Strumigenys louisianae

A highly-variable widely-distributed New World species that is one of the most commonly encountered members of the genus. It also has the lowest chromosome number of any Neotropical ant (2n = 4) and one of the lowest known for any ant (Alves-Silva et al., 2014).

Identification
A member of the Strumigenys louisianae-group. See the description below.

Longino (Ants of Costa Rica) - Apical fork of mandible with two tiny intercalary teeth; one conspicuous preapical tooth, but no other preapical denticles; outstanding setal pairs on sides of head and mesonotum spatulate, outstanding seta pair on humeral angles flagelliform, bent and coachwhip-like; gaster finely granular, opaque; erect setae on gaster strongly spatulate; posterior face of petiolar node rounded in posterior view. [The above applies to Costa Rican populations.] Details of setal characters and gaster sculpture vary geographically.

Distribution
Bolton (2000) - The distribution area of louisianae is vast. Brown (1953a, 1953d, 1961) noted its presence in many southern states of the U.S.A. (Alabama, Arizona, Florida, Georgia, Louisiana, Mississippi, North Carolina, Tennessee, Texas), as well as from Mexico, Guatemala, Costa Rica, Cuba, Haiti , Puerto Rico, Brazil , Surinam, Colombia, Bolivia and Argentina. The present survey has added localities in Nicaragua, Venezuela, Ecuador and Paraguay.

Distribution based on Regional Taxon Lists
Nearctic Region: United States. Neotropical Region: Argentina, Bahamas, Barbados, Bolivia, Brazil, Colombia, Costa Rica, Cuba, Dominican Republic, Ecuador, Galapagos Islands, Greater Antilles, Guatemala, Haiti, Honduras, Mexico, Nicaragua, Paraguay, Puerto Rico, Suriname, Venezuela.



Habitat
The species prefers synanthropic and/or seasonally dry habitats.

Abundance
It is more abundant in the peripheral parts of its range (Costa Rica northward, southern Brazil southward) than in the center (Panama southward to Amazonian Brazil) (Brown 1961).

Biology
Wilson (1954):

"studied seven colonies of this species from several localities in southern and central Alabama were studied over a period of a little less than a year. As noted in my preliminary paper (1950), the principal food appears to be certain groups of Collembola, although a few other arthropods are taken occasionally, especially if they are dead or injured.

By far the most preferred prey of this species are entomobryoid and symphypleonan Collembola, and these were used to maintain the colonies in the present study. Other arthropods were treated in a somewhat desultory fashion by the ants. When a half dozen or so termites were placed in the food chamber of a nest, they were attacked at once, and although one or two usually succeeded in escaping and hiding under debris, the majority were killed within several hours. Some of the corpses were carried to the brood chambers and consumed, while others were left undisturbed in the food chamber. Psocids and small flies were usually either ignored or half-heartedly attacked by the ants, and those carried to the brood chambers were then only occasionally partly consumed. Japyx were vigorously attacked by the ants, which appeared to have an especial antipathy toward these insects, and like the termites they rarely survived more than several hours in the food chamber of a sizeable colony. Legs and antennae were often consumed by the adults and larvae, but the bodies were almost never disturbed.

Only several mites were accepted over the entire period of observation. Since the debris in the food chambers almost always swarmed with a substantial number of species in several families, the possibility must be considered that Strumigenys regularly accept certain forms but reject the majority. There is some evidence that mites are normally accepted in nature (see below). Symphyla were consistently accepted and eaten, to the extent that they seemed to be preferred almost as much as Collembola. Small ant larvae were also readily accepted, possibly suggesting that the Strumigenys may on occasion behave as thief ants, although no evidence of this was seen in the field.

A spot check of the food-chamber experiments was attempted by placing a Janet nest containing a colony in a terrarium partly filled with fresh humus and leaf litter, which were taken from several areas and replaced periodically. Occasionally bits of wood containing termites were also added. The ants were allowed access to this material through galleries leading to the outside of the nest. Their prey, checked daily over a two-week period, consisted at least of eleven entomobryids, seven isotomids, four mites, and one symphylan. Except for the unexpected large proportion of mites, this sample is consonant with the prey specificity exhibited by colonies confined to the artificial nests.

This species were bolder and more direct in their manner of stalking prey than the other dacetines studied. This trait is perhaps a result of their more efficient mandibles, which are extremely long and supplied with prominent apical teeth. They can be opened to almost a 180° angle and operate very much as miniature animal spring traps. When approaching a collembolan, the worker moves slowly and cautiously spreading its mandibles to the maximum angle and exposing two long hairs which arise from the paired labral lobes. These hairs extend far forward of the ant's head and apparently serve as tactile range finders for the mandibles. When they first touch the prey, its body is well within reach of the apical teeth. A sudden and convulsive snap of the mandibles literally impales it on the teeth, and drops of haemolymph often well out of the punctures. If the collembolan is small or average in size, the ant lifts it into the air and curls its gaster forward in an attempt to sting, a surprisingly awkward motion in this species and the other dacetines studied, for in many catches observed it did not appear that the sting entered the body of the prey at all. Nevertheless, all but the largest Collembola are quickly immobilized by this action, and struggling is feeble and short-lived. Apparently the prey are only paralyzed and not killed outright, since most can be prodded into feebly moving even after they have been carried into the brood chamber.

A great deal of individual deviation from the typical hunting behavior occured in the nests under observation. Sometimes workers were much less stealthy in their approach and seemed almost to charge their prey. Frequently they seized Collembola when these insects blundered into them. Dead Collembola were not stalked but were directly picked up after a brief inspection.

Nothing resembling concerted or coordinated stalking on the part of several workers was observed. Hunting in general appeared to be rather haphazard, a condition accentuated by the apparent inability of the ants to recognize the presence of insects more than about a millimeter away. Occasionally workers were seen to stalk and strike Collembola which had already been captured by other workers and were being transported to the brood chambers.

Field observations on the food habits of these tiny and secretive ants are understandably meager. At two localites in Alabama, Gulf State Park and Mobile, stray workers were observed carrying entomobryids. On one occasion, in Mobile, a worker was found carrying a small Campodea. As mentioned in my preliminary paper on this species, one nest collected at Tuscaloosa, Ala., contained recognizable insect remains in the galleries; these were identified as belonging to three entomobryoid genera, Entomobrya, Proisotoma, and Isotoma.

Larvae are fed by being placed directly on the prey. As many as ten or more may be piled at first on the same collembolan, and since they are active feeders they may wholly consume a small individual within several hours. The final fragments of the insect are held aloft, ponerine fashion. No case of ingluvial feeding of the larvae by the workers was ever observed, although occasionally workers were seen passing food in this manner."

Brown (1962) - Unaccountably absent from certain well-collected areas within this range, such as parts of the Canal Zone, Trinidad and British Guiana, although plentiful in Costa Rica and at least some localities in Colombia. This species tolerates much drier conditions and will live in plantations and other cultivated situations, so perhaps it is found mostly in habitats outside the primary forest in the central parts of its range.

Deyrup (1997) - Nests are often in rotten wood, colonies were found in Nassau in a pile of rotting coconut husks. Although Brown (1962) found this species tolerant of relatively dry conditions, I have usually found it in mesic forest, swamp forest, or even the edge of salt marshes.

Longino (Ants of Costa Rica) - The feeding habits of louisianae have been studied by Wilson (1950, 1954) and Brown (1962). Workers take live prey, mostly small arthropods in litter and soil. Preferred prey are entomobryoid and symphypleonan Collembola; poduroid collembolans are not taken.

Nomenclature

 *  louisianae. Strumigenys louisianae Roger, 1863a: 211 (w.) U.S.A. Emery, 1895c: 326 (q.); Haug, 1932: 170 (m.); Wheeler, G.C. & Wheeler, J. 1955a: 136 (l.). Senior synonym of unispinulosa: Emery, 1895c: 326; Brown, 1953g: 28; of laticephala: Smith, M.R. 1951a: 826; Brown, 1953g: 28; of bruchi, costaricensis, cubaensis, guatemalensis, longicornis, obscuriventris, soledadensis: Brown, 1953g: 28; of clasmospongia, fusca, unidentata: Brown, 1961a: 64; of producta: Bolton, 2000: 524; of wani: Bolton, Sosa-Calvo, et al. 2008: 62. See also: Wilson, 1954: 481; Bolton, 2000: 525.
 * unidentata. Strumigenys unidentata Mayr, 1887: 575 (w.) BRAZIL. Junior synonym of louisianae: Brown, 1961a: 64.
 * unispinulosa. Strumigenys unispinulosa Emery, 1890b: 67, pl. 7, fig. 5 (w.q.) COSTA RICA. [Also described as new by Emery, 1894k: 58.] Junior synonym of louisianae: Emery, 1895c: 326; Brown, 1953g: 28.
 * fusca. Strumigenys fusca Emery, 1894c: 215, pl. 1, fig. 8 (w.) BRAZIL. Junior synonym of louisianae: Brown, 1961a: 64.
 * longicornis. Strumigenys unispinulosa var. longicornis Emery, 1894c: 214 (w.) BOLIVIA. Junior synonym of louisianae: Brown, 1953g: 28.
 * obscuriventris. Strumigenys louisianae var. obscuriventris Wheeler, W.M. 1908a: 145, pl. 12, fig. 14 (w.) PUERTO RICA. Junior synonym of louisianae: Brown, 1953g: 28.
 * bruchi. Strumigenys bruchi Forel, 1912e: 197 (w.m.) ARGENTINA. Junior synonym of louisianae: Brown, 1953g: 28.
 * cubaensis. Strumigenys eggersi var. cubaensis Mann, 1920: 430 (w.) CUBA. Junior synonym of louisianae: Brown, 1953g: 28.
 * laticephala. Strumigenys (Strumigenys) louisianae subsp. laticephala Smith, M.R. 1931c: 690, pl. 1, fig. 2 (w.) U.S.A. Haug, 1932: 170 (m.). Junior synonym of louisianae: Smith, M.R. 1951a: 826; Brown, 1953g: 28.
 * costaricensis. Strumigenys (Strumigenys) louisianae subsp. costaricensis Weber, 1934a: 39 (w.) COSTA RICA. Junior synonym of louisianae: Brown, 1953g: 28.
 * guatemalensis. Strumigenys (Strumigenys) louisianae subsp. guatemalensis Weber, 1934a: 39 (w.) GUATEMALA. Junior synonym of louisianae: Brown, 1953g: 28.
 * soledadensis. Strumigenys (Strumigenys) louisianae subsp. soledadensis Weber, 1934a: 38 (w.q.) CUBA. Junior synonym of louisianae: Brown, 1953g: 28.
 * clasmospongia. Strumigenys clasmospongia Brown, 1953e: 2 (w.) BRAZIL. Junior synonym of louisianae: Brown, 1961a: 64.
 * producta. Strumigenys producta Brown, 1953e: 3 (w.) BOLIVIA. Junior synonym of louisianae: Bolton, 2000: 524.
 * wani. Pyramica wani Makhan, 2007a: 6, figs. 11, 12 (w.) SURINAM. Combination in Strumigenys: Bolton, Sosa-Calvo, et al. 2008: 62. Junior synonym of louisianae: Bolton, Sosa-Calvo, et al. 2008: 62.

Bolton (2000) - As mentioned in the introduction to the Strumigenys louisianae-group, louisianae, as presently constituted, is an extremely variable species. It is in need of a detailed investigation that is beyond the scope of the current study. There is considerable variation in distribution, density and intensity of sculpture on the alitrunk, postpetiole and gaster; in relative dimensions of head, mandible and scape; in shape and relative and absolute size of spongiform appendages; in shape, size and density of cephalic ground pilosity; in length and shape of gastral pilosity; and in colour, though this may be altitude-related as in many other dacetines. Brown (1953a, 1961) concluded that all the variation was indicative of a single extremely plastic species, but I suspect that this may not be the case.

Because of the consistency of the character combination used above to diagnose the species, and its duplication in the type-series of producta, I have added this name to the synonymy of louisianae. I have removed Strumigenys infidelis from Brown's synonymy and reinstated it as a valid species because it lacks flagellate hairs at the pronotal humeri.

Worker
Bolton (2000) - TL 1.8 - 3.4, HL 0.46 - 0.70, HW 0.38 - 0.54, CI 71 - 92, ML 0.23 - 0.48, MI 50 - 70, SL 0.24 - 0.45, SI 63 - 85, PW 0.24 - 0.36, AL 0.45 - 0.74 (includes all measurements made by Brown, 1953a, 1953d, 1961; 35 more measured to provide values omitted from these papers). Mandible without a denticle proximal to the preapical tooth. With head in profile postbuccal groove a narrow and deeply incised notch. Apicoscrobal hair short and stiff, remiform. Cephalic dorsum with a single pair of short standing hairs, close to occipital margin. Pronotal humeral hair fine and flagellate; mesonotal standing hairs remiform. Ventral surface of petiole without trace of spongiform tissue.

Type Material
Bolton (2000):

Syntype workers, U.S.A.: Louisiana (location of types not known).

Strumigenys unidentata Mayr, 1887: 570 (in key), 575. Lectotype worker (designated by Brown, 1961: 68), BRAZIL: Santa Catarina [examined].

Strumigenys unispinulosa Emery, 1890: 67, pl. 7, fig. 5. Syntype workers and queen, COSTA RICA: Alajuela [not seen].

Strumigenys unispinulosa var. longicornis Emery, 1894a: 214. Holotype queen, BOLIVIA: Cantoni di Coroico, Chilumani Yungas (Balzan) (MCSN) [not seen].

Strumigenys fusca Emery, 1894a: 215, pl. 1, fig. 8. Holotype worker, BRAZIL: Amazonas, Manicore (MCSN) [not seen].

Strumigenys louisianae var. obscuriventris W.M. Wheeler, 1908: 145, pl. 12, fig. 14. Syntype workers, PUERTO RICO: Utuado and Coamo Springs (W.M. Wheeler) [examined].

Strumigenys bruchi Forel, 1912b: 197. Syntype workers and males, ARGENTINA: La Plata (Bruch) [examined].

Strumigenys eggersi var. cubaensis Mann, 1920: 430. Syntype workers, CUBA: Cienfuegos, 1917 (W.M. Mann) (AMNH, ) [examined].

Strumigenys louisianae subsp. laticephala M.R. Smith, 1931: 690, pl. 1, fig. 2. Syntype workers, U.S.A.: Mississippi, Longview (M. R. Smith) (USNM) [examined].

Strumigenys (Strumigenys) louisianae subsp. soledadensis Weber, 1934a: 38. Syntype workers and queen, CUBA: Soledad, Cienfuegos, Harvard Bot. Gdns, 11, 15 and 29.viii. 1933 (N.A. Weber) (AMNH, USNM) [examined].

Strumigenys (Strumigenys) louisianae subsp. guatemalensis Weber, 1934a: 39. Syntype workers, GUATEMALA: Zacapa, 13.xii.1911 (W.M. Wheeler) (, AMNH) [not seen].

Strumigenys (Strumigenys) louisianae subsp. costaricensis Weber, 1934a: 39. Syntype workers, COSTA RICA: Cartago, 15.xii.1911 (W.M. Wheeler) (MCZ, AMNH) [examined].

Strumigenys ciasmospongia Brown, 1953d: 2. Holotype and paratype workers, BRAZIL: Rio de Janeiro, Petropolis, viii.1922, nr. 5884 (T. Borgmeier) (, MCZ, USNM) [examined].

Strumigenys producta Brown, 1953d: 3. Holotype and paratype workers, BOLIVIA: Beni, Rurrenabaque, x.1921, Mulford BioI. Exp. (W.M. Mann) (USNM, MCZ, MZSP) [examined].