Eciton burchellii parvispinum

Identification
Jack Longino:

Minor worker: head, mesosoma, and metasoma black; occipital tooth small but present; petiolar teeth in the form of short nearly right-angle flanges, either extending onto posterior face of propodeum as separate short carinae, or joined horizontally and forming scoop-like structure, with no descending carina; petiole relatively short and hump-shaped, with no anterodorsal elevated flange; fourth abdominal tergite with short, sparse appressed pubescence beneath erect setae.

Major worker: head and variable extent of mesosoma yellow red, grading to darker red brown to black metasoma; face densely micropunctate, matte; long sickle-shaped mandibles simple, without tooth on inner margin; other characters as in minor.

Similar species: Eciton burchellii foreli.

There are numerous subspecies of Eciton burchellii. In Costa Rica there are two, Eciton burchellii foreli and Eciton burchellii parvispinum. The workers of foreli have a red brown metasoma, such that minor workers appear bicolored in the field, while the workers of parvispinum have a black metasoma, and the minor workers are entirely black. I can find no other morphological or behavioral feature that correlates with the color difference. The color forms could easily rest on a very small genetic difference, perhaps a single gene. However, the males also exhibit a difference. Males from the Atlantic slope, corresponding to the range of foreli, have a number of long flexuous setae on the scutellum; males from the Pacific slope and the range of parvispinum have the scutellum bare. In most cases this character difference is discrete, but I collected one male that was intermediate. Among a series of males from Monteverde, which is close to the zone of contact of the two forms, one had long setae on one half of the scutellum and the other half was bare (differing across a sagittal plane). All the rest of the males and all of the workers I have seen from Monteverde have the parvispinum phenotype.

Distribution
Jack Longino: The distributions of Eciton burchellii foreli and Eciton burchellii parvispinum are very sharply parapatric and do not seem to correlate with particular habitats or thermal environments. Eciton b. parvispinum has the broadest distribution, occurring across all of the Pacific slope, from the dry forests of Guanacaste to the rainforest of the Osa Peninsula, from sea level up into the mountains, and dropping a short distance down onto the Atlantic slope, where it meets foreli. For example, on the Barva Transect of Braulio Carrillo National Park, foreli is common from La Selva up to 500m elevation. Above that elevation Eciton burchellii has lower density, but I have three collections from 1400-1500m elevation, about 10km further up-slope, and they are parvispinum. In the Peñas Blancas Valley, east of Monteverde, only foreli occurs at Refugio Eladio, at 800m elevation. At Refugio El Aleman, 950m elevation and 5km further up the valley, I have only collected parvispinum. West of El Aleman, continuing up to Monteverde, only parvispinum is found. Like other examples of step-clines in morphological or genetic characters, it begs the question of what mechanism is maintaining the sharp boundary between these two forms, especially for such large, nomadic organisms.

Distribution based on Regional Taxon Lists
Neotropical Region: Belize, Costa Rica, Guatemala, Honduras, Mexico, Nicaragua, Panama.

Biology
Baudier et al. (2015) studied thermal tolerances of a variety of army ant workers. Eciton burchellii parvispinum was the single above ground army ant species sampled.

Castes

 * Worker

Nomenclature

 *  parvispinum. Eciton burchelli var. parvispina Forel, 1899c: 23 (w.) GUATEMALA. Currently subspecies of burchellii: Borgmeier, 1953: 8. Senior synonym of infumatum: Borgmeier, 1955: 178.
 * infumatum. Eciton burchelli var. infumatum Wheeler, W.M. 1909b: 231 (s.w.) MEXICO. [Misspelled as infuscatum by Santschi, 1925d: 222.] Junior synonym of parvispinum: Borgmeier, 1955: 178.