This is a new one for me - like a lot of people I didn't even know such things existed.
Last month I was surveying a site in Surrey, just outside of Greater London, and had found a nice south facing slope with a thermophilic invertebrate community present. This included mining bees Andrena sp., plus the cleptoparasitic nomad bees Nomada sp. and bee-flies Bombylius sp., and the wasp Dolichovespula media. Also present were the ants Formica fusca, so I spent a bit of time on my knees seeing what other species were present (only Lasius niger s. str. and Myrmica scabrinodis as it turned out).
Whilst I was on the ground a solitary bee flew by, followed closely by this fairly nondescript looking fly. The bee stopped, and the fly stopped a few centimetres behind. The bee flew a little further and stopped, and the fly did likewise. It slowly dawned on me that the fly was actually stalking the bee!
The bee started to move down amongst some grass whilst the fly waited around, so I took the opportunity to take the photographs below. Okay, so they will win no prizes, but they were the only two I could take before I got too close and scared the fly away.
It's taken me a while to work out what this thing was, but I'm now pretty certain it was a satellite fly Leucophora sp. Like Nomada and Bombylius, Leucophora are cleptoparasitic, laying their eggs in the burrows of the bees they hunt. According to the Society for the study of flies there are only eight species of Leucophora in the UK, none of which have been commonly recorded if their maps are any judge (though these are likely to be very incomplete).
However, other than this I've been unable to find out any information. Remarkably, none of the British species of Leucophora have conservation status, which implies that they were missed or ignored the last time these flies were reviewed, as some of the eight species must be rare. Perhaps too little was known about them to make an accurate judgement of their status. Honestly, if it hadn't been behaving so unusually I'd have ignored this individual.
If anyone else has any useful information on these beasts I'd be very interested to hear, even if it is to say that they are actually really common!