The Bee Fly lxys its eggs on the larvae of a Solitary Bee. When it hatches it eats the bee larvae.
Goerge Allen & myself saw one in my garden, then on my finger. If I had realised what a rarity it was I would have photographed it before helping it out of the greehouse. I took the camera out of my pocket with my other hand, altered the zoom setting to macro, only to witness the Bee Fly fly away.