Eciton burchellii parvispinum Forel 1899

Ecitoninae, Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia

major face view

major lateral view

major petiole, lateral view

male face view

male lateral view

Additional images: minor worker, lateral view (small, large).

Range

Species: throughout the Neotropics. Subspecies: southern Mexico to Panama, type locality Costa Rica. Costa Rica: all of Pacific slope, sea level to montane, dropping down a short distance onto Atlantic slope.

Identification

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: burchellii foreli.

Natural History

The name E. burchellii is frequently encountered in ecological and behavioral studies of army ants. Members of this species are the most intensively studied army ants, and most of the studies elucidating the cycle of nomadic and statary phases have been done with E. burchellii. In Costa Rica this is the only species of the genus Eciton whose raids are in the form of large carpet-like swarms of ants. Other species of Eciton always hunt in files. However, some species of Labidus also hunt in carpet-like swarms, and so may be confused in the field. Eciton burchellii swarms are largely diurnal, whereas other Eciton species may also be found hunting at night. Eciton burchellii swarms take a broad range of prey types, including other social Hymenoptera (other ants, in particular), Dictyoptera (cockroaches and mantids), spiders, scorpions, and Orthoptera. Some arthropods they ignore, such as heavily sclerotized beetles, spiny or hairy caterpillars, and long-legged phalangids (which seem able to stand above a raid as workers pass beneath them).

There are numerous subspecies of E. burchellii. In Costa Rica there are two, E. b. foreli and E. b. 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.

The distributions of foreli and 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 Peľas 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.

Comments

Eciton burchellii or burchelli? The species was originally published as burchellii. Taxonomists later (later being the 1800's) decided that the double-i was unecessary. But recently the taxonomic rules have become more strict about sticking to the form of the original publication. In his 1995 catalog Bolton was particularly careful about checking names in the original publications. Eciton burchellii it is.


Page author:

John T. Longino, The Evergreen State College, Olympia WA 98505 USA.longinoj@evergreen.edu


Date of this version: 30 June 2007.
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