateral margin of clypeus yellow brown to brown; scape brown, pedicel and flagellum yellow brown to yellow. Alitrunk brown to dark brown with legs brown; tibiae and tarsi yellow brown to yellow. Petiole, postpetiole, and gaster brown to dark brown. All setae white.

Male
DuBois (1986) - Head: (representing different localities; N= 10) HL 0.65-0.74 (0.71), HW 0.69-0.78 (0.75), SL 0.22-0.30 (0.26), IOD 0.22-0.26 (0.24), OD 005-0. 12 (0.09), EL 0.28-0.34 (0.32), MOD 0.15-0.24- (0.19). Structure—Head slightly broader than long, CI 105-109 (107), distinctly longer than scape, SIL 31-42 (36); SIW 29-38 (34-). In full frontal view, head broadest at or slightly above eyes; side straight to slightly convex (usually obscured by compound eye); occiput rounded laterally, summit flat (usually obscured by posterior ocelli). Eye large. Scape not reaching occiput. Mandible with 3 teeth; maxillary palp 2-segmented; labial palp 2—segmented. Clypeal teeth reduced or absent (blunt if present). Frontal carinae diverging slightly posteriorly. Pilosity—Setae erect to decumbent over entire surface of head with erect to suberect setae predominating near clypeus, gular region, mandible, and occiput. Antennal pilosity as in queen. Sculpture—Entire head covered with dense, moderate to large, non-piliferous punctures. Small to moderate, parallel, longitudinal rugae beginning all along lateral margin of clypeus, continuing to level of antennal insertion and fusing with large, concentric, semicircular rugae surrounding antennal insertion. Alitrunk. PW 0.70-0.81 (0.76), PL 0.44-0.62 (0.52), WL 1.66-1.90 (1.75). Structure—Mesonotum lacking notal furrow. Pronotal-scutal suture on dorsal 1/3 of alitrunk. Scutum and scutellum not depressed. Mesopleural suture deflected ventrally at posterior end and deflected dorsally at anterior end (resulting in an S-shaped suture) with small pit on posterior end. Metanotum (in lateral view) reaching or exceeding level of propodeum and scutellum. Propodeum angular, basal face 2X as long as declivitous face. Wings present; venation typical; 5-7 hamuli on hindwing; stigma located directly above vannal emargination. Priority—Dorsal outline of alitrunk with more than 30 erect to suberect setae; setae absent from side (except near sutures). Leg pilosity as in queen. Sculpture—Entire surface smooth and shining except as follows: small to moderate piliferous punctures on dorsum of alitrunk; small to moderate non-piliferous punctures loosely distributed on side; lower 1/2 of side of propodeum (below spiracle) with small, parallel, longitudinal rugae; entire surface of propodeum often covered with small, non-piliferous punctures. Petiole: Dorsum of node flat to emarginate. Subpetiolar process reduced, anterior to node. Setae erect to decumbent on anterior surface of node, erect to suberect on side and posterior surface of node, absent elsewhere (including dorsum of node). All surfaces smooth and shining except as follows: small to moderate piliferous punctures on anterior and posterior surfaces of node; side with small to moderate non-piliferous punctures. Postpetiole. Dorsum of node flat to slightly emarginate. Anterior subpostpetiolar process absent. Setae erect to decumbent on anterior surface, side, and posterior surface, absent elsewhere. Dorsum of node smooth and shining; remainder of node covered with moderately dense, non-piliferous punctures. Gaster. Pilosity as in queen. All surfaces smooth and shining with small to moderate piliferous punctures. Genitalia: Eighth sternite with emergination lacking setae, approximately as deep as wide. Ninth sternite with 6-8 erect setae. Aedeagus with 12-15 teeth; toothed margin rounded. Volsella with curved digitus and reduced cuspis; cuspis with 3 setae. Color: Head brown to dark brown, mandible and lateral margin of clypeus yellow brown to yellow. Alitrunk brown to yellow brown; leg coloration as in queen, Petiole, postpetiole, and gaster brown to dark brown. Genitalia brown to yellow brown. All setae white.

Type Material
DuBois (1986) - None known to exist (Creighton, 1950). Wheeler (1902) stated the probable type locality as Austin, Texas (or vicinity) since Buckley (1867) failed to mention a specific locality. Although a few of Buckley’s specimens remain (Academy of Natural Sciences, Philadelphia and collection of Gustav Mayr, Vienna Museum of Natural History, Vienna, Austria), none could be located that were collected in the provided the basis for his description.

Since two species of Monomorium, Monomorium cyaneum and M. minimum, could occur in the Austin, Texas, vicinity, I designate the following queen as neotype of Monomorium minimum to reduce confusion: Texas. Bastrop Co., Bastrop State Park, June 9, 1954, W. Clayd T-119. Since Buckley failed to mention a specific locality in his original description, I followed Wheeler’s suggestion in designating a neotype from the Austin, Texas, vicinity. This specimen bears a red, handwritten label: Monomorium minimum (Buckley) Neotype M. DuBois 1983.