Neivamyrmex nigrescens

This common species is by far the most widespread species in the United States. As a result of this wide range it is also by far the most studied and best known of the Neivamyrmex.

Identification
Smith (1942) – Worker The shape of the mandibles and petiole of the major worker are highly characteristic, as are also the nature of the body sculpturing and color. Also noteworthy are the long scape, the prominent pronotal carina, and the absence of a broad flange in front of the antennal socket. The major worker varies considerably in sculpture and color. The pilosity may also vary in length, but this is often due to wear. The general color ranges from light reddish brown through dark reddish brown into a deep infuscation that approaches black. The sculpturing on the postpetiole is sometimes so delicate as to give this region a slightly shining appearance. The foveolate impressions may vary from a few scattered ones on some specimens to numerous ones on others. Individuals with only a few shallow impressions resemble Neivamyrmex opacithorax; those those with coarser and more numerous impressions approach Neivamyrmex sumichrasti in appearance. There are intergradations in sculpturing between these extremes. The worker is most likely to be confused with that of opacithorax, which it closely resembles in structure and pilosity but from which it differs noticeably in shape of mandibles and nature of sculpturing. The head and usually the postpetiole of nigrescens are heavily sculptured and opaque. The same regions in opacithorax are shining and almost free of conspicuous sculpturing.

Queen There is variation in size of body shape of mandibles, depth and shape of petiolar impressions, length and abundance of hair, color, and amount of sculpture. The mandibles of some specimens are shaped much like those of opacithorax but are broader in porportion to their length. In general, the superior border of the mandible approaches the inferior border toward the apex, making a longer and somewhat more pronounced curve before forming the apical point. The median longitudinal impression on the petiole of most specimens is rather deep, extending the length of the petiole and somewhat widening posteriorly; the Colorado specimen, though, has a very shallow impression. The hair on the body is generally longer and more abundant than that of opacithorax, but this is not always true. The color may range from light ferruginous brown to dark ferruginous brown. The sculpture, though generally similar to that of opacithorax, is usually much coarser, the sculpture on the head being especially rougher.

Male Typical specimens agree very closely with the description, especially in possessing strongly developed frontal carinae; ridges over antennal sockets; rather robust antennae; heavily sculptured body of which the dorsum of the head and thorax is noticeably subopaque owing to the coarser punctation and general nature of the ground surface; blackish head and thorax with slightly lighter gaster; and light-yellowish or grayish pile covering the body. The males are highly variable, different individuals varying with regard to length and robustness of mandibles; depth of groove between frontal carinae, as well as shape of carinae; amount of development of the longitudinal ridge mesad of each eye, and of the ridge above antennal socket; and length of space between lateral ocellus and eye. The thorax may be higher in proportion to its length and possess a weaker transverse prothoracic impression. The sculpturing may be feebler so that parts of the body are less dull, this being especially true of the sides of the thorax. The color of the pilosity may range from light yellowish or grayish to an almost golden yellow; and that of the wings from subhyaline through slightly infuscated to almost blackish.

Identification Keys including this Taxon
Key to the Neivamyrmex species of the United States

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

Habitat
N. nigrescens shows an extremely wide habitat range. Rarely encountered in desert environments it is nonetheless present, apparently largely confined to canyons and hillsides. Ward (1999) listed the elevational range from sea level to 1460m in California, but we have records up to 2200m in areas outside of California. (Snelling and Snelling 2007)

Biology
Snelling and Snelling (2007) :

Colonies studied by Wheeler (1900) consisted of “thousands” of individuals, while Schneirla (1958) estimated 150,000 to 250,000 workers per nest. Neivamyrmex nigrescens has a nomadic/statary cycle like Neotropical army ants such as Eciton. The nomadic phase of the cycle begins when pupae eclose to workers. The whole colony then moves along a trail, usually during night hours, capturing any insects they encounter and raiding the nests of other ant species encountered. Columns may be 90m long and are headed by scouts. The colony bivouacs before dawn, using natural cavities or nests of other species, which they have pillaged. The following night they again move and raid. This nomadic cycle lasts for about three weeks or until the larvae in the colony (which they transport each night) begin to pupate. The statary phase then begins and the ants nest in subterranean cavities, either under stones or in abandoned ant nests for about 18 days (Schneirla, 1958). Raids continue but are less extensive than during the nomadic phase.

New colonies of N. nigrescens are formed when “a daughter queen leaves the parental nest, accompanied by a number of workers. A mature colony is capable of producing a small number of females, some of which may be fertilized in the nest by their brothers, but this does not preclude mating outside the nest, or with males of other colonies. Since females are never winged, they can make no nuptial flight.” (Smith 1965). Recent very preliminary data for this species suggest that N. nigrescens may, at least at times, have more than one functional queen present in the colony.(D. Kronauer, pers. comm.)

Other ants form an important part of the diet of N. nigrescens. Mallis (1938) observed this species carrying larvae and pupae of Tetramorium caespitum (Linné), as well as click beetles, mayflies, water boatmen and crickets. Wheeler & Long (1901) found larvae of Solenopsis geminata (Fabr.) and three species of Pheidole, as well as dead carabid beetles, in nests they studied in Texas. Ward (1999) further notes that in California Veromessor andrei (Mayr), Pheidole californica Mayr, Pheidole hyatti Emery, Solenopsis molesta (Say) and Formica moki Wheeler are also prey items of this species. In Arizona N. nigrescens has been observed regularly raiding Pheidole obtusospinosa Pergande (as P. subdentata) and Pheidole desertorum Wheeler. Neece & Bartell (1982) noted the presence of unidentified mites of the family Trachyaropodidae in colonies of N. nigrescens. The blind snake, Leptotyphlops dulcis, is able to follow the pheromone trails of N. nigrescens to locate columns and feed on the ant brood (Watkins et al., 1967). When the army ants attack the snake it forms a protective ball-like coil and smears a cloacal fluid on its body, which discourages further ant attacks (Watkins et al., 1972).

Several species of scuttle flies (Diptera: Phoridae) are known to parasitize adults of N. nigrescens. These include species in the genera Dacnophora and Cremersia (B. V. Brown, pers. comm.), and Xanionotum (Rettenmeyer and Akre 1968). The diapriid wasp, Ecitovagus gibbus Masner has been found as a parasitoid of N. nigrescens in southeastern Arizona (Masner 1977). Myrmecophilous Staphylinidae (Coleoptera) associated with this ant in areas other than California include: Microdonia laticollis Brues, M. nitidiventris Brues, M. occipitalis Casey, Ecitoxenidia brevicornis Seevers, E. brevipes Brues, Dinocoryna carolinensis Seevers, and Ecitonidia wheeleri Wasmann (Seevers 1965). At least two species in the carabid beetle genus Helluomorphoides (H. ferrugineus Casey and H. latitarsis LeConte) are specialized predators on both the booty and brood of N. nigrescens in southeastern Arizona: “The beetles were observed running in army ant columns or standing off to the sides of the columns, behind rocks or beneath clusters of leaf litter. During their predatory activities, beetles ran along the trails in both directions, 'plowing' through the continuous two-way ant traffic. When a beetle of either species contacted a worker ant bringing booty back to her bivouac, the ant usually dropped the booty. On some occasions, if the booty was a larval or pupal individual of another ant species, the beetle immediately ate it and continued on the trail. On other occasions the beetle picked up the dropped booty, left the raiding column, and proceeded to a nearby rock. There, the beetle quickly ate the larva or pupa, returned to the column, and resumed running along the trail” (Topoff, 1969). Beetles were observed to forcibly take booty from the ants. The beetles were also seen to feed on brood caches of the Neivamyrmex colony with which they became associated.

For further information on the biology and behavior of this species, see Ward (1999).

Wheeler (1903) - A fine colony of Eciton schmitti which I kept a few years ago, exhibited this restlessness in a striking and ludicrous manner. The colony at first confined in a tall glass jar on a square board surrounded by a water moat. The ants kept going up and down the inside of the jar in files for many hours. Finally removed the lid. The file at advanced over the rim and descended on the outer surface till it reached the circular base of the jar where it turned to the left at a right angle and proceeded completely around the base till it met the column at the turning point. To my surprise it kept right on over the same circumference which was long enough to accommodate the whole colony. The ants continued going round and round the circular base of the jar, following one another like so many sheep, without the slightest inkling that they were perpetually traversing the same path. They behaved exactly as they do on one of their predatory expeditions. They kept up this gyration for 46 hours before the column broke and spread over the board to the water’s edge and clustered in the manner so characteristic of this and the allied species (E. opacithorax, sumicrasti, etc.). I have never seen a more astonishing exhibition of the limitations germanice "Bornirtheit" of instinct. For nearly two whole days these blind creatures, so dependent on the contact-odor sense of their antennae, kept palpitating their uniformly smooth, odoriferous trail and the advancing bodies of the ants immediately preceding them, without perceiving that they were making no progress but only wasting their energy, till the spell was finally broken by some more venturesome members of the colony. Recently I have found a remarkable observation of the same kind recorded by Fabre in the 6th volume of his incomparable “Souvenirs Entomologiques.” He describes an army of caterpillars of the “processionnaire du pin” (Cnethocampa pityocampa) going round and round the outside of a large vase x.35 m. in circumference for seven days! During this period the caterpillars were on the march 84 hours altogether, stopping to rest on their path only when overtaken by the cold, and not actually deviating till the eighth day. Fabre estimated that the caterpillars crawled around the vase 335 times! In this case the insects were not guided by contact-odor like the Ecitons, but by the silken thread spun by each individual over the surface traversed.

In the mountains of southern Arizona, two army ant species, Neivamyrmex nigrescens and Neivamyrmex rugulosus, prey on (Miranda et al. 1980, LaPolla et al. 2002). In Tamaulipas, Mexico, Neivamyrmex texanus was observed raiding a colony of Trachymyrmex saussurei (Rabeling & Sanchez-Peña, unpublished data). Based on these few observations, army ants seem to be important predators of at least some Trachymyrmex species, and their raids may result in a significant brood loss and partial destruction of the fungus garden (LaPolla et al. 2002).

Nomenclature

 *  nigrescens. Labidus nigrescens Cresson, 1872: 194 (m.) U.S.A. Wheeler, W.M. 1900a: 563 (q.); Smith, M.R. 1942c: 551 (w.); Wheeler, G.C. & Wheeler, J. 1984: 273 (l.). Combination in Eciton: Dalla Torre, 1893: 5; in E. (Labidus): Emery, 1895c: 261; in E. (Acamatus): Emery, 1900a: 187; in E. (Neivamyrmex): Smith, M.R. 1942c: 550; Gregg, 1963: 286; in Neivamyrmex: Borgmeier, 1955: 494; Watkins, 1972: 349. Senior synonym of schmitti: Smith, M.R. 1938b: 160; Borgmeier, 1955: 494. See also: Watkins, 1972: 358; Smith, D.R. 1979: 1331; Petralia & Vinson, 1980: 378; Ward, 1999a: 84; Snelling, G.C. & Snelling, R.R., 2007: 484.
 * schmitti. Eciton (Acamatus) schmitti Emery, 1894c: 183 (diagnosis in key) (w.) U.S.A. Emery, 1895c: 258 (w.); Wheeler, W.M. & Long, 1901: 161 (m.); Wheeler, G.C. 1943: 331 (l.). Junior synonym of nigrescens: Smith, M.R. 1938b: 160; Borgmeier, 1955: 494.

Worker
Smith (1942) - Major. Length 4-5 mm.

Head scarcely longer than broad, narrowed posteriorly; posterior border emarginate, forming very distinctly produced, sharp, angular corners which are often somewhat outwardly curved but not so pronouncedly as with wheeleri. Eye ocellus-like, convex, usually very distinct because of the opaque appearance given head by sculpturing. Mandible with basal tooth on superior border lacking or very faintly indicated; margin between where his tooth should be and masticatory border convex, instead of straight or excised as in some of the other species; discal area of exterior surface flattened. Scape approximately three and one-half times as long as wide, extending beyond posterior border of eye a distance almost equivalent to greatest width of scape; funiculus with segments 2 to 4 inclusive at most scarcely broader than long. Frontal carina not forming a broad, distinct flange in front of antennal socket as in wheeleri. Dorsum of thorax, from above, less convex laterally than in wheeleri, thus giving thorax a more compressed appearance. Side of prothorax extending above fore coxae as a prominent, somewhat reflexed lobe. Promesonotum, in profile, appearing as an arch which merges into basal surface of epinotum without forming as abrupt an angular termination as in some species. Basal surface of epinotum meeting declivity in a rather rounded, obtuse angle. Petiole, from above, approximately two-thirds as broad as long; anteroventral surface of peduncle with bluntly rounded tooth. Postpetiole, from above, subtrapezoidal, slightly shorter than petiole, approximately as long as broad; almost anterior half somewhat laterally margined on each side.

Head, thorax, petiole, and postpetiole opaque, covered with dense granulate punctures, interspersed with coarse foveolate impressions; head and thorax most heavily sculptured, petiole and postpetiole least of all; legs subopaque or faintly shining, gaster smooth and shining. Discal surface of mandible bearing fine rugulae and coarse, scattered punctures which give an opaque appearance; borders of mandible more shining.

Hairs yellowish, rather abundarit, of various lengths, suberect to erect, many unusually long.

Head, thorax, petiole, and postpetiole usually deep reddish brown, sometimes almost blackish; gaster and legs slightly lighter. Eye yellow or amber.

Queen
Smith (1942) - Length 10-14 mm.

Head approximately as Ions as broad, broadest anteriorly; posterior border emarginate. Eye ocelluslike, rather large, much larger and more distinct than that of opacithorax. Mandible of somewhat similar shape to that of opacithorax but usually more robust. Scape curved, rather robust, approximately one-half length of head. Region adjacent to and also somewhat posterior to frontal area rather angularly produced anteriorly. A conspicuous median groove extending posteriorly from clypeus toward vertex, becoming feebler posteriorly. Dorsal surface of clypeus concave, middle of anterior border broadly but shallowly excised. Dorsal surface of head with deep median impression near occipital border and a groove leading from this toward front of head, also with a distinct impression on each side of head in front of posterior corners; these impressions giving back of head an extended effect and causing posterior corners to have an unusually angulate or tuberculate appearance. Thorax, from above, more than twice as long as wide, gradually increasing in width posteriorly to metanotum; epinotum not so wide as head. Dorsal thoracic sutures distinct. Pronotum approximately as broad as long, marginate anteriorly and laterally. Mesonotum with a somewhat angular anterior border, and a more broadly angular posterior border. Epinotum broader than long, with bluntly angular posterior corners. Mesonotum and epinotum with conspicuous longitudinal, median impression. Petiole, in profile, of approximately same height as epinotum but not so long; peduncle with large, convex protuberance beneath; from above, not one and a half times as broad as long, scarcely broader in front than behind, and with somewhat subparallel sides, and a deep median, longitudinal impression which widens posteriorly.

Head and thorax opaque, owing to the dense 'granulate shagreening and the scattered, deep punctures. Petiole more finely sculptured than head or thorax.

Hairs fairly abundant on head, thorax, petiole, and appendages; clypeus, gula, mandibles, and sea pes with longer hairs of variable length.

Light or deep ferruginous brown.

Male
Smith (1942) - Length 11.25-13 mm.

Head approximately one and eight-tenths times as broad as long; posterior border rounded. Eye rather small, moderately convex, and protuberant. Ocelli very small, placed on low protuberance which is only slightly elevated above general surface of head; summit of protuberance concave; lateral ocellus far removed from eye, this space often greater than the space between the two lateral ocelli. Frontal carinae converging behind, with distinct but somewhat feeble groove between them leading to anterior ocellus. Ridge over antennal socket remarkably well developed, forming a large, thick welt which tends partly to obscure the posterior border of the head, when the head is viewed anteriorly. Antenna distinctly more robust than that of opacithorax; scape slightly shorter than combined length of first 3 funicular segments; segments 2 to 6 inclusive much more bro:!dened than those of opacithorax, all segments except first clearly longer than broad. Mandible moderately elongate to rather elongate, with subparallel superior and inferior borders basally, superior border converging with inferior border somewhere between apical half to third of mandible and forming a rather blunt point. Head, from above, with very prominent frontal carinae and a very strong, protuberant ridge above each antennal socket; a transverse groove behind each ridge. Head not noticeably extended behind eyes, posterior corners well rounded, the curvature blending into that of eyes. Eye, in profile, narrowed above, not occupying all of side of head, there being an area mesad and ventrad of it larger than similar areas of opacithorax, and a much larger area posterodorsad than the two areas mentioned; head behind ocelli convex, without occipital flange. Thorax, in profile, approximately one and one-half times as long as high, not extended anteriorly above head. Prothorax anteriorly with a distinct, transverse impression. Epinotum truncate in appearance but really weakly concave. Mesonotum with distinct anteromedian and parapsidal lines. Tarsal claws feebly toothed. Gaster rather robust, more noticeably so than in opacithorax; with pronounced constrictions between segments, and a transverse impression near base of sixth gastric tergum. Intermediate tooth of seventh gastric sternum small, often not clearly seen.

Body more opaque than that of opacithorax, especially on dorsum of head and thorax, where the coarse punctation and general ground surface of these regions obscure the shining effect. Gaster also more coarsely sculptured than that of opacithorax.

Hairs light yellowish or grayish to deeper yellow, sometimes almost golden; rather closely appressed on all parts of body except antennal scapes, head, and ventral surfaces of thorax and petiole, where they are longer and more nearly erect.

Head and thorax almost black; gaster lighter brown; funiculi and tarsi usually lighter. Wings ranging from subhyaline through slightly infuscated to deeply infuscated; veins and stigma brownish to blackish.

Type Material
Smith (1942) - Texas, G. W. Belfrage. Holotype in Academy of Natural Sciences of Philadelphia.

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