Spilled coffee grounds? was my first reaction when I caught this brownish residue out of the corner of my eye one morning, walking out to feed the chickens. It didn't take long to realize the error—this was life on the move. We had one of these irruptions* inside the house late last summer, possibly the same species, in a breezeway newly converted to civilization.
* An irruption is a sudden explosion in population, or sometimes a shift in presence in a given area, so this is the wrong word, but I've yet to think of a better one. The event last fall was a bit different, a true irruption of sorts, as winged ants were attempting to strike out to colonize new areas.
It wasn't long before I realized my error—this wasn't moving out, this was spring cleaning! (You needn't be surprised that I didn't recognize the phenomenon; it's rare around here!) I did manage (being close to the house) to press the Colorado quarter into duty as a scale object this time, but you'll have to click to see subtle activity along the edge of the paver in this photo.
My guess was the ants had chosen this fine day to remove fallen comrades from the nest. A long winter had, no doubt, induced a fair complement of colony mortality, and "ceremonies," however perfunctory, were in order.
This behavior is called necrophoric (from roots for dead and carrying) and is, like most events in the life of an ant or an entire colony, mediated by chemicals. If you smell like a dead ant, you are presumed to be a corpse and can expect to be treated accordingly. In fact, according to the researchers, "It was soon established that bits of paper treated with acetone extracts of Pogonomyrmex [ harvester ant] corpses were treated just like intact corpses" by worker ants. Separation of components of the extract later revealed (again through "behavioral assay," that is the workers' response to test chemicals) that long-chain fatty acids, in particular oleic acid, were the critical substances.
According to Hölldobler and Wilson:
"The transport of dead nestmates is one of the most conspicuous and stereotyped patterns of behavior exhibited by ants. ... Thus the worker ants appear to recognize corpses on the basis of a limited array of chemical breakdown products. They are, moreover, very "narrow-minded" on the subject. Almost any object possessing an otherwise inoffensive odor is treated as a corpse when daubed with oleic acid."
Even live worker ants were carried to the refuse pile "unprotesting" after being treated with oleic acid. "After being deposited, they clean themselves and return to the nest." And the penalty for inadequate cleaning is... you guessed it! Another trip to the refuse pile. A whole new concept of the living dead. Perhaps I should have saved this post for Halloween.
Sure enough, the next morning, the pavers were littered with broken, immobile ant bodies. I failed to capture the "after" shot before morning breezes blew the departed insects away.
[Forgive my delay. The actual date of this event was April 11, 2009.]