Created by accident in the genetic engineering laboratories of Vault 38.
Multiplies by stinging a host and injecting eggs. The host then has several Devil fly larvae growing within, which at some point takes control of his body via a nerve-link to the spinal column. The Devil flies have shown intelligence by forcing a host to move to a strategic position before release the new batch of flies.
There was some evidence to the effect that Devil flies retain part of memories and personality of the person within whom they grew as larvae, and a similar transfer may have been possible from fly larva to host as well.
Since the vault’s destruction none have been seen.