Slow Death from the Skies: Phorids and Ants (part 1)

In Argentina, an ant-decapitating fly (Pseudacteon sp.) attempts to separate a fire ant (Solenopsis sp.) from her nestmates:

More photos- and the story behind them- below.

An ant burdened with prey is the easiest target of all:
The ants aren't defenseless. The classic "run-and-hide" works well enough:

What's the deal with the dreaded Ant-Decapitating Flies? The University of Texas Fire Ant Project explains:

"Female phorid flies are attracted to fire ants swarming over a disturbed mound or foraging along a trail to food. They hover over ants looking for a preferred individual. (Each phorid species has a particular size range of fire ant workers which it prefers.) When the hapless victim is chosen, the phorid darts in, injects an egg into the ant's body, and explodes away at warp speed. The attack takes a fraction of a second and leaves the ant partly paralyzed and disoriented for a minute or so before she staggers off to join her sisters!

"The injected egg develops in the ant's thorax until after about ten days the ant dies as the larva moves into the ant's head. The head falls off and the larva eventually pupates in the safety of the hard chitin shell that once housed the ant's jaw muscles and brain. Ant pieces are tossed on ant trash piles or middens and adult flies emerge from pupae about 45 days after the original attack. That's the direct effect of mortality that these decapitating flies impose on ants.

"The other thing that phorids 'do' to ants is probably the most significant from the standpoint of biocontrol. As phorids fly above ants looking for victims, the ants respond by hiding, pilling on top of one another, retreating into the nest, and posturing in various odd ways. This fly harassment disrupts the economy of provisioning the nest with food and protecting home and territory."

While I'd like to say I carefully planned the photo session- for images with commercial potential like these I often do- these shots were the result of a chance encounter, and one that only happened as a bureaucratic quirk.

Jo-anne and I arrived one sunny morning at Parque Nacional El Palmar to gather population genetic samples of a local ant, Linepithema micans, only to find that our research group's collection permits had expired in January. The renewal would take at least a day.

What to do while we waited? Photography, of course. Lemonade from the lemons of an aborted work day.

I didn't have any particular objective as we headed out into the park's trail system, but not fifteen minutes later we noticed several of the little flies terrorizing a stretch of a fire ant foraging column. Perfect! An hour later I'd snapped about 80 exposures to get half a dozen usable shots.

Technical details:

Lens: Canon MP-E 1-5x macro lens, at about 2x
Body: Canon EOS 20D
Flash: Canon MT-24EX twin-flash, diffused through tracing paper
Settings: ISO 100, f/13, 1/250 sec

The top two photos are cropped in slightly (about 15% and 25%, respectively), the last one is uncropped, and all have undergone slight levels adjustments in photoshop.

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Great photos, Alex! I suppose it was the flash that "froze" the action so well. Tracing paper as diffuser? How long did you have to wait to get these shots?

Hi Alex, great blog here. I used to be quite a fan of ants during my teen years. I had my own ant farms. Fascinating creatures they are. I am now getting into the world of macro photography and came across your blog though links. I will be reading this one regularly. Cheers.

Thanks guys. You're right, Larry, that the flash was a key ingredient for these shots. Ambient light would have left the insects a blur. At about 1/250 sec the insects are frozen but there's still plenty of blur in the wings to suggest motion.

Excellent photos! You really captured the action! It's like the flies are in the Matrix...Sorry. Just watched the Matrix; it's on my brain.


I'd read about decapitating flies but always assumed, from the description of these flies as "tiny," that they were very small relative to the ants too. That always made me wonder how an ant would recognize the thing to be a threat. Now I understand a bit better. I'd be freaked, too, to have something almost the size of my head bearing down on me like that! Glad to be enlightened--and I hope the ants under the leaf escaped.

Very cool.

You obviously have a talent for this sort of photography.

Keep up the good work.

If you've ever met a fire ant in any of the southern US states, you'll understand why the University of Texas is interested in a tiny Argentine fly -- and why the many of us who know fire ants have no sympathy at all for whatever unpleasant things happen to that nasty little critter.

Hi Alex:

Stunning photos! BTW, I'm pretty sure based both on the locality and the appearance of the beast, that the fire ant in question is Solenopsis macdonaghi.

By James C. Trager (not verified) on 14 Apr 2009 #permalink

The fly in the second photo with the ant carrying something looks like it is Pseudacteon solenopsidis. Nice photo please call if you want more details about the biology (352 374-5914)

By Sanford Porter (not verified) on 17 Apr 2009 #permalink

Hi Alex:

Stunning photos! BTW, I'm pretty sure based both on the locality and the appearance of the beast, that the fire ant in question is Solenopsis macdonaghi.

thanks.. alex