About 40 days after the egg is deposited, the phorid fly larva develops into a pupa that more close resembles an adult phorid fly (shown in a file photo).

Although the flies only kill a small fraction of ants this way, the ants seem to be so afraid of the flies that their mere presence prevents the ants from collecting food, said Donald Feener, an ant ecologist at the University of Utah.

Such interruptions slowly reduce the rate at which new fire ant colonies form.