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@JoePablo
They have really complex reproductive behaviors. You can sometimes see the females, like the one in the great photo from @Jehannum , bobbing up and down over sandy or dusty soil dipping their abdomen in the dust. They then fly low over the ground looking for the nest holes of solitary bees and wasps. When they find one they do a low pass over the nest hole and flick their dust-covered eggs into the hole. If they are successful, their larvae hatches, eats the larvae of the solitary bee or wasp and then eats all the provisions (nectar/pollen) that the adult bee or wasp had left for it's own larvae. The Bee Fly larvae then pupates and the new adult emerges next spring to start the cycle over again. The Bee Flies do all this timed to the nesting of their hosts, i.e. now.
There are a couple of species in the UK but only the Dark-edged Bee Fly (Bombylius major) is common. Having said that, it's worth keeping an eye out for the Dotted Bee Fly (Bombylius discolor) (dots on the wings and not the black bar, usually a rich red brown 'fur' and pale cream at the wing bases) if you are in southern UK - I've seen a couple this year so far.
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