Mayflies resembled a spindly Earth mayfly, hence the name. They started life as eggs which would be implanted inside a host. They needed an extremely healthy host to survive, which meant that they would produce a chemical which would cure every known disease. Once the host was cured, the larvae would start eating each other until only one was left. If the host died in the meantime, the larvae could leave the body and find a new host. Eventually, they would reach a length of about a foot and try to leave the body. The adults could reach several metres long.
The Pharm eventually found a Mayfly and was using it and its offspring to develop an ultimate cure, "Reset". Already dying as a result of the Pharm's experiments, it was euthanised by Torchwood Three with the closing of the facility. (TV: Reset)