Many flies, especially Muscid flies, are affected by this Entomophthora fungus. Spores from the fungus attach themselves to adult flies, work their way into the body, and kill them from within. Afflicted flies climb to the tops of plants or tips of leaves, spread their wings and stay there. The fly sticks its proboscis into the plant to enable it to hang on and there it stays until dead. This one has been dead awhile because you can see the fungal spores that have come out of the flies body and landed on the leaf and along its legs. This is a severely abbreviated explanation! A bit gruesome.