This P.salinus worker was found dead approx. 4 m / 10 ft from a nest in central WA. It is likely that this ant succumbed to the heat while foraging, and was unable to make it back home (the ventrally 'curled up' posture is typical of ants when they die). The everted sting (or 'stinger') is clearly visible, shown protruding from the tip of the gaster (the terminal body part of the ant). Normally (for most Pogos) the sting is not visible, unless in use.
Only females (workers and reproductives) can sting, as the sting apparatus is a modified ovipositor, that males do not possess.
Typically, a stinging Pogonomyrmex ant will hold tightly to the victim with its mandibles while applying the sting, and injecting venom. Pogo venoms contain a wide variety of proteins, enzymes, histamines, hemolytic agents, and other chemicals.
A Pogo's venom is often much less effective against other insects/arthropods than it is against vertebrates, and seems to have evolved primarily as a defense against larger mammalian intruders (such as rodents looking to plunder the ants' seed-cache) [1]. This quality makes Pogo stings particularly painful, and sometimes dangerous for humans - in fact, Pogo venoms are some of the most toxic known in the insect world [2]. Species such as Pogonomyrmex rugosus and P. occidentalis will sting viscously with little provocation.
Recently (Schmidt & Snelling 2009), work has been done on the venom of Pogonomyrmex anzensis, an unusual species known only from a few locations in Anza-Borrego Desert State Park, in San Diego County, CA. These ants (unlike many other Pogo species) do not defend themselves or their colonies aggressively - and when they do sting a human, the effects are very mild. It was found that while P. anzenis' venom is extremely lethal to mammals, the individual workers produce very little of it. The resulting sting is less than half as potent as that of a Africanized honey bee. Interestingly, the venom reservoirs of P. anzensis can hold quantities of venom much greater than the ants actually produce. In a harsh environment where their nests are not likely to be plundered by mammals, this reduced venom production is likely an energy and resource saving adaptation [3]. [scroll down for additional notes/references]
ADDITIONAL NOTES/REFERENCES:
[1]
In addition to the fact that Pogo venoms often have very little effect on insects, the workers of some Pogo species have barbs on the stings, so that the sting detaches from the ant and remains inserted in the victim - sometimes continuing to pump venom (sting autotomy, as in the honeybees), and resulting in the ant's death. This type of defensive tactic is effective against large (often single, or few in number) vertebrate intruders, but not in colony defense against raiding insects (such as enemy ants).
·Herman, H.R. 1971. Sting Autotomy, a Defensive Mechanism in Certain Social Hymenoptera. Insectes Sociaux Vol. 18, No.2 , June 1971.
·Holldobler, B. & E.O. Wilson.1990. The Ants. Cambridge, MA, Belknap/Harvard Press
·Taber, S.W. 1998. The World of the Harvester Ants. College Station, TX, Texas A&M University Press
[2]
A venom's toxicity is usually measured by its LD50 value (a lower LD50 value indicates a potentially more toxic/lethal venom). The LD50 for Pogonomyrmex maricopa is .12 mg / kg - making this venom lethal enough to kill a mouse with a few stings, or a 2 kg / 4.4 lb rat with just 12 stings. P.maricopa is usually cited as having the most powerful venom in the insect world. Other Pogo species do not have venoms quite as potent as P.maricopa's, but even an average Pogo venom is much more toxic than that of a yellowjacket (Vespula squamosa) or a paper wasp (Polistes canadensis). The LD50 values for the above mentioned insects are 3.5 mg / kg, and 2.4 mg / kg, respectively, and in one study, an average value for 10 Pogo species was determined to be .66 mg / kg.The amount of venom injected by a Pogo with each sting is so small, however, that most humans and other large animals are not in danger of dying from a Pogo attack (though it is an extremely painful deterrent). Human deaths occasionally do result - but this is usually due to an anaphylactic (severe allergic) reaction to being stung.
Note: As of 4/10 I have been stung twice by P.salinus workers. In both cases I was stung on the forearm. First, there was an intense pain at the sting site, followed by a mild numbness throughout the arm (this lasted a few hours). A small red welt at the actual sting site remained for a day or two, and then disappeared completely. In both cases, the ants appeared to have retained their stings - they were not autotomized and left at the sting site. -D.L. Quinn
·Klotz, J.H., Schmidt, J.O., Pinnas, J.L., & Klotz, S.A. 2005. Consequences of Harvester Ant Incursion into Urbanized Areas: A Case History of Sting Anaphylaxis. Socialbiology Vol. 45, No. 3, 2005
·Meyer, W.L. 1996. University of Florida Book of Insect Records, Chapter 23, Most Toxic Insect Venom. Dept. of Entomology & Nematology University of Florida, Gainesville, FL 32611, May 1996
·Taber, S.W. 1998. The World of the Harvester Ants. College Station, TX, Texas A&M University Press
[3]
·Schmidt, J.O. & G.C. Snelling. 2009. Pogonomyrmex anzensis Cole: Does an Unusual Harvester Ant Species Have Unusual Venom? Journal of Hymenoptera Research, Vol. 18(2), 2009, pp. 322-325