I am sure this beetle must derive some benefit from having fat thighs but it is not immediately obvious to me what that might be. It is a feature of the males only so it may be some advantage when mating? There are various common names for this insect, including the swollen-thighed beetle, the knobbly-kneed beetle, the false-oil beetle and the thick-legged flower beetle but my insect field guide gives only its scientific name, Oedemera nobilis.
It is common insect from May through until August on all sorts of pollen laden flowers but it is particularly noticeable on emerging bramble flowers. Despite its long antennae it is not a long horned beetle, it is more closely related to wood boring beetles but it does not bore holes.
This is a very common species in the Mediterranean region and has gradually spread north and since 1995 it has been expanding its range in Britain and growing in numbers. It is not a pest and so is a welcome addition to our fauna.