Stubbs & Drake's British Soldierflies & Their Allies' excellent identification guide. The relatively rare Bombylius discolor is centre-left, and the very common Bombylius major is centre-right.
For years I have examined hundreds of the common Bombylius major Bee-flies in Bedfordshire hoping to come across Bombylius discolor....but to no avail. Here on the Isle of Wight there have been large numbers of Bombylius major Bee-flies holding territory and buzzing all over Golden Hill Country Park in the sunshine over the past week...but no Bombylius discolor, though I have been told that the latter are fairly common on the Island.
Then today, at Walter's Copse, Newtown, I came across this during a brief patch of warmer weather:
I spent the next few minutes observing the fly nectaring on Primrose, it's amazingly long proboscis reaching right to the base of the petal tube as this video shows:
The long mouth-parts mean that it's not receiving any pollen in exchange, so the flower is probably being cheated! (There were also Platycheirus albimanus hoverflies feeding at the flowers - they would have been unable to reach the nectar but would be specifically after the pollen!).
What I will be looking out for now is the amazing sight of the female Bee-flies flicking their eggs onto the ground where the larvae will crawl into solitary bee holes and parasitize the larvae. These species of Bee-fly even have a sand-chamber at the base of their abdomen which they use to coat the eggs....no doubt giving them a bit more weight for launching and, maybe, to give the eggs a bit of camouflage and protection before hatching out!
Here's a bit more video to enjoy:
Notice that white stripe running down the centre of the backside! When the bees are in flight I reckon that might be an easier way for distinguishing the two species rather than trying to work out if the wings have spots which I have been seeking to do up to now.