Decapitated Ant Fly 2015 01 08
A female ant-decapitating fly (Dohrniphora longirostrata). Image: Brown et al, 2015.

There are more than four thousand species of Phorid flies, tiny critters that at their biggest measure just six millimetres in length. They may be tiny, but they’re a terrifying family, with members whose names sound more like crappy horror movies than actual insects: coffin flies, bee-killing flies ... and ant-decapitating flies.

Ant-decapitating flies have been known for some time, but researchers from the Natural History Museum of Los Angeles have discovered an entirely new kind of ant decapitation, previously unknown to science. 

In most instances, the tiny flies lay their eggs inside the heads of dead or dying ants. Then, once the larvae break out of their eggs, they eat the tiny ant brains, causing the head to fall off. Thus the ants become decapitated from the inside out. In some species, the larvae also release an enzyme that serves to sever the attachments between the ant's head and body, speeding up the decapitation process. In all these cases, it’s the larvae that are responsible for the decapitation.

But for three types of flies in the genus Dohrniphora, entomologist Brian Brown and his colleagues discovered a whole new mechanism of ant decapitation (almost always large trap-jaw ants like those in the genus Odontomachus).

Here’s how it works. The flies, which usually arrive while they’re having sex, see an injured ant (in this case, the researchers intentionally injured the ants to lure the flies). If they see a suitable ant, the pair lands and the female is left behind while the male takes off. The first thing the female does is determine just how injured the ant is. It does this by drumming on the leaves surrounding the ant, sometimes by moving in to poke it and occasionally by yanking on its legs or antennae. If the target is too feisty, the fly moves on in search of one more suitably incapacitated.

That’s a good tactic, because healthy trap-jaw ants can make quick work of the flies, gobbling them up with tremendous haste. (That’s something the researchers verified by trapping the flies with healthy ants.) 

Watch the fly circle the ant, occasionally poking at it, to determine just how injured it really is:

Once the flies decide the ants are injured enough, they "[climb] on the ant body, and [begin] to probe with their mouthparts". They move their heads in a back-and-forth manner, as if sawing the ant’s body in half, as well as employing rotational head movements, jumbling up the ant’s insides. The goal is to sever the ant’s digestive tract and its nervous system. Sometimes, though, a fly might decide she needs a bit of privacy for her mouthpart probing; on such occasions, the authors noted that the flies would haul the entire ant host away, "apparently to deal with it in a more secluded location".

Fly Proboscis 2015 01 08
The tip of a female's proboscis … Eli Roth could not have come up with a more sinister-looking torture device. Image: Brown et al, 2015.

A close look at the female's proboscis reveals how she is able to make such a mess of the ants. The proboscis, which is nearly as long as the rest of the fly’s body, is basically a trident, with a spiky poke in the centre and two serrated blades on the sides.

Eventually, all that sawing makes the ant’s head loose enough for the female to yank off. The whole process can occur in as little as eight minutes, but that’s only if she’s left alone to do her work in peace, something that rarely occurs. 

Watch the ant just left of centre. The fly works to saw its head off, ultimately succeeding in decapitating it:

Once the head has been severed, she drags it up to several metres away to feast on what the researchers refer to as the "head capsule contents".

This is the only case known to science where the female does the decapitation, rather than her larvae. Still, the researchers assumed that what the flies were after was a suitable place to deposit eggs. But here's the puzzling part: the flies didn’t have any mature eggs inside their ovaries to deposit in the first place! Brown and his colleagues suspect that the nutrition the female gains from eating is somehow necessary for the development or maturation of her eggs.

Brown also discovered that the flies are uniquely attracted to injured trap-jaw ants, a tremendously narrow specialisation. They actually ignored all the injured grasshoppers, katydids and termites that the researchers offered them. That means there must be enough injured trap-jaw ants in the Central and South American tropics that an entire suite of fly species evolved to rely on them as sustenance for themselves and for their larvae.

What is even more surprising is that Brown, an expert on Phorid flies, has been studying them in Central and South America for 30 years. If this is something he and his colleagues have only just noticed, what other secrets are lurking in the leaf litter of the neotropical rainforests?

Top header image: Brown et al, 2015.