As many of you know, I work on phorid flies, including those known as “ant decapitating flies”, genus Apocephalus. These flies dart down at their hosts, laying an egg in the host body (usually the head). The egg hatches, and the resulting larva feeds inside the ant’s head capsule, consuming about all of the contents of the head. Now, the inside of ants head is not like yours or mine, full of brains; instead, they have huge muscles inside their head capsules to move their massive mandibles (mouthparts). The insect brain is an insignificant little shred of material compared to ours. Anyway, the larva feeds, and eventually the ants head falls off, sometimes before the rest of the body finishes walking around – hence the name “ant decapitating fly.”

There are almost 300 described species of these flies, but we think there are perhaps 200 or 300 more. Therefore, we’re down here collecting, and trying to understand some life history information. We were able to solve one little mystery on this trip.

A couple of years ago, we saw workers of a species of the ant genus Pheidole being attacked by an Apocephalus species, in which the flies seemed to be darting at ant larvae, rather than the adults. We saw the same thing here, and studied it in a little more detail. We even got some video of the flies 

We cut open an old dead log and found a nest of Pheidole. In short order, we had dozens of flies attracted to this exposed colony. We saw the flies darting at the ants when they were carrying larvae, as in our previous observation. We saw, however, that the flies were actually attacking the ants, and not the larvae.

The ants seem to be quite aware of the presence of the flies, and keep their mandibles in the air apparently trying to snap and grab the flies. The ants are highly agile and fast, so this seems to be a formidable defense. Apparently, the flies wait until the ants pick up a larva or pupa before attacking them, as their defense is neutralized when carrying a burden. This seems to me an amazing behavioral adaptation, but it might be commonplace, given our poor understanding of ant/parasitoid interactions.