This is a fungus - Entomophthora sp. that attacks blue bottle flies. It's rather like the Gibellula fungus that preys on spiders.
Unlike spiders, flies are gregarious and the fungus could maximise its chances of infecting others by arranging for its host to die in a gathering spot. I found four of these within a meter of each other.
Previously I had only ever seen this fungus in close proximity with stink horns, leading me to google things like "do stink horns kill flies?" with no success.










