A team of scientists working in tropical forests in Brazil has observed a previously unknown type of behavior in tiny, humble-looking flies, and it’s pretty darn disgusting. Several different species within a particular group of phorid flies were found to exhibit “headhunting,” in which females slowly chopped the heads off injured ants, which were significantly larger than the beheaders, using their sharp mouthparts. Talk about adding insult to injury. The fly then drags the head off to a secluded dining area and either gobbles down its delicious contents, or lays an egg next to it so it can feed her hungry offspring. The grisly observations are described in Biodiversity Data Journal.

The Phoridae are a huge and incredibly diverse family of tiny (0.4-6.0 mm), hump-backed flies that look a bit like fruit flies. More than 4,000 species of phorid flies have so far been described, belonging to 230 genera. Although many members are poorly described, some species are well-known for the bizarre, and rather disgusting, behaviors they exhibit. Coffin flies, for example, are so named because they are remarkably talented at getting into sealed places containing decaying matter, including human coffins, and will then use the corpse as a breeding site and nursery ground.

Another truly horrible group of phorid flies is the “ant-decapitating flies.” No prizes for guessing why. But rather than going around chopping ants heads off, these flies inject their eggs into the host’s body, which then hatch and feed inside the ant’s head. Eventually, the poor ant’s head falls off. It’s also thought that the flies could be releasing an enzyme to dissolve the attachment between the head and the rest of the body, thus assisting in the decapitation process. Yummy.

Now, scientists have discovered another related group of phorid flies that also like to decapitate flies, but they do it in a different way. Adult females from the Dohrniphoria genus, which are native to South and Central America, start out the gruesome process by picking on an injured trap-jaw ant. “Ants are nasty, belligerent creatures that often fight each other to the death and leave the injured to die,” said lead author Brian Brown. “These flies have found a small niche to exploit that’s been highly productive.”

The researchers aren’t too sure how these flies become attracted to the crippled ants, but it’s probably due to pheromones wafting from the wounded warrior. After spending several minutes “apparently assessing the degree of incapacitation of the ants,” the fly climbs on the ant and begins the beheading. The fly uses its sharp mouthparts to hack away at the gut, nerve cord and other connections of the head to the rest of the body. After sawing away for several minutes, the fly tugs the ant’s head off and drags it to a secluded spot, up to several meters away, where it will then either dine on its contents or lay an egg nearby so that the larvae can eventually feed on it.

Since the majority of females inspected by the team were egg-free, the researchers think that the head could be helping fuel egg development.

[Via Biodiversity Data Journal, Pensoft Publishers and Live Science]