Fighting ants and Freeloading flies

Harvester ants fight while a freeloader fly awaits the spoils of war.

I snapped this picture of fighting harvester ants in Arizona, and it wasn’t until I could see the images under the camera’s magnification that I was able to notice a few cool details.  First, please note that this is not in fact, two harvester ants fighting, but rather two and a half ants fighting.  These ladies are tenacious in battle.  Second, as I glanced through the series of pictures of this encounter I realized that one of the ants had a passenger.  (Or, perhaps, was showing off her lovely living hat.)  Paul Lenhart clued me in to the really interesting bit of biology playing out here.

Harvester ant tug-of-war, with freeloader fly looking on.

The fly riding out the fight atop one of the ants’ heads belongs to the family Milichiidae, the freeloader flies.  These very small flies are best known for the members of the family which make their living as commensals or kleptoparasites of predatory insects.  The adult flies hang out near predators, such as spiders or assassin bugs, and sometimes even ride along with them (attendance) awaiting the opportunity to sip up fluids exuded by wounded prey.

Freeloader fly rides out a fierce ant tussle.

Much like mosquitoes and blood-feeding, in many species only the female exhibits this parasitic behavior, presumably because she needs a high protein diet to lay her eggs.  Some myrmecophilous species (ant-loving) have developed such a specialized relationship they even directly solicit ants for food via regurgitation or have larvae tended by worker ants in the nest.  In some cases this interaction takes the form of a “mugging.”  The flies pursue an ant, and, if they can successfully grab hold of the head by gripping the end of the antennae, the ant will then freeze and the fly can extend its proboscis into the ant’s mouth and trigger the regurgitation reflex (Wild & Brake, 2009).  As always, Alex’s pictures are impressive, so if you can’t access the paper figures definitely check the subset in his gallery.

• See also: Gruesome leafcutter ant battle (video)

If you give an ant a cookie…

Pogonomyrmex and Aphaenogaster ants wrestle for a cookie crumb.

At the Southwest Ant Course last summer, I got the chance to participate in some cool biodiversity assays of the local ants, comparing assemblages before and after a flash flood.  Among other things, we pit-trapped, counted nest entrances, and scoured the ground for wandering ants.  To encourage ant activity, quadrants were sprinkled with cookie crumbs.  We were using crumbled up pecan sandies because, and I quote, “those are ants’ favorite cookies.”

I can’t speak for any given ant (I, personally, prefer Thin Mints), but the desert ants of Arizona certainly did seem to like pecan sandies.  I’m also going to go out on a limb and speculate that these cookies are useful as assay tools for their combination of sugars, proteins, and lipids in order to attract ants with a broad variety of diets.

Two different species of desert ants continue to wrestle for a cookie crumb.

Pogonomyrmex (harvester ants) and Aphaenogaster (BugGuide calls these the “spine-waisted ants”) were by far the dominant species in the area.  The ants pictured above wrestling over a cookie crumb were in between two nests, and each was determined to get the prize back to her own colony.  I watched the ants play tug-of-war for several minutes, sometimes even picking up the cookie crumb with the other ant attached (as in the photo above).  Finally, the Aphaenogaster was joined by two of her nestmates, and the out-numbered pogo almost immediately surrendered the prize and fled the scene.  A rousing victory for Aphaenogaster-kind, who I assume went home to celebrate with cookies and tea.

Aphaenogaster ants team up to wrestle a cookie crumb away from a lone Pogonomyrmex ant.

FYI, I believe these were Aphaenogaster albisetosa and Pogonomyrmex maricopa but don’t quote me on that.

Spiders raiding ant nest

Spiders slip inside a sleeping harvester ant nest to prey on the worker ants.

Last week I had the pleasure of attending the new Ants of the Southwest course at the Southwest Research Station in Arizona, and I thought I’d jump right and share with you my favorite entomological encounter.  While black lighting in the desert we happened across a harvester ant nest that was being raided by dozens of small hungry spiders.  The spiders would slip inside the nest entrance individually and emerge some time later, dragging worker ant prey.   It was pretty impressive to watch, particularly as there was no evidence that the spiders’ raiding was provoking any kind of response from the colony.  Harvester ants (or “pogos,” from Pogonomyrmex) are a group of ants with a fairly vicious sting, and the workers dwarfed their tiny spider predators.

I’ve included more pictures as well as a video below, with bonus excited chattering commentary.

A spider drags its harvester ant prey from the sleeping nest.

I don’t have an ID for the spider as of yet.  Several group of spiders are known to mimic ants, either to help them obtain prey or to help them avoid predators.  If these spiders don’t look like ants to you, it’s possible they may smell enough like ants to fool the colony.  Cosmophasis jumping spiders use this scent-disguise tactic to enter weaver ant nests and prey on workers and larvae.  Meanwhile certain spiders in the genus Masoncus take this a step further and live only inside harvester ant nests, where they prey on another nest symbiote, collembolans.

A spider hangs from a grass blade, feeding on a harvester ant many times bigger than itself.

Update:

Thanks for the feedback here and over at Bugguide.  These spiders appear likely to be members of the family Theridiidae and the genus Euryopis.  Many members of this genus appear to be specialist predators of ants.  I poked around in the literature and there are a couple of papers about the species E. coki, a specialist predator of another species of harvester ants, Pogonomyrmex owyheei.  (I believe the harvester ants pictured above were P. maricopa.)  There were a number of similarities to the behavior I observed.  E. coki was observed to lurk outside nest entrances and ambush ant workers, first tacking down a leg with silk and then biting the ant.  When bushes or grasses were nearby the spiders employed a dangling feeding behavior (Porter and Eastman 1982).  Dale Ward has a great shot of a male Euryopis mating with a female spider near the nest entrance of P. rugosus.  Tetramorium’s Ants of Arizona page also has some great shots and info.  I haven’t been able to find any literature on thespiders actually entering the ant nests, though.