Myopa - a Thick-Headed Fly

Flies in the genus Myopa, also known as Thick-headed Flies, have larvae which are endoparasites of certain bees and wasps. Typically, the adult flies wait on flowers and then "hijack" passing hosts, grappling with them and then inserting a single egg into their bodies. The developing Myopa larva consumes the host bee/wasp from the inside eventually killing it. Parasites that kill their hosts are known as parasitoids.

I found this example sitting on a dandelion (Taraxacum officinale) flower in the local churchyard. I have seen one other, which was beside a colony of the Mining Bee Andrena nigroaenea. These strange-looking flies have a head that is "thicker" than it's body - hence the name.