I came across this fly attached to the underside of some bramble that I had just harvested for the children's stick insects. On closer inspection I found it to be unresponsive and attached to the leaf by what looks like a fungus. Certain parasites modify host behaviour to their advantage and this occurs across many insect orders, notably in the ant species Camponotus leonardi by the fungus Ophiocordyceps unilateralis. [Andersen SB et al. The Life of a Dead Ant: The Expression of an Adaptive Extended Phenotype. Am Nat 2009; 174: 424-433]
Has this fly been compelled to rest in the optimum position for spore dispersal just prior to death? Is this a new phenomenon?
My initial reaction was Entomophthora muscae, but as I had never seen Calliphora in the list of hosts for that species I did some further searching and found two Entomophthoraceae on calliphorids: Entomophthora calliphorae and Furia vomitoriae (names used according to http://www.speciesfungorum.org/). I don't know which of the two this is, but as far as I know the species infested by this type of fungi indeed adopt a behaviour that will increase the likelyhood that it can spread to a new host.