Messor pergandei worker in Arizona, USA.

With a range including portions of Arizona, southern California, southern Nevada, and parts of Mexico, these black harvester ants are often found in very hot, dry (xeric) habitats.

M. pergandei workers can be said to be 'primitively polymorphic', or more correctly 'monaphasically allometric' with the largest workers reaching a length of about 8-9mm, and the smallest ones about 1/2 that size. In M. pergandei, there does not seem to be a strong correlation between worker size and specialized tasks within the colony (as is the case with 'true polymorphism'). Rather, the differently sized workers are the results of annual fluctuations in resource availability, and other factors.

Colonies of this species are often founded by multiple queens (pleometrosis), and can be enormous - containing up to 50,000 workers in total, with 30,000 of these acting as foragers.

M. pergandei is a granivore, and subsists mainly on a variety of seeds and other plant parts - though they are known to occasionally scavenge dead arthropods as well.

The huge foraging columns extending from the nest can contain many thousands of ants, but the workers will also forage individually in times of seed/resource scarcity. A column 40m/131ft in length was once observed, and was estimated to contain 17,000 ants. Over time, the direction of the foraging column is rotated around the nest (like the hand of a clock), so the ants are continually encountering and exploiting fresh patches of seeds. With the column direction changing as often as every day, a complete 'rotation' might take 2 - 3 weeks. Quantities of seeds are also stored within the nest, to be used when needed. When few seeds are available, M. pergandei workers will harvest less desirable plant parts.

Like some other desert dwelling ants, Messor pergadei workers posses psammophores. These basket-like structures (on the underside of the head) are composed of special setae (hairs), and aid in the transport of excavated sand/soil.

M. pergandei nests usually take the form of a low sand or gravel crater - often ringed with discarded plant material (chaff).

[IMAGE: M. pergandei worker in southern Arizona USA [scroll down for additional note/references]

References for entire page :

·Cole, A.C. 1966. Ants of the Nevada Test Site. Brigham Young University Science Bulletin, Biological Series - Vol. VII, Number 3, June 1966
·Deneubourg, J.L. & S Goss. 1989. The Self-Organising Clock Pattern of Messor Pergandei (Formicidae, Myrmicinae). Insectes Sociaux, Paris, 1989, Volume 36, No. 4, pp. 339-346
·Hölldobler, B. & E.O. Wilson.1990. The Ants. Cambridge, MA, Belknap/Harvard Press
·Johnson, R.A. 2000. Seed Harvesting Ants (Hymenoptera: Formicidae) of North America: An Overview of Ecology and Biogeography. Sociobiology Vol.36, No. 1, 2000
·Rissing, S.W. 1986. Annual Cycles in Worker Size of the Seed Harvesting Ant Veromessor pergandei (Hymenoptera: Formicidae). Behavioral Ecology and Sociobiology (1987) 20:117-124