These small defensive spines can be found on many species of Pogonomyrmex ants, and on many other types of ants as well. These structures are also referred to as epinotal spines - because the part of the body they are attached to, the propodeum, is alternatively called the epinotum - the terms are interchangeable, but the former is preferred [1]. The terms proprodeum, and propodeal spines will be used on this site.

As can be seen in the image, the ant's narrow, wasp-like 'waist', the petiole (attached to the propodeum by the very slender stalk-like peduncle), is located just behind the propodeal spines. This is a particularly vulnerable part of the ant's anatomy, and is easily 'snipped' by the mandibles of an attacking ant [2] - resulting in certain death for the victim. The spines can afford some measure of protection, especially, it seems, if the petiole is rotated forward - pressing the attacker's head into the two sharp points, or by preventing any access at all to the petiole by the aggressor.

The length, thickness, and how sharply pointed the spines are, varies greatly between the different species (and even within a single species), as is the case with Pogonomyrmex salinus [3].
[IMAGE: P. salinus worker in central Oregon - white arrow indicates the propodeal spines, red arrow points to the petiole, and the green arrow shows the peduncle] [scroll down for additional notes/references]

ADDITIONAL NOTES/REFERENCES:

[1]
·Fisher, B.L. & S.P. Cover. 2007. Ants of North America: A Guide to the Genera. Berkeley and Los Angeles, CA, University of California Press
·Hölldobler, B. & E.O. Wilson.1990. The Ants. Cambridge, MA, Belknap/Harvard Press

[2]
·Taber, S.W. 1998. The World of the Harvester Ants. College Station, TX, Texas A&M University Press

[3]
·Cole, A.C. 1968. Pogonomyrmex Harvester Ants: A Study of the Genus in North America. Knoxville, University of Tennessee Press
·Shattuck, S.O. 1987. An Analysis of Geographic Variation in the Pogonomyrmex occidentalis Complex (Hymenoptera: Formicidae) Psyche Vol. 94
·Taber, S.W., J.C. Cokendorfer & O.C. Franke.1987. Scanning Electron Microscopic Study of North American Pogonomyrmex (Hymenopter:Formicidae). Proceedings of the Entomological Society of Washington. 89(3), 1987, pp. 512-526