While watching Andrena nigroaenea mining bees at the Spetchells the other day I noticed this strange looking insect flying around rather furtively and taking an interest in the bees' nest holes. I guessed it was a parasite of some kind and thought it might be a parasitic wasp. It turns out to be a cuckoo bee, a cleptoparasite of mining bees. It is Nomada goodeniana - many thanks to Phil for help with identification. This is the female. She has no scopa (pollen collecting hairs on the hind leg) of course because she has no need to collect pollen.
Cuckoo bees parasitise bees in the way cuckoos do birds. The female nomada bee sneaks into the nest while the female mining bee is away foraging and lays her own egg. Either she or her larva eats the mining bee egg and the deception is complete. Fortunately there were many hundred mining bees and only a few cuckoos. I guess it would have to be that way - there have to be plenty of hosts for the cuckoo to try to take advantage of.