Like most flies, members of the genus Pseudacteon lay their eggs inside a food source to give their growing maggots plenty to eat. But this particular group of flies doesn’t go looking for rotting meat, overripe fruit, or dung to deposit their little darlings. These flies go looking for live ants. And when they find them, the results aren’t pretty.
There are 110 species of Pseudacteon in South America; all of them are parasitoids of ants, which means that although the adult flies aren’t parasitic, their larvae are. Like parasitic wasps, the flies lay their eggs inside living hosts (in this case, ants), which serve as both larder and incubator for the growing larvae. The ultimate effect on the ants is clear from Pseudacteon’s common name: the ant-decapitating fly.
When a gravid female fly spots ants out in the open, she launches an immediate aerial attack on the foraging workers, dive-bombing them to stab them with her hypodermic ovipositor. Each time she succeeds, she lays a single egg inside an ant’s thorax. When the egg hatches, the maggot crawls into the ant’s head and spends two weeks eating ant hemolymph, muscle, and brains. Eventually, the maggot has hollowed out the ant’s head, and it falls off its body and becomes a pupation chamber for the fly. After another couple of weeks, the adult fly emerges and leaves the dead ant’s head through its mouth.
One of the genera of ants vulnerable to fly attack is Solenopsis, commonly known as fire ants. And that little fact has transformed these tiny flies from a gory bit of natural trivia into potential biological control agents. States from Texas to Florida are looking for weapons to aim at the fire ants that were accidentally introduced there, and ant-decapitating flies have become part of their arsenal. The flies don’t actually make a big dent in a fire ant colony through parasitism, but when female flies start dive-bombing the colony, the worker ants hide in their nest and stop foraging. Recent research has shown that the presence of ant-decapitating flies cuts fire ant foraging in half, and less food in the colony means fewer fire ants overall.
Porter, S. D. et al. 1995. Growth and development of Pseudacteon phorid fly maggots (Diptera: Phoridae) in the heads of Solenopsis fire ant workers (Hymenoptera: Formicidae). Environ. entomol. 24(2): 475-479.
Photo by Dr. Larry Gilbert.